Overview.
The electrical control of magnetism presents intriguing scientific questions and
potentially offers an enabling technology for many applications. Today, this subject
represents a “holy grail” simultaneously in the fields of physics, materials science and
electronics. We propose to exploit the properties of 2D layered structures based on van
der Waals (vdW) materials to control magnetism through new mechanisms based on
electrical gating of exchange interactions between magnetic ions or nanoparticles coupled
by vdW layers.
Our scientific goal is to produce novel spin response in analogy with the novel electronics
of graphene. Our technological goal is the creation of new three-terminal devices based
on electrical control of spin-dependent scattering by gating exchange interactions, a
mechanism analogous to that of giant magnetoresistance (GMR). We will explore the
possibility of a new class of devices based on electrically controlled magnetism, which
could enable new sensing, non-volatile memory and information processing applications.
The exploration of the physics of graphene has been a major focus in recent years. Its
honeycomb arrangement of carbon atoms leads to an unusual Dirac spectrum in which
energy varies linearly with momentum. Given its unique dispersion relation and wealth of
unusual physical properties, including edge states and novel topological phases, it is not
surprising that many generalizations of graphene have already been explored. Some
involve alternate geometries: nanoribbons whose zig-zag or armchair edges affect
materials properties, or multiple graphene layers. More substantial modifications include
chemical tailoring via metal adatoms, or the addition of nitrogen or boron.
The present project moves beyond graphene into the rich realm of van der Waals coupled,
2D layered materials and heterostructures [1, 2]. These materials offer new and unique
possibilities for incorporating magnetic ions and integrating magnetic nanoparticles
(NPs) so as to couple spin through exchange interactions with the electron environment,
i.e., Ruderman-Kittel-Kasuya-Yoshida (RKKY) interactions. Electrical control of the
Fermi wavelength through gating of the electron density will allow control of the
magnitude and sign the RKKY coupling.
Two key and unique features of vdW materials for this study are: 1) the ability to
intercalate magnetic ions within the vdW interfaces and 2) the pristine nature of vdW
surfaces due to in-plane bonding within the 2D layers. These features allow the
incorporation magnetic ions and nanoparticles into arrays with regular (uniform) spacing,
thereby providing virtually identical coupling between all elements in the array.
We will focus on transition-metal dichalcogenides including the sulfides MoS2, NbS2;
the selenides Bi2Se3, MoSe2, NbSe2; and the tellurides Bi2Te3, MoTe2, NbTe. This
materials matrix was chosen to provide compositional tunability of bandgap and spin
orbit coupling, two important parameters in our study. The materials will be grown by
molecular beam epitaxy (MBE), which offers demonstrated and potential advantages for
the growth of high-quality, highly tailorable vdW materials including heterostructures
and structures intercalated with 3d transition metal ions such as Mn, Cr, Co.
1
Magnetic NP arrays will be fabricated by drop casting magnetic NP (e.g., Fe3O4) directly
on the MBE-grown surface and by drop casting non-magnetic NP masks for patterning
underlying magnetic thin-films. Field-effect transistor structures will be fabricated by ebeam lithography and a variety of clean-room nanofabrication processes.
Magnetic properties of the structures will be characterized by vibrating sample
magnetometry (VSM), alternating gradient Magnetometry (AGM), magneto-optical Kerr
effect (MOKE) with focused beam (NanoMOKE), and superconducting quantum
interference device (SQUID) magnetometry. Scanning electron microscopy (SEM),
Raman spectroscopy, and a combination of topographical, conductive, magnetic scanning
probe microscopy (AFM, C-AFM, MFM) will also be used. The electrical characteristics
of devices will be measured as a function of temperature and magnetic field.
Another important part of this study is its theoretical component in which the selection of
materials and interpretation of results will be coordinated with theoretical considerations
based on Density Functional Theory (DFT) calculations and Quantum Monte Carlo
(QMC) simulations. In particular, the compositionally tunable bandstructure of the
semiconductor host will be used in synergy with the varying atomic and bonding
characteristics of different magnetic atoms to adjust exchange coupling.
Context and plan of the proposal. [RK will revise]
The topic of RKKY exchange coupling between ferromagnetic islands in a
semiconductor has been of interest for some time. This topic gained considerable
attention in the study of dilute magnetic semiconductors, where magnetic behavior was
attributed to the formation of MnAs precipitates in GaMnAs 1 2, 3. In direct contrast to
that work, in which precipitates were formed at random positions and with little control
of size and inter-particle spacing, this study is conceived and organized to create
optimized materials, structures and interactions. The plan is to follow a systematic path to
demonstrate the basic physics and device concept via a set of complementary approaches.
2
Materials growth studies
We will first explore the MBE growth of epitaxial layers of non-magnetic 2D materials,
on which magnetic nanoparticles will subsequently be deposited at UC-Davis for gating
studies. This stage of work will include the growth of 2D chalcogenides of Bi, Mo and
Nb, which were specifically selected because they span a wide range of energy gaps and
exhibit a wide progression of spin-orbit interactions (see Table I). We already have
considerable experience (see Figs. 1 and 2) with vdW MBE growth and characterization
of Bi2Se3, Bi2Te3,1,2 MoSe2,3 and MoTe2, which we will adapt to the needs of the
proposed work with magnetic nanoparticles. Additionally, we propose to extend our vdW
MBE growth to compounds based on two new elements, sulfur (S) and niobium (Nb).
Sulfur-based systems are chosen for this work, because this will allow us to have a
complete set of chalcogenides spanning the values of spin-orbit interaction from the
lowest (sulfides) to the highest (tellurides)45-6, allowing us establish the role of this
interaction in mediating magnetic exchange in the 2D systems under consideration.
Proceeding to sulfide formation will require the installation of a sulfur effusion cell
system in our MBE machine. Accordingly, funds for the sulfur source are requested in
the budget for the Notre Dame subcontract.
To our knowledge very little is known about the MBE growth of the proposed sulfide
systems. In developing the growth methodology of the 2D chalcogenindes, it will be
particularly interesting to explore the effects of atomic size on the growth process. For
example, among the systems listed above, in the case of MoTe2 both atoms are of
approximately the same size, while in MoS2 the atomic sizes of the transition metal and
of the chalcogen are drastically different. In this context it is also worth noting that
having the sulfide capability will automatically allow us to fabricate layers of Bi2S3, thus
allowing us to compare the properties of this 2D system with Bi2Se3 and Bi2Te3, both
structurally and electrically. By developing and understanding the MBE growth processes
of the above materials, it is expected that information obtained in the process of this work
will also have a broader impact by contributing to the general understanding of the
formation of 2D materials during epitaxy.
In contrast to the materials mentioned above, Nb chalcogenides are metals7. Extending
our proposed work on MBE growth to these metallic 2D systems will thus bring the
presence of RKKY interactions into the picture. We expect this feature to be significant
in determining the role of magnetic nanoparticles deposited on the nonmagnetic metallic
2D Nb chalcogenide layers.
As our experience grows in the formation of the above nonmagnetic layers, we will be
able to extend our efforts to the growth of 2D magnetic systems formed by intercalation
of magnetic transition metal ions (Mn, Cr, Co, and others) in the 2D systems explored
above, starting with NbS2. A great deal of work has been done on similar systems in
bulk form, particularly on transition metal sulfides8,9,-10, showing that such intercalation
is made possible by the van der Waals forces involved in the formation of such 2D
structures (see Fig. 3). While MBE is a “natural” process for controlling and
manipulating growth of such systems, and to the formation of layered structures, to our
knowledge this has not been explored. As a start, our MBE growth of these magnetic 2D
3
layers will thus be guided by the work already done on bulk materials. Specifically, we
propose to explore the formation of NbS2 alloyed with 4th row magnetic transition metal
ions (identified as feasible in the bulk by Parkin and Friend11) by MBE. It is particularly
important to note that, among the materials being proposed, the Nb chalcogenides are
metallic (in which the RKKY process is therefore likely to occur), while the Mo- and the
Bi-based systems are not (SEE Table 1). This therefore opens important opportunities for
exploring 2D magnetism and its relation to the types of magnetic exchange which are
dominant in specific media; as well as for exploring magnetic coupling between layers in
2D multilayer systems.
Based on our experience with NbS2, we propose to extend the studies of magnetic ion
intercalation to selected selenides and tellurides developed above. In this activity we will
build on guidance from our theoretical team at UC-Davis and from our colleagues at IBM.
We note that we already have experience with incorporating magnetic ions (Mn) in the
2D systems Bi2Se3 and Bi2Te3,12 where Mn, instead of distributing itself randomly
throughout the system, leads to the formation of a new phase incorporated in the parent
nonmagnetic bismuth chalcogenide matrix (see Fig. 2).
Throughout the course of the above effort, systemic growths of 2D films will be
performed under various highly controlled conditions, and will be carefully characterized
in order to identify optimal growth temperatures, flux ratios, specific surface passivation
methods, and growth initiation procedures. In particular, control of the quality of
interfaces, layer thickness, and compositional uniformity will be of special importance in
carrying out this program. In investigating various growth approaches we will explore
using vicinal substrates applying advanced MBE techniques such as atomic layer epitaxy
(ALE) and migration-enhanced epitaxy (MEE) by monitoring the growth using RHEED
intensity.13 As an example, special effort will be made to form high quality single-layer
structures of the proposed materials on specific substrates, in order to apply both front
and back gates that will enable device research on monolayer systems.
Having addressed the challenges identified above in connection with epilayer formation
described above, we will then proceed to the formation of heterostructures based on
selected 2D systems, guided by experience and by the input from the theoreticians
involved in the EFRI-2DARE Program. Specifically, we propose to undertake fabrication
of multilayer structures, such as quantum wells and superlattices, including short-period
superlattices. We note parenthetically that we have already succeeded in the growth of
the following multilayer systems: Bi2Se3/MoSe2 superlattices; Bi2Se3-MoSe2-Bi2Se3
trilayers; and MoS2-MoSe2-MoTe2 multilayers; and Bi2Se3-MoSe2-NbSe2 multilayers.
Of special interest in the present proposal will be to extend this work to integration of
nonmagnetic systems with the magnetic systems such as Cr1/3NbS2.
4
EFRI 2-DARE: Electrically Controlled Magnetic 2D Layered Structures
Table I. Physical properties of proposed 2D materials. In the case of semiconductors Egi and Egi indicate
direct and indirect energy gaps, respectively.
S
Se
Te
Nb
NbS2
Metal
Hex: P63/mmc
a=3.310Å, c=11.89Å [1]
NbSe2
Metal
Hex: P63/mmc
a=3.450Å, c=12.55Å [2]
NbTe2
Metal
Mono: C2/m
Mo
MoS2
Semiconductor
Egi ~ 1.23eV; Egd ~ 1.69eV
[3]
Hex: P63/mmc
a=3.160Å, c=12.29Å [1,2]
MoSe2
Semiconductor
Egi ~ 1.09eV; Egd ~ 1.35eV
[3]
Hex: P63/mmc
a=3.288Å, c=12.93Å [2]
Bi2Se3
Semiconductor
Egd = 0.32eV [7]
Bi2Te3
Semiconductor
Egd = 0.15 [7] or 0.17eV [8]
Hex: R3m
a=4.138Å, c=28.64Å [7]
Hex: R3m
a=4.383Å, c=30.487Å [7]
Bi
Bi2S3
Semiconductor
Egd = 1.3 eV [6]
Ortho: Pmcn
MoTe2
Semiconductor
Egi ~ 1.0eV; Egd ~ 1.1 eV [4]
Hex: P63/mmc
a=3.518Å, c=13.97Å [5]
Fig. 1 High-angle annular-dark-field (HAADF) image of the Bi2Te3 layer grown on a GaAs (001) substrate.
5
EFRI 2-DARE: Electrically Controlled Magnetic 2D Layered Structures
Fig. 2 HAADF images of (A) 2.5% Mn-doped and (B) 4.2% Mn-doped Bi2Se3 thin films grown on GaAs
substrates. The arrows point out the Mn-rich layers. Note that the Mn-containing layers are
conspicuously thicker than the non-magnetic layers.
Fig. 3 (a) Structure of M1/3NbS2, (M=V, Cr, Mn), showing the relationship of the 3d metal ions to the NbS2
layers. [1,2] (b) Magnetic susceptibility of Cr1/3NbS2, both parallel and perpendicular to the c axis, in a
field of 330 G.[3] (c) Resistivity in the layers, and Hall coefficient for current in the layers and magnetic
field perpendicular to them in Cr1/3NbS2.[4]
[1] F. Jellinek, G. Brauer, H. Müller, Molybdenum and Niobium Sulphides, Nature (London), 185, 376
(1960).
[2] V. L. Kalikhman, [Need title] Izv. Akad. Nauk SSSR Neorg. Mater., 19, 1060 (1983).
6
EFRI 2-DARE: Electrically Controlled Magnetic 2D Layered Structures
[3] K. K. Kam and B. A. Parklnson, Detailed Photocurrent Spectroscopy of the Semiconducting Group VI
Transition Metal Dichalcogenides, J. Phys. Chem. 86, 463-467 (1982).
[4] A. J. Grant, T. M. Griffiths, G. D. Pitt and A D Yoffe, The electrical properties and the magnitude of the
indirect gap in the semiconducting transition metal dichalcogenide layer crystals, J. Phys. C: Solid State
Phys., 8, L17 (1975).
[5] O. Knop, R. D. MacDonald, Chalkogenides of the Transition Elements: III. Molybdenum Ditelluride,
Can. J. Chem. 39, 897 (1961).
[6] Madelung O., Semiconductors Data Handbook. Berlin: Springer; 2004 p. 691. Available at:
http://www.amazon.com/Semiconductors-Data-Handbook-Otfried-Madelung/dp/3540404880
[7] Jian-Min Zhang, Wenmei Ming, Zhigao Huang, Gui-Bin Liu, Xufeng Kou, Yabin Fan, Kang L.Wang, and
Yugui Yao, Stability, electronic, and magnetic properties of the magnetically doped topological insulators
Bi2Se3, Bi2Te3, and Sb2Te3, Phys. Rev. B 88, 235131 (2013).
[8] E. Kioupakis, M. L. Tiago, and S. G. Louie, Quasiparticle electronic structure of bismuth telluride in the
GW approximation, Phys. Rev. B 82, 245203 (2010).
7
EFRI 2-DARE: Electrically Controlled Magnetic 2D Layered Structures
[1] R. H. Friend, A. R. Beal & A. D. Yoffe (1977) Electrical and magnetic properties of some first row
transition metal intercalates of niobium disulphide, Philosophical Magazine, 35:5, 1269-1287.
[2] S. S. P. Parkin, E. A. Marseglia and P. J. Brown, Magnetisation density distribution in Mn1/4TaS2:
observation of conduction electron spin polarization, J. Phys. C: Solid State Phys., 16 (1983) 2749-2764.
[3] S. S. P. Parkin & R. H. Friend (1980) 3d transition-metal intercalates of the niobium and tantalum
dichalcogenides. I. Magnetic properties, Philosophical Magazine Part B, 41:1, 65-93, DOI:
10.1080/13642818008245370.
[4] S. S. P. Parkin & R. H. Friend (1980) 3d transition-metal intercalates of the niobium and tantalum
dichalcogenides. II. Transport properties, Philosophical Magazine Part B, 41:1, 95-112, DOI:
10.1080/13642818008245371.
8
EFRI 2-DARE: Electrically Controlled Magnetic 2D Layered Structures
Magnetic behavior studies.
Method:
In this project, it is critical to distinguish dipolar vs. exchange interactions in the
nanoparticles arrays. We will use a recently developed first-order reversal curve (FORC) method
by Liu, Scalettar and colleagues (Verosub and Zimanyi), which is sensitive to such different
magnetic interactions.4-8 The method employs hundreds of partial hysteresis curves, called
FORC’s. After positively saturating the sample the applied field is reduced to a given reversal
field HR, the magnetization M is then measured back to positive saturation thereby tracing out a
FORC (Fig. 10a). This process is repeated for more negative reversal fields until negative
saturation is reached. A mixed second order derivative of the magnetization M (H, HR) is used to
generate the FORC distribution,
 (H, HR)/2HHR,
(1)
which captures the irreversible switching events.9 The FORC distribution can also be represented
by local coercivity (HC) and bias field (HB) according to:
𝐻𝐶 =
(𝐻−𝐻𝑅 )
2
,
𝐻𝐵 =
(𝐻+𝐻𝑅 )
2
.
(2)
The FORC method is a versatile yet simple “magnetic fingerprinting” technique that yields
extremely detailed information about any magnetic heterogeneities or different reversal
mechanisms in the sample.5, 10-13
To expose the fundamental interactions present, the FORC method will be coupled with
the ΔM method13, 14 to obtain
ΔM(H)=MDCD(H)/MR+2MIRM(H)/MR-1,
(3)
where MDCD(H), MIRM(H), and MR are the dc demagnetization, isothermal, and saturation
remanence, respectively. ΔM(H) is expected to be positive for interactions dominated by
exchange and negative for those with dipolar origins.
Prior studies
We have carried out a series of FORC studies on a wide variety of nanomagnets,
including nanoparticles,8 nanodots,10, 11, 15 nanowires,16 perpendicular magnetic anisotropy
systems,5, 12, 17 exchange spring magnets,6 exchange biased systems,18 complex oxides,7 and
Figure 10. (a) Schematic of a first-order reversal curve as described in the text. Examples for FORC distributions for (b)
an unbiased Fe film at 100K, (c) exchange biased Fe/FeF2 film at 50K, (d) arrays of non-interacting Fe nanodots at
300K, and (e) arrays of Ni nanowires with strong dipolar interaction.
9
EFRI 2-DARE: Electrically Controlled Magnetic 2D Layered Structures
magnetic recording media 19-21. Our results have been presented in several recent conference
invited talks, including the 2008 SPIE, 2008 MRS, 2009 & 2011 APS March Meetings, 2011
ACerS Meeting, 2012 MRS and 2012 ACNS Meeting. Of particular relevance to the proposed
work is the ability of FORC to “fingerprint” magnetic systems and distinguishing dipolar vs.
exchange interactions.
1.1 “Fingerprinting” magnetic systems
Fig. 10 illustrates a few representative FORC distributions. Fig. 10(b) shows an unbiased
Fe (27nm thick) thin film at 100K. The FORC distribution is essentially a -function with narrow
local coercivity HC distribution and centered on zero bias field HB. Once the same Fe film is
exchange biased by an antiferromagnetic FeF2, the FORC feature is displaced along the HB-axis,
as shown in Fig. 10(c). When a similar Fe film is patterned into arrays of non-interacting
nanodots, the FORC distribution is illustrated in Fig. 10(d) as a horizontal ridge along the HCaxis.10 The HC spread manifests the size and anisotropy variation in the array, while the lack of
any appreciable distribution along the bias field HB-axis confirms the negligible inter-dot
interaction. Fig. 10(e) shows the FORC distribution of an array of perpendicular Ni nanowires. 16
The vertical ridge is caused by a strong demagnetizing dipolar interaction within the arrays,
along with a narrow distribution of nanowire coercivity. These examples illustrate that different
types of magnetic characteristics can lead to qualitatively different FORC diagrams, which can
be used as “fingerprints” of magnetic systems.
Dipolar vs. exchange interaction
We have used the FORC method to investigate CoPtCr-SiO2 perpendicular recording media
grown on MnRu and Ru underlayers to distinguish predominantly dipolar vs. exchange
interactions in the media.21 We have found that using an antiferromagnetic MnRu (vs.
conventional Ru) underlayer can simultaneously enhance the anisotropy of the recording media
Figure 11. Families of FORC of 4 nm thick CoPtCr-SiO2 grown on Mn67Ru33 and Ru underlayer are shown in (a) and (b),
respectively. Corresponding FORC distributions are shown in (c) and (d), and TEM plan-view images are shown in (e)
and (f), respectively. ΔM plots for the two samples are shown in (g).
10
EFRI 2-DARE: Electrically Controlled Magnetic 2D Layered Structures
and lower the inter-granular exchange coupling.
