Magnetic properties of directionally solidified MnBi-Bi eutectics

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Abstract
Samples consisting of approximately parallel arrays of ferromagnetic MnBi rods in a Bi matrix have been grown by directional solidification of eutectic Mn-Bi alloys. The rods are of order 1μm in diameter, and very long in comparison to their diameter. Room-temperature intrinsic coercive fields up to 30 kOe have been attained, suggesting that the rods show single-domain behavior. The measured magnetizations are substantially higher than expected from the known moment of MnBi and the predicted volume fraction of MnBi in the eutectic; this means that the MnBi in our samples has an abnormally high moment, or (more likely) that the MnBi content is higher than predicted from the phase diagram. Magnetic measurements on samples cooled to 77 K show a stepped hysteresis loop which is most simply interpreted as a superposition of two loops, indicating the presence of two different magnetic phases. One phase has low coercive field (< 1 kOe) and is probably "normal" MnBi, which is known to have low anisotropy at 77 K. The other phase has a Curie point near 270 K, and is presumably the phase found in similarly prepared samples by Noothoven van Goor and Zijlstra. In some samples, we find this phase has a coercive field of 120 kOe, and is not saturated at our maximum field of 145 kOe (all at 77 k). We have not yet been able to separate this phase from "normal" MnBi, and so do not know its morphology or magnetic properties. It seems likely that it has a magnetization at least as large as that of normal MnBi, and much higher anisotropy.