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 Nel 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.
森 道康; 前川 禎通
Applied Physics Letters, 122(4), p.042202_1 - 042202_5, 2023/01
We show that ultrasound can induce the Shapiro steps (SS) in the charge-density-wave (CDW) state. When ultrasound with frequency and a dc voltage are applied, the SS occur at the current with integer . Even and odd multiples of SS are represented by two couplings between the CDW and ultrasound. Although an ac voltage bias with frequency induces the SS at , the ultrasound bias enhances the odd multiples more strongly than the even ones. This is the difference between the ultrasound and the ac voltage. Since the SS cause abrupt peaks in the , the extreme changes in the curve will be applied to a very sensitive ultrasound detector.
出田 真一郎*; Johnston, S.*; 吉田 鉄平*; 田中 清尚*; 森 道康; 安齋 太陽*; 井野 明洋*; 有田 将司*; 生天目 博文*; 谷口 雅樹*; et al.
Physical Review Letters, 127(21), p.217004_1 - 217004_6, 2021/11
Hybridization of Bogoliubov quasiparticles (BQPs) between the CuO layers in the triple-layer cuprate high-temperature superconductor BiSrCaCuO is studied by angle-resolved photoemission spectroscopy (ARPES). In the superconducting state, an anti-crossing gap opens between the outer- and inner-BQP bands, which we attribute primarily to interlayer single-particle hopping with possible contributions from interlayer Cooper pairing. We find that the -wave superconducting gap of both BQP bands smoothly develops with momentum without abrupt jump in contrast to a previous ARPES study. Hybridization between the BQPs also gradually increases in going from the off-nodal to the anti-nodal region, which is explained by the momentum-dependence of the interlayer single-particle hopping. As possible mechanisms for the enhancement of the superconducting transition temperature, the hybridization between the BQPs, as well as the combination of phonon modes of the triple CuO layers and spin fluctuations are discussed.
森 道康; 前川 禎通
Applied Physics Express, 14(10), p.103001_1 - 103001_4, 2021/10
A superconducting quantum interference device (SQUID) comprising 0- and -Josephson junctions (JJs), called -SQUID, is studied by the resistively shunted junction model. The -SQUID shows half-integer Shapiro-steps (SS) under microwave irradiation at the voltage = , with angular frequency and half-integer /2 in addition to integer . We show that the -SQUID can be a -qubit with spontaneous loop currents by which the half-integer SS are induced. Making the 0- and -JJs equivalent is a key for the half-integer SS and realizing the -qubit.
Yao, Y.*; Cai, R.*; Yang, S.-H.*; Xing, W.*; Ma, Y.*; 森 道康; Ji, Y.*; 前川 禎通; Xie, X.-C.*; Han, W.*
Physical Review B, 104(10), p.104414_1 - 104414_6, 2021/09
We report the experimental observation of half-integer Shapiro steps in the strong ferromagnetic Josephson junction (Nb-NiFe-Nb) by investigating the current-phase relation under radio-frequency microwave excitation. The half-integer Shapiro steps are robust in a wide temperature range from T = 4 to 7 K. The half-integer Shapiro steps could be attributed to co-existence of 0- and -states in the strong ferromagnetic NiFe Josephson junctions with the spatial variation of the NiFe thickness. This scenario is also supported by the high-resolution transmission electron microscopy characterization of the Nb/NiFe/Nb junction.
奥谷 顕*; 大西 弘明; 木村 尚次郎*; 竹内 徹也*; 木田 孝則*; 森 道康; 三宅 厚志*; 徳永 将史*; 金道 浩一*; 萩原 政幸*
Journal of the Physical Society of Japan, 90(4), p.044704_1 - 044704_9, 2021/04
The high-field multifrequency electron spin resonance (ESR), the magnetization, and the specific heat of single crystals of BaCoVO (a quasi-one-dimensional Ising-like antiferromagnet with ) were measured in two different transverse magnetic fields: and . A softening of the lowest ESR mode was observed near the saturation field of the magnetization around 40 T in , whereas it occurs around 10 T in , far below the saturation field. These observations were reproduced by numerical calculations using the density-matrix renormalization group methods, including the staggered magnetic fields induced by a uniform external magnetic field and the interchain magnetic exchange interaction. We found that the directional dependence of the lowest mode originates mainly from the remarkable angular dependence of the induced magnetic field. The interchain interaction is also quantitatively important for reproducing the experimental results. The directional dependence of the lowest mode is demonstrated by the particle-hole excitation of spinons using the Jordan-Wigner transformation.
Journal of the Physical Society of Japan, 89(11), p.114705_1 - 114705_7, 2020/11
The nuclear magnetic relaxation time in a ferrimagnetic insulators is calculated within the mean-field approximation for the magnetic exchange interactions and the Raman process involving the hyperfine interaction. We find that the value of 1/ on one type of site increases rapidly near the compensation temperature , whereas that on the other type of site does not increase up to Curie temperature . This is due to the fact that the soft-magnon bandwidth becomes comparable to . An increase in 1/ below is found also in another type ferrimagnet, which shows a hump structure in the temperature dependence of magnetization instead of compensation. Also in that case, we find the rapid increase in 1/ below , even though the magnetization does not show compensation. The coexistence of soft and hard magnons leads to these remarkable properties of ferrimagnets.
遠山 貴己; 森 道康; 曽田 繁利*
Physical Review B, 97(23), p.235137_1 - 235137_7, 2018/06
We investigate the dynamical spin and charge structure factors of the -- four-leg ladder by using dynamical density-matrix renormalization group. The dynamical charge structure factor along the momentum direction from to clearly shows low-energy excitations corresponding to the stripe order in hole doping. On the other hand, the stripe order weakens in electron doping, resulting in less low-energy excitations in the charge channel. In spin channel, we find incommensurate spin excitations near forming a hour-glass behavior in hole doping, while in electron doping we find clearly spin-wave-like dispersions starting from . Along the - direction, the spin excitations are strongly influenced by the stripes in hole doping, resulting in two branches that form a discontinuous behavior in the dispersion. In contrast, the electron-doped systems show a downward shift in energy toward .
