Chen, Y.*; 塩見 雄毅*; Qiu, Z.*; 新関 友彦*; 埋田 真樹*; 齊藤 英治
Scientific Reports (Internet), 9, p.19052_1 - 19052_8, 2019/12
In superconductors, a topological configuration of the superconducting order parameter called a superconducting vortex carries magnetization. Such a magnetic topological object behaves like a minute particle generating a magnetic flux. Since the flux is localized with a nanometer scale, the vortex provides a nano-scale probe for local magnetic fields. Here we show that information of magnetic stripes in insulators can be read out by using vortices in an adjacent superconductor film as a probe. The orientation and width of magnetic micro stripes are both transcribed into resistance change of the superconductor through the modulation of vortex mobility affected by local magnetization. By changing the direction of external magnetic fields, zero-field resistance changes continuously according to the stripe orientation, and its modulation magnitude reaches up to 100%. The width of the stripes can also be estimated from the oscillatory magnetoresistance. Our results demonstrate a new possibility for non-volatile analog memory devices based on topological objects.
Hou, D.*; Qiu, Z.*; 齊藤 英治
NPG Asia Materials, 11, p.35_1 - 35_6, 2019/07
Spin transport is a key process in the operation of spin-based devices that has been the focus of spintronics research for the last two decades. Conductive materials, such as semiconductors and metals, in which the spin transport relies on electron diffusion, have been employed as the channels for spin transport in most studies. Due to the absence of conduction electrons, the potential to be a spin channel has long been neglected for insulators. However, since the demonstration of spin transmission through a ferromagnetic insulator, it was realized that insulators with magnetic ordering can also serve as channels for spin transport. Here, the recent progress of spin transport in antiferromagnetic insulators is briefly described with an introduction to the experimental techniques. The observations regarding the temperature dependence of spin transmission, spin current switching and the negative spin Hall magnetoresistance are discussed. We also include the challenges for developing the functionality of antiferromagnetic insulators as well as the unresolved problems from the experimental observations.
Qiu, Z.*; Hou, D.*; Barker, J.*; 山本 慧; Gomonay, O.*; 齊藤 英治*
Nature Materials, 17(7), p.577 - 580, 2018/05
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.
日置 友智*; 井口 亮*; Qiu, Z.*; Hou, D.*; 内田 健一*; 齊藤 英治
Applied Physics Express, 10(7), p.073002_1 - 073002_4, 2017/06
We have investigated the magnetic-field-induced suppression of the longitudinal spin Seebeck effect (LSSE) by using a time-resolved measurement technique at room temperature. The result manifested two distinctive time domains: the short-time domain where the observed voltage is insensitive to the magnetic fields, and the long-time domain where the both response time and the magnitude of the observed voltage decreased simultaneously by the magnetic fields. We estimated the magnon propagation length by fitting the transient LSSE response. The propagation length shows a strong dependence on the applied magnetic field, indicating the importance of long-range and low-frequency magnons in the LSSE.
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.
Hou, D.*; Qiu, Z.*; Barker, J.*; 佐藤 浩司*; 山本 慧; Vlez, S.*; Gomez-Perez, J. M.*; Hueso, L. E.*; Casanova, F.*; 齊藤 英治
Physical Review Letters, 118(14), p.147202_1 - 147202_6, 2017/04
Spin Hall magnetoresistance (SMR) has been investigated in Pt/NiO/YIG structures in a wide range of temperature and NiO thickness. The SMR shows a negative sign below a temperature that increases with the NiO thickness. This is contrary to a conventional SMR theory picture applied to the Pt/YIG bilayer, which always predicts a positive SMR. The negative SMR is found to persist even when NiO blocks the spin transmission between Pt and YIG, indicating it is governed by the spin current response of the NiO layer. We explain the negative SMR by the NiO "spin flop" coupled with YIG, which can be overridden at higher temperature by positive SMR contribution from YIG. This highlights the role of magnetic structure in antiferromagnets for transport of pure spin current in multilayers.
Seo, Y.-J.*; 針井 一哉; 高橋 遼*; 中堂 博之; 大柳 洸一*; Qiu, Z.*; 小野 崇人*; 塩見 雄毅*; 齊藤 英治
Applied Physics Letters, 110(13), p.132409_1 - 132409_4, 2017/03
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.
Qiu, Z.*; Li, J.*; Hou, D.*; Arenholz, E.*; N'Diaye, A. T.*; Tan, A.*; 内田 健一*; 佐藤 浩司*; 岡本 聡*; Tserkovnyak, Y.*; et al.
