Wang, H.*; Hou, D.*; Qiu, Z.*; 吉川 貴史*; 齊藤 英治; Jin, X.*
Journal of Applied Physics, 122(8), p.083907_1 - 083907_6, 2017/08
An electric method for measuring magnetic anisotropy in antiferromagnetic insulators (AFIs) is proposed. When a metallic film with strong spin-orbit interactions, e.g., platinum (Pt), is deposited on an AFI, its resistance should be affected by the direction of the AFI Nel vector due to the spin Hall magnetoresistance (SMR). Accordingly, the direction of the AFI Nel vector, which is affected by both the external magnetic field and the magnetic anisotropy, is reflected in resistance of Pt. The magnetic field angle dependence of the resistance of Pt on AFI is calculated by considering the SMR, which indicates that the antiferromagnetic anisotropy can be obtained experimentally by monitoring the Pt resistance in strong magnetic fields. Calculations are performed for realistic systems such as Pt/CrO, Pt/NiO, and Pt/CoO.
吉川 貴史*; 鈴木 基寛*; 岡林 潤*; 内田 健一*; 菊池 大介*; Qiu, Z.*; 齊藤 英治
Physical Review B, 95(21), p.214416_1 - 214416_7, 2017/06
Magnetic moments in an ultrathin Pt film on a ferrimagnetic insulator YFeO (YIG) have been investigated at high magnetic fields and low temperatures by means of X-ray magnetic circular dichroism (XMCD). We observed an XMCD signal due to the magnetic moments in a Pt film at the Pt and edges. By means of the element-specific magnetometry, we found that the XMCD signal at the Pt edge gradually increases with increasing the magnetic field even when the field is much greater than the saturation field of YIG. Importantly, the observed XMCD intensity was found to be much greater than the intensity expected from the Pauli paramagnetism of Pt when the Pt film is attached to YIG. These results imply the emergence of induced paramagnetic moments in Pt on YIG and explain the characteristics of the unconventional Hall effect in Pt/YIG systems.
Ramos, R.*; 吉川 貴史*; Anadn, A.*; Lucas, I.*; 内田 健一*; Algarabel, P. A.*; Morelln, L.*; Aguirre, M. H.*; 齊藤 英治; Ibarra, M. R.*
AIP Advances (Internet), 7(5), p.055915_1 - 055915_7, 2017/05
We report temperature dependent measurements of the spin Seebeck effect (SSE) in multilayers formed by repeated growth of a FeO/Pt bilayer junction. The magnitude of the observed enhancement of the SSE, relative to the SSE in the single bilayer, shows a monotonic increase with decreasing the temperature. This result can be understood by an increase of the characteristic length for spin current transport in the system, in qualitative agreement with the recently observed increase in the magnon diffusion length in FeO at lower temperatures. Our result suggests that the thermoelectric performance of the SSE in multilayer structures can be further improved by careful choice of materials with suitable spin transport properties.
Yagmur, A.*; 内田 健一*; 井原 和紀*; 井岡 郁夫; 吉川 貴史*; 小野 円佳*; 遠藤 純一*; 柏木 王明*; 中島 哲也*; 桐原 明宏*; et al.
Applied Physics Letters, 109(24), p.243902_1 - 243902_4, 2016/12
吉川 貴史*; Shen, K.*; Flebus, B.*; Duine, R. A.*; 内田 健一*; Qiu, Z.*; Bauer, G. E. W.*; 齊藤 英治
Physical Review Letters, 117(20), p.207203_1 - 207203_6, 2016/11
Sharp structures in the magnetic field-dependent spin Seebeck effect (SSE) voltages of Pt/YFeO at low temperatures are attributed to the magnon-phonon interaction. Experimental results are well reproduced by a Boltzmann theory that includes magnetoelastic coupling. The SSE anomalies coincide with magnetic fields tuned to the threshold of magnon-polaron formation. The effect gives insight into the relative quality of the lattice and magnetization dynamics.
