Tuesday 9 July 2013, Pentland Room, 11:00-13:00
Thin film magnetism, nano magnetism and molecular magnetism
(invited) Probing the magnetic behaviour of perpendicular magnetic recording media using small-angle polarised
neutron scattering
S Lee1, S Lister2, V Venkataramana 2, T Thomson3, J Kohlbrecher4, K Takano5, H Ikeda5, L J Heyderman4 and G Heldt4
1
University of St. Andrews, UK, 2SUPA, UK, 3University of Manchester, UK, 4Paul Scherrer Institute,
Switzerland, 5Hitachi San Jose Research Center, USA
Perpendicular magnetic recording media are found in all modern magnetic hard drives, the data storage technology
that continues to be of tremendous commercial and technological importance. These media are advanced
functional multilayered materials, containing an active recording layer of only around 10 nm in thickness. This
recording layer is compositionally segregated into 8 nm-sized grains of a magnetic CoCrPt alloy separated by a thin
oxide shell, typically SiO2. These media have their magnetic moments oriented perpendicular to the plane of the
film. Determining the local magnetic structure and reversal behaviour is key to understanding the performance of
perpendicular media in recording devices. SANS is a very effective tool to measure these materials at a sub-10nm
length scales. The experiments are challenging due both to the small scattering volume available and to the smallangle scattering from other layers in the structure, some of which are also magnetic. Polarised SANS has proved a
particularly effective technique with which to study these materials. We will present a summary of some recent
results on recording media, including measurements of the grain-sized dependent switching with and without the
presence of an exchange spring. We will also briefly mention experiments that demonstrate the viability of extending
this approach to measurement for lithographically defined structures similar to those for application in bit-patterned
media.
In-situ neutron reflectometry during thin film growth by sputter deposition
B Wiedemann1, W Kreuzpaintner1, T Mairoser2, A Schmehl2, A Herrnberger2, J-F Moulin3, M Haese-Seiler3, M Pomm3,
P Böni1 and J Mannhart3
1
Technische Universität München, Germany, 2Zentrum für elektronische Korrelation und Magnetismus, Universität
Augsburg, Germany, 3Planck-Institut für Festkörperforschung, Germany
Thin magnetic layers and heterostructures are the basic building blocks of a large number of magneto-electronic
devices whose performance relies on the morphology and microstructure of the individual layers and on the
coupling between them. Since these parameters can change during the process of growth, it is important for the
understanding and optimization of the devices to accurately monitor the structural and magnetic thin film properties
during the deposition process. While the structural characterization of thin films during growth by various techniques
is common practice, the in-situ measurement of the magnetic properties of films using polarised neutron
reflectometry is a challenging task. At FRM II, within a collaboration of Technische Universität München, Universität
Augsburg and MPI Stuttgart, we operate a sputtering facility at the neutron reflectometer REFSANS for the growth
and in-situ monitoring of magnetic multilayers. In our contribution, the experimental setup and first proof of principle
results will be presented.
ICNS 2013 International Conference on Neutron Scattering
Giant proximity effect and critical opalescence in magnetic materials
T Charlton1, S Ramos2 and J Quintanilla3
1
Science and Technology Facilities Council, UK, 2Diamond Light Source, UK, 3ISIS Facility, UK/University of Kent, UK
The proximity effect is a phenomenon where an ordered state (e.g. magnetism or superconductivity) “leaks” from a
material into an adjacent one over some finite distance, ξ. For superconductors, the characteristic range ξ is of the
order of the coherence length. Nevertheless a much longer, "giant" proximity effects have been observed in cuprate
junctions. This surprising effect can be understood as a consequence of critical opalescence. Since critical
opalescence occurs in all second order phase transitions giant proximity effects are expected to be very general
and, in particular, they should also be present in magnetic systems. Compared to its superconducting counterpart,
the ferromagnetic proximity effect has the advantage that its order parameter (magnetization) can be observed
directly.
We have fabricated Co/EuS thin film samples, where both materials undergo ferromagnetic transitions but at rather
different temperatures (bulk TC of 1400K for Co and 16.6K for EuS). A dramatic increase in the extent of the
proximity effect is expected to occur near the TC of EuS. We will present the results of our measurements of the
magnetization profiles as a function of temperature, carried out using the complementary techniques of low energy
muon relaxation and polarized neutron reflectivity.