FORC analyses of the two samples are shown in Figs. 11(a)-11(d). While both FORC
distributions show a single peak, characteristic of a reversal via single domain rotation,10 the
distribution is significantly broader in the sample with the Mn67Ru33 underlayer [Fig. 11(c)]. The
increased width along the HC-axis indicates a broad coercivity distribution while the enhanced
width along the HB-axis indicates strong dipolar-like interactions between well isolated grains
[similar to Fig. 10(e)], as confirmed by the plan-view TEM image shown in Fig. 11(e), which
shows that the MnRu can indeed promote SiO2 segregation to the grain boundaries. Conversely,
the TEM image of the sample with the Ru underlayer [Fig. 11(f)] shows poor grain separation,
which results in strong exchange coupling between grains and a narrower FORC distribution
along both the HC and HB axis as shown in Fig. 11(d). This is further confirmed by the ΔM plots
shown in Fig. 11(g). A positive peak is found in the ΔM plot for the sample grown on Ru,
indicative of strong exchange coupling in the sample. In contrast, the sample deposited on the
Mn67Ru33 exhibits a negative peak in the ΔM plot, indicating dipolar interactions between the
grains of the media. For this project, we will use NanoMOKE in conjunction with the FORC and
ΔM methods to probe the nature of intra-array interaction, and whether gating can introduce any
appreciable and qualitative change of the interactions. This will be studied as the nanoparticle
size, spacing and layout are changed, which will tune both the dipole interactions as well as the
expected exchange interactions.
11
EFRI 2-DARE: Electrically Controlled Magnetic 2D Layered Structures
Electronic transport studies.
The giant magnetoresistance (GMR) effect has led to spectacular successes in both
nanotechnology and spintronics 22. Magnetic sensors based on GMR, and the related effect of
tunneling magnetoresistance (TMR), have revolutionized magnetic recording 23 and hold
promise for other mass-market applications, such as bio-sensing 24. We will explore the creation
of a new class of three-terminal magnetoresistance devices based on a gated spin-dependent
scattering of electrons. These devices can be described as field-effect transistors in which the
gate potential controls exchange interactions between magnetic entities (magnetic ions or
nanoparticles) coupled through the electron channel.
Figure 1 shows a schematic diagram of a generic structure for back-gated exchange coupling via
a vdW electron channel. Electron current flows through the channel in the vdW layer that is
coupled to a magnetic NP array. The local spin of electrons in the channel is controlled by the
magnetic moment of the nearby NP, as indicated in the figure. The electron density in the
channel is controlled by back-gate bias, thereby controlling the exchange interaction.
In this example, we show a device in which the magnetization is in the plane of the surface.
(Devices with perpendicular magnetization are also possible.) The magnetic NPs are taken to be
closely spaced along rows going into the plane, which are separated by a somewhat larger interrow spacing s. In this case, the spacing into the plane is chosen to be small enough to insure that
the spins of NPs along each row align in a direction either into or out of the plane, as indicated
by crosses and circles in the figure. The magnetization of the NPs transfer their spin polarization
to nearby itinerant electrons in the channel by an exchange interaction, thereby influencing the
GMR-like spin scattering transport through the channel from the source S to the drain D. The
potential on the back-gate electrode G controls the electron density and Fermi wavelength in the
channel, thereby controlling the sign and magnitude of a second exchange interaction: the RKKY
indirect exchange interaction between NP mediated by the electron channel. With appropriate
design, this electrically controlled RKKY exchange interaction can serve to switch the rows of
NP between antiparallel (shown) and parallel (not shown) magnetic alignments, thereby
modulating the channel resistance in a manner similar to that GMR devices.
The basic operation of the device is illustrated in Fig. 2. As the gate voltage is increased i) the
2DEG electron density n increases, ii) the Fermi wavelength λF decreases, iii) the exchange
coupling J oscillates and iv) G the conductance oscillates. Applying a magnetic field Hs
sufficient to force parallel alignment of the ferromagnetic nanoparticles will suppress the G
oscillation. Thus the G-Vg characteristic contains a clearly identifiable signature of the gated
exchange interaction: an oscillation that diminishes with increased in-plane magnetic fields.
Our plan of study is to begin with structures comprised of drop-cast 2D arrays of Fe3O4 NP on
graphene as a control structure for the dichalcogenides. This will build on our pre-proposal work
in which we have established chemical protocols and fabrication processes suitable achieving
electronic coupling between the Fe3O4 NPs and graphene (see Fig. X).
Hanle
magnetoresistance characterization and related techniques will be used to optimize exchange
coupled transfer of spin to dichalcogenides, using graphene as a comparative baseline. We will
then focus on demonstrating electrical control of magneto-resistance by field-effect gating
transition metal dichacogenides coupled to arrays of magnetic NPs. Studies of field effect gating
12
EFRI 2-DARE: Electrically Controlled Magnetic 2D Layered Structures
in 3d-intercalated dichalcogenides will begin in parallel with the NP array structures somewhat
later in the project, depending on the results of the magnetometry studies of these materials.
As part of the preliminary work for this proposal, a wide variety of test structures have been
designed to characterize the electrical behavior of the devices. A complete set of masks have
been designed and fabricated for these experiments. The mask design shown in Fig 6 includes a
variety of top and bottom gated FETs, van der Pauw, and Hall bar structures, in addition to large
gated test structures for MOKE characterization. The mask design includes process monitors for
characterizing etching, metal deposition, isolation and other in-process and post-process tests.
