Diffusion Thermopower of(Ga,Mn)As/GaAsTunnel Junctions

Abstract

We report the observation of tunneling anisotropic magnetothermopower, a voltage response to a temperature difference across an interface between a normal and a magnetic semiconductor. The resulting voltage is related to the energy derivative of the density of states in the magnetic material, and thus has a strongly anisotropic response to the direction of magnetization in the material. The effect will have relevance to the operation of semiconductor spintronic devices, and may indeed already play a role in correctly interpreting the details of some earlier spin injection studies.PACS numbers: 75.50. Pp, 75.30.Gw, 73.50.Jt, The most pressing issues of modern information technologies are power consumption and heat dissipation. As such, a proper understanding of the relationship between electrical and thermal effects in devices is essential to their design and operation. When these devices are magnetic in nature, the study of this relationship is the topic of the field of spin caloritronics [1], which was strongly inspired by the initial reports of the spin Seebeck effect in metals [2] and subsequently in magnetic semiconductors [3].As a simple example of the importance of thermal effects on device operation, consider the problem of the injection of spin polarized current into a nonmagnetic semiconductor. Spin injection is often measured using a four contact nonlocal technique [4,5], where current is passed between two adjacent contacts, and the chemical potential resulting from spin accumulation is detected at two further contacts which are situated next to, but not between the current contacts. Ideally, the signal from such measurements should produce a voltage at the detection leads, which is equal but opposite in sign depending on the relative orientations of the contacts. In practice this is almost never the case, as the measurements typically include a constant voltage offset, which it has become customary to neglect as an unimportant background contribution [5].This background voltage is a thermoelectric effect [6], which in most cases is isotropic to magnetic field and thus independent of the magnetization direction of the contacts. The subtraction of such a constant background is thus inconsequential to the experimental conclusions.As we show in this Letter however, in materials where strong spin-orbit coupling links the magnetic properties to the density of states [7], a spin caloritronic phenomenon, which we dub tunneling anisotropic magnetotermopower (TAMT), appears. This not only may have interesting implications for device applications, but can also lead to misinterpretations if one fails to take it into consideration. Indeed, we show that thermal voltages detected in such materials, e.g. the ferromagnetic semiconductor (Ga,Mn)As, closely resemble typical nonlocal spin-valve signals [8], even when no spin injector is used.The configuration we study looks at the thermopower across a tunnel junction between ferromagnetic and nonmagnetic regions [9]. Specificly, our experiment examines diffusion thermop...

Thermal spin current and magnetothermopower by Seebeck spin tunneling

“…32 Anisotropy of the tunnel magnetothermopower was also reported. 33 Last but not least, it was predicted that thermal gradients give rise to thermal spin-transfer torques in magnetic heterostructures 9,34 and tunnel junctions 35 and experimental evidence for thermal torques has been presented. 36,37 Le Breton et al 24 described the salient features of Seebeck spin tunneling by numerical evaluation of a free-electron model.…”

Galvanomagnetic and thermogalvanomagnetic transport effects in ferromagnetic fccCoxPd1−xalloys from first principles

“…While the AMR and the closely related planar Hall effect have been extensively studied, there are relatively few experimental investigations on the ASE and planar Nernst effect (PNE) to be found in the literature [1][2][3][4], and, to our knowledge, so far only one first-principles study is available [5], which deals with the magnetic anisotropy of the transmission through a Cu|Co|Cu trilayer system and its enhancement due to the symmetry breaking at the Co|Cu interface. To a much greater extent investigations have been carried out on a closely related class of phenomena, namely the magneto-thermopower or -Seebeck effect and its variations (spin-dependent, tunneling, tunneling anisotropic) occurring in various types of heterostructures [6][7][8][9].Concerning the AHE [10-13] and ANE [13,14], strong interest has arisen in recent years driven by progress in the understanding of the microscopic origins of transverse transport effects and by the (re)discovery of the spin Hall effect [15][16][17]. The latter also has its thermoelectric analog, the spin Nernst effect [18][19][20].…”

Tunneling anisotropic thermopower and Seebeck effects in magnetic tunnel junctions

“…2). Both models have been successful in describing TAMR experiments [26], while the latter one has been used for describing TAMT experiments as well [37]. A version of the rectangular barrier model in the absence of SOI has recently been applied to the theoretical [100]…”

“…Hence, for the case of MTJs this quantity is dubbed tunneling magnetothermopower. Naydenova et al [37] have measured an anisotropic dependence of the thermopower on the magnetization orientation of the ferromagnetic electrode with respect to a reference crystallographic axis. As in the TAMR case, the anisotropy is likely due to the effect of the strong SOI on the DOS of the ferromagnetic semiconductor.…”