Magnetic properties of the low-temperature phase of MnBi

Abstract

MnBi forms peritectically at ∼450 °C. Preparation of MnBi employing conventional techniques such as arc melting and induction melting results in the segregation of manganese. In order to avoid this segregation, we followed the procedure recommended by Guo et al. [X. Guo, A. Zaluska, Z. Altounian, and J. O. Strom-Olsen, J. Mater. Res. 5, 2646 (1990)] and prepared a low-temperature phase of MnBi by melt spinning, followed by heat treatment. Fine powder of MnBi was prepared by ball milling the melt-spun ribbons for various lengths of time. Magnetic properties of these powders were determined. In particular, the temperature dependent coercivity was studied from room temperature to 360 °C for the powