Boston Regional
Nanomagnetism Workshop
One-day Workshop on Magnetism at the Nanoscale
Sponsored by:
IEEE Magnetics Society and Northeastern University
Location and Date:
Northeastern University
Boston, MA.
April 4th, 2014
Logo design: Ian McDonald & Nina Bordeaux
Agenda
AGENDA
Boston Regional Nanomagnetism Workshop
Date: April 4th, 2014
Location: Room 378, 140 The Fenway Building
Northeastern University, Boston, MA.
Reception – Coffee – Poster set up (9:15 AM – 10:00 AM)
Morning Session (10:00 AM – 12:30 PM)
Time
Topic
Speaker
Affiliation
10:00 – 10:10
Greetings
Prof. Laura H. Lewis
Chem. E., NEU
Focus: Nanomagnetism from Perspective of Fundamental Science [Moderator: Dr. Radhika Barua]
10:12 – 10:24
Inducing Ferromagnetism in Topological Insulators Using
Heterostructures of TI and Magnetic Insulator
Dr. Ferhat Katmis
Physics, MIT
10:24 – 10:36
Domain Walls Driven by Interfacial Phenomena: Pushing the
Boundaries of Magnetics
Dr. Satoru Emori
ECE, NEU
10:36 – 10:48
Exchange Bias of Ion Beam Etched NiFe/IrMn Nanostructures
Mr. Frank Liu
MSE, MIT
10:48 – 11:00
SnTe-EuS-SnTe Trilayers Grown by MBE on Si: A Materials
Characterization Report.
Mr. Badih Assaf
Physics, NEU
Break (11:00 AM – 11:20 AM)
11:24 – 11:36
Quantum Anomalous Hall Effect in the Magnetic Topological
Insulator Films
Dr. Chang Cui-Zu
Physics, MIT
11:36 – 11:48
Square Array of BiFeO3-CoFe2O4 Mutiferroic Nanocomposite
Templated by Triblock Terpolymer
Dr. Hong Kyoon
Choi
MSE, MIT
11:48 – 12:00
Magneto-Ionic Control of Interfacial Magnetism
Mr. Uwe Bauer
MSE, MIT
12:00 – 12:12
Correlation of Electronic Structure and Magnetic Properties in
Fe-doped Titania Nanotubes
Dr. Pegah M.
Hosseinpour
Chem. E., NEU
Lunch and Poster Session (12:15 PM – 2:00 PM)
A-1
Maximizing Hysteretic Losses in Magnetic Ferrite Nanoparticles via ModelDriven Synthesis and Materials Optimization
Mr. Ritchie Chen
MSE, MIT
A-2
Tailoring AlFe2B2 Magnetism-Structure Relationships for Magnetocaloric
Applications
Mr. Brian Lejune
Chem. E., NEU
B-1
Templated Self-assembly of Perovskite/Spinel Nanocomposite Thin Films
Mr. Nicholas Aimon
MSE, MIT
B-2
Towards Exchange-Biased Permanent Magnets: Structure-Property
Correlations in FeMn Alloys
Mr. Ian McDonald
Chem. E., NEU
C-1
Interlayer Coupling in Ultra-Thin L10-FePt/MgO/[Co/Pd]30 Magnetic Tunnel
Junctions
Dr. Pin Ho
MSE, MIT
C-2
Half-Metallic Antiferromagnets: A New Class of Materials for Spintronic
Devices
Ms. Michelle Jamer
Physics, NEU
D-1
Large-area Periodic Magnetic Microstructures for Controlling Magnetic
Micro-particles
Ms. Minae Ouk
MSE, MIT
i
Agenda
D-2
Understanding L10-phase Formation in the FeNi System through the Study
of FePd(Ni) Compounds
Ms. Ana MontesArango
Chem. E., NEU
E-1
Nanoscale-driven Crystal Growth of Hexaferrite Architecture for
Magnetoelectrically Tunable Microwave Semiconductor Integrated Devices
Ms. Bolin Hu
ECE, NEU
E-2
Sub-100 nm Magnetic Wires with Low Edge Roughness
Ms. Saima Siddiqui
MSE, MIT
F-1
Thermomagnetic Behavior of L10 FeNi (Tetrataenite) from Meteorites
Ms. Nina Bordeaux
Chem. E., NEU
F-2
Formation and Current Effects on 360° Domain Walls in Magnetic
Nanowires
Dr. Larysa Tryputen
MSE, MIT
G-1
Transofmation Kinetics in Epitaxial FeRh Thin Films
Dr. Melissa Loving
Chem. E., NEU
G-2
Dzyaloshinskii-Moriya Interaction Influence on Stochastic Spin Orbit Torque
Switching
Mr. Seong-Hoon
Woo
MSE, MIT
H-1
Detection of Field and Current Effects on 360˚ Domain Walls by Anisotropic
Magnetoresistance Measurements
Mr. Jinshuo Zhang
MSE, MIT
H-2
Resonant modes of coupled magnetic nanodisks
Mr. Maximilian
Albert
U. of
Southampton
Afternoon Session (2:00 PM – 3:00 PM)
Focus: Nanomagnetism: Towards Magnetic Devices [Moderator: Dr. Pegah M. Hosseinpour]
Time
Topic
Speaker
Affiliation
2:00 – 2:12
Tailoring the FeRh Magnetostructural Response: Simultaneous
Effects of Pressure and Magnetic Field
Dr. Radhika Barua
Chem. E., NEU
2:12 – 2:24
Magnetothermal Multiplexing
Mr. Michael
Christiansen
MSE, MIT
2:24 – 2:36
Quantification of Strain and Charge Co-Mediated
Magnetoelectric Coupling on Ultra-thin Permalloy/PMN-PT
Interface
Mr. Tianxiang Nan
ECE, NEU
2:36 – 2:48
Nanoscale Magnetic Materials for Energy-Efficient Spin-Based
Transistors and Logic
Ms. Jean Currivan
MSE, MIT;
Physics, Harvard
2:48 – 3:00
Fabrication of Magnetically Hard Cobalt Carbide Nanoparticles
via Wet Chemical Synthesis
Dr. Mehdi
Zamanpour
Interdiscip. Eng.,
NEU
3:00 – 3:12
Integration of Self-Assembled Nanocomposite on Si substrate
Dr. Dong Hun Kim
MSE, MIT
Break (3:15 PM – 3:30 PM)
Keynote Speaker, IEEE Distinguished Lecture [Moderator: Professor Laura H. Lewis]
3:30 to 4:30
Topological Effects in Nanomagnetism:
From Perpendicular Recording to Monopoles
Prof. Hans-Benjamin Braun
End of the Workshop
ii
University College Dublin,
Ireland
Greetings from the Organizers
Greetings from the Organizers of the
Boston Regional Nanomagnetism Workshop
“When you engage with people, you build your own insight into what’s being discussed.
Someone else’s understanding complements yours, and together you start to weave an
informed interpretation. You tinker until you can move on.”
- Marcia Conner, Author of the book, “The New Social Learning”
In keeping with the spirit of this quotation, Northeastern University is pleased to
host the first Boston Regional Nanomagnetism Workshop (RNW) on April 4th 2014.
The primary objective of this forum is to facilitate exchange of ideas and active
collaborations between research groups working in the field of fundamental and
applied magnetism in the Greater Boston area. The Workshop features a series of
oral talks and poster presentations by graduate students and post-doctoral research
associates from local universities namely Northeastern University, Harvard
University and Massachusetts Institute of Technology (MIT). The key-note speech for
the event will be given by the IEEE Magnetics Society 2014 Distinguished Lecturer,
Prof. Hans-Benjamin Braun, University College Dublin (UCD), Ireland.
Abstracts of the talks and posters are available in this booklet and online at:
http://www.northeastern.edu/nanomagnetism. To enable long term interaction
between the attendees of the Workshop, contact information of the authors of the
abstracts is provided in this document as well. We hope that you have a wonderful
time networking with your peers today!
Sincerely,
Radhika Barua, Ph. D.
Pegah M. Hosseinpour, Ph. D.
Co-Organizers, Boston Regional Nanomagnetism Workshop
iii
Keynote Speaker
Keynote Speaker
Professor Hans-Benjamin Braun
School of Physics, University College Dublin, Ireland.
(IEEE Magnetics Society 2014 Distinguished Lecturer)
Biography: Professor Hans-Benjamin Braun is currently Associate
Professor for Theoretical Physics at University College Dublin
(Ireland). After studies in Physics and Mathematics he received
his diploma degree from the University of Basel (Switzerland) and in 1991 he earned his PhD in
Theoretical Physics at ETH in Zurich. After postdoctoral research at the Physics Department and
the Center for Magnetic Recording Research at the University of California at San Diego he was
awarded a NSERC International Fellowship to work at Simon Fraser University in Vancouver
(Canada). Subsequently he returned to Switzerland to take up a position as Senior Scientist at
the Paul Scherrer Institute (PSI). He joined the Faculty of the School of Physics at University
College Dublin (UCD) in 2004, where he founded and leads the group in Condensed Matter
Theory supported by the Science Foundation of Ireland. Professor Braun developed the theory
for non-uniform thermally activated magnetization reversal in nanowires which now forms the
basis for the design of perpendicular magnetic recording media. Well before it was recognized
experimentally, he theoretically predicted quasi one-dimensional behavior in magnetic
nanowires and he introduced the now widely used notion of domain wall chirality. His work led
to the prediction of the spontaneous emergence of spin currents in quantum spin chains, an
effect that he and his collaborators subsequently observed via spin polarized neutron
scattering.
