Lock-in thermoreflectance as a tool for investigating spin caloritronics

We report lock-in thermoreflectance (LITR) measurements of spin-caloritronic phenomena in magnetic hybrid structures. In the LITR measurements, the temperature modulation signals due to the spin Peltier effect (SPE) and anomalous Ettingshausen effect (AEE) were detected through the temperature dependence of the optical reflectivity of the sample surface. By using the lock-in technique, the reflectivity modulation in response to an alternating charge current applied to the samples can be detected sensitively, which allows us to clarify the transient responses of the SPE and AEE and gives a clue to separate these phenomena. We applied this method to a junction system comprising a ferromagnetic metal film formed on a magnetic insulator substrate and found that the transient response unique to the SPE can be observed even when the SPE and AEE coexist. We also checked that the lock-in signals of the reflected light intensity exhibit no light polarization rotation; the observed temperature modulation is free from parasitic magneto-optical effects. The SPE and AEE can thus be measured not only by the conventional laser-based LITR but also by light-emitting-diode-based LITR, which enables versatile and cost-effective measurements.