13
EFRI 2-DARE: Electrically Controlled Magnetic 2D Layered Structures
KIEHL FIGS
n
Figure 6: Overview of a single chip. Close-up of the devices suitable for MOKE measurements is shown. These devices
were selected due Vto a large enough active area for nano-structure patterning enabling an overall measureable MOKE
g
signal.
λF
VDS
s
Vg
x=s
J
S
FM NP
van der Waals
layer
D
2DEG
Vg
depleted
G
VGS
SiO2
p-Si
H = HS
G
H=0
Vg
Figure
2.
Operating
principle.
As the gate
voltage is increased ii) n the
2DEG
electron
density
increases, ii) the Fermi
wavelength decreases, iii) J
the
exchange
coupling
oscillates and iv) G the
conductance
oscillates.
Applying a magnetic field Hs
sufficient to force parallel
alignment
of
the
ferromagnetic nanoparticles
will suprpresses the G
oscillation.
Figure 1. Schematic diagram of a device for investigating
field-effect controlled exchange interactions.
The
ferromagnetic (FM) nanoparticle (NP) clusters are closely
spaced along rows going into the plane and have an
inter-row spacing s.
Circles and crosses indicate
magnetization out of and into the plane. The spin of each
NP is transferred to the 2D electron gas (2DEG) in the
van der Waals channel layer via an exchange interaction,
resulting in GMR-like spin scattering for transport
through the 2DEG from the source S to the drain D.
Back-gating through the substrate electrode (G) controls
the electron density and Fermi wavelengthTopography
in the 2DEG,
thereby controlling the magnitude and sign of the RKKY
exchange interaction between the NP rows. The NP
design and layout are chosen to minimize magnetostatic
dipole interactions. Gating the 2DEG density controls the
amplitude and sign of the exchange interaction, thereby
switching the rows between antiparallel (shown) and
parallel (not shown) alignment..
Current
Figure
X.
Conductive
atomic-force-microscopy
(C-AFM)
topography and current images for Fe3O4 nanoparticles on singlelayer graphene demonstrating the achievement of electronic
coupling between the nanoparticles and graphene in this sample
by thermal annealing and other processing..
14
EFRI 2-DARE: Electrically Controlled Magnetic 2D Layered Structures
15
EFRI 2-DARE: Electrically Controlled Magnetic 2D Layered Structures
Theoretical studies
Theory work will include Density Functional Theory (Pickett-Jones), Quantum Monte Carlo
generalization of the model work in the MacDonald PRB 87, 144410 (Scalettar).
While model Hamiltonians illuminate the origins of many phenomena, material-specific
predictions that can be compared directly with experimental data to interpret the origin of the
behavior and then to predict where specific phenomena may occur, are provided by density
functional theory (DFT). DFT is reliable for structural information, and provides theoretical data
for magnetic properties (magnetic moments, magnetocrystalline anisotropy, exchange coupling)
as well as possible orbital ordering and coupling between lattice, charge, spin, and orbital
degrees of freedom. Pickett’s group has experience in predicting the ground state properties of
transition metal oxide interfaces [1WEP,2WEP,3WEP] and the effects of overlayers that provide
the metallic contacts. [4WEP,5WEP]
Examples of systems that will be studied in this project are Mn0 doped and Mn-intercalated
Bi2Ti3 and MoS2. These systems will be modeled using the all-electron codes Wien2k [6WEP]
and FPLO [7WEP], often after structural relaxation using the efficient VASP code.[8WEP]
Particularly relevant for connecting to the experimental data are the magnitude and directions of
the Mn moments, the magnetocrystalline anisotropy, and the coupling of the Mn moments in the
presence of various changes such as substrate gating. Magnetic coupling will be determined by
comparing energetics of different magnetic configurations, and noncollinear spin calculations
will be employed if necessary.
An important system, following the study of clusters of magnetic atoms on graphene,
[MacDonald, others] is an analogous study of the coupling of Mn or Co clusters on transition
metal sulfides or selenides (TMSs), such as MoS2. These materials can be exfoliated as ultrathin,
single-unit-cell layers, providing an alternative to the graphene examples that have been studied
heretofore.[9WEP]. Moreover, single-unit-cell layers can potentially be grown directly by MBE.
Whereas graphene provides a single band (per atom) platform and linear dispersion, gapped
massive small gap semiconductors, such as the materials studied in this project, hold greater
promise for the manipulation of spins and hence magnetic properties.
We plan to initially study Mn atoms and clusters (i.e. high spin moments, since spin-exchange
creates the RKKY magnetic coupling) on MoS2. The underlying band structure will be identified
by DFT, enabling the building of realistic model Hamiltonians that can host the effects of
interactions [10WEP] and disorder [11WEP]. In this way, the induced spin density reflecting the
RKKY coupling to itinerant electrons can be calculated directly on a nm scale.
A preliminary spin density functional theory study of Co on MgO(001) by Jones’ group (senior
investigator/industrial partner, this proposal) illustrates the potential for such systems to provide
the desired magnetic behavior. The large magnetic anisotropy of the Co moment results from a
very large orbital contribution that both enhances the total moment and anchors its direction
through spin- orbit coupling. The geometry (Co on top of O) and the shape of the Co
magnetization is illustrated in Fig. ZZ. This result points out the importance, for large orbital
moments a large anisotropy, of placing the magnetic atom atop a polarizable anion (viz., S, Te or
Se in our study).
