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論文

Magnetic structures and spin-wave excitations in rare-earth iron garnets near the compensation temperature

森 道康; Ziman, T.*

IEEE Transactions on Magnetics, 59(11), p.1300505_1 - 1300505_5, 2023/11

We introduce a simple model for the ferrimagnetic non-collinear "magnetic umbrella" states of rare-earth iron garnets (REIG), common when the rare-earth moments have non-zero orbital angular momentum. The spin-wave excitations are calculated within linear spin wave theory and temperature effects via mean-field theory. By the symmetry of the model, only one rare earth mode hybridizes, inducing a gap at zero wave number and level repulsion at finite frequency. At the compensation point, the hybridization gap closes and finally, as we approach the N$'e$el temperature, the hybridization gap appears to reopen. The chirality of the lowest mode changes its sign around the frequency at which the level repulsion occurs. This is important to estimate the spin current generation in REIGs.

論文

Observation of magnon polarization

南部 雄亮*; Barker, J.*; 沖野 夕貴*; 吉川 貴史*; 塩見 雄毅*; Enderle, M.*; Weber, T.*; Winn, B.*; Graves-Brook, M.*; Tranquada, J. M.*; et al.

Physical Review Letters, 125(2), p.027201_1 - 027201_6, 2020/07

 被引用回数:35 パーセンタイル:93.71(Physics, Multidisciplinary)

We measure the mode-resolved direction of the precessional motion of the magnetic order, i.e., magnon polarization, via the chiral term of inelastic polarized neutron scattering spectra. The magnon polarization is a unique and unambiguous signature of magnets and is important in spintronics, affecting thermodynamic properties such as the magnitude and sign of the spin Seebeck effect. However, it has never been directly measured in any material until this work. The observation of both signs of magnon polarization in Y$$_3$$Fe$$_5$$O$$_{12}$$ also gives direct proof of its ferrimagnetic nature. The experiments agree very well with atomistic simulations of the scattering cross section.

論文

Skew scattering from correlated systems; Impurities and collective excitations in the spin Hall effect

Ziman, T.*; Gu, B.; 前川 禎通

Journal of the Physical Society of Japan, 86(1), p.011005_1 - 011005_8, 2017/01

 被引用回数:1 パーセンタイル:12.14(Physics, Multidisciplinary)

We examine the enhancement of the spin Hall effect due to resonant skew scattering induced by electron correlations. For single-impurity scattering, local Coulomb correlations may significantly change the observed spin Hall angle. There may be additional effects because of the special atomic environment close to a surface: extra degeneracies compared to the bulk, enhanced correlations that move the relative $$d$$- or $$f$$-levels, and interference effects coming from the lower local dimension. Our results may explain the very large spin Hall angle observed in CuBi alloys. We discuss the impact on the spin Hall effect from cooperative effects, firstly in an itinerant ferromagnet where there is an anomaly near the Curie temperature originating from high-order spin fluctuations. The second case considered is a metallic spin glass, where exchange via slowly fluctuating magnetic moments may lead to the precession of an injected spin current.

論文

What determines the sign of the spin Hall effects in Cu alloys doped with 5d elements?

Xu, Z.; Gu, B.; 森 道康; Ziman, T.*; 前川 禎通

Journal of Magnetism and Magnetic Materials, 400, p.184 - 187, 2016/02

 被引用回数:4 パーセンタイル:21.44(Materials Science, Multidisciplinary)

We perform a systematical analysis of the spin Hall effect (SHE) in the Cu alloys doped with a series of 5d elements, by the combined approach of density functional theory and Hartree-Fock approximation. We find that not only the spin orbit interactions (SOI) in both the 5d and 6p orbitals, but also the local correlations in the 5d orbitals of the impurities, are decisive on the sign of the spin Hall angle (SHA). Including all of these three factors properly, we predict the SHA for each alloy in the series. The signs of CuIr and CuPt are sensitive to perturbation of the local correlations. This observation is favorable for controlling the sign of the transverse spin Hall voltage.

