Interfacial Spin Filtering at Copper/GaMnAs Contacts*

We determine the spin injection efficiency using a single ferromagnetic film without the need for a spin-detection layer. This is accomplished by studying the temperature dependence of the specific contact resistance (AR$_{C})$ of a copper/GaMnAs contact using a circular transmission line method. AR$_{C}$ is as low as 5x10$^{-8} \quad \Omega $cm$^{2}$, and decreases slowly with decreasing temperature T. However, as T approaches Curie temperature T$_{C}$, AR$_{C}$ abruptly jumps to about double its initial value. We suggest that this behavior arises from the suppression of one of the two spin conduction channels, which results in substantial spin polarization.