16
EFRI 2-DARE: Electrically Controlled Magnetic 2D Layered Structures
Bonding occurs through the dz2 which carries no orbital moment, and the atop site maintains
approximate circular symmmetry, allowing SOC to form and occupy m = ±1, m = ±2 complex
orbitals instead of the circular-symmetry-broken real orbitals
Our group has long-standing expertise in the development and application of Quantum Monte
Carlo methods to low dimensional magnetic materials. This includes work on fundamental
models of magnetic phenomena and how they interplay with metal-insulator transitions, charge
order, and superconductivity [12–17], as well as on specific magnetic materials [18–21]. Figure 2
provides results from a recent quantum simulation study [22] of the competition between
antiferromagnetic exchange induced spin correlations and those produced by the RudermannKittel-Kasuya-Yoshida (RKKY) interactions. Both are key players in the materials to be studied
in this EFRI project.
Our proposed QMC work will, firstly, use appropriate tight binding Hamiltonians to compute the
coupling between magnetic clusters on top of metallic or semiconductor substrates. Figure 2 is
for a full magnetic layer, but our codes are written such that general geometries, including
spatially separated clusters, can be easily simulated. Likewise, the choice of interatomic
hybridizations is very flexible, allowing for the study of quite general (parameterized) band
structures. Appropriate hopping matrix elements and interaction stengths will be taken from the
literature, or from the partner DFT calculations undertaken in this project. These QMC studies
complement DFT by treating the electron-electron interactions more exactly. They are, however,
limited in the accessible temperatures and system sizes (roughly 103 electrons). A second QMC
effort will explore correlations between disordered magnetic impurities in a host material. The
interplay of randomness and electron interactions has been a long-standing interest of our group
[14, 23–31].
The work of the MacDonald group [41] combined density functional theory computations with a
kinetic-exchange model to understand the gate-tunable exchange coupling of cobalt clusters on
graphene. More specifically, the kinetic exchange model combines a Hamiltonian describing the
hopping of electrons on the substrate with energies which characterize the effect of the
superimposed magnetic clusters. The calculations reported in [41] treated the effects of the
magnetic clusters perturbatively, and focussed on the Dirac Hamiltonian for graphene. We
propose to generalize the kinetic energy exchange approach to the clusters and substrates studied
in this EFRI project. Our methodology will be the numerical diagonalization of these models
which, because of the absence of electron-electron interactions, can be done on very large lattices
which can capture quite complex substrates and cluster geometries
17
EFRI 2-DARE: Electrically Controlled Magnetic 2D Layered Structures
SCARETTAR FIGS
Figure 2: Short and long range spin correlations for two fixed
values of the localized electron-conduction band hybridization V ,
which controls the strength of the RKKY interaction, as a
function of the localized band hybridization tf , which controls the
antiferromagnetic exchange J ∼ t2f /Uf . Top and bottom rows are
two different inverse temperatures β = 1/T = 12, 36. Units are such
that if the conduction electron bandwidth were 3 eV , the
temperatures would be T = 375°K and T = 125°K.
Figure 3: Antiferromagnetic structure factor of the half-filled
Hubbard model on a 1/5 depleted square lattice at inverse
temperature β = 20 (in units of the larger of the two
hybridizations). In the limits t′/t << 1 and t/t′ << 1 the system
breaks into independent plaquettes and dimers respectively.
Saf is small and does not grow with system size. Close to t
 t´ the growth of Saf with number of sites suggests long
range order might be present.
Figure ZZ: (a) Illustration of the conventional means of probing the
geometry and polarization of a magnetic atom (or cluster) on a
semiconducting surface by scanning tunneling microscope. (b)
Density contour plot of the Co-atop-oxygen geometry. The lower
figures provide two views of the magnetization density on Co, with
some being induced on nearby oxygen ions.
18
EFRI 2-DARE: Electrically Controlled Magnetic 2D Layered Structures
Experimental activities and plans. [RK will revise this based on PI’s table info – PI’s
please add your input in Tables at end of this doc]
In the early stage of the project, systematic variations will be made to the FM nanoparticle and
2DEG substrate designs to explore the parameter space for gated exchange interactions.
Variations will be made in the nanoparticle material, diameter, shape and spacing in order to
vary coercivity, magnetization, anisotropy, interparticle dipole interactions and other parameters.
The focus will be on Fe and Co EBL-patterned nanoparticles in the 30 to 300 nm size regime.
Three different types of FM/SC structures having complementary aspects will be examined. The
study will begin with structures based on Fe epitaxially grown on GaAs by MBE. This structure
has already been optimized by one of the investigators and shown to provide excellent spin
injection. A modified structure grown on an n-type substrate will be designed to allow back
gating of the 2DEG. Back-gated structures based on Co deposited on p-InAs substrates will also
be designed and tested. This approach takes advantage of Fermi-level pinning to create a 2DEG
at the InAs surface and has also been shown to exhibit good spin injection. As a third alternative,
we will fabricate and characterize structures based on NiFe and other FM metals deposited on
graphene, which is itself on the surface of a SiO2/p-Si substrate. While spin injection into
graphene is a new subject, graphene supports a surface 2DEG that may conveniently be backgated and theoretical studies show graphene to be a highly promising candidate spin transfer 25.