内田 健一*; 安立 裕人; 吉川 貴史*; 桐原 明宏*; 石田 真彦*; 萬 伸一*; 前川 禎通; 齊藤 英治*
Proceedings of the IEEE, 104(10), p.1946 - 1973, 2016/10
The spin Seebeck effect (SSE) refers to the generation of a spin current as a result of a temperature gradient in magnetic materials including insulators. The SSE is applicable to thermoelectric generation because the thermally generated spin current can be converted into a charge current via spin-orbit interaction in conductive materials adjacent to the magnets. The insulator-based SSE device exhibits unconventional characteristics potentially useful for thermoelectric applications, such as simple structure, device-design exibility, and convenient scaling capability. In this article, we review recent studies on the SSE from the viewpoint of thermoelectric applications.
Yagmur, A.*; 軽部 修太郎*; 内田 健一*; 近藤 浩太*; 井口 亮*; 吉川 貴史*; 大谷 義近*; 齊藤 英治
Applied Physics Letters, 108(24), p.242409_1 - 242409_4, 2016/06
The longitudinal spin Seebeck effect (SSE) in Bi O/Cu/yttrium-iron-garnet (YIG) devices has been investigated. When an out-of-plane temperature gradient is applied to the BiO/Cu/YIG device, a spin current is generated across the Cu/YIG interface via the SSE and then converted into electric voltage due to the spinO/Cu interface. The sign of the SSE voltage in the BiO/Cu/YIG devices is opposite to that induced by the conventional inverse spin Hall effect in Pt/YIG devices. The SSE voltage in the BiO/Cu/YIG devices disappears in the absence of the BiOlayer and its thermoelectric conversion efficiency is independent of the Cu thickness, indicating the important role of the BiO/Cu interface. This result demonstrates that not only the bulk inverse spin Hall effect but also the spin-orbit coupling near the interface can be used for SSE-based thermoelectric generation.
吉川 貴史*; 内田 健一*; 大門 俊介*; 齊藤 英治
Journal of the Physical Society of Japan, 85(6), p.065003_1 - 065003_2, 2016/06
The spin Seebeck effect (SSE) in a Pt-film/YFeO (YIG)-slab junction system has been investigated at low temperatures and under various magnetic fields in the longitudinal configuration. We found that, by increasing applied magnetic fields, the SSE signal gradually decreases and converges to zero without showing sign reversal. The complete field-induced suppression of the SSE is interpreted in terms of the effect of the Zeeman gap in magnon excitation.
Ramos, R.*; 吉川 貴史*; Aguirre, M.*; Lucas, I.*; Anadn, A.*; 小宅 教文*; 内田 健一*; 安立 裕人; 塩見 淳一郎*; Algarabel, P. A.*; et al.
Physical Review B, 92(22), p.220407_1 - 220407_5, 2015/12
Spin Seebeck effects have been investigated in highly crystalline magnetic multilayer films. Voltage as well as power generated by the spin Seebeck effect were found to be significantly enhanced with increasing the number of layers, . This voltage enhancement defies the simple understanding of the spin Seebeck effect and suggests that spin current flowing between the magnetic layers in the thickness direction plays an important role in the multilayer SSE systems and the observed voltage enhancement.
内田 健一*; 吉川 貴史*; 関 剛斎*; 小宅 教文*; 塩見 淳一郎*; Qiu, Z.*; 高梨 弘毅*; 齊藤 英治
Physical Review B, 92(9), p.094414_1 - 094414_6, 2015/09
The anomalous Nernst effect (ANE) has been investigated in alternately stacked multilayer films comprising paramagnetic and ferromagnetic metals. We found that the ANE is enhanced by increasing the number of the paramagnet/ferromagnet interfaces and keeping the total thickness of the films constant, and that the enhancement appears even in the absence of magnetic proximity effects; similar behavior was observed not only in Pt/Fe multilayers but also in Au/Fe and Cu/Fe multilayers free from proximity ferromagnetism. This universal enhancement of the ANE in metallic multilayers suggests the presence of unconventional interface-induced thermoelectric conversion in the Fe films attached to the paramagnets.
Qiu, Z.*; Hou, D.*; 吉川 貴史*; 内田 健一*; 齊藤 英治
Applied Physics Express, 8(8), p.083001_1 - 083001_3, 2015/08
We report the observation of longitudinal spin Seebeck effects (LSSEs) in an all-oxide bilayer system comprising an IrO film and an YFeO film. Spin currents, which are generated by a temperature gradient across the IrO YFeO interface, were detected as a voltage via the inverse spin Hall effect in the conductive IrO layer. This voltage is proportional to the magnitude of the temperature gradient; its magnetic field dependence is consistent with the characteristics of LSSEs. This demonstration may lead to the realization of low-cost, stable, transparent spin-current-driven thermoelectric devices.
