Spin caloritronics, which focuses on the interconversion between spin and heat currents, is rising in popularity due to newly discovered physical effects. The spin Seebeck effect (SSE), referring to thermal spin-current generation in a magnet, may provide new mechanisms for solid-state thermal-to-electrical energy conversion with applications for waste heat recycling and temperature sensors.

Oyanagi et al. investigate the magnetic field dependence of the SSE in a nonlocal geometry. The nonlocal SSE, as affected by the distance between the heat source and spin current detector, can enable sensitive probes for monitoring spin-transport property in magnets. The researchers sought to understand the effect’s dependence on separation distance, magnetic field, as well as temperature and excitation current.

The researchers placed two platinum wires on a ferrimagnetic yttrium iron garnet (YIG, with chemical formula Y3Fe5O12) film. One of the wires was a heat source as a conventional charge current through the wire increased the temperature locally in the YIG film. Thermal magnons generated by the heat source diffused through the YIG film away from the first platinum wire and the resulting magnon spin current is detected at the other wire via spin to charge conversion generated by the inverse spin Hall effect.

In addition to clarifying the origin of magnetic-field-induced suppression in nonlocal SSE at low temperatures, they discovered a non-linear effect in the nonlocal SSE – the voltage deviates from the conventional linear heater-power dependence with increasing power. Such nonlinear nonlocal SSE signal remained almost unchanged when exposed to a high magnetic field at low temperatures.

The results add information regarding the magnetic field response and nonlinear effect in spin transport by magnons at low temperatures.

Source: “Magnetic field dependence of the nonlocal spin Seebeck effect in Pt/YIG/Pt systems at low temperatures,” by Koichi Oyanagi, Takashi Kikkawa, and Eiji Saitoh, AIP Advances (2020). The article can be accessed at http://doi.org/10.1063/1.5135944.