The field-effect devices and associated test structures will be fabricated using a combination of
optical lithography and e-beam lithography. EBL will be used to define nanoparticle arrays in
small active regions approximately 10 to 100 microns on a side by ion milling and lift off
techniques optimized for nanoscale patterning. Source, drain and gate structures will be targeted
to the active regions using a previously designed optical lithography mask set optimized for
back-gated FETs on conductive substrates.
Initial characterization of the device structures will be carried out by MOKE and C-AFM. These
room-temperature characterization techniques will provide rapid feedback for optimization of the
structure design and fabrication processes. C-AFM will be used to examine the electronic
coupling between the nanoparticles and 2DEG. MOKE will be used to characterize the magnetic
properties of the nanoparticle arrays and to look for signatures of dipole and exchange
interactions using the FORC technique.
The electrical characteristics of the most promising devices will then be fully examined as a
function of gate voltage, magnetic field and temperature.
Qualifications of the investigators.
The principal investigators are well qualified for carrying out this project. Kiehl brings expertise
in the design, fabrication and characterization of III-V heterostructure FETs. His group will also
provide needed support in scanning probe microscopy. Liu brings expertise in magnetometry for
spintronic devices. His group will support the identification of gated exchange interactions
through FORC analysis.
Palmstrom brings expertise in the growth of epitaxial
ferromagnetic/semiconductor spintronic structures by MBE. His group will provide FM/SC
structures with well-established spin injection properties for this study.
19
EFRI 2-DARE: Electrically Controlled Magnetic 2D Layered Structures
Concluding remark on importance and outlook.
This project will serve to generally advance the field of spintronics by providing new results on
spin transfer in novel structures and materials. If fully successful, the results could be very far
reaching. The discovery and development of 2-terminal GMR devices was rapid and had a
revolutionary impact on the information storage industry. This project could lead to 3-terminal
GMR devices with enhanced capabilities, such as ultra-high sensitivity sensing via gate feedback
control and new capabilities, such as field-effect transistors with non-volatile states. Threeterminal GMR could enhance the capabilities for current applications and open the door to new
ones.
RESULTS FROM PRIOR NSF SUPPORT
“Materials World Network: Magnetic Nanostructures with Perpendicular Anisotropy”,
Kai Liu, DMR-1008791, $371,500; 8/1/10-7/31/14:
Intellectual Merit. Nanostructures with uniform and graded perpendicular anisotropy have been
realized, where the reversal can be influenced by both the vertical magnetic anisotropy gradient
and the lateral feature size. Structural integrity, deposition order and amount of disorders are
found to sensitively influence the magnetic properties. Ion irradiation has been shown as a new
method to achieve anisotropy gradient. Depth-dependent magnetization profiles and magnetic
anisotropy have been investigated by polarized neutron reflectivity. One notable accomplishment
is the realization of (001) oriented L10 (Fe1-xCux)55Pt45 thin films, with perpendicular magnetic
anisotropy up to 3.6×107 erg/cm3, using atomic-scale multilayer sputtering and post annealing at
400 °C for 10 seconds.26 These are significantly reduced thermal cycles compared to earlier
studies. The relatively convenient synthesis conditions, along with the tunable magnetic
properties, make such materials highly desirable for future heat-assisted magnetic recording
technologies. Broader Impacts. A Materials World Network has been established with
colleagues at National Tsing Hua University (NTHU) in Taiwan, NIST, and additional partners
in Spain, Germany, Japan and China. UCD graduate students worked at NTHU for extended
periods. The PI has co-organized three workshops and a conference symposium. He has
participated as faculty advisor in the California Professoriate for Access to Physics Careers
program, dedicated to mentoring CSU students, especially under-represented minorities, toward
obtaining advanced degrees in physics. One graduate student, Dustin Gilbert, won the 1st Prize
Margaret Burbidge Award at the 2013 APS-Far West Section meeting. This project has so far led
to 21 journal publications.15, 20, 26-44 The PI has given over 30 invited talks on results from this
award, including those at 2011 APS March Meeting, 2012 MRS and ACNS Meetings, and 2013
MMM Conference.
20
EFRI 2-DARE: Electrically Controlled Magnetic 2D Layered Structures
Broader Impacts.
The proposed project will have broad impacts on engineering, education, and society. Its success
could lead to new interdisciplinary research directions and enable new technologies with
applications ranging from information processing to bio-sensing in medicine. The educational
aspect of the proposal will be especially valuable for preparing students to be experts in
spintronic and semiconductor topics that are at the forefront of several disciplines and will
provide valuable ingredients for developing future academic and industrial leaders to carry out
innovative research, thus helping to establish the U.S. leadership position in this emerging field.
The graduate students will participate in regularly held joint group meetings. The PIs have been
actively integrating research activities into teaching and have active outreach programs. Kiehl
has a graduate student and post-doctoral scholar in his group who are both women. Liu has been
serving as a mentor for under-represented minority students (MURRPS program) and NSFsponsored as well as other REU students. The Davis campus also has a strong tradition in
involving undergraduate students in extramurally-funded research projects.
21
EFRI 2-DARE: Electrically Controlled Magnetic 2D Layered Structures
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EFRI 2-DARE: Electrically Controlled Magnetic 2D Layered Structures
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