Furthermore he proposed and interpreted a series of experiments on
nanolithographic arrays that led to the discovery of emergent monopoles in artificial spin ice
together with colleagues from PSI and UCD. In addition to numerous publications in top
research journals he also authored popular articles for the French and German versions of
Scientific American and he holds two patents.
1
Keynote Speaker
Keynote Presentation
Topological Effects in Nanomagnetism: From Perpendicular Recording to Monopoles
Prof. Hans-Benjamin Braun, University College Dublin (School of Physics), Ireland
Similar to knots in a rope, the magnetization in a material can form particularly robust
configurations. Such topologically stable structures include domain walls, vortices and
skyrmions which are not just attractive candidates for future data storage applications
but are also of fundamental importance to current memory technology. For example,
the creation of domain wall pairs of opposite chirality delimits the thermal stability of
bits in present high anisotropy perpendicular recording media. The ever increasing
demand for higher data storage density forces us to understand topological defects at
ever decreasing length scales where thermal and quantum effects play an increasingly
important role.
This talk will be adapted to the interests of the audience and will start with an
overview over topological defects in magnetic systems. As a practical application it is
shown how thermal domain wall nucleation affects the design of perpendicular
magnetic recording media. In a second part, it is demonstrated how the geometric
Berry’s phase allows micromagnetics to be extended to include quantum effects. As an
important consequence it will be shown how the chirality of a classical domain wall
translates into quantum spin currents which in turn can be used for information
transport. All concepts will be illustrated by state of the art experiments, which
encompass the techniques of polarized neutrons and synchrotron x-rays. The final part
of the talk will discuss how magnetic monopoles emerge as topological defects in
densely packed arrays of nanoislands which effectively interact as dipoles, a system also
known as ‘artificial spin ice’. In contrast to conventional thin films, where magnetization
reversal occurs via nucleation and extensive domain growth, magnetization reversal in
2D artificial spin ice is restricted to an avalanche-type formation of 1D strings. These
objects constitute classical versions of Dirac strings that feed magnetic flux into the
emergent magnetic monopoles. It is demonstrated how the motion of these magnetic
charges can be individually controlled experimentally and used to perform simple logic
operations.
References:
[1] H.B. Braun, "Topological effects in nanomagnetism: from superparamagnetism to
chiral quantum solitons", Adv. Phys. 61, 1–116 (2012).
[2] E. Mengotti, L.J. Heyderman, A. Fraile Rodriguez, F. Nolting, R.V. H Hügli and H.B.
Braun, "Real space observation of Dirac strings and magnetic monopoles in artificial
kagome spin ice” Nature Physics 7, 68–74 (2011).
2
Oral Presentations
(Morning Session)
Oral Presentations (Morning Session)
Inducing Ferromagnetism in Topological Insulators using Heterostructures of TI and
Magnetic Insulator
F. Katmis1,2, V. Lauter3, B. A. Assaf4, P. Wei2, D. Heiman4, and J. S. Moodera1,2
1
Department of Physics, MIT, Cambridge, MA, USA
Francis Bitter Magnet Laboratory, MIT, Cambridge, MA, USA
3
NSSD, Oak Ridge National Laboratory, Oak Ridge, TN, USA
4
Department of Physics, Northeastern University, Boston, MA, USA
2
The short-range nature of magnetic proximity coupling with a ferromagnetic insulator
(FI) induces ferromagnetic interactions on a topological insulator (TI) surface state. In
the present study we investigate Bi2Se3/EuS (bi-layer) heterostructures and elucidate
the mechanism to induce the ferromagnetic order onto the surface of Bi2Se3 thin films
by using EuS as a FI layer. SQUID measurements demonstrated a magnetic moment that
is excessive for the EuS film alone, thus indicating that EuS induces a significant
magnetic moment on the surface of the Bi2Se3 film. Polarized neutron reflectometry
(PNR) allows for a direct measurement of the magnetization depth profile in the bi-layer
films. Using PNR we reveal that EuS induces a significant magnetic moment in TI films.
These results indicate that the interface region of TI becomes magnetized. Thus, it
creates broken time-reversal symmetry and should appear as a magnetic signature in
electrical transport. Both the ferromagnetism of EuS and coupling between EuS and
Bi2Se3 has to be strong to induce surface magnetism. These findings contribute to
emergent quantum coherent phenomena; the local time-reversal symmetry breaking is
essential for observing predicted new effects such as quantized topological
magnetoelectric response.
4
Oral Presentations (Morning Session)
Domain Walls Driven by Interfacial Phenomena: Pushing the Boundaries of Magnetics
Satoru Emori1,2, and Geoffrey S. D. Beach1
1
Massachusetts Institute of Technology, Cambridge, USA
2
Northeastern University, Boston, USA
A domain wall in a ferromagnetic material is a boundary between differently
magnetized regions, and its motion provides a convenient scheme to control the
magnetization state of the material. Domain walls can be confined and moved along
nanostrips of magnetic thin films, which are proposed platforms for next generations of
solid-state magnetic memory-storage and logic devices. In these devices, domain walls
must be moved by electric current, rather than by magnetic field, to achieve scalability
and lower-power operation. Recent studies have reported efficient domain-wall motion
driven by current in out-of-plane magnetized multilayer films with strong spin-orbit
coupling. In particular, extraordinary current-driven domain-wall motion has been
observed in atomically-thin ferromagnets sandwiched between a nonmagnetic heavy
metal and an insulator.
Through experimental studies on various sputtered magnetic multilayers, we
elucidate the mechanism of such anomalous domain-wall dynamics driven by the spin
Hall effect: a charge current in the nonmagnetic heavy metal generates a spin current,
which exerts a torque on spins in the adjacent ferromagnet. This spin Hall torque drives
domain walls forward if the domain-wall spins are parallel to the nanostrip axis with a
fixed chirality. We reveal that the Dzyaloshinskii-Moriya interaction, arising from spinorbit coupling and asymmetric interfaces, stabilizes homochiral domain walls in ultrathin
ferromagnets. Our findings not only provide a route to bolster current-driven domainwall dynamics, but also enable new chiral magnetic textures in magnetic
heterostructures for device applications.
5
Oral Presentations (Morning Session)
Exchange bias of ion beam etched NiFe/IrMn nanostructures
F. Liu and C.A. Ross
Massachusetts Institute of Technology, USA
Exchange bias between ferromagnet/antiferromagnet (FM/AFM) layers creates a bias
field that is promising for locally pinning the magnetic orientation of ferromagnetic
layers in nanodevices. However, in structures where part of the ferromagnetic layer is
pinned and another part is unpinned, such as a magnetic wire with exchange pinning at
the ends, it is necessary to develop a fabrication process that provides adequate local
pinning. One such method is to deposit the bilayer stack then selectively remove the
AFM material by ion beam etching. However, to minimize the amount of etching and
protect the unbiased parts of the structure, an alternative method will be described in
which the FM is deposited and patterned, then a second lithography step is done to
open windows where the AFM is required. A short etch followed by optionally
deposition of a thin FM then an AFM layer completes the process. This presentation
compares results from these processing methods. Fig. 1 shows the effect on the
hysteresis loops of ion beam etching of a NiFe 10 nm/IrMn 20 nm bilayer. 10 nm NiFe
was deposited using DC magnetron sputtering, then ion beam etched for varying
amounts of time before 20 nm IrMn was deposited. Thickness, coercivity, and exchange
bias were obtained using vibrating sample magnetometry. Results from patterned
nanostructures will be presented including the effect of feature size on exchange bias
and coercivity.
Figure. A comparison between hysteresis loops of 120 nm diameter dots and that of
continuous films under different periods of ion beam etching time.
