Low Temperature Hysteresis Loops Research Articles

Magnetic anisotropy in MnBi particles grown by directional solidification of the Mn-Bi eutectic

Directional solidification of the eutectic Mn-Bi alloy (2 at% Mn) produces an aligned array of ferromagnetic Mn-Bi needles, 0.1 to 1.5 μm in diameter, in a matrix of diamagnetic Bi. Samples grown at about 50 cm/hr and then annealed at 200 °C contain two magnetic phases. One phase is normal equilibrium MnBi, known in the recent literature as the low-temperature phase (LTP). The second phase is crystallographically similar to the high-temperature phase (HTP) Mn1.08Bi, but has a much lower Curie temperature of 240 K. Low-temperature hysteresis loops measured parallel and perpendicular to the axis of freezing are of unusual form. If the magnetization due to the normal MnBi phase is subtracted the resulting loop in the parallel direction has Hci≳100 kOe, suggesting single-domain behavior. The resulting loop in the perpendicular direction can be accounted for by the presence of a relatively large fourth-order anisotropy.