Tunneling spin polarization in magnetic tunnel junctions near the Curie temperature

Temperature dependence of the tunneling magnetoresistance and magnetization have been measured for spin-valve magnetic tunnel junctions where the free magnetic electrode is a Cu38Ni62 alloy with a Curie temperature of roughly 240 K. The temperature dependence of the normalized magnetoresistance is found to be independent of the voltage bias applied across the junction at all temperatures. By extracting the temperature dependence of the tunneling spin polarization of the CuNi electrode using the Julliere formula, we show that the tunneling spin polarization as a function of temperature can be related to the temperature-dependent magnetization; the anticipated linear relation is not obtained. This nonlinear polarization versus magnetization relation, in addition to the linear relation obtained previously by other tunneling methods, are both reproduced by a theoretical model that incorporates multiple s-d hybridized bands into the tunneling process.