6
Oral Presentations (Morning Session)
SnTe-EuS-SnTe Trilayers Grown by MBE on Si: A Materials Characterization Report
B. A. Assaf1, F. Katmis2, 3, P. Wei2, B. Satpati4, J. S. Moodera2, 3, D. Heiman1
1
Physics. Dept. Northeastern University, Boston, MA 02115, USA
2
Francis Bitter Magnet Lab, MIT, Cambridge, MA 02139, USA
3
Physics. Dept. MIT, Cambridge, MA 02139, USA
4
Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 64, India
The realization of functional devices, combining a topological material with a
ferromagnet is of capital importance, as it allows one to probe and manipulate
topological surface states without altering the bulk [1,2]. We thus report the growth and
characterization of heterostructures consisting of a 2nm-thick ferromagnetic insulator –
EuS – buried between two 10nm-thick films of topological crystalline insulator (TCI)
SnTe. Trilayers are grown by MBE on Si. X-ray diffraction, atomic force microscopy,
cross-sectional transmission electron microscopy and X-ray absorption spectroscopy are
used to characterize the structural properties of the films. SQUID magnetometry and
transport measurements are used to investigate the magnetic properties of the trilayers
and the possible appearance of proximity-induced magnetism onto the surface states of
SnTe. Our results show that the magnetic properties of EuS are not dramatically altered
and the EuS layer remains strongly ferromagnetic. A change in the lattice constant of
EuS is observed along with a slight reduction of the moment per Eu2+. These changes
could either be a result of strain or Te diffusion into EuS. It is interesting that proximityinduced magnetism is only observed in the trilayer having the largest interface
roughness. These results provide the basis for future studies on the behavior of TCI
surface states under the effect of proximity-induced magnetism.
Work supported by NSF-DMR-0907007 and partly by NSF-DMR-1207469, ONR-N0001413-1-0301, and MIT MRSEC under NSF-DMR-0819762.
References:
[1] P. Wei, et al. Phys. Rev. Lett. 110, 186807 (2013).
[2] C.H. Li, et al. Nature Nanotech. 16, 218 (2014).
7
Oral Presentations (Morning Session)
Quantum Anomalous Hall Effect in the Magnetic Topological Insulator Films
Cui-Zu Chang
Francis Bitter Magnetic Lab, MIT, Cambridge, U.S.A (czchang@mit.edu)
Anomalous Hall effect (AHE) was discovered by Edwin Herbert Hall in 1880. In this talk,
we report the experimental observation of the quantized version of AHE, the quantum
anomalous Hall effect (QAHE) in thin films of Cr-doped (Bi0.1Sb0.9)2Te3 magnetic
topological insulator. At zero magnetic field, the gate-tuned anomalous Hall resistance
exhibits a quantized value of h/e2 accompanied by a significant drop of the longitudinal
resistance. The longitudinal resistance vanishes under a strong magnetic field whereas
the Hall resistance remains at the quantized value. The experimental realization of the
QAHE paves a way for developing low-power-consumption electronics.
References:
[1] Cui-Zu Chang et al. Adv. Mater. 25, 1065 (2013).
[2] Cui-Zu Chang et al. Science 340, 167 (2013)
8
Oral Presentations (Morning Session)
Square Array of BiFeO3-CoFe2O4 Mutiferroic Nanocomposite Templated by Triblock
Terpolymer
Hong Kyoon Choi, Nicolas Aimon, Dong Hun Kim, Xue Yin Sun, Caroline Ross
Department of Materials Science and Engineering, Massachusetts Institute of
Technology, Cambridge, MA, USA
BiFeO3/CoFe2O4 (BFO/CFO) composite grown on SrTiO3 (STO) substrate forms selfassembled columnar nanostructure. This BFO/CFO nanocomposite shows
magnetoelectric coupling induced from epitaxial strain along to the vertical interface. In
order to apply this multiferroic columnar nanostructure for devices application,
controlling the location of pillars in ordered array is desirable. In this work, we present
an effective process to fabricate square array BFO/CFO nanocomposite by using ABC
linear triblock terpolymer as a template. As described in Figure 1, square array of pits on
Nb:STO substrate were generated by pattern transfer form PI-b-PS-b-PFS triblock
terpolymer. CFO nuclei and thin BFO film were then selectively grown in pit and mesa
respectively by taking advantage of different wetting behavior of etch phases. Finally
thick BFO/CFO nanocomposite was deposited from guidance of thin BFO/CFO layer.
Compare to previously reported top down patterning methods, this patterning process
based on block copolymer can provide smaller periodicity of square symmetry over
large area with short process time.
Figure. (a) Schematic illustration of the fabrication process of a templated BFO/CFO
nanocomposite. (b) Self-assembled square symmetry of holes from PI-b-PS-b-PFS triblock
terpolymer. (c) Square array of pits patterned on Nb:STO substrate. (d) CFO nuclei selectively
grown in patterned pits. (e) Thin layer of BFO/CFO nanocomposite. (f) Thick BFO/CFO
nanocomposite grown from thin layer of BFO/CFO nanocomposite. Scale bars are 300nm.
9
Oral Presentations (Morning Session)
Magneto-Ionic Control of Interfacial Magnetism
U. Bauer, S. Emori and G.S.D Beach
Massachusetts Institute of Technology, Cambridge, USA
Voltage control of magnetism could bring about revolutionary new spintronic memory
and logic devices. Here, we examine domain wall (DW) dynamics in ultrathin Co films
and nanowires under the influence of a voltage applied across a gadolinium oxide gate
dielectric that simultaneously acts as an oxygen ion conductor. We investigate two
electrode configurations, one with a continuous gate dielectric and the other with a
patterned gate dielectric which exhibits an open oxide edge right underneath the
electrode perimeter. We demonstrate that the open oxide edge acts as a fast diffusion
path for oxygen ions and allows voltage-induced switching of magnetic anisotropy at the
nanoscale by modulating interfacial chemistry rather than charge density. At room
temperature this effect is limited to the vicinity of the open oxide edge, but at a
temperature of 100˚C it allows complete control over magnetic anisotropy across the
whole electrode area, due to higher oxygen ion mobility at elevated temperature. We
then harness this novel ‘magneto-ionic’ effect to create unprecedentedly strong
voltage-induced anisotropy modifications of 5000 fJ/Vm and create electrically
programmable DW traps with pinning strengths of 650 Oe, enough to bring to a
standstill DWs travelling at speeds of at least 20 m/s.
10
Oral Presentations (Morning Session)
Correlation of Electronic Structure and Magnetic Properties in Fe-doped Titania
Nanotubes
Pegah M. Hosseinpour, Félix Jiménez-Villacorta and Laura H. Lewis
Department of Chemical Engineering, Northeastern University, Boston, MA
Iron-doped titania nanotube arrays are proper candidates for applications such as
photocatalytic and potential spintronic devices. Considering the effect of crystallinity,
magnetic properties and electronic structure on the functionality of titania-based
nanotubes, understanding the influence of the crystal structure and magnetic impurities
on the electronic structure of titania as a function of dopant composition and processing
conditions is of paramount importance. In this work, arrays of iron-incorporated titania
nanotubes are electrochemically synthesized followed by annealing at 450 °C in oxygen
to crystalize. The crystal structure, magnetic properties and electronic structure of these
nanotubes in the as-anodized and annealed states are characterized using x-ray
diffraction, SQUID magnetometry and x-ray absorption spectroscopy (XAS). Results
show that annealing the nanotubes yields crystallization into the anatase structure, and
that Fe-doped nanotubes have a slightly larger unit cell volume as compared to the pure
nanotubes. In addition, higher amount of near surface and bulk anatase formation and
larger crystallite size is observed in the Fe-doped nanotubes. Iron in the doped
nanotubes is in the ionized state and mostly with a local structure resembling that of the
α-Fe2O3. Furthermore, the magnetic moment of the Fe-doped nanotubes, larger than
that of the pure nanotubes, is suggested to have its origin in the diluted iron in form of
Fe3+. The attained information on the crystallography, electronic structure and magnetic
characteristics of the Fe-doped titania nanotubes results in a change in the functional
properties of these materials and highlights the path toward modification of these
structures for energy-related applications.
This work if funded by the National Science Foundation NSF (Grant No. DMR-0906608).
11
Poster Presentations
Poster Presentations
Maximizing Hysteretic Losses in Magnetic Ferrite Nanoparticles via Model-Driven
Synthesis and Materials Optimization
R. Chen, M. Christiansen, and P. Anikeeva
Massachusetts Institute of Technology, Cambridge, MA
Using magnetic and structural nanoparticle characterization, we identify key synthetic
parameters in the thermal decomposition of organometallic precursors that yield
optimized magnetic nanoparticles over a wide range of sizes and compositions. The
developed synthetic procedures allow for gram-scale production of magnetic
nanoparticles stable in physiological buffer for several months. Our magnetic
nanoparticles display some of the highest heat dissipation rates, which are in qualitative
agreement with the trends predicted by a dynamic hysteresis model of coherent
magnetization reversal in single domain magnetic particles. By combining physical
simulations with robust scalable synthesis and materials characterization techniques,
our work provides a pathway to a model-driven design of magnetic nanoparticles
tailored to a variety of biomedical applications ranging from cancer hyperthermia to
remote control of gene expression.
