Spin dynamics and spin-orbit interactions in magnetic nano-structures
Timothy Ziman1, Gerrit Bauer2, Michiyasu Morii3
1
2

DInIstitut Laue Langevin and Universite Grenoble-Alpes

Delft University of Technology and IMR, Tohoku University,
3

Advanced Science and Research Center, JAEA

Abstract

We study the influence of the interplay of spin-orbit interactions and interactions on the dynamics of magneticstructures, such as domain walls and spin currents in alloys different magnetically ordered structures:
ferromagnets, antiferromagnets and more complicated magnetic order. We study the interaction of the magnetic
order parameter with phonon and photon fields in the search for strong coupling phenomena.

1. Research Objectives

Spin-orbit interactions constitute the microscopic mechanism for many key effects in both pure and appliedmagnetism, involving coupling of orbital and spin degrees of freedom. Many of the useful and interesting effects
are limited by their size. In spin-dependent skew scattering, the cornerstone of efforts to produce spin currents,
the generated currents are small because the spin-orbit interaction is small in all except the heaviest elements
and because the asymmetry depends on higher order interference effects. Ways around this limitation include
enhancement by correlations and by resultant resonant scattering effects.In the search for larger effects we canalso look to inclusion of scattering of low frequency collective excitations. This lead to examination, forexample, of skew scattering effects where there is critical slowing down, the simplest example being that
occurring near the Curie temperature of a ferromagnet. Such investigations lead to the discovery of aninteresting anomaly and a novel probe of non-linear fluctuations near such a critical point. This suggests that
other anomalous effects may occur in glassy systems where the hierarchy of low energy fluctuations may persist
over a wider range of temperatures.
Spin-orbit effects are also basic to the effects of crystalline anisotropy in magnetic systems and the coupling
between magnetism and lattice effects. Part of the project was to investigate in detail such couplings in, forexample, multifunctional materials such as multi-ferroics and in the effects of Dzyaloshinsky-Moriya
interactions in domain wall motion. The spin-orbit interaction also causes the magnetoelastic interaction andthereby couples lattice vibrations (phonons) with spin waves (magnons) in magnetic materials. In favourable
cases the coupling can be strong and cause remarkable effects. Several experiments motivated us to interpretthese couplings and analyse what can be learned from the observations.

2. Research Contents

We investigated systematically correlation effects on skew scattering by both impurities in alloys and bycollective excitations in various forms of cooperatively ordered solids including ferromagnets,
antiferromagnets and magnetic glasses. We were guided by choices of experimental colleagues at the ASRC
and elsewhere in order to make direct comparisons. The methods were a combination of state of the art
numerical techniques, for example a combined approach of density functional methods and quantum Monte
Carlo and simpler Hartree-Fock approximations, and more phenomenological approaches using as input
correlations as observed by a variety of experimental techniques for materials, such as neutron scattering, Xray scattering, and muons, as well as device characteristics. We also studied the effects of magnetoelastic
coupling on the transport properties of magnetic insulators such as yttrium iron garnet (YIG).
3. Research results

As part of a wider program [1] to understand quantitatively Spin Hall
angles in alloys and bulk magnetic solids we analysed [3] of the spin Hall
effect in the Cu alloys doped with a series of 5d elements, by the combined
approach of density functional theory and Hartree–Fock approximation. We
found (see Fig.1) that the local correlations in the 5d orbitals of theimpurities are decisive in determining the sign of the spin Hall angle (SHA).

Fig 1：Predicted Spin Hall with and without 5d correlation.
FIncluding all

R f [3]Correlations and the spin-orbit

strengths fully we predict the SHA for each alloy in the series. The signs ofCuIr and CuPt are sensitive to perturbation of the local correlations. Thisobservation may suggest mechanisms for controlling the sign of thetransverse spin Hall voltage.
We proposed[2] a method to realize diluted magnetic semiconductors(DMSs) with both p- and n-type carriers by choosing host semiconductors
with a narrow band gap with starting point the recent observations of Mndoped BaZn2As2. By density functional and quantum Monte Carlosimulation, we suggested the possibility of using Mn-doped BaZn2As2. We
also predicted a nontoxic DMS, with As replaced by Sb, could have an evenhigher Curie temperature.
We develop a Boltzmann transport theory for magnon-polarons anddetermined transport coefficients and spin diffusion length, as shown in Fig.
2.. Magnon polaron formation causes anomalous features in the magnetic
field and temperature dependence of the spin Seebeck effect when thedisorder scattering in the magnetic and elastic subsystems is sufficientlydifferent. Experimental data by Kikkawa et al. [PRL 117, 207203 (2016)]
on YIG was shown to be explained if the acoustic quality is much better
than the magnetic quality of the material.
We also organized the 37th Reimei Workshop on Frontiers of Correlated

Quantum Matters and Spintronics in conjunction with the DMREF-FCMP
Fig 2：The sample configuration (left) and calculated spincurrent–temperature

curvesfor

diferentin-plane

Symposium, the 37th Reimei meeting on “Mott Transitions andComputational Approaches: was organized in Tokai and Tokyo inconjunction with the Reimei program of Y.Uemura. and we contributed to“Spin Mechanics IV” (39th Reimei) in British Columbia.

4. Conclusion
We found new results in the theory of skew scattering from impurities and collectively ordering magneticsystems as summarized in Ref. [1,3]. New ideas were developed in narrow band gap semiconducting systems that
may assist in the search for new higher temperatures diluted semiconductors.[2] New concepts in the coupling ofmagnons and polarons should be important in the burgeoning fields of spin Seebeck devices [4,5]. Apart from theregular collaboration between different members of the Reimei project, there has been great benefit in bringing
together people with different perspectives and contacts with different experimental groups – in Tokai, Tohoku,
Grenoble and Delft. We organized a joint meeting in Tokyo and Tokai with Y. Uemura’s muon Reimei project .
5. References[1] 'Skew Scattering from Correlated Systems: Impurities and Collective Excitations in the Spin Hall Effect',
Timothy Ziman, Bo Gu, and Sadamichi Maekawa, J. Phys. Soc. Jpn. 86, 011005 (2017).
[2] 'Diluted magnetic semiconductors with narrow band gaps', Bo Gu and Sadamichi Maekawa, Phys. Rev. B94, 155202 (2016).
[3] 'What determines the sign of the spin Hall effects in Cu alloys doped with 5d elements?', Z. Xu, B. Gu, M.

Mori, T. Ziman, and S. Maekawa J. Magn. Magn. Mater. 400 184–187 (2016).
[4]'Magnon-Polarons in the Spin Seebeck Effect', T. Kikkawa, K. Shen, B. Flebus, R. A. Duine, K. Uchida, Z.

Qiu, G.E.W.Bauer, and E. Saitoh, Phys. Rev. Lett. 117, 207203 (2016).
[5] D. Hou, Z. Qiu, R. Iguchi, K. Sato, E. K. Vehstedt, K. Uchida, G. E.W. Bauer, and E. Saitoh, Observationof temperature-gradient induced magnetization, Nat. Commun. 7, 12265 (2016)