Giant magneto-optic rotation in amorphous uranium antimonide (invited)

Amorphous U-Sb alloys in the vicinity of the composition U 3Sb4 have magneto-optic Faraday rotations of + 2 × 106 deg/cm. This is the highest rotation of any amorphous ferromagnetic material. These alloys also have relatively high Curie temperatures of about 140 K with magnetic moments per uranium of up to 1.4μB. These properties are unexpected since it is known from intermetallic compounds that U atoms must be isolated from each other in order to develop a magnetic moment. In a concentrated random amorphous alloy, no magnetic moment on the uranium would be expected because of the high probability of uranium near neighbors. In the U-Sb amorphous alloys, however, there appears to be chemical short-range order which tends to favor Sb nearest neighbors for uranium. The amorphous U-Sb alloys have Kerr rotations of about -3°. This value is comparable in magnitude but opposite in sign to that of the single-crystal rock salt compound USb in the visible. In these amorphous alloys the Hall angle from the spontaneous Hall effect can be as much as 17°. Similar large magneto-optic and Hall coefficients have also been found in amorphous U-Bi, U-Sb-Mn, and U-Sb-Co alloys.