Qualitative evaluation of the temperature dependence of dynamical spin injection in CoFeB/Pt/CoFeB trilayer thin films

Abstract

Dynamical spin injection based on ferromagnetic resonance (FMR) has been intensively investigated owing to its simple structure with variable applications [1]. When the FMR is excited in ferromagnetic-metal (FM)/nonmagnetic-metal (NM) bilayer thin films, the spin current flows from the FM layer into the NM layer. Although the mechanism of the dynamical spin injection is mainly explained by spin pumping, FMR-heating could be a possible reason for the dynamical spin injection [2].To investigate the dominant contribution for the dynamical spin injection, we measured the temperature dependence of the inverse spin Hall signal in a CoFeB/Pt/CoFeB trilayer structure. Fig.1 shows the schematic illustration of the measurement system. Here, the bottom Cu layer acts as the coplanar wave guide for genrating a high frequency magnetic field, resulting in the efficient excitation of the FMR of the trilayer structure. In addition, since the Cu layer with sufficiently low electrical resistivity is directly connected to the trilayer, we are able to purely observe inverse spin Hall signal in the Pt due to the dynamical spin injection with eliminate the spurious signals such as the rectified anisotropic magnetoresistance and anomalous Nernst effects.[3] The observed inverse spin Hall signals at room temperature (297K) and low temperature(77K) are shown in Fig.2. It should be emphasized that the sign of the inverse spin Hall signal at 77 K is opposite from that at 297 K. This can be explained by the change of the thermal gradient in CoFeB/Pt/CoFeB/Cu/Sub, indicating that the FMR heating is a dominant mechanism for the dynamical spin injection. **