Investigating spin-transfer torques induced by thermal gradients in magnetic tunnel junctions by using micro-cavity ferromagnetic resonance

Abstract

Similar to electrical currents flowing through magnetic multilayers, thermal gradients applied across the barrier of a magnetic tunnel junction may induce pure spin-currents and generate ‘thermal’ spin-transfer torques large enough to induce magnetization dynamics in the free layer. In this study, we describe a novel experimental approach to observe spin-transfer torques induced by thermal gradients in magnetic multilayers by studying their ferromagnetic resonance response in microwave cavities. Utilizing this approach allows for measuring the magnetization dynamics on micron/nano-sized samples in open-circuit conditions, i.e. without the need of electrical contacts. We performed first experiments on magnetic tunnel junctions patterned into 6 × 9 µm2 ellipses from Co2FeAl/MgO/CoFeB stacks. We conducted microresonator ferromagnetic resonance (FMR) under focused laser illumination to induce thermal gradients in the layer stack and compared them to measurements in which the sample was globally heated from the backside of the substrate. Moreover, we carried out broadband FMR measurements under global heating conditions on the same extended films the microstructures were later on prepared from. The results clearly demonstrate the effect of thermal spin-torque on the FMR response and thus show that the microresonator approach is well suited to investigate thermal spin-transfer-driven processes for small temperatures gradients, far below the gradients required for magnetic switching.

Tunnel magneto-Seebeck effect

“…The TMS in combination with a spin-transfer torque (STT) [23] has the potential to thermally assist the magnetic switching of MTJs [24]. After the first theoretical descriptions of the thermal STT in spin valves [25,26] and MTJs [27,28], the first evidence of an experimental confirmation can be found in [29][30][31][32][33]. While the electrically induced STT (magnetic switching by spin-polarized charge currents) is already utilized in commercially available STT-based magnetoresistive random-access memory devices, the thermal STT is not yet explored in a way that it can be used for commercial devices.…”

Bolometric Properties of a Spin-Torque Diode Based on a Magnetic Tunnel Junction

“…The magneto-Seebeck effect and the thermal initiating spintransfer torques in MTJ are an important part of spincaloritronic studies [1][2][3][4][5][6][7] and are of a high significance for development of nonvolatile memory and temperaturesensitive devices [8][9][10]. Spin-torque diode effect in a magnetic tunnel structure is the voltage rectification effect via the microwave modulation of the active component of the MTJ resistance due to the spin-transfer torque [11].…”

“…After the linearization of (5), one can find the active part of the small deviation of the magnetization = 0 from the equilibrium position and calculate Δ ( ) according to (4). Taking into account the linearity of the thermal spin-transfer torques ‖(⊥) with respect to amplitude Δ 1 ( ) of the first-harmonic component of temperature drop, the Δ ( ) will be described by the following formula in the case of a small oscillation of m near m 0 = e Y :…”

Special issue on spin caloritronics

“…Gomonay et al [21] predict that the spin Peltier effect should exist in an antiferromagnetic insulator. Moreover, magnetic nanostructures (Asam et al [22], Michel et al [23], Cansever et al [24]) as well as III-V semiconductors (Göbbels et al [25]) become accessible. Theoretical aspects of spin caloritronics in exotic materials are studied theoretically by Wang and Wang [26] (topologically non-trivial spin lattices), Tatara et al [27] (insulating chiral magnets) and Kurpas et al [28] (black phosphorus and phosphorene).…”