Incipient Orbital Order in Half-Metallic $Ba_2FeReO_6$

Largely unquenched Re 5d orbital magnetic moments in half-metallic $Ba_2FeReO_6$ drive a symmetry lowering transition from a cubic paramagnet to a compressed tetragonal (c/a < 1) ferrimagnet below TC\sim 305 K, with a giant linear magnetoelastic constant and the spins lying spontaneously along the unique tetragonal axis. The large orbital magnetization and degree of structural deformation indicate proximity to a metal-insulator transition. These results point to an incipient orbitally ordered state in the metallic ferrimagnetic phase