Optical detection of transverse spin-Seebeck effect in permalloy film using Sagnac interferometer microscopy

Abstract

Macroscopic spatially spin distribution caused by the application of an in-plane thermal gradient in a conducting ferromagnetic film, known as transverse spin-Seebeck effect (TSSE), is in many cases overshadowed by thermoelectric and magneto-thermoelectric effects when using the conventional electrical detection via the inverse spin Hall effect.Here we report an optical method for the detection and characterization of TSSE response in permalloy films using magneto-optical Kerr effect with an ultrasensitive fiber-optic Sagnac interferometer microscope that is free of magneto-thermoelectric artefacts, which also allows measurements with field direction parallel and perpendicular to the film surface. We found a substantial anisotropy in the permalloy TSSE coefficient, where the 'in-plane magnetization' coefficient is much larger than that in the 'out-of-plane magnetization'. †These authors contributed equally to this work. *To whom correspondence should be addressed: val@physics.utah.edu ‡ Current address: Department of Physics, North Carolina State University, Raleigh, NC, 27695,The young field of 'Spin Caloritronics' seeks to exploit the strong coupling between spin currents and heat currents with application opportunities in novel devices [1,2]. It has been shown that a temperature gradient in a ferromagnetic (FM) metal [3,4] and alloys [5], FM semiconductor [6,7], or FM insulator [8][9][10][11][12][13][14] in the presence of a magnetic field B spontaneously generates a position-dependent accumulation of spin-polarized carriers that forms spin current (J S ) in an adjacent non-magnetic overlayer; this was dubbed the spin-Seebeck effect (SSE) [2].The SSE has been shown to exist even in the paramagnetic phase above the FM phase-transition temperature in some cases [15,16]. Among the spin caloritronics processes, longitudinal SSE response (LSSE, where J S is parallel to the direction of thermal gradient) in magnetic insulators have been extensively studied [9][10][11][12][13][14], whereas attempts to measure the transverse SSE response (TSSE, where J S is perpendicular to the direction of thermal gradient) [3,[6][7][8] have raised a number of questions [17][18][19][20][21]. In particular, the steady state TSSE in conducting FM films has not been thoroughly explored.So far the SSE has been exclusively detected by depositing onto the FM film a "spin-detection layer" that consists of a non-magnetic metal (NM) having strong spin-orbit coupling (SOC) such as Pt, which converts the spin current in the FM layer into charge current in the NM overlayer via the inverse spin Hall effect (ISHE) [3]. We note, however that the ISHE voltage that is used to detect the SSE in the FM substrate is quite small (of the order of several μV/K) and phenomenologically similar to a number of thermoelectric and magneto-thermoelectric artefacts such as the regular Seebeck effect, the proximity/anomalous/planar Nernst effect and the LSSE induced by an unintentional out-of-plane thermal gradient [17][18][19][20][21]. Therefore it has be...

Long-range transverse spin Seebeck effect in permalloy stripes using Sagnac interferometer microscopy

“…While measuring the temperature transient increase using thermocouples after the heating power is turned on at the right end of the copper block, we note that the sample holder reaches thermal equilibrium in about 10 minutes. For It is thus due to the TSSE effect, similar to our previous observations in a continuous NiFe film on SiN substrate [28]. We also confirmed, as a control experiment that the change in  at each NiFe stripe is reversed when the direction of the magnetic field is reversed, as shown in Figures 2(c) and (d).…”

“…The sample was exposed to an out-of-plane DC magnetic field (B ~ 800 mT) in a configuration suitable for probing the polar MOKE effect (TSSEPM, BΔT) as shown in Figure 1(a). The corresponding M-B loop in this configuration has been measured by the Sagnac interferometer at room temperature [28]. We note that the NiFe magnetization is aligned along the out-of-plane direction when B > 150 mT.…”

“…substrates [28]. The samples were connected to two copper blocks with a separation of 5 mm using thermally conductive glue.…”

“…Moreover, the MOKE technique can directly probe the spin accumulation in conducting FM slabs without involving additional top metal layers required to measure spin current via ISHE, so that the possible unintentional out-of-plane thermal gradient can be greatly suppressed. Two different experimental MOKE measurements have been used to date, namely the picosecond time resolve reflectivity via the technique of pump-probe correlation [23,25], and a steady state technique using a Sagnac interferometer with superior sensitivity [28]. In our previous report [28] we presented a continuous-wave (CW) optical study of the TSSE response in a conducting FM slab, namely permalloy, over a macroscopic length scale of several mm's, which is conceptually close to the original TSSE report [3].…”

“…Two different experimental MOKE measurements have been used to date, namely the picosecond time resolve reflectivity via the technique of pump-probe correlation [23,25], and a steady state technique using a Sagnac interferometer with superior sensitivity [28]. In our previous report [28] we presented a continuous-wave (CW) optical study of the TSSE response in a conducting FM slab, namely permalloy, over a macroscopic length scale of several mm's, which is conceptually close to the original TSSE report [3]. We measured the optical TSSE coefficients of NiFe film deposited on three distinct substrates and confirmed that the observed spin accumulation is generated via TSSE since SDSE response does not depend on the choice of different substrates.…”