Ferromagnetism and Metamagnetism in Copper-doped Germanium Clathrate

Abstract

Cu-doped type-I germanium clathrate can exhibit dilute magnetism, including ferromagnetism, antiferromagnetism, and metamagnetic transitions up to 90 K. 63Cu NMR measurements confirm that these transitions are due to a dilute composition of magnetic defects coupled by conduction electrons, behavior similar to that of magnetic semiconductors. Magnetic measurements indicate localized magnetic moments, attributed to clusters of magnetic ions, with competition between ferromagnetic and antiferromagnetic exchange, and also indications of glassy behavior in the ferromagnetic phase. NMR Knight shifts and relaxation times show that the conduction band is metallic with a large Korringa ratio. Comparison to a mean-field theory for the ordering behavior gives a good accounting for the ferromagnetic transition.