Element specific and depth-resolved interface magnetism in BiFeO3/ La0.33Sr0.67MnO3 thin films
J Bertinshaw1, S Brück2, H Fritzsche3, Y Khaydukov4, O Soltwedel4, T Keller4, E Goering5, P Audehm5, D Lott6, W
Hutchison1, R Maran1, N Valanoor1, F Klose2 and C Ulrich1
1
University of New South Wales/ANSTO, Australia, 2Bragg Institute, Australia, 3Chalk River Laboratories, Canada,
Max-Planck-Institut für Festkörperforschung, Germany, 5Max-Planck-Institut für Intelligente Systeme, Germany,
6
Helmholtz-Zentrum Geesthacht, Germany,
4
In recent years a significant research effort has been conducted to probe the emergent physics at transition metal
oxide layered systems [1,2]. It is evident that the spin, orbital and charge state of electrons in the interfacial region
largely define the exhibited properties, such as the tunnelling potential in multiferroic tunnel junctions [3]. A largely
neglected experimental characteristic concerns the stoichiometry at the interfacial region that may drastically
impact the electronic and magnetic properties.
In this work we investigate the effect of interfacial stoichiometry modification on the magnetic properties of bi-layers
of multiferroic BiFeO3/ferromagnetic La0.33Sr0.67MnO3 (LSMO). Polarised Neutron Reflectivity (PNR) was conducted at
ANSTO, Sydney; Chalk River, Canada; and FRM-II, Munich to measure the absolute magnetic moment through the
bi-layer, and revealed a region of depleted magnetisation, extending ~25Å into the LSMO at the bi-layer interface
region, significantly larger than the atomic scale interface roughness. X-ray Resonant Magnetic Reflectivity
measurements performed at BESSY II in Berlin provided element specific magnetic information as well as a precise
chemical profile, which confirmed the results of the PNR study, and indicated a corresponding region of modified
stoichiometry at the interface. Thus, using the combined X-ray synchrotron and PNR techniques, we were able to
determine the precise chemical and magnetic properties of the interfacial region.
[1]
[2]
[3]
H. Y. Hwang et al., Nature Materials 11, 103 (2012).
J. Mannhart & D. G. Schlom, Science 327, 1607 (2010).
M. Hambe et al., Adv. Func. Mat. 20, 2436 (2010).
ICNS 2013 International Conference on Neutron Scattering
(invited) The origin of the single molecule magnet behaviour in a Co(II) tetramer investigated using inelastic neutron
scattering spectroscopy
S Ochsenbein1, M Murrie2, G Chaboussant3 and A Farrell2
1
Paul Scherrer Institut, Switzerland, 2University of Glasgow, UK, 3Laboratoire Léon Brillouin, France
The molecular spin cluster (C(NH2)3)8[Co4(cit)4] (Co4) is a so-called single molecule magnet (SMM). The term SMM
refers to the class of molecules that display magnetic bistability due to an energy barrier for the reversal of the
magnetisation direction, which in principle allows the storage of information on a single molecule. The above
molecule contains divalent, six-coordinate cobalt ions, which have a ground state with non-zero orbital angular
momentum. This orbital angular momentum contribution leads to anisotropic exchange interactions between the
four Co(II) ions in Co4. Presumably, these anisotropic exchange interactions are at the origin of the energy barrier
for reversal of the magnetisation direction, but these compounds are less well understood than their spin-only
counterparts. Inelastic neutron scattering (INS) spectroscopy is the ideal tool to determine the energy level diagram
governed by these interactions, and thus to gain an understanding of the origin of the SMM behaviour.
We performed polarised INS experiments on Co4 to differentiate between magnetic and vibrational excitations, and
were able to measure the exchange splitting in this SMM. With the findings from INS we were able to model the
results from other experimental methods, such as magnetometry and magnetic circular dichroism spectroscopy.
Furthermore, diffuse polarised scattering allowed us to distinguish the various contributions to the scattering
intensity of Co4 (nuclear-spin incoherent, coherent and isotope incoherent, magnetic).
ICNS 2013 International Conference on Neutron Scattering