Single-Crystal Growth of (Ca_1-xMg_x)₂Co₁₂P₇ and its Itinerant-Electron Magnetism

Abstract

The itinerant ferromagnetism is reported in A₂T₁₂P₇ (A = lanthanoid, Group 2-4 element, T = transition metal) with Zr₂Fe₁₂P₇-type structure. In order to investigate the behavior of electrons in this structure, we have synthesized single crystals of (Ca_1-xMg_x)₂Co₁₂P₇ and measured magnetizations. From magnetic susceptibility, we found that Ca₂Co₁₂P₇ shows a ferromagnetic transition at T_C = 78 K, which is attributed to the Co-3d itinerant electrons. We also found the Curie temperature decreases and eventually disappears around x~0.6, when Ca is substituted by Mg. It has been suggested that the electronic density of states changes and the magnetic state of this system crosses the quantum critical point due to the positive chemical-pressure effect of the Ca substitution by Mg.