Nature Communications (Internet), 7, p.12670_1 - 12670_6, 2016/08
Spin fluctuation and transition have always been one of the central topics of magnetism and condensed matter science. Experimentally, the spin fluctuation is found transcribed onto scattering intensity in the neutron-scattering process, which is represented by dynamical magnetic susceptibility and maximized at phase transitions. Importantly, a neutron carries spin without electric charge, and therefore it can bring spin into a sample without being disturbed by electric energy. However, large facilities such as a nuclear reactor are necessary. Here we show that spin pumping, frequently used in nanoscale spintronic devices, provides a desktop microprobe for spin transition; spin current is a flux of spin without an electric charge and its transport reflects spin excitation. We demonstrate detection of antiferromagnetic transition in ultra-thin CoO films via frequency-dependent spin-current transmission measurements, which provides a versatile probe for phase transition in an electric manner in minute devices.
Hou, D.*; Qiu, Z.*; 井口 亮*; 佐藤 浩司*; Vehstedt, E. K.*; 内田 健一*; Bauer, G. E. W.*; 齊藤 英治
Nature Communications (Internet), 7, p.12265_1 - 12265_6, 2016/07
Applying magnetic fields has been the method of choice to magnetize non-magnetic materials, but they are difficult to focus. The magneto-electric effect and voltage-induced magnetization generate magnetization by applied electric fields, but only in special compounds or heterostructures. Here we demonstrate that a simple metal such as gold can be magnetized by a temperature gradient or magnetic resonance when in contact with a magnetic insulator by observing an anomalous Hall-like effect, which directly proves the breakdown of time-reversal symmetry. Such Hall measurements give experimental access to the spectral spin Hall conductance of the host metal, which is closely related to other spin caloritronics phenomena such as the spin Nernst effect and serves as a reference for theoretical calculation.
Geprgs, S.*; Kehlberger, A.*; Coletta, F.*; Qiu, Z.*; Guo, E.-J.*; Schulz, T.*; Mix, C.*; Meyer, S.*; Kamra, A.*; Althammer, M.*; et al.
Nature Communications (Internet), 7, p.10452_1 - 10452_6, 2016/02
Magnons are the elementary excitations of a magnetically ordered system. Here, we show that the spin Seebeck effect is sensitive to the complexities of the magnon spectrum. The spin Seebeck effect is caused by a thermally excited spin dynamics that is converted to a voltage by the inverse spin Hall effect at the interface to a heavy metal contact. By investigating the temperature dependence of the spin Seebeck effect in the ferrimagnet gadlinium iron garnet, with a magnetic compensation point near room temperature, we demonstrate that high-energy exchange magnons play a key role in the spin Seebeck effect.
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.
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.
Qiu, Z.*; 賣市 幹大*; Hou, D.*; 内田 健一*; 山本 浩史*; 齊藤 英治
AIP Advances (Internet), 5(5), p.057167_1 - 057167_7, 2015/05
Spin-current injection into an organic semiconductor -(BEDT-TTF)Cu[N(CN)]Br film induced by the spin pumping from an yttrium iron garnet (YIG) film. When magnetization dynamics in the YIG film is excited by ferromagnetic or spin-wave resonance, a voltage signal was found to appear in the -(BEDT-TTF)Cu[N(CN)]Br film. Magnetic-field-angle dependence measurements indicate that the voltage signal is governed by the inverse spin Hall effect in -(BEDT-TTF)Cu[N(CN)]Br. We found that the voltage signal in the -(BEDT-TTF)Cu[N(CN)]Br/YIG system is critically suppressed around 80 K, around which magnetic and/or glass transitions occur, implying that the efficiency of the spin-current injection is suppressed by fluctuations which critically enhanced near the transitions.
Wray, L. A.*; Huang, S.-W.*; Jarrige, I.*; 池内 和彦*; 石井 賢司; Li, J.*; Qiu, Z. Q.*; Hussain, Z.*; Chuang, Y.-D.*
Frontiers in Physics (Internet), 3, p.32_1 - 32_11, 2015/05
In resonant inelastic X-ray scattering (RIXS), core hole resonance modes are used to enhance coupling between photons and low energy electronic degrees of freedom. Resonating with shallow core holes accessed in the extreme ultraviolet (EUV) can provide greatly improved energy resolution at standard resolving power, but has been found to often yield qualitatively different spectra than similar measurements performed with higher energy X-rays. This paper uses experimental data and multiplet-based numerical simulations for the M-edges of Co-, Ni-, and Cu-based Mott insulators to review the properties that distinguish EUV RIXS from more commonly performed higher energy measurements. Key factors such as the origin of the strong EUV elastic line and advantages of EUV spectral functions over soft X-ray RIXS for identifying intrinsic excitation line shapes are discussed.
Qiu, Z.*; Hou, D.*; 内田 健一*; 齊藤 英治
Journal of Physics D; Applied Physics, 48(16), p.164013_1 - 164013_5, 2015/04
The longitudinal spin-Seebeck effect (LSSE) has been investigated for Pt/yttrium iron garnet (YIG) bilayer systems. The magnitude of the voltage induced by the LSSE is found to be sensitive to the Pt/YIG interface condition. We observed large LSSE voltage in a Pt/YIG system with a better crystalline interface, while the voltage decays steeply when an amorphous layer is introduced at the interface artificially.