論文

Strong suppression of the spin hall effect in the spin glass state

新見 康洋*; 木俣 基*; 大森 康智*; Gu, B.; Ziman, T.*; 前川 禎通; Fert, A.*; 大谷 義近*

Physical Review Letters, 115(19), p.196602_1 - 196602_5, 2015/11

 被引用回数:11 パーセンタイル:63.21(Physics, Multidisciplinary)

We have measured spin Hall effects in spin glass metals, CuMnBi alloys, with the spin absorption method in the lateral spin valve structure. Far above the spin glass temperature $$T_{g}$$ where the magnetic moments of Mn impurities are randomly frozen, the spin Hall angle of a CuMnBi ternary alloy is as large as that of a CuBi binary alloy. Surprisingly, however, it starts to decrease at about 4$$T_{g}$$ and becomes as little as 7 times smaller at 0.5$$T_{g}$$. A similar tendency was also observed in anomalous Hall effects in the ternary alloys. We propose an explanation in terms of a simple model considering the relative dynamics between the localized moment and the conduction electron spin.

論文

Enhanced spin Hall effect by electron correlations in CuBi alloys

Gu, B.; Xu, Z.; 森 道康; Ziman, T.*; 前川 禎通

Journal of Applied Physics, 117(17), p.17D503_1 - 17D503_4, 2015/05

 被引用回数:7 パーセンタイル:31.13(Physics, Applied)

We find that the SHA can be dramatically enhanced by Bi impurities close to the Cu surface. The mechanisms of this enhancement are two-fold. One is that the localized impurity state on surface has a decreased hybridization and combined with Coulomb correlation effect. The other comes from the low-dimensional state of conduction electrons on surface, which results in a further enhancement of skew scattering by impurities. Furthermore, we note that a discrepancy in sign of SHA between the experiment and previous theories is simply caused by different definitions of SHA. This re-establishes skew scattering as the essential mechanism underlying the spin Hall effect in CuBi alloys.

論文

Analysis of the spin Hall effect in CuIr alloys; Combined approach of density functional theory and Hartree-Fock approximation

Xu, Z.; Gu, B.; 森 道康; Ziman, T.*; 前川 禎通

Journal of Applied Physics, 117(17), p.17D510_1 - 17D510_4, 2015/05

 被引用回数:4 パーセンタイル:18.73(Physics, Applied)

We analyze the spin Hall effect in CuIr alloys in theory by the combined approach of the density functional theory (DFT) and Hartree-Fock (HF) approximation. The spin Hall angle (SHA) is obtained to be negative without the local correlation effects. After including the local correlation effects of the 5$$d$$ orbitals of Ir impurities, the SHA becomes positive with realistic correlation parameters and consistent with experiment. Moreover, our analysis shows that the DFT + HF approach is a convenient and general method to study the influence of local correlation effects on the spin Hall effect.

論文

Sign change of the spin Hall effect due to electron correlation in nonmagnetic CuIr alloys

Xu, Z.; Gu, B.; 森 道康; Ziman, T.*; 前川 禎通

Physical Review Letters, 114(1), p.017202_1 - 017202_5, 2015/01

 被引用回数:19 パーセンタイル:73.56(Physics, Multidisciplinary)

Recently a positive spin Hall angle (SHA) of 0.021, was observed experimentally in nonmagnetic CuIr alloys and attributed predominantly to an extrinsic skew scattering mechanism, while a negative SHA was obtained from ab initio calculations, using consistent definitions of the SHA. We reconsider the SHA in CuIr alloys, with the effects of the local electron correlation U in 5d orbitals of Ir impurities, included by the quantum Monte Carlo method. We found that the SHA is negative if we ignore such local electron correlation, but becomes positive once U approaches a realistic value. This may open up a way to control the sign of the SHA by manipulating the occupation number of impurities.

