Anisotropic magnetothermoelectric power of ferromagnetic thin films

Abstract

In this article, we report the measurements of the magnetothermoelectric power (MTEP) in metallic ferromagnetic thin films of Ni80Fe20 (Permalloy; Py), Co and CrO2 at temperatures in the range of 100K to 400K. In 25nm thick Py films and 50nm thick Co films both the anisotropic magnetoresistance (AMR) and MTEP show a relative change in resistance and thermoelectric power (TEP) of the order of 0.2% when the magnetic field is reversed, and in both cases there is no significant change in AMR or MTEP after the saturation field has been reached. Surprisingly, both Py and Co films have opposite MTEP behaviour although both have the same sign for AMR and TEP. The data on half metallic ferromagnet CrO2 films show a different picture. Films of thickness of 100nm were grown on TiO2 and on sapphire. The MTEP behavior at low fields shows peaks similar to the AMR in these films, with variations up to 1%. With increasing field both the MR and the MTEP variations keep growing, with MTEP showing relative changes of 1.5% with the thermal gradient along the b-axis and even 20% with the gradient along the c-axis, with an intermediate value of 3% for the film on sapphire. It appears that the low-field effects are due to the magnetic domain state, and the high-field effects are intrinsic to the electronic structure of CrO2 and intergarian tunnelling magnetoresistance that contributes to MTEP as tunnelling-MTEP. Our results will stimulate the research work in the field of spin dependent thermal transport in ferromagnetic materials to further develop spin-Caloritronics.