13
Poster Presentations
Designing ‘Cool’ Magnetic Materials for Efficient Refrigeration:
Tailoring AlFe2B2 Magnetism-Structure Relationships for Magnetocaloric Applications
Brian Lejeune, Radhika Barua, and L. H. Lewis
Department of Chemical Engineering, Northeastern University, Boston, United States
The magnetocaloric effect (MCE) can be utilized for magnetic refrigeration to achieve
higher efficiencies than compression cooling cycles potentially reducing society’s energy
usage.1,2 The MCE is the reversible adiabatic temperature change of a magnetic material
upon the application or removal of a magnetic field.3 In AlFe2B2 it exceeds all other
intermetallic borides and is comprised of lightweight, inexpensive earth-abundant
elements in contrast to the prototypical MCE material Gd, providing motivation for this
research.4 The intermetallic compound AlFe2B2 serves as a test bed to provide insight
into the phase transition driving the MCE behavior of transition-metal borides. Synthesis
was achieved via arc melting elements in a 3 Al:2 Fe:2 B stoichiometric ratio. Attainment
of the layered orthorhombic crystal structure (a= 2.923Å b= 11.038Å c= 2.871Å) was
confirmed using Cu Kα x-ray diffraction. AlFe2B2 possesses a saturation magnetization
value of ~37.2 emu/g and exhibits a ferromagnetic to paramagnetic phase transition at a
Curie temperature (TC )~ 300 K.1,3 The presence of a 10 K thermal hysteresis (∆Tt) upon
heating and cooling through the magnetic phase transition confirms it is
thermodynamically first-order in nature. A 5 K/T magnetic field-dependence (dTC/dH )
shifts TC to higher values upon increasing the applied magnetic field. It is anticipated
that correlation of: crystal structure, microstructure, magnetism, and thermal behavior
will facilitate a better understanding of the magnetic phase transition underlying the
MCE response of the AlFe2B2 system, allowing for tuning the TC and the associated MCE.
References:
[1] ElMassalami et al. J Magn Magn Mater 323, 2133-2136 (2011).
[2] Gschneidner, K., Pecharsky, V. Physical review letters 12, 1-19 (1997).
[3] Tan et al. Journal of the American Chemical Society 135, 9553-9557 (2012).
[4] Franco, V. Annual review of materials research 42, 305-342 (2012).
[5] Lewis, L. Jiménez-Villacorta, F. Metall. Mater. Trans. A. 44, 2-20 (2013).
14
Poster Presentations
Templated Self-assembly of Perovskite/Spinel Nanocomposite Thin Films
Nicolas M. Aimon, Hong Kyoon Choi, XueYin Sun, Dong Hun Kim, Caroline A. Ross
Massachussetts Institute of Technology, Cambridge, MA, USA
Vertically aligned self-assembled nanocomposites of a perovskite and a spinel phases,
like BiFeO3 (BFO) and CoFe2O4 (CFO) have been extensively studied because of the
improved magnetoelectric coupling due to the increased interfacial area between the
piezoelectric and the magnetoelastic phases, as well as to the reduced substrate
clamping. Since the early reports of such nanostructures1, additional interesting
properties have been discovered at the interfaces between the two phases such as local
conduction2 and enhanced magnetization due to coupling between the CFO and BFO
spins3. For devices that leverage the magnetoelectric coupling and these interfacial
functionalities, the accurate control of the location of the pillars and interfaces will be
required, ideally using fabrication methods that take advantage of the autoorganization. Our recent work4 describes how topographical features written in Nbdoped SrTiO3, either using top-down lithography (Focused Ion Beam) or by etching
through a self-assembled mask (triblock terpolymer), can be used to selectively nucleate
the spinel phase in controlled locations, providing a handle on the location of the
growth of the pillars. Using this method, pillars of CFO, MgFe2O4, NiFe2O4, and mixed
composition CoxNi1-xFe2O4 were arranged in arrays of various symmetries (square,
rectangular and hexagonal) and arbitrary shapes. We will discuss the magnetic and
electrical properties of these templated films as measured by scanning probe
microscopy-based techniques, confirming their multiferroic character.
References:
[1] Zheng et al., Science, 303 (5658) 661–3. (2004).
[2] Hsieh et al., Advanced Materials, 24 (33) 4564–8 (2012).
[3] Chen et al., Nanoscale, 5 (10) 4449–53 (2013).
[4] Aimon et al., Advanced Materials (2014).
Figure. Top view SEM image of a template nanocomposite, where the spacing between CFO
pillars was 80 nm and the film thickness 100 nm
15
Poster Presentations
Towards Exchange-Biased Permanent Magnets:
Structure-Property Correlations in FeMn Alloys
Ian McDonald, Luke G. Marshall, Laura H. Lewis
Department of Chemical Engineering, Northeastern University, Boston, MA, USA
Efforts to realize novel types of permanent magnet materials have been motivated by
the need for greater control over technical magnetic properties in inexpensive and
readily-accessible materials. In particular, development and control of magnetic
anisotropies in such systems has shown to be a suitable route for the attainment of
enhanced energy products and performance. Towards this goal, FeMn-based alloys
exhibit a range of tailorable magnetic anisotropies and are thus promising candidates
for permanent magnetic materials. Depending upon the precise composition, alloys in
the Fe-Mn system can exhibit ferromagnetism or antiferromagnetism, facilitating the
development of enhanced coercivity donated by exchange anisotropy in multiphase
FeMn systems.
In this study, alloys of composition Fe100-xMnx (x = 10, 30) have been solidified in
a non-equilibrium manner using melt spinning to produce FeMn nanocomposites:
Fe90Mn10 is primarily ferromagnetic while Fe70Mn30 is primarily antiferromagnetic.
Evolution of the multiphase microstructure upon systematic annealing is correlated with
intrinsic and extrinsic magnetic attributes of these alloys, providing unique and
controllable testbed systems to study the impact of various anisotropies
(magnetocrystalline, exchange, shape, etc.) on the technical magnetic properties of
these nanocomposite FeMn alloys.
(This work is supported by ONR Grant # N00014-10-1-0553)
16
Poster Presentations
Interlayer coupling in ultra-thin L10-FePt/MgO/[Co/Pd]30 magnetic tunnel junctions
P. Ho1,2, G. C. Han3 , G. M. Chow1 , C. A. Ross2 and J. S. Chen1
1
Department of Materials Science and Engineering, National University of Singapore,
Singapore
2
Department of Materials Science and Engineering, Massachusetts Institute of
Technology, MA.
3
A*STAR Data Storage Institute, Singapore, Singapore
Spin valves and magnetic tunnel junctions (MTJs) with perpendicular magnetic
anisotropy (PMA) are favorable for spin transfer torque magnetic random access
memory (STT-MRAM) as they promise a reduction in the critical current density and an
improvement in areal density while maintaining thermal stability. L10-FePt and Co/Pd
multilayers have been extensively studied as suitable candidates for the magnetic layers
due to their high PMA [1-3]. A device structure which consists of an ultra-thin (< 4 nm)
L10-FePt layer as the free layer and Co/Pd multilayers as the fixed layer has the potential
in fulfilling these requirements while minimizing high temperature deposition process.
In this work, we investigate the feasibility of such ultra-thin L10-FePt/MgO/[Co/Pd]30
MTJs, with emphasis placed on the study of interlayer coupling effects in these MTJs.
The interlayer coupling within the MTJ was attributed mainly to the magnetostatic
coupling and direct coupling due to pinholes [Figure 1]. The magnitude of the interlayer
coupling field Hint in the L10-FePt/MgO/[Co/Pd]30 MTJ was insignificant compared to the
L10-FePt based pseudo spin valves reported earlier [4-5]. The improved interlayer
decoupling in the MTJs was presumably due to the reduced interlayer diffusion with the
room temperature deposition of MgO spacer and top fixed [Co/Pd]30 layers. There was
also no sign of dipolar stray field coupling, owing to the single domain characteristic
property of the fixed [Co/Pd]30 layer. The MTJ films are further patterned into nanodevices to provide an insight into the influence of interlayer coupling on the spin
transport properties.
References:
[1] P. Ho, G. C. Han, R. F. L. Evans, R.W. Chantrell, G. M. Chow, and J. S. Chen, Appl. Phys.
Lett. 98, 132501 (2011).
[2] P. Ho, G. C. Han, K. H. He, G. M. Chow, and J. S. Chen, Appl. Phys. Lett. 99, 252503
(2011).
[3] P. F. Carcia, A. D. Meinhalt, and A. Suna, Appl. Phys. Lett. 47, 178 (1985).
[4] P. Ho, G. C. Han, G. M. Chow, and J. S. Chen, Appl. Phys. Lett. 98, 252503 (2011).
[5] P. Ho, G. C. Han, K. H. He, G. M. Chow, and J. S. Chen, J. Appl. Phys. 111, 083909
(2012).