関 剛斎*; 内田 健一*; 吉川 貴史*; Qiu, Z.*; 齊藤 英治; 高梨 弘毅*
Applied Physics Letters, 107(9), p.092401_1 - 092401_4, 2015/08
We experimentally observed the inverse spin Hall effect (ISHE) of ferromagnetic FePt alloys. Spin Seebeck effect due to the temperature gradient generated the spin current () in the FePtYFeO (YIG) structure, and was injected from YIG to FePt and converted to the charge current through ISHE of FePt. The significant difference in magnetization switching fields for FePt and YIG led to the clear separation of the voltage of ISHE from that of anomalous Nernst effect in FePt. We also investigated the effect of ordering of FePt crystal structure on the magnitude of ISHE voltage in FePt.
吉川 貴史*; 内田 健一*; 大門 俊介*; Qiu, Z.*; 塩見 雄毅*; 齊藤 英治
Physical Review B, 92(6), p.064413_1 - 064413_9, 2015/08
The longitudinal spin Seebeck effect (LSSE) in Pt/YFeO (YIG) junction systems has been investigated at various magnetic fields and temperatures. We found that the LSSE voltage in a Pt/YIG-slab system is suppressed by applying high magnetic fields and this suppression is critically enhanced at low temperatures. The field-induced suppression of the LSSE in the Pt/YIG-slab system is too large at around room temperature to be explained simply by considering the effect of the Zeeman gap in magnon excitation. This result requires us to introduce a magnon-frequency-dependent mechanism into the scenario of LSSE; low-frequency magnons dominantly contribute to the LSSE. The magnetic field dependence of the LSSE voltage was observed to change by changing the thickness of YIG, suggesting that the thermospin conversion by the low-frequency magnons is suppressed in thin YIG films due to the long characteristic lengths of such magnons.
塩見 雄毅*; 半田 優*; 吉川 貴史*; 齊藤 英治
Physical Review B, 92(2), p.024418_1 - 024418_4, 2015/07
We report anomalous Hall effects exhibiting a hysteresis loop as large as about 10 T in a ferromagnetic superlattice comprising LaSrMnO and SrRuO layers. The superlattices grown by pulsed laser deposition exhibit a strong antiferromagnetic interlayer coupling below 110 K, where both LaSrMnO and SrRuO layers show anomalous Hall effects. With increasing magnetic-field strength, the anomalous Hall resistivity in the superlattices changes its sign depending on the magnetization directions of the LaSrMnO and SrRuO layers. As a consequence of competition among the antiferromagnetic interlayer coupling, the Zeeman effect, and magnetic anisotropies, the width of the hysteresis loop in the anomalous Hall resistivity in the superlattices becomes larger than 8 T at 10 K, clearly greater than those observed in LaSrMnO and SrRuO single layer films.
内田 健一*; 大江 純一郎*; 吉川 貴史*; 大門 俊介*; Hou, D.*; Qiu, Z.*; 齊藤 英治
Physical Review B, 92(1), p.014415_1 - 014415_8, 2015/07
The magnetic-field dependence of the longitudinal spin Seebeck effect (LSSE) in a Pt/YFeO (YIG)-slab junction system was found to deviate from a bulk magnetization curve of the YIG slab in a low field range. In this paper, we show that the deviation originates from the difference between surface and bulk magnetization processes in the YIG slab and that it appears even when removing possible extrinsic magnetic anisotropy due to surface roughness and replacing the Pt layer with different materials. This result indicates that the anomalous field dependence of the LSSE is due to an intrinsic magnetic property of the YIG surface. Our numerical calculation based on the Landau-Lifshitz-Gilbert equation shows that the deviation between the LSSE and bulk magnetization curves is qualitatively explained by introducing easy-axis perpendicular magnetic anisotropy near the surface of YIG.
