Investigating spin-transfer torques induced by thermal gradients on magnetic tunnel junctions using microcavity ferromagnetic resonance

Similar to electrical currents flowing through magnetic multilayers [1], it has been predicted that thermal gradients applied across the spacer of a spin-valve or a magnetic tunnel junction may induce pure spin currents and generate ‘thermal’ spin-transfer torques (T-STTs) large enough to induce magnetization dynamics [2-3]. Nevertheless, providing detailed experimental studies in this direction has so far proved elusive, due to difficulties in generating sufficiently large thermal gradients for such effects to be observed. Here, we describe a different approach, which focuses on observing and quantifying spin-transfer torques induced by thermal gradients in magnetic multilayers by means of ferromagnetic resonance (FMR) response under open circuit conditions. The FMR response is measured using specially designed planar microresonators, which generate ac fields perpendicular to the plane of the layers [4]. Such microresonators, with loop diameters of 10 and 20 μm were optimized at a fixed frequency of 14 GHz. Magnetic multilayers with different compositions were patterned using electron-beam lithography into micron-sized pillars with different cross-sections. Microresonators were fabricated using UV lithography such that the magnetic device lies in the center of the loop. An example is shown in Fig 1. For laser heating, we used a diode laser with 51 mW power (5-10 μm focus in diameter). Fig 2 shows a set of FMR measurements performed on an 8x10 μm elliptical shape Py/Cu/Py magnetic multilayer under laser heating, with different laser powers. A clear change is observed at higher than 30 mW laser power, with the FMR line exhibiting changes in resonance field and linewidth. These changes likely arise from a combination of the induced TSTT and the heating of the whole device. The results are analyzed by means of conventional FMR modeling and the thermal gradients are estimated from COMSOL simulations.This project is funded by DFG Priority Programme SPP 1538 Spincaloritronics (SpinCat) and supported by the Nanofabrication Facilities at Ion Beam Center.