(Figure Next Page)
17
Poster Presentations
450
(a) -1 kOe
450 -1 kOe
Magnetization (emu/cc)
Magnetization (emu/cc)
500
(1)
400
350
300
-3
-2
-1
0
1
2
3
250
200
(2)
300
250
-3
-2
-1
0
1
2
3
200
150
60
(c) -20 kOe
40
(1)
-150
-200
Hint (Oe)
Magnetization (emu/cc)
350
Field (kOe)
Field (kOe)
-250
-300
(b) -5 kOe
100
150
-100
400
-3
-2
-1
0
1
2
3
20
0
-20
-350
Magnetostatic
+ Pinholes
(2)
-400
-40
-450
-20
Field (kOe)
-16
Magnetostatic
+ Pinholes
-12
-8
-4
Field (kOe)
0
Figure. Minor hysteresis loops of the L10-FePt (4 nm)/MgO (2.5 nm)/[Co (0.3 nm)/Pd (0.8
nm)]30 MTJ recorded under the influence of the different magnetization states of the top fixed
[Co/Pd]30 layer, created through an applied field of (a) -1 (b) -5 and (c) -20 kOe. The dotted line
indicates the center of the minor hysteresis loop; the arrow indicates the direction of the shift
of the minor hysteresis loop. Insets indicate schematically the influence of the top fixed
[Co/Pd]30 layer on the reversal of the bottom free L10-FePt. (d) Interlayer coupling field Hint
within the MTJ with respect to the applied field. Hint is the difference between the coercivity in
the first and second quadrants of the minor hysteresis loop.
18
Poster Presentations
Half-Metallic Antiferromagnets: A New Class of Materials for Spintronic Devices
M. E. Jamer, B. A. Assaf, and D. Heiman
Department of Physics, Northeastern University, Boston, MA, USA
Half-metallic antiferromagnets (HMAF) are a theorized class of materials that would be
beneficial for applications in processors for quantum computers and nonvolatile RAM
memory devices. HMAF materials can generate a spin-polarized current without
generating a disrupting magnetic field. These materials are expected to exhibit
antiferromagnetism at room temperature, which makes them well positioned for
practical devices. In this project, V3Al and Mn3Al were synthesized to investigate their
possible HMAF properties. The preliminary data has shown the V3Al displays
antiferromagnetic behavior below TN~570 K (Neel temperature). The electrical transport
measurements show that the compound’s carriers are holes and displays metallic
behavior at room temperature. Current progress on these compounds and other
possible HMAF compounds will be presented, including X-ray magnetic linear dichroism
and circular dichroism results from Brookhaven National Laboratory. The long term
outcome of this project would be to find possible materials exhibiting HMAF properties
for future electronic memory devices.
19
Poster Presentations
Large-area Periodic Magnetic Microstructures for Controlling Magnetic Micro-particles
Minae Ouk, and G.S.D. Beach
Massachusetts Institute of Technology, Cambridge, USA
Superparamagnetic microbeads (SBs) are widely used to capture biological entities in a fluid
environment. Chip-based magnetic actuation provides a means to transport SBs in lab-on-a-chip
technologies. This is usually accomplished using the stray field from patterned magnetic
microstructures [1], or domain walls in magnetic nanowires [2]. However, lithographic
patterning over a large area is costly and impractical using conventional techniques such as
electron beam lithography. Here we use a simple floating-transfer technique [3] for large-area
self-assembly of polystyrene microspheres on a Si wafer to produce lithographic masks texturing
a substrate. Hexagonal patterns are used as lift-off and etching masks to create magnetic dot
and anti-dot arrays in Co thin films, with a size and spacing that can be tuned via sphere diameter
and RIE etch time. Using a rotating magnetic fields, we show that these magnetically-patterned
substrates can transport SBs across large distances on the wafer surface, opening the possibility
to augment or replace microfluidic actuation for long distance transport. Supported by the MIT
Deshpande Center.
References:
[1] B. Yellen, et al., Lab Chip, 7, 1681 (2007)
[2] E. Rapoport and G. S. D. Beach, APL 100, 082401 (2012)
[3] X. Ye and L. Qi, Nano Today 6, 608 (2011)
20
Poster Presentations
Understanding L10-phase Formation in the FeNi System through the Study of FePd(Ni)
Compounds
A. M. Montes-Arango1, N. Bordeaux1, K. Barmak2, L. H. Lewis1
1
Department of Chemical Engineering, Northeastern University, Boston, MA
Department of Applied Physics and Applied Mathematics, Columbia University, New
York, NY
2
The development of next-generation permanent magnets for energy conversion
requires realization of novel materials with lower cost and reduced environmental
impact. To move beyond rare-earth-based magnetic materials, research interest has
shifted to compounds comprised of earth-abundant elements. With high uniaxial
magnetocrystalline anisotropy and large saturation magnetization, ferrous compounds
with the tetragonal chemically-ordered L10 structure are promising for permanent
magnet applications. Among these, L10-FePt and FePd have attracted considerable
attention, but their high cost limits their use to thin film applications. In contrast, the
low cost and good availability of the constituent elements make L10-FeNi ideal for bulk
permanent magnet construction. However, L10 phase formation in this system is highly
kinetically limited and thus it is only found naturally in meteorites that have cooled over
billions of years. To date, bulk laboratory synthesis has not been achieved, with very
limited amounts produced using irradiation techniques.
To gain insight into the development of chemical ordering in the FeNi system, Ni
additions to the model FePd compound were studied. Fe50Pd50-xNix (x=0-7at%) alloys
annealed for 100h at 500℃ were analyzed by x-ray diffraction, vibrating sample
magnetometry and differential scanning calorimetry. The amount of L10-phase formed
decreased with increasing Ni additions, diminishing substantially the magnetic
anisotropy. A reduction in the order-disorder transition temperature and the associated
enthalpy in Ni-containing alloys leads to believe that Ni produces unfavorable
characteristics for the formation of an L10 phase. These results highlight the need to
utilize nonconventional processing techniques that enhance diffusion to attain the L10phase in FeNi.
(This work is supported by NSF CMMI Division Grant # 1129433)
21
Poster Presentations
Nanoscale-driven Crystal Growth of Hexaferrite Architecture for Magnetoelectrically
Tunable Microwave Semiconductor Integrated Devices
Bolin Hu1, Zhaohui Chen1, Zhijuan Su1, Andrew Daigle1, Parisa Andalib1, Xian Wang2,
Jason Wolf 3, Michael E. McHenry3, and Yajie Chen1, , Vincent G. Harris1
1
Center for Microwave Magnetic Materials and Integrated Circuits, and Department of
Electrical and Computer Engineering, Northeastern University, Boston, MA, USA
2
School of Optical and Electronic Information, Huazhong University of Science and
Technology, Wuhan, Hubei, China
3
Materials Science and Engineering, Carnegie Mellon University, Pittsburgh,
Pennsylvania, USA
Thick barium hexaferrite films, i.e. Ba2Co2Fe12O22 (Co2Y), were epitaxially grown on c-axis
oriented GaN/Al2O3 substrates by a low temperature process in which the growth
temperature was substantially reduced by the use of Co2Y nanoparticles thus precluding
the need for flux. X-ray diffraction showed (00l) crystallographic texture while pole
figure analyses confirmed epitaxial growth. Saturation magnetization, 4πMs, was
measured for as-grown films to be 2.5 ± 0.1 kG with an out of plane magnetic anisotropy
field,
, 32 kOe (the easy magnetic polarization aligns in the film plane). The
ferromagnetic resonance (FMR) spectrum measured at 9.53 GHz had an FMR line width
H, of 280 Oe with an in-plane 6-fold symmetric magnetic anisotropy field
, of 55 Oe.
These properties demonstrate an innovative, scalable and cost effective pathway to
growing thick high quality ferrite films that enable the integration of ferrite microwave
passive devices with active semiconductor circuit elements for potential ME application.
22
Poster Presentations
Sub-100 nm Magnetic Wires with Low Edge Roughness
S. Siddiqui1, J. Currivan2,3, S. Ahn4, G. Beach, M. Baldo1 and C. Ross4
1
Electrical Engineering and Computer Science, Massachusetts Institute of Technology, USA
2
Physics, Massachusetts Institute of Technology, Cambridge, USA
3
Physics, Harvard University, Cambridge, USA
4
Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, USA
Patterning of thin films into <100 nm wide structures is essential for device scaling, and
low edge roughness is required for reproducibility of the magnetic switching
characteristics, since edge roughness in the nanostructures can act as domain wall traps.
We have patterned sub-100 nm ferromagnetic wires with very low edge roughness
using a removable bilayer poly(methyl methacrylate) (PMMA) and hydrogen
silsesquioxane (HSQ) resist mask. All patterning was done on silicon substrates with a
native oxide. 10 nm of polycrystalline Co60Fe20B20 was deposited using UHV DC
magnetron sputter deposition. 2% PMMA in Anisole and then 2% HSQ in methyl isobutyl
ketone were spun on the CoFeB. The HSQ was exposed at 125 kV electron energy. After
development, an O2 reactive ion etch (RIE) was used to remove the PMMA except under
the HSQ, resulting in a bilayer removable mask. The RIE power and time determined the
wire width. The CoFeB was ion milled using Ar ion etching at base pressure 1×10–6 Torr
with 10 mA beam current. After etching the pattern, the PMMA/HSQ mask was
removed by NMP along with sonication. SEM imaging gives a low average edge
roughness, less than 4% of the wire width. Examples of patterned films with both in
plane and perpendicular anisotropy will be given.
