Tuning magnetic properties in nanostructures on different length scales
Final Report Abstract
The aim of the joint binational project was an investigation of the possibility to tune magnetic properties of nanostructures via modification of interfaces on different length scales. This can be achieved, for instance, by variation of growth conditions, by self-organization of chemical and magnetic clusters during annealing, or via fabrication of lateral structures. The German team of Prof. Zabel investigated magnetic heterostructures with competing ferromagnetic, antiferromagnetic, and superconducting properties. These heterostructures were grown by various sputtering techniques and by molecular beam epitaxy. Electron beam lithography and optical lithography were used for laterally patterning the magnetic heterostructures. Resonant magnetic x-ray scattering and polarized neutron reflectivity were employed for advanced analysis of the heterostructures, in addition to more standard laboratory-based techniques, such as MOKE, SQUID, MFM. Using these methods we studied combinations of lateral patterns with magnetic nanoparticles. Assisted self-assembly of magnetic nanoparticles is of interest for potential high-density magnetic storage devices. We were interested to find out how trenches prepared by lithographic means in a Si substrate could assist the self-assembly process. While spin-coating of the nanoparticles and filling of the trenches was successfully executed, Bragg reflections verifying the filling factor of the trenches showed only little effects. This raised the question of how the filling factor of the trenches is expressed in x-ray structure factors. Extensive theoretical analysis showed that with a filling factor of 2/3 the structure factor is basically identical for filled and unfilled trenches. While this result can explain our data, the theoretical analysis is more general and can assist further analysis. The Russian team of Prof. Uzdin developed a theoretical approach that is capable to describe spontaneous transitions between magnetic states due to thermal fluctuations. The model encompasses magnetic nanostructures of size ranging from clusters containing a few atoms up to micro-magnetic systems, which have several stable magnetic states. Finding minimal energy paths (MEP) between these stable magnetic configurations allows calculating the lifetime of magnetic states. Maximum energy along the MEP determines the barrier between the states, whereas the curvature of the energy surface near saddle points and local minima yield an analytical expression for the pre-exponential factor in an Arrhenius-like law for transition rates. This new model provides a powerful method for analyzing magnetization reversal processes in magnetic nanoparticles.
Publications
- Magnetization reversal process at atomic scale in systems with itinerant electrons. J. Phys.: Condens. Matter 24, 176002 (2012)
V.M. Uzdin, A. Vega
- “Noncollinear Fe spin structure in (Sm-Co)/Fe exchange-spring bilayers: Layerresolved 57Fe Mossbauer spectroscopy and electronic structure calculations”, Phys. Rev. B 85, 024409 (2012)
V. M. Uzdin, A. Vega, A. Khrenov, W. Keune, V. E. Kuncser, J. S. Jiang, and S. D. Bader
(See online at https://doi.org/10.1103/PhysRevB.85.024409) - Potential Energy Surfaces and Rates of Spin Transitions, Zeitschrift für Physikalische Chemie. 227, N 11, 1543 (2013)
P. F. Bessarab, V. M. Uzdin, and H. Jónsson
- Size and Shape Dependence of Thermal Spin Transitions in Nanoislands, Phys. Rev. Lett., 110, 020604 (2013)
P. F. Bessarab, V. M. Uzdin, and H. Jónsson
- Manipulation by exchange coupling in layered magnetic structures. J. Appl. Phys., 115, 053913 (2014)
M.A. Moskalenko, V.M. Uzdin, H. Zabel
(See online at https://doi.org/10.1063/1.4864429) - Template assisted self-assembly of iron oxide nanoparticles: An x-ray structural analysis. J. Appl. Phys., 115, 054104 (2014)
D. Mishra, H. Zabel, S.V. Ulyanov, V.P. Romanov, V.M. Uzdin
(See online at https://doi.org/10.1063/1.4864129)
