Visualization of anomalous Ettingshausen effect in a ferromagnetic film: Direct evidence of different symmetry from spin Peltier effect

Abstract

Spatial distribution of temperature modulation due to anomalous Ettingshausen effect (AEE) is visualized in a ferromagnetic FePt thin film with in-plane and out-of-plane magnetizations using the lock-in thermography technique. Comparing the AEE of FePt with the spin Peltier effect (SPE) of a Pt / yttrium iron garnet junction provides direct evidence of different symmetries of AEE and SPE. Our experiments and numerical calculations reveal that the distribution of heat sources induced by AEE strongly depends on the direction of magnetization, leading to the remarkable different temperature profiles in the FePt thin film between the in-plane and perpendicularly magnetized configurations. (99 words) * Spin-current (J s )-mediated interconversion between electric charge current (J c ) and heat current (J q ), research field of which is frequently called "spin caloritronics" [1], has largely fascinated us from the viewpoint of not only fundamental physics but also potential applications. Spin Seebeck effect (SSE) [2-4] and spin Peltier effect (SPE) [5,6] are representative phenomena of spin caloritronics. SSE enables us to convert a temperature gradient (∇T) to pure J s owing to the collective magnetization dynamics activated by ∇T [7,8]. On the other hand, SPE is the reverse process of SSE, in which the flow of J s produces J q due to the transfer of spin angular momentum and energy from conduction electron spins to local spins, and vice versa, and the resultant non-equilibrium states of the magnon and electron systems [5]. Eventually, the SPE induces ∇T along J s . In both cases of SSE and SPE, junctions consisting of ferromagnetic and paramagnetic materials are often studied [2,3,5,6,9], e.g. a ferrimagnetic insulator yttrium iron garnet (YIG) and a paramagnetic metal Pt. For SSE, the J s due to the non-equilibrium spin state at the ferromagnet / paramagnet interface is observed as electric voltage in the paramagnet via the spin-orbit interaction, i.e. a spin Hall effect (SHE) [10]. The SPE has recently been observed in junctions with YIG and Pt by using microfabricated thermopiles [5] and active infrared emission microscopy called lock-in thermography (LIT) [6,11,12]. J s was generated via the SHE of Pt by the J c flow, and the interaction of J s and spontaneous magnetization (M) of YIG modulated the temperature of the junction.In addition to the SSE and SPE, anomalous Nernst effect (ANE) and anomalous Ettingshausen effect (AEE) are famous thermoelectric phenomena in ferromagnets that have been known for a long time [13], in

Transient response of the spin Peltier effect revealed by lock-in thermoreflectance measurements

“…11 Importantly, we found that the transient response of the SPE signal is different also from that of the AEE signal, although SPE and AEE are believed to exhibit the similar thermoelectric conversion symmetry in in-plane magnetized conditions. 17 To demonstrate the difference, we measured the f dependence of Aodd for AEE (gray circles) by using the sample in which the Pt/YIG layer is replaced with a 13.3-nm-thick Ni film, where the AEE signal appears in the Ni layer instead of the SPE signal.…”

Spin-mediated charge-to-heat current conversion phenomena in ferromagnetic binary alloys

“…For the measurements of the AMPE and the AEE, the slabs were cut into a U-shaped structure using wire electric discharge machining. The length along the x direction, total width along the z direction, width of the leg parts along the z direction, and thickness of the U-shaped slabs along the y We measured the temperature modulation induced by the AMPE and the AEE by means of the LIT method [18][19][20][24][25][26][27][28][29][39][40][41]. In the LIT measurements, while applying a square-wave-modulated AC charge current with the frequency f (1-25 Hz) to a sample, the temperature change oscillating with the same frequency as the charge current is extracted.…”

Systematic Investigation of Anisotropic Magneto–Peltier Effect and Anomalous Ettingshausen Effect in Ni Thin Films

“…This situation is being changed with rapid developments of spin caloritronics [24,32], where these phenomena are attracting renewed attention. In this stream, the observation of the anisotropic magneto-Peltier effect (AMPE) [37,41,45], the reciprocal of the AMSE, and the anomalous Ettingshausen effect (AEE) [39,[41][42][43], the reciprocal of the ANE, in ferromagnetic metals has recently been reported.The AMPE is a phenomenon that the Peltier coefficient depends on the angle θ M between the directions of a charge current J c and magnetization M in a ferromagnet. Due to the AMPE, the Peltier coefficient  in an isotropic ferromagnet exhibits the θ M dependence satisfying the following equation:where  ||() is the Peltier coefficient for the M || J c (M  J c ) configuration.…”

“…The AEE generates a heat current in the direction of the cross product of J c and M:where j q,AEE , j c , m, and  AEE denote the heat-current density driven by the AEE, charge current density applied to a ferromagnet, unit vector of M, and anomalous Ettingshausen coefficient, respectively [ Fig. 1(a)] [39,[41][42][43].Therefore, when AEE is isotropic in a ferromagnet, the temperature gradient induced by the AEE should be proportional to sinθ M . The AEE may also bring novel thermal management technologies because it enables magnetic control of a heat-current direction.The understanding of the microscopic mechanism of the AMPE and AEE in ferromagnetic conductors and improvement of their thermoelectric conversion efficiency are crucial not only for the fundamental physics of spin caloritronics and thermoelectrics but also for the realization of nano-scale thermal energy engineering.…”

“…The role of the substrates in the AEE experiments is also yet to be investigated systematically. The purpose of this study is to establish a procedure for the detection of the AMPE and AEE contributions in thin films and to clarify the dependences of the AMPE and AEE on the thickness of the films and thermal properties of the substrates.In this paper, we report the observation of the AMPE and AEE in Ni thin films with changing the film thickness and substrate materials by means of the lock-in thermography (LIT) method [38][39][40][41][42][43][44][45][47][48][49]. We have demonstrated that the pure AMPE and AEE contributions, separated from other thermoelectric effects, can be extracted even in the thin films by measuring the magnetic-field-angle dependence of the LIT images.…”