23
Poster Presentations
Thermomagnetic Behavior of L10 FeNi (Tetrataenite) from Meteorites
N. Bordeaux1, A. Mubarok2, E. Poirier3, K. Barmak4, J. I. Goldstein2, F. Pinkerton3 and L. H. Lewis1
1
Department of Chemical Engineering, Northeastern University, Boston, USA
2
University of Massachusetts, Amherst, USA
3
GM R&D Center, Warren, USA
4
Columbia University, New York, USA
Chemically-ordered ferrous L10-structured compounds with tetragonal symmetry have
high magnetocrystalline anisotropy and high magnetization suitable for application as
rare-earth-free permanent magnet materials. L10-structured FeNi (tetrataenite), which
is a metastable phase in the Fe-Ni system, is especially attractive due to the availability
and low cost of its constituents; however the magnetic character and kinetic parameters
of the order-disorder phase transformation of tetrataenite have not been well-studied.
In this work, tetrataenite extracted from a meteorite was utilized as a natural source of
the chemically-ordered phase (43 at% Ni composition). Characterization of the chemical
disordering transformation was studied using magnetometry and differential scanning
calorimetry (DSC) in the temperature range 25-700 °C.
Meteoritic tetrataenite features an apparent Curie temperature TC at ~534 °C
consistent with DSC results that show an endothermic peak with an onset temperature
of ~534 ºC and a transformation enthalpy of 3.8 kJ/mol corresponding to the L10→A1
chemical order-disorder phase transformation. After heating, the coercivity value
measured at 5 K decreases from 1075 to 3 Oe, and the magnetization at 5 T increases by
14%. The coupled magnetic-structural phase transformation temperature is well above
the reported order/disorder temperature of 320 ºC signifying that the disordering
process is kinetically limited [1,2]. The DSC results will be discussed in the context of 1-D
interface-controlled and 1-D diffusion-controlled phase growth models [3].
Establishment of the disordered phase growth mechanism and transformation enthalpy
provides guidance for planned laboratory synthesis of the L10 FeNi phase.
(This work is supported by ARPA-E REACT Grant # 0472-1537)
Reference:
[1] J. Paulevé, D. Dautreppe, J. Laugier, and L. Néel, J. Phys. Radium 23, 841-843 (1962).
[2] K.B. Reuter, D.B. Williams, and J.I. Goldstein, Metall. Trans. 20A, 711-718 (1989).
[3] C. Michaelsen, K. Barmak, and T.P. Weihs, J. Phys. D: Appl. Phys. 30, 3167-2186
(1997).
24
Poster Presentations
Formation and Current Effects on 360° Domain Walls in Magnetic Nanowires
L. Tryputen, J. Zhang, J. A. Currivan, F. Liu, D. Bono, C. A. Ross
Department of Materials Science and Engineering, MIT, Cambridge, MA, USA
(tryputen@mit.edu, zhangjs@mit.edu, currivan@mit.edu, frankliu@mit.edu, dbono@mit.edu,
caross@mit.edu)
The dynamic behavior of 360° domain walls (360DWs) is of intense interest as it differs
significantly from the behavior of the 180° domain walls (180DWs) currently used in
several proposed memory devices. A study of the effects of nanosecond current pulses
and magnetic fields on 360DWs in curved NiFe nanostructures is presented. The
360DWs are first formed in a wire attached to a circular injection pad by applying a
saturating magnetic field perpendicular to the wire to form a 180DW, followed by a
smaller reverse field to inject a second 180DW of opposite sense, which combines with
the first 180DW to produce a 360DW [1]. Higher order walls such as 540DWs can be
generated with additional field cycling. The formation and equilibrium structure of
360DWs in the wire was verified by MFM measurements. An array of wire/pad
structures was made and after field cycling and MFM, electrical contacts were made to
selected wire/pad structures enabling anisotropic magnetoresistance to be used to
detect 360DWs. A coplanar waveguide was used to inject current pulses with ns
duration. Micromagnetic simulations [2] predict that current pulses will either translate
a 360DW or lead to its destruction, with the annihilation threshold varying with applied
field. However, fields alone do not translate 360DWs, but instead compress or
dissociate them. The comparison between experimental results of current pulsing and
the micromagnetic predictions are discussed. This work could provide insight into the
behavior of 360DWs in racetrack devices and the possibility of new magneto-electronic
applications using 360DWs.
References:
[1] Y. Jang, S. R. Bowden, M. Mascaro, J. Unguris, and C. A. Ross, Applied Physics Letters
100, 062407 (2012).
[2] M. D. Mascaro and C. A. Ross, Physical Review 82, 214411 (2010).
25
Poster Presentations
Transformation Kinetics in FeRh Thin Films
Melissa Loving1, F. Jimenez-Villacorta1, C.J. Kinane2, S. Langridge2,
C. H. Marrows3 and L. H. Lewis1
1
Department of Chemical Engineering, Northeastern University, Boston, MA, USA
2
ISIS Rutherford Appleton Laboratory, UK
3
School of Physics and Astronomy, University of Leeds, Leeds, UK
Equiatomic α´-FeRh undergoes a first-order phase transformation (FOPT) from
antiferromagnetic (AF) to ferromagnetic (FM) character.1 FOPTs proceed by nucleation
and growth processes which may be monitored through measurement of a macroscopic
property of the transforming phase. Conventionally, FOPTs are monitored using
calorimetric approaches; however, calorimetry is not generally applicable to thin film
systems, due to the difficulty detecting the inherently small calorimetric signals. To
transcend this difficulty, we have employed a magnetization-based approach to gain a
comprehensive understanding of the kinetics underlying the FOPT in FeRh thin films. In
this manner, connections between the microstructure and the phase transformation
character of the FeRh films may be studied and tuned.
Epitaxial FeRh films were deposited onto (001)-MgO and annealed in-situ at 973
K to promote CsCl-type chemical order. The FOPT was examined with magnetometry
and magnetic force microscopy (MFM). Temperature- (T) and time- (t) dependent
magnetization (M) measurements were collected through the FOPT. The M(T) data
display an abrupt FOPT upon heating and thermal hysteresis upon cooling. The M(t)
measurements are sigmoid-shaped, characteristic of FOPT nucleation and growth
processes, and are best fit with the Johnson-Mehl-Avrami-Kolmogorov model for
crystallization kinetics 2-4. Temperature-dependent MFM images provide visualization of
the evolution of FM domains: the FM phase nucleates and grows upon heating, from
single domain islands emerged in an AF sea, and shrinks upon cooling. Overall, this study
provides a deeper understanding of the dynamics and transformation geometry of the
FeRh film FOPT.
(Acknowledgements: NSF: DMR-0908767 and UK-EPSRC:EP/G065640/1)
References:
[1] J. S. Kouvel and C. C. Hartelius, J. Appl. Phys. 33, 1343 (1962).
[2] M. Avrami, J. Chem. Phys. 7, 1103 (1939).
[3] M. Avrami, J. Chem. Phys. 8, 212 (1940).
[4] W. Johnson and R. Mehl, Transactions AIME 135 (1939).
26
Poster Presentations
Dzyaloshinskii-Moriya Interaction Influence on Stochastic Spin Orbit Torque Switching
S. Woo1, N. Pérez2, E. Martinez2, L. Torres2, S. Emori1 and G. S. D. Beach 1
1
Massachusetts Institute of Technology, Cambridge, USA
2
Universidad de Salamanca, Salamanca, Spain
We studied the effect of the Dzyaloshinskii-Moriya interaction (DMI) in the magnetic
switching of a perpendicularly magnetized oxide / ferromagnet / heavy metal trilayer
both experimentally and through micromagnetic simulations. We report the generation
of helical magnetization stripes for a sufficiently large DMI strength in the switching
region, giving rise to intermediate states in the magnetization confirming the essential
role of the DMI on switching processes. Using both experiments and simulations, we
show the presence of helical magnetization intermediate states in current pulses
switching in Pt/CoFe/MgO, while hysteresis loops in Ta/CoFe/MgO are clean,
demonstrating the contribution of the DMI. Although the study of current-induced
magnetization dynamics in these multilayers is still in its early stages, our results also
point out the possibility of engineering complex magnetization patterns such as helices
or skyrmions which present promising perspectives for high-performance spintronics
applications.