杉本 貴則*; 森 道康; 遠山 貴己; 前川 禎通
Physical Review B, 97(14), p.144424_1 - 144424_10, 2018/04
Frustrated spin ladders show magnetization plateaux depending on the rung-exchange interaction and frustration defined by the ratio of first and second neighbor exchange interactions in each chain. This paper reports on its magnetic phase diagram. Using the variational matrix-product state method, we accurately determine phase boundaries. Several kinds of magnetization plateaux are induced by the frustration and the strong correlation among quasiparticles on a lattice. The appropriate description of quasiparticles and their relevant interactions are changed by a magnetic field. We find that the frustration differentiates the triplet quasiparticle from the single tone in kinetic energy.
Journal of the Physical Society of Japan, 86(12), p.124705_1 - 124705_7, 2017/12
We study magnetic excitations in a bilayer of antiferromagnetic (AF) insulator and correlated metal, in which the double occupancy is forbidden. Effective action of AF spinwave in the AF insulator is derived by using the path integral formula within the second order of an inter-plane coupling. The electrons correlation in the metallic plane is treated by the Gutzwiller approximation, which renormalizes the hopping integrals as proportional to hole density. Remarkable is that the AF magnons are broaden at low energies, only when the metallic planes includes correlation effect. The present results will be useful also to another bilayer of metal and ferrimagnet.
松田 雅昌*; 大西 弘明; 奥谷 顕*; Ma, J.*; Agrawal, H.*; Hong, T.*; Pajerowski, D. M.*; Copley, J. R. D.*; 奥西 巧一*; 森 道康; et al.
Physical Review B, 96(2), p.024439_1 - 024439_8, 2017/07
BaCoVO consists of Co chains, in which Co ion carries a fictitious spin with Ising anisotropy. We performed elastic and inelastic neutron scattering experiments in BaCoVO in magnetic field perpendicular to the axis which is the chain direction. With applying magnetic field along the axis at 3.5 K, the antiferromagnetic order with the easy axis along the axis, observed in zero magnetic field, is completely suppressed at 8 T, while the magnetic field gradually induces an antiferromagnetic order with the spin component along the axis. We also studied magnetic excitations as a function of transverse magnetic field. The lower boundary of the spinon excitations splits gradually with increasing magnetic field. The overall feature of the magnetic excitation spectra in magnetic field is reproduced by the theoretical calculation based on the spin antiferromagnetic chain model, which predicts that the dynamic magnetic structure factor of the spin component along the chain direction is enhanced and that along the field direction has clear incommensurate correlations.
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
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.
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
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.
杉本 貴則*; 森 道康; 遠山 貴巳*; 前川 禎通
Physics Procedia, 75, p.861 - 867, 2015/12
The quasi-spin correlations in a frustrated quantum spin ladder with one-half magnetization are theoretically studied by using the density-matrix renormalization-group method and the quasi- spin transformation. In this model, the frustration induces a gapless-to-gapful phase transition with a strong rung coupling. The gapful state is observed as the one-half magnetization plateau in the magnetization curve. In the magnetization-plateau state, we find that the quasi-spin dimer has a large expectation value with long-ranged correlations. This result does not only comes in useful to clarify the magnetization-plateau state, but gives a crucial information to understand the magnetization curve of the real compound BiCuPO, whose effective spin model corresponds to ours.
杉本 貴則*; 森 道康; 遠山 貴巳*; 前川 禎通
JPS Conference Proceedings (Internet), 8, p.034005_1 - 034005_6, 2015/09
The magnetization curve in a frustrated two-leg spin-ladder system is theoretically studied using both analytical and numerical methods. This spin system is mapped onto a hard-core boson system, which is composed of bond-operators describing the triplon excitation. By the analytical method using the mean-field theory on the bond-operators, we show that cusp singularities emerge in the magnetization curve as a function of a magnetic field due to the strong frustration. This originates from a change of number of Fermi points in the bosonic dispersion relation with the applied field. It is analogous to Lifshitz transition. This singularity is clarified by numerical calculation with the density-matrix renormalization-group method. Our results will be useful to understand the magnetization process observed in the frustrated two-leg spin-ladder compound BiCuHOPOHO.
杉本 貴則*; 森 道康; 遠山 貴巳*; 前川 禎通
Physical Review B, 92(12), p.125114_1 - 125114_6, 2015/09
The quantum phase transitions induced by a magnetic field are theoretically studied in a frustrated two-leg spin ladder. Using the density-matrix renormalization-group method, we find some magnetic phase transitions and plateaux in two different cases of strong and weak rung couplings. With the strong rung coupling, the three magnetization plateaux are found at 1/3, 1/2, and 2/3 due to the frustration. Those can be understood in terms of a quasi-spinon reconstructed from the singlet and the triplets of spins on a rung. The plateau at 1/2 corresponds to the valence bond solid of the quasispinons, while the plateaux at 1/3 and 2/3 can be associated with the array of quasi-spinons such as soliton lattice. This is different from the usual Bose-Einstein-condensation picture of triplons. Our results will be useful to understand magnetization curves in BiCuPO.
遠山 貴巳*; 筒井 健二; 森 道康; 曽田 繁利*; 柚木 清司*
Physical Review B, 92(1), p.014515_1 - 014515_8, 2015/07
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
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.
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
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.
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
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.