論文

Negatively charged muonium as a detector of electron spin polarization; A Puzzle and a possible theory

Gu, B.; Ziman, T.*; 前川 禎通

JPS Conference Proceedings (Internet), 2, p.010301_1 - 010301_5, 2014/03

We explore theoretical explanations for laser pumped experiments, that have established that muonium can be a tool in spintronics for following non-equilibrium spin polarizations of the conduction electrons. In the original experiments on Si the sensitivity was argued to be due to spin exchange with the bound electron in muonium, but in GaAs an effect is seen even in the case of negatively charged muonium. This is more problematic, as spin exchange with a third electron should be weak given the spin singlet state of the two bound electrons of the negative muonium ion. We discuss a mechanism to explain the sensitivity of negatively charged muonium ions to the spin-polarization of semiconductors in terms of the coherent mixing of charge states induced by hybridization with the semiconducting host.

論文

Theory of the spin Hall effect, and its inverse, in a ferromagnetic metal near the Curie temperature

Gu, B.; Ziman, T.*; 前川 禎通

Physical Review B, 86(24), p.241303_1 - 241303_5, 2012/12

 被引用回数:11 パーセンタイル:46.8(Materials Science, Multidisciplinary)

We give a theory of the inverse spin Hall effect (ISHE) in ferromagnetic metals based on skew scattering via collective spin fluctuations. This extends Kondo's theory of the anomalous Hall effect (AHE) to include short-range spin-spin correlations. We find a relation between the ISHE and the four-spin correlations near the Curie temperature $$T$$$$_{rm c}$$. Such four-spin correlations do not contribute to the AHE, which relates to the three-spin correlations. Thus our theory shows an essential difference between the AHE and ISHE, providing an essential complement to Kondo's classic theory of the AHE in metals. We note the relation to skew-scattering mechanisms based on impurity scattering. Our theory can be compared to recent experimental results by Wei et al. for the ISHE in ferromagnetic alloys.

論文

The Spin Hall effect as a probe of nonlinear spin fluctuations

Wei, D. H.*; 新見 康洋*; Gu, B.; Ziman, T.*; 前川 禎通; 大谷 義近*

Nature Communications (Internet), 3, p.1058_1 - 1058_5, 2012/09

 被引用回数:33 パーセンタイル:80.78(Multidisciplinary Sciences)

The spin Hall effect (SHE) and its inverse have key roles in spintronic devices as they allow conversion of charge currents to and from spin currents. The conversion efficiency strongly depends on material details, such as the electronic band structure and the nature of impurities. Here we show an anomaly in the inverse SHE in weak ferromagnetic NiPd alloys near their Curie temperatures with a shape independent of material details, such as Ni concentrations. By extending Kondo's model for the anomalous Hall effect (AHE), we explain the observed anomaly as originating from the second-order nonlinear spin fluctuation of Ni moments. This brings to light an essential symmetry difference between the SHE and the AHE, which reflects the first-order nonlinear fluctuations of local moments. Our finding opens up a new application of the SHE, by which a minuscule magnetic moment can be detected.

論文

Giant spin Hall effect of Au films with Pt impurities; Surface-assisted skew scattering

Gu, B.; Ziman, T.*; Guo, G.-Y.*; 永長 直人; 前川 禎通

Journal of Applied Physics, 109(7), p.070502_1 - 070502_3, 2011/03

表面で発生するスキュー散乱の効果を考えることにより、室温で巨大なスピンホール効果が現れることを理論的に示した。密度汎関数理論と量子モンテカルロ法を組合せ、さまざまな金の中におけるプラチナ不純物によるスピンホール効果を調べた。バルクの金ではスピンホール角は小さいにもかかわらず、金の(111)面において、スピンホール角が0.1よりも大きくなりうること、また金(001)面上では約半分に減少することを示した。この量子モンテカルロによる結果は、プラチナ不純物のスピン軌道相互作用が金の(001)面と(111)面において増大していることを示している。この現象は、プラチナの5dレベルは電荷ゆらぎによってフェルミレベルまで上昇するため、加えてバルクの金や金(001)面上においてはプラチナに対して1つのチャンネルだけであるが、金(111)面上では、2つのチャンネルが存在するために生じることを明らかにした。