27
Poster Presentations
Detection of Field and Current Effects on 360° Domain Walls by Anisotropic
Magnetoresistance Measurements
J. Zhang, L. Tryputen, J. Currivan, and C. Ross
Massachusetts Institute of Technology, Cambridge, MA, USA
360DWs are metastable structures formed by combination of two 180DWs. To
experimentally study the behavior of a 360DW is of great importance in the study of DW
dynamics as well as in DW based memory or logic devices. Modeling predicts that a
moderate magnetic field applied along the long axis of the stripe does not translate a
360DW [1], unlike 180DW. When the field is higher than a critical value, a 360DW is
annihilated in place by forming a vortex core at the stripe edge when the field is
antiparallel with the center magnetization of the 360DW. When the field is parallel with
the center magnetization of the 360DW, it is dissociated into two 180DWs. A moderate
DC will drive a 360DW to move with a constant velocity similar to that of an 180DW.
However, when the current is high enough, a 360DW will be annihilated. The presence
of 360DW can be inferred from resistance decrease due to anisotropic
magnetoresistance which can also distinguish 180DW from 360DW. 360DWs were
generated by field cycling in y direction of a narrow magnetic strip attached to an
injection pad as in Fig. 1a. The field was then applied along x. For 180DW, a resistance
jump of 0.05 Ohm is observed, corresponding to the 180DW being pushed out of the
region of the arc between the inner two contacts. For a 360DW, a larger resistance jump
of 0.07 Ohm is observed but at higher fields, corresponding to the dissociation and
annihilation of the 360DW, respectively.
Reference: [1] Mascaro, Mark D., and C. A. Ross. "AC and DC current-induced motion of
a 360° domain wall." Physical Review B 82.21 (2010): 214411.
Figure. (a) SEM of the DW generation pattern and contacts for AMR measurement. (b)
Resistance vs. field applied antiparallel to the 360DW core magnetization. (c) Resistance vs.
field applied parallel to the 360DW core magnetization.
28
Poster Presentations
Resonant Modes of Coupled Magnetic Nanodisks
Maximilian Albert1, Luzanne Fadahunsi1, Weiwei Wang1, Marc-Antonio Bisotti1,
Dmitri Chernychenko1, Marijan Beg1, Peter Metaxas2, Hans Fangohr1
1
Faculty of Engineering and the Environment, University of Southampton, United Kingdom
2
School of Physics, University of Western Australia, WA, Australia
Interacting magnetic nano-elements with tailored magnetic configurations [1] have a
wide range of applications, from magnetic logic [2] to radio-frequency and microwave
signal generation, especially if they are incorporated in spin-torque nano-oscillators
(STNOs). [3,4] We study the stability and resonant modes of metastable states in pairs
of coupled magnetic nanodisks in view of these applications. Resonant modes are
computed using an analytical eigenvalue method [5]. We investigate the dependence of
these modes on the inter-disk separation, both for double-vortex states and coupled
metastable uniform states. Frequency splitting is observed for closely spaced disks in
both cases.
References:
[1] S. Jain et al., Nanotech., 21, 285702 (2010)
[2] M. Dvornik et al., J. Appl. Phys. 109, 07B912 (2011)
[3] N. Locatelli et al., Appl. Phys. Lett., 98, 062501 (2011)
[4] A.D. Belanovsky et al., Phys. Rev. B, 85, 100409R (2012)
[5] D'Aquino et al., J. Comp. Phys., 228, 17, 6130-6149 (2009)
29
Oral Presentations
(Afternoon Session)
Oral Presentations (Afternoon Session)
The Magnetostructural Response of FeRh-based Compounds:
Tailoring with Temperature, Pressure and Magnetic Field
Radhika Barua1, Ian McDonald2, Félix Jiménez-Villacorta1,3 and L. H. Lewis1
1
Department of Chemical Engineering, Northeastern University, Boston, MA
Department of Electrical and Computer Engineering, Northeastern University, Boston, MA
3
Materials Science Institute of Madrid (ICMM-CSIC), Juana Inés de la Cruz 3, 28049 Madrid, Spain
2
In its bulk form, Fe1-xRhx (0.47  x  0.53) possesses a B2-ordered crystal structure with
an abrupt antiferromagnetic (AFM) to ferromagnetic (FM) phase transition upon heating
to Tt ~ 350 K, accompanied by a unit cell volume increase of 1%. Strong coupling
between the magnetic spins and the lattice allow subtle variations of magnetic field,
pressure and/or composition to control and tune the transition for potential magnetic
devices such as magnetic refrigerators and sensors. Here, an arc-melted (Fe47.5Ni1.5)Rh51
alloy, serves as a test bed for understanding the simultaneous effects of temperature (2400 K), magnetic field (up to 5 T) and pressure (up to 10 kbar) on the magnetostructural
response of FeRh-based systems.
At zero applied pressure and magnetic field, (Fe47.5Ni1.5)Rh51, exhibits a
magnetostructural transition at Tt ~150 K. Application of an external magnetic field in
the ambient pressure state causes Tt to decrease at a rate much higher than that of the
parent FeRh compound ((dTt/dH )FeRhNi = -25 K/T; (dTt/dH )FeRh = -8 K/T). Field-induced
lowering of Tt is accompanied by an unexpected metastable retention of a fraction of
the high-temperature ferromagnetic phase below Tt and broadening of the thermal
hysteresis width (ΔTt). At Happ > 3 T, complete stabilization of the ferromagnetic phase is
noted. When pressure is applied at zero magnetic field to the Ni-modified sample, a
pronounced increase in the transition temperature ((dTt/dP) FeRhNi = 15.6 K/kbar) and a
decrease in the thermal hysteresis width (ΔTt) of the sample are noted. At high pressure,
large magnetic fields are required to completely suppress the magnetostructural
transition. At the current time, the unusual magnetostructural behavior of Ni-doped
FeRh systems at low temperatures is tentatively ascribed to the critically slow dynamics
of the phase transformation process at low temperatures.
31
Oral Presentations (Afternoon Session)
Magnetothermal Multiplexing
M.G. Christiansen, R. Chen, and P.O. Anikeeva
Massachusetts Institute of Technology, Cambridge, USA
Heat dissipation by single domain magnetic nanoparticles (SDMNPs) in the presence of
an alternating magnetic field (AMF) has long been studied for the biomedical application
of cancer hyperthermia.[1] More recently, SDMNPs in AMFs have been used to trigger
the response of individual biochemical pathways such as action potentials [2] and gene
transcription.[3] In heat mediated biological signaling, a technique to selectively heat
different types of collocated SDMNPs by changing the driving conditions of the AMF
would offer multiple signaling channels. This concept could be termed “magnetothermal
multiplexing.” Using a dynamic hysteresis model [4] that accounts for the effect of the
applied field on the anisotropy barrier, unlike typical treatments by linear response
theory (LRT),[5] we suggest how magnetothermal multiplexing could be accomplished.
We then experimentally illustrate a simple bimodal system with a set of 24nm Fe3O4
particles and 14nm MnxFe3-xO4 (x =0.04) particles. The larger particles are selectively
heated at 100 kHz and 35kA/m, whereas the smaller particles are selectively heated at
2.55MHz and 5kA/m. Multiplexing in this manner should be possible for a wide variety
of material types and field conditions, and could prove useful for numerous
applications.
Figuer. SLP multiplexing is illustrated for a 24nm Fe3O4 particle and a 14nm MnxFe3-xO4
(x=0.04) particle. For a 100kHz, 35kA/m AMF (
experimental,
dynamic hysteresis
simulation) the particle in the ferromagnetic regime heats more dramatically than the one in
the superparamagnetic regime. At 5kA/m and 2.55MHz (
experimental,
linear
response theory approximation including Brownian relaxation) the heat dissipation rates of
the two particles are swapped.
32
Oral Presentations (Afternoon Session)
Quantification of Strain and Charge Co-Mediated Magnetoelectric Coupling on UltraThin Permalloy/PMN-PT Interface
T. Nan1, Z. Zhou1, M. Liu2, X. Yang1, Y. Gao1, B. A. Assaf 3, H. Luo4, D. Heiman3,
B. M. Howe2, G. J. Brown2, and N. X. Sun1
1
Department of Electrical and Computer Engineering, Northeastern University, Boston, MA.
Materials & Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson, OH.
3
Department of Physics, Northeastern University, Boston, MA.
4
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, China
2
Strain and charge co-mediated magnetoelectric coupling are expected in ultra-thin
ferromagnetic/ferroelectric multiferroic heterostructures, which could lead to
significantly enhanced magnetoelectric coupling. It is however challenging to observe
the combined strain charge mediated magnetoelectric coupling, and difficult in
quantitatively distinguish these two magnetoelectric coupling mechanisms. We
demonstrated in this work, the quantification of the coexistence of strain and surface
charge mediated magnetoelectric coupling on ultra-thin Ni0.79Fe0.21/ PMN-PT interface
by using a Ni0.79Fe0.21/Cu/PMN-PT heterostructure with only strain-mediated
magnetoelectric coupling as a control. The NiFe/PMN-PT heterostructure exhibited a
high voltage induced effective magnetic field change of 375 Oe enhanced by the surface
charge at the PMN-PT interface. Without the enhancement of the charge-mediated
magnetoelectric effect by inserting a Cu layer at the PMN-PT interface, the electric field
modification of effective magnetic field was 202 Oe. By distinguishing the
magnetoelectric coupling mechanisms, a pure surface charge modification of magnetism
shows a strong correlation to polarization of PMN-PT. A non-volatile effective magnetic
field change of 104 Oe was observed at zero electric field originates from the different
remnant polarization state of PMN-PT. The strain and charge co-mediated
magnetoelectric coupling in ultra-thin magnetic/ferroelectric heterostructures could
lead to power efficient and non-volatile magnetoelectric devices with enhanced
magnetoelectric coupling.
