Direct observation of topological magnon polarons in a multiferroic material

Bao, S.*; Gu, Z.-L.*; Shangguan, Y.*; Huang, Z.*; Liao, J.*; Zhao, X.*; Zhang, B.*; Dong, Z.-Y.*; Wang, W.*; 梶本 亮一; et al.

Nature Communications (Internet), 14, p.6093_1 - 6093_9, 2023/09

https://doi.org/10.1038/s41467-023-41791-9

Magnon polarons are novel elementary excitations possessing hybrid magnonic and phononic signatures, and are responsible for many exotic spintronic and magnonic phenomena. Despite long-term sustained experimental efforts in chasing for magnon polarons, direct spectroscopic evidence of their existence is hardly observed. Here, we report the direct observation of magnon polarons using neutron spectroscopy on a multiferroic FeMoO possessing strong magnon-phonon coupling. Specifically, below the magnetic ordering temperature, a gap opens at the nominal intersection of the original magnon and phonon bands, leading to two separated magnon-polaron bands. Each of the bands undergoes mixing, interconverting and reversing between its magnonic and phononic components. We attribute the formation of magnon polarons to the strong magnon-phonon coupling induced by Dzyaloshinskii-Moriya interaction. Intriguingly, we find that the band-inverted magnon polarons are topologically nontrivial. These results uncover exotic elementary excitations arising from the magnon-phonon coupling, and offer a new route to topological states by considering hybridizations between different types of fundamental excitations.

Magnetic properties and electronic configurations of Mn ions in the diluted magnetic semiconductor BaK(ZnMn)As studied by X-ray magnetic circular dichroism and resonant inelastic X-ray scattering

鈴木 博人*; Zhao, G.*; 岡本 淳*; 坂本 祥哉*; Chen, Z.-Y.*; 野中 洋亮*; 芝田 悟朗; Zhao, K.*; Chen, B.*; Wu, W.-B.*; et al.

Journal of the Physical Society of Japan, 91(6), p.064710_1 - 064710_5, 2022/06

https://doi.org/10.7566/JPSJ.91.064710

The magnetic properties and the electronic excitations of the new diluted magnetic semiconductor BaK(ZnMn)As have been studied by X-ray magnetic circular dichroism (XMCD) and resonant inelastic X-ray scattering (RIXS). The sum rule analysis of the XMCD spectra indicates that the Mn atoms are in the high-spin configurations of , whereas the presence of competing ferromagnetic and antiferromagnetic interactions between the Mn ions reduces the net spin moment. Based on a comparison of the RIXS line shapes with those of GaMnAs, it is concluded that the ground state of Mn in BaK(ZnMn)As consists of both the and electron configurations.

Li(Cd,Mn)P; A New cadmium based diluted ferromagnetic semiconductor with independent spin & charge doping

Han, W.*; Chen, B. J.*; Gu, B.*; 前川 禎通*; 他14名*

Scientific Reports (Internet), 9, p.7490_1 - 7490_6, 2019/05

https://doi.org/10.1038/s41598-019-43754-x

We report a new diluted ferromagnetic semiconductor Li(Cd,Mn)P, wherein carrier is doped via excess Li while spin is doped by isovalence substitution of Mn into Cd. The extended Cd 4-orbitals lead to more itinerant characters of Li(Cd,Mn)P than that of analogous Li(Zn,Mn)P. A higher Curie temperature of 45 K than that for Li(Zn, Mn)P is obtained in Li(Cd,Mn)P polycrystalline samples by Arrott plot technique. The p-type carriers are determined by Hall effect measurements. The first principle calculations and X-ray diffraction measurements indicate that occupation of excess Li is at Cd sites rather than the interstitial site. Consequently holes are doped by excess Li substitution. More interestingly Li(Cd,Mn)P shows a very low coercive field (100 Oe) and giant negative magnetoresistance (similar to 80%) in ferromagnetic state that will benefit potential spintronics applications.

Enhanced orbital magnetic moment in FeCo nanogranules observed by Barnett effect

緒方 裕大; 中堂 博之; Gu, B.; 小林 伸聖*; 小野 正雄; 針井 一哉; 松尾 衛; 齊藤 英治; 前川 禎通

Journal of Magnetism and Magnetic Materials, 442, p.329 - 331, 2017/11

https://doi.org/10.1016/j.jmmm.2017.06.101

The gyroscopic g factor, , of FeCo nanogranules embedded in a matrix of MgF (FeCo-MgF) was determined by measuring the magnetic-field generation from a rotating sample due to the Barnett effect. The value of the FeCo-MgF is estimated to be 1.76 0.11. The orbital contribution to the magnetic moment in the FeCo nanogranules was found to be quite large compared with that in bulk FeCo, being consistent with a density-functional-theory calculation that shows that the orbital magnetic moment may increase at the FeCo/MgF interfaces. The result suggests that the orbital magnetic moment is enhanced by symmetry breaking at the surface of the FeCo nanogranules.

