Magnetic and structural ordering in Fe3O4-Metal interfaces

G. E. MURGIDA; M. A. BARRAL; J. PEREA ACOSTA; A. M. LLOIS; M. H. AGUIRRE

Upsala

Simposio; Joint European Magnetic Symposia 2019; 2019

The ability to transform heat currents into spin currents (Spin Seebeck Effect, SSE) has been intenselyinvestigated in the last years with the driving force of both, the development of efficient thermoelectricpower generators and, on the other hand, the achievement of a fundamental explanation of thisphenomenon, which still presents controversies and an important lack of understanding. The SSE waspreviously reported in several ferromagnetic materials, including semiconductors, metals, and insulators.To detect the SSE, thin films of non-ferromagnetic metals with large spin orbit coupling are deposited onthe ferromagnetic material (FM). The spin current generated by thermal gradient is pumped from themagnet into the metal, where a transverse charge current, that can be detected and measured, due tothe inverse spin Hall Effect (ISHE). The understanding of the structural and magnetic coupling at theFM-Metal interface is crucial in order to clarify the involved mechanisms and to develop and optimizeSSE effect based devices. In this work, using the framework of Density Functional Theory (DFT), westudy the electronic, magnetic and structural ordering in magnetite (Fe 3 O 4 ), which is one of the mostpromising materials together with YIG to exploit the SSE, and at the interfaces of Magnetite withdifferent non magnetic metals.We use DFT in the GGA+U approach, considering a Hubbard term for the electrons localized at dorbitals and taking into account the spin orbit coupling. The charge localization process in the Fe 2+polarons of magnetite is studied, showing that it can be achieved still in the cubic phase without previousstructural deformations, and allowing the conduction by polaron hopping in such cubic phase.Reconstructions of the (001) surface termination of cubic magnetite, are simulated to compare theirstructural stability and to realize the surface effects on the electronic and magnetic properties. Finally,Fe 3 O 4 -Metal interfacial properties are analyzed considering thin films of non-magnetic metals with largespin orbit coupling (Pt, Au, Nb, Ta, and W) deposited on the (001) magnetite surface. We find that thesemetallic over-layers grow preferentially in the (001) direction, on top of the oxygen ions. The totalmagnetization density as a function of z (along the (001) direction) shows that, depending on eachmetal, the first or the first two atomic metallic layers on the magnetite surface present a moderatemagnetization, and this magnetization vanishes within the next atomic layers. The Au over-layer exhibitsthe lowest magnetization, with 0.01 μ B per atom versus magnetizations ranging from 0.1 to 0.2 μ B in Pt,Nb, Ta and W. The metals with more than half of the d orbitals occupied (Pt and Au) coupleferromagnetically to the upper layer of magnetite, while the metals with less than five d electrons (Nb, Taand W) show an antiferromagnetic coupling with the magnetite surface. We think these results canprovide insight into the fundamentals and contribute to the development of efficient devices for spincaloritronics.