Finite tunneling spin polarization at the compensation point of rare-earth-metal-transition-metal alloys

We show, using superconducting tunneling spectroscopy and tunneling magnetoresistance measurements, that ferrimagnetic alloys of Co and Gd can exhibit both positive and negative spin polarization depending on temperature and composition. These observations can be understood by considering the relative contributions of independent spin-polarized tunneling currents from the rare-earth-metal and transition-metal subnetwork magnetizations, which are coupled antiferromagnetically. At the compensation point of the alloy, where the subnetwork magnetizations are equal and the alloy has nearly zero net magnetization, nevertheless large tunneling spin polarization is observed. © 2005 The American Physical Society.