論文

Surface-assisted spin Hall effect in Au films with Pt impurities

Gu, B.; 管井 勇*; Ziman, T.*; Guo, G. Y.*; 永長 直人; 関 剛斎*; 高梨 弘毅; 前川 禎通

Physical Review Letters, 105(21), p.216401_1 - 216401_4, 2010/11

 被引用回数:71 パーセンタイル:90.45(Physics, Multidisciplinary)

We show, both experimentally and theoretically, a novel route to obtain giant room temperature spin Hall effect due to surface-assisted skew scattering. In the experiment, we report the spin Hall effect in Pt-doped Au films with different thicknesses $$t_N$$. The giant spin Hall angle $$gamma_S$$ = $$0.12 pm 0.04$$ is obtained for $$t_N$$ = 10 nm at room temperature, while it is much smaller for $$t_N$$ = 20 nm sample. Combined ${it ab initio}$ and quantum Monte Carlo calculations for the skew scattering due to a Pt impurity show $$gamma_S$$ $$cong$$ 0.1 on the Au (111) surface, while it is small in bulk Au. The quantum Monte Carlo results show that the spin-orbit interaction of the Pt impurity on the Au (111) surface is enhanced, because the Pt 5$$d$$ levels are lifted to the Fermi level due to the valence fluctuation. In addition, there are two spin-orbit interaction channels on the Au (111) surface, while only one in bulk Au.

論文

Quantum renormalization of the spin hall effect

Gu, B.; Gan, J.-Y.*; Bulut, N.*; Ziman, T.*; Guo, G.-Y.*; 永長 直人*; 前川 禎通

Physical Review Letters, 105(8), p.086401_1 - 086401_4, 2010/08

 被引用回数:31 パーセンタイル:79.18(Physics, Multidisciplinary)

金に含まれる鉄不純物のスピン軌道相互作用による輸送特性を、多軌道アンダーソン模型に量子モンテカルロ法を用いて研究を行った。そして、初めてスピン軌道相互作用がスピン揺らぎにより強い変更を受けることを明らかにした。この仕組みにより、最近観測された金の中の鉄不純物による巨大なスピンホール効果を説明することができた。一方、この効果は異常ホール効果には現れないものである。加えて、スピン軌道相互作用がクーロン相互作用によっても大きな変更を受けることを示した。これらの結果により、金の中の鉄不純物の軌道角運動量に関するこれまでの矛盾を解決することができた。

論文

Dzyaloshinski-Moriya interaction in the 2D spin gas system SrCu$$_{2}$$(BO$$_{3}$$)$$_{2}$$

C$'e$pas, O.*; 加倉井 和久; Regnault, L. P.*; Ziman, T.*; Boucher, J. P.*; 阿曽 尚文*; 西 正和*; 陰山 洋*; 上田 寛*

Physical Review Letters, 87(16), p.167205_1 - 167205_4, 2001/10

 被引用回数:102 パーセンタイル:92.09(Physics, Multidisciplinary)

Dzyaloshinski-Moriya(DM)相互作用が直交ダイマー系SrCu$$_{2}$$(BO$$_{3}$$)$$_{2}$$の磁気的フラストレーションを一部解消することを明らかにした。ESRや中性子非弾性散乱で観察された第一トリプレット励起状態の微細構造及び異常な磁場依存性がこの静的DM相互作用で説明できることを示し、第一トリプレット励起の分散の主なる原因であることを主張した。またESR遷移の強度を説明するためにスピン-フォノン相互作用による動的DM相互作用の可能性も指摘した。

口頭

Giant spin Hall effect of Au films with Pt impurities; Surface-assisted skew scattering