Magnetization dynamics and its scattering mechanism in thin CoFeB films with interfacial anisotropy

岡田 篤*; He, S.*; Gu, B.; 金井 駿*; Soumyanarayanan, A.*; Lim, S. T.*; Tran, M.*; 森 道康; 前川 禎通; 松倉 文礼*; et al.

Proceedings of the National Academy of Sciences of the United States of America, 144(15), p.3815 - 3820, 2017/04

http://doi.org/10.1073/pnas.1613864114

Studies of magnetization dynamics have incessantly facilitated the discovery of fundamentally novel physical phenomena, making steady headway in the development of magnetic and spintronics devices. The dynamics can be induced and detected electrically, offering new functionalities in advanced electronics at the nanoscale. However, its scattering mechanism is still disputed. Understanding the mechanism in thin films is especially important, because most spintronics devices are made from stacks of multilayers with nanometer thickness. The stacks are known to possess interfacial magnetic anisotropy, a central property for applications, whose influence on the dynamics remains unknown. Here, we investigate the impact of interfacial anisotropy by adopting CoFeB/MgO as a model system. Through systematic and complementary measurements of ferromagnetic resonance (FMR) on a series of thin films, we identify narrower FMR linewidths at higher temperatures. We explicitly rule out the temperature dependence of intrinsic damping as a possible cause, and it is also not expected from existing extrinsic scattering mechanisms for ferromagnets. We ascribe this observation to motional narrowing, an old concept so far neglected in the analyses of FMR spectra. The effect is confirmed to originate from interfacial anisotropy, impacting the practical technology of spin-based nanodevices up to room temperature.

Diluted magnetic semiconductors with narrow band gaps

Gu, B.; 前川 禎通

Physical Review B, 94(15), p.155202_1 - 155202_8, 2016/10

AA2016-0395.pdf:0.38MB

https://doi.org/10.1103/PhysRevB.94.155202

The diluted magnetic semiconductors (DMSs) have received considerable attention owing to potential applications based on the use of both charge and spin degrees of freedom in electronic devices. Historically, (Ga,Mn)As has received the most attention in DMSs, and so far the highest Curie temperature in (Ga,Mn)As has been = 190 K in the experiment. The substitution of divalent Mn atoms into trivalent Ga sites introduces hole carriers; thus, (Ga,Mn)As is a -type DMS. Here, we propose a method to realize DMSs with - and -type carriers by choosing host semiconductors with a narrow band gap. By employing a combination of the density function theory and quantum Monte Carlo simulation, we demonstrate such semiconductors using Mn-doped BaZnAs, which has a band gap of 0.2 eV. In addition, we found a nontoxic DMS Mn-doped BaZnSb, of which the Curie temperature is predicted to be higher than that of Mn-doped BaZnAs, the of which was up to 230 K in a recent experiment.

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

https://doi.org/10.1103/PhysRevLett.115.196602

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 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 and becomes as little as 7 times smaller at 0.5. 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.

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

https://doi.org/10.1063/1.4914531

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 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.

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

https://doi.org/10.1103/PhysRevLett.114.017202

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.

Diluted ferromagnetic semiconductor Li(Zn,Mn)P with decoupled charge and spin doping

Deng, Z.*; Zhao, K.*; Gu, B.; Han, W.*; Zhu, J. L.*; Wang, X. C.*; Li, X.*; Liu, Q. Q.*; Yu, R. C.*; 後神 達郎*; et al.

Physical Review B, 88(8), p.081203_1 - 081203_5, 2013/08

https://doi.org/10.1103/PhysRevB.88.081203

We report the discovery of a diluted magnetic semiconductor, Li(Zn,Mn)P, in which charge and spin are introduced independently via lithium off-stoichiometry and the isovalent substitution of Mn for Zn, respectively. Isostructural to (Ga,Mn)As, Li(Zn,Mn)P was found to be a -type ferromagnetic semiconductor with excess lithium providing charge doping. First-principles calculations indicate that excess Li is favored to partially occupy the Zn site, leading to hole doping. Ferromagnetism with Curie temperature up to 34 K is achieved while the system still shows semiconducting transport behavior.