塩見 雄毅*; 半田 優*; 吉川 貴史*; 齊藤 英治
Applied Physics Letters, 106(23), p.232403_1 - 232403_4, 2015/06
Transverse thermoelectric effects in response to an out-of-plane heat current have been studied in an external magnetic field for ferromagnetic superlattices consisting of LaSrMnO and SrRuO layers. The superlattices were fabricated on SrTiO substrates by pulsed laser deposition. We found that the sign of the transverse thermoelectric voltage for the superlattices is opposite to that for LaSrMnO and SrRuO single layers at 200 K, implying an important role of spin Seebeck effects inside the superlattices. At 10 K, the magnetothermoelectric curves shift from the zero field due to an antiferromagnetic coupling between layers in the superlattices.
新関 智彦*; 吉川 貴史*; 内田 健一*; 岡 美嶺人*; 鈴木 和也*; 柳原 英人*; 喜多 英治*; 齊藤 英治
AIP Advances (Internet), 5(5), p.053603_1 - 053603_6, 2015/05
The longitudinal spin-Seebeck effect (LSSE) has been investigated in cobalt ferrite (CFO), an exceptionally hard magnetic spinel ferrite. A bilayer of a polycrystalline Pt and an epitaxially-strained CFO(110) exhibiting an in-plane uniaxial anisotropy was prepared by reactive rf sputtering technique. Thermally generated spin voltage in the CFO layer was measured via the inverse spin-Hall effect in the Pt layer. Externalmagnetic-field () dependence of the LSSE voltage () in the Pt/CFO(110) sample with was found to exhibit a hysteresis loop with a high squareness ratio and high coercivity, while that with shows a nearly closed loop, reflecting the different anisotropies induced by the epitaxial strain. The magnitude of has a linear relationship with the temperature difference (), giving the relatively large / of about 3 V/K for CFO(110) which was kept even at zero external field.
Sola, A.*; Kuepferling, M.*; Basso, V.*; Pasquale, M.*; 吉川 貴史*; 内田 健一*; 齊藤 英治
Journal of Applied Physics, 117(17), p.17C510_1 - 17C510_4, 2015/05
In the framework of the longitudinal spin Seebeck effect (LSSE), we developed an experimental setup for the characterization of LSSE devices. This class of device consists in a layered structure formed by a substrate, a ferrimagnetic insulator (YIG) where the spin current is thermally generated, and a paramagnetic metal (Pt) for the detection of the spin current via the inverse spin Hall effect. In this kind of experiments, the evaluation of a thermal gradient through the thin YIG layer is a crucial point. In this work, we perform an indirect determination of the thermal gradient through the measurement of the heat flux. We developed an experimental setup using Peltier cells that allow us to measure the heat flux through a given sample. In order to test the technique, a standard LSSE device produced at Tohoku University was measured. We find a spin Seebeck S coefficient of 2.810 V K.
内田 健一*; Qiu, Z.*; 吉川 貴史*; 井口 亮*; 齊藤 英治
Applied Physics Letters, 106(5), p.052405_1 - 052405_5, 2015/02
The temperature dependence of spin Hall magnetoresistance (SMR) in Pt/YFeO (YIG) bilayer films has been investigated in a high temperature range from room temperature to near the Curie temperature of YIG. The experimental results show that the magnitude of the magnetoresistance ratio induced by the SMR monotonically decreases with increasing the temperature and almost disappears near the Curie temperature. We found that, near the Curie temperature, the temperature dependence of the SMR in the Pt/YIG film is steeper than that of a magnetization curve of the YIG; the critical exponent of the magnetoresistance ratio is estimated to be 0.9. This critical behavior of the SMR is attributed mainly to the temperature dependence of the spin-mixing conductance at the Pt/YIG interface.
Schreier, M.*; Bauer, G. E. W.*; Vasyuchka, V.*; Flipse, J.*; 内田 健一*; Lotze, J.*; Lauer, V.*; Chumak, A.*; Serga, A.*; 大門 俊介*; et al.
Journal of Physics D; Applied Physics, 48(2), p.025001_1 - 025001_5, 2015/01
We carried out a concerted effort to determine the absolute sign of the inverse spin Hall effect voltage generated by spin currents injected into a normal metal. We focus on yttrium iron garnet (YIG)|platinum bilayers at room temperature, generating spin currents by microwaves and temperature gradients. We find consistent results for different samples and measurement setups that agree with theory. We suggest a right-hand-rule to define a positive spin Hall angle corresponding to the voltage expected for the simple case of scattering of free electrons from repulsive Coulomb charges.