Gu, B.; Ziman, T.*; Guo, G.-Y.*; 永長 直人*; 前川 禎通

no journal, , 

We show theoretically a novel route to obtain giant room temperature spin Hall effect (SHE) using surface-assisted skew scattering. By a combined approach of density functional theory and the quantum Monte Carlo (QMC) method, we have studied the SHE due to a Pt impurity in different Au hosts. We show that the spin Hall angle (SHA) could become larger than 0.1 on the Au (111) surface, and decreases by about a half on the Au (001) surface, while it is small in bulk Au. The QMC results show that the spin-orbit interaction (SOI) of the Pt impurity on the Au (001) and Au (111) surfaces is enhanced, because the Pt 5d levels are lifted to the Fermi level due to the valence fluctuations. In addition, there are two SOI channels on the Au (111) surface, while only one for Pt either on the Au (001) surface or in bulk Au.

口頭

Theory of anomalous Hall and spin Hall effects in ferromagnetic metals

Gu, B.; Ziman, T.*; 前川 禎通

no journal, , 

We give a unified theory of the anomalous Hall effect (AHE) and the spin Hall effect (SHE) in ferromagnetic metals (FM). We do this by extending Kondo theory of the AHE in FM and including the short range spin-spin correlations. We find a novel relation between the SHE and the second order nonlinear spin fluctuation in FM near Curie temperature Tc, which has been hidden for about 50 years, since Kondo gave a relation between the AHE and the first order nonlinear spin fluctuation in pure FM near Tc in 1962. Our results show an essential difference between the AHE and SHE in terms of the symmetry. Our theory can be applied to the recent SHE experiment in FM near Tc by Y. Otani et al.

口頭

Theory of the spin Hall effect in ferromagnetic metals; Nonlinear behaviors around the Curie temperature

Gu, B.; Ziman, T.*; 前川 禎通

no journal, , 

We give a theory of the spin Hall effect in ferromagnetic metals near the Curie temperature. We do this by extending Kondo's theory of the anomalous Hall effect in ferromagnetic metals to include the short range spin-spin correlations. We find a novel relation between the spin Hall effect and a second order nonlinear spin fluctuation in ferromagnetic metals near Curie temperature Tc. In 1962 Kondo gave a relation between the anomalous Hall effect and the first order nonlinear spin fluctuation in pure ferromagnetic metals near Tc. Thus our results show an essential difference between the anomalous and spin Hall effects in terms of the nonlinear spin correlations that are probed. Our theory can be compared to experiments in ferromagnetic alloys near Tc by Y. Otani et al.

口頭

Theory of the inverse spin hall effect in ferromagnetic metals near the curie temperature

Gu, B.; Ziman, T.*; 前川 禎通

no journal, , 

We give a theory of the Inverse Spin Hall Effect in ferromagnetic metals near the Curie temperature. We do this by extending Kondo's theory of the Anomalous Hall Effect in ferromagnetic metals to include the short range spin-spin correlations. We find a novel relation between the Inverse Spin Hall Effect and a second order nonlinear spin fluctuation in ferromagnetic metals near Curie temperature Tc, which has been hidden for about 50 years, since 1962. In that year Kondo gave a relation between the Anomalous Hall Effect and the first order nonlinear spin fluctuation in pure ferromagnetic metals near Tc. Our results show an essential difference between the Anomalous and Inverse Spin Hall Effects in terms of the spin correlations that are probed. Our theory can be compared to recent experiments in ferromagnetic alloys.

口頭

Theory of the spin Hall effect, and its inverse, in a ferromagnetic metal near the Curie temperature

Gu, B.; Ziman, T.*; 前川 禎通

no journal, , 

We give a theory of the spin Hall effect (SHE) and its inverse in ferromagnetic metals based on skew-scattering via collective spin fluctuations. This extends Kondo's theory of the anomalous Hall effect (AHE) to include short range spin-spin correlations. We find a novel relation between the SHE and the four-spin correlations near the Curie temperature Tc. Such four-spin correlations were defined 50 years ago in Kondo's theory, but do not contribute to the AHE, which relates to the three-spin correlations. Thus our theory shows an essential difference between the AHE and SHE. Our theory can be compared to recent experimental results by Wei et al for the SHE in ferromagnetic alloys.

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