33
Oral Presentations (Afternoon Session)
Nanoscale Magnetic Materials for Energy-Efficient Spin-Based Transistors and Logic
J. A. Currivan1,2, S. Siddiqui1, S. Dutta1, M. A. Baldo1 and C. A. Ross1
1
Massachusetts Institute of Technology, Cambridge, USA,
2
Harvard University, Cambridge, USA
Energy wasted as heat dissipation is the most serious problem confronting modern
electronics. Scientists can step in and develop creative solutions to overcome this power
dissipation problem. We investigated the switching of magnetic moments in nanoscale
soft ferromagnets as a means to build logic gates and circuits. Unlike charge flowing
through a channel, spins in a material can switch collectively, thus transistors encoded
using spin have the potential to be more energy efficient than complementary metaloxide-semiconductor (CMOS) transistors.
The ferromagnetic logic gates we are building use current-induced domain wall
motion to write the logic state of the device and a magnetic tunnel junction to read it
out. We developed and modeled the device, and are fabricating a prototype, as shown
in Figure 1. Our modeling results showed that this device satisfies all the requirements
of beyond-CMOS logic: it has gain and concatenability; individual devices are scalable;
operating voltages are 100 mV – 10 mV; and switching energies could scale below those
of contemporary CMOS. Furthermore, the device performs as a non-volatile universal
gate with a complete set of Boolean operations. It can support its own circuits or be
integrated with CMOS. Initial fabrication uses electron-beam lithography, UHV sputter
deposition, and etching techniques.
References:
[1] J.A. Currivan, Y. Jang, M.D. Mascaro, M.A. Baldo, and C.A. Ross, IEEE Magnetic
Letters, 3 (2012).
[2] J.A. Currivan, S. Siddiqui, S. Ahn, L. Tryputen, G.S. Beach, M.A. Baldo, and C.A. Ross.
Journal of Vacuum Science & Technology B 32 (2), 021601 (2014).
[3] B. Behin-Aein, D. Datta, S. Salahuddin, S. Datta, Nature Nanotechnology 5 (2010).
Figure. Initial fabrication of device prototype. Here, a 70 nm wide, 10 nm thick NiFe arc is
exchanged biased on the ends by IrMn antiferromagnetic pads (a). Using 4-point
measurement techniques, we observe the domain wall motion back and forth along the wire
by an abrupt change in resistance (b).
34
Oral Presentations (Afternoon Session)
Fabrication of Magnetically Hard Cobalt Carbide Nanoparticles via Wet Chemical
Synthesis
Mehdi Zamanpour1, Yajie Chen1 and Vincent G. Harris1, 2
1
Centre for Microwave Magnetic Materials and Integrated Circuits (CM3IC),
Northeastern University, Boston, USA
2
Department of Electrical and Computer Engineering, Northeastern University, Boston,
USA
CoxC magnetic nanoparticles were successfully synthesized via a modified polyol process
without using a rare-earth catalyst during the synthesis process. The present results
show admixtures of Co2C and Co3C phases possessing magnetization values exceeding
45 emu/g and coercivity values exceeding 2.3 kOe at room temperature. Moreover,
these experiments have illuminated the important role of the reaction temperature, and
the reaction duration on the crystallographic structure and magnetic properties of CoxC,
while tetraethylene glycol was employed as a reducing agent. The role of the ratios of
Co2C and Co3C phases in the admixture on magnetic properties is discussed. The
crystallographic structure and particle size of the CoxC nanoparticles were characterized
by
X-ray diffractometry and scanning
electron
microscopy. Vibrating
sample
magnetometry was used to determine magnetic properties. Scale-up of synthesis to
more than 5 grams per batch was demonstrated with no significant degradation of
magnetic properties.
35
Oral Presentations (Afternoon Session)
Integration of Self-Assembled Nanocomposite on Silicon Substrate
Dong Hun Kim, Nicolas M. Aimon, X. Sun, and C. A. Ross
Massachusetts Institute of Technology, Cambridge, MA, USA
Self-assembled nanocomposite thin films such as BaTiO3-CoFe2O4, BiFeO3-CoFe2O4 (BFOCFO), and BiFeO3-NiFe2O4, in which a ferrimagnetic spinel phase grows epitaxially as
pillars within an immiscible ferroelectric perovskite phase, have been studied intensively
as new multiferroic materials.[1-2] Vertical epitaxial nanocomposites have been
exclusively grown on single crystal oxide substrates which limits their utility in
microelectronic devices. Integration of nanocomposites on a Si platform would provide
a path towards large scale and low cost devices such as multiferroic memory and logic
device. We have found previously that Sr(Ti1-xFex)O3 (STF) films can be grown epitaxially
on CeO2/YSZ-buffered (001) Si.[3-5] In that work, films grown in vacuum with x = 0.1 ~ 0.5
exhibited room-temperature magnetism and a strong out-of-plane anisotropy of
magnetoelastic origin, but STF deposited in oxygen did not show significant roomtemperature magnetism, attributed to the lower lattice strain and oxygen vacancy
concentration. When nanocomposite films grow on STF layer STF not only guides the
BFO-CFO epitaxial growth but also contributes to the magnetic properties of the film
stack. Cubic or retangular factted CFO pillars in BFO matrix grew epitaxially (Fig. 1 (a))
but abnormal CFO pillar growth orientation was observed on rough STF layer (Fig. 1 (b)).
We have also epitaxially grown BFO-CFO nanocomposites on MBE grown (001) SrTiO3
film coated Si substrate. BFO-CFO nanocomposites on both STF and STO layer showed
strong out-of-plane anisotropy due to the combination of shape anisotropy and
magnetoelastic anisotropy. Removal of the BFO matrix relaxed the strain and lowered
the anisotropy.
References: [1] H. Zheng et al., Science, 303, 661 (2004); [2] S. C. Liao et al., ACS Nano, 5, 4118
(2011); [3] D. H. Kim et al., Phys. Rev. B, 84, 014416 (2011); [4] D. H. Kim et al., J. Appl. Phys.,
111, 07A918 (2012); [5] D. H. Kim et al., J. Phys.: Condens. Matter, 25, 026002 (2013).
Figure. Top view SEM image of nanocomposite on (a) smooth and (b) bumpy
STF35/CeO2/YSZ/Si. Insets are top view SEM image of STF layer. The circled CFO pillars in
Figure (b) are 45˚ rotated cube on-cube epitaxial growth on STF35. Scale bars correspond to
100 nm and scale of inset is the same.
36
List of Presenters
Presenters
Aimon, Nicolas M.
Assaf, Badih A.
Barua, Radhika
Bauer, Uwe
Bordeaux, Nina
Chang, Cui-Zu
Chen, Ritchie
Chen, Yajie
Christiansen, Michael G.
Currivan, Jean Anne
Emori, Satoru
Ho, Pin
Hosseinpour, Pegah
Hu, Bolin
Jamer, Michelle E.
Katmis, Ferhat
Kim, Dong Hun
Lejune, Brian
Liu, Frank
Loving, Melissa
McDonald, Ian
Montes-Arango, Anna Maria
Nan, Tianxiang
Ouk, Minae
Siddiqui, Saima
Tryputen, Larysa
Woo, Seonghoon
Zamanpour, Mehdi
Zhang, Jinshuo
Email Address
naimo@mit.edu
assaf.b@husky.neu.edu
barua.r@husky.neu.edu
ubauer@mit.edu
bordeaux.n@husky.neu.edu
czchang@mit.edu
rchen627@mit.edu
y.chen@neu.edu
mgc@mit.edu
currivan@mit.edu
s.emori@neu.edu
hopin@mit.edu
hosseinpour@coe.neu.edu
hu.b@husky.neu.edu
jamer.m@husky.neu.edu
Katmis@mit.neu
rogercop@mit.edu
lejeune.b@husky.neu.edu
frankliu@mit.edu
melissa.loving@gmail.com
mcdonald.i@husky.neu.edu
montesarango.a@husky.neu.edu
nan.t@husky.neu.edu
minaeouk@mit.edu
afroz@mit.edu
tryputen@mit.edu
shwoo@mit.edu
zamanpour.m@husky.neu.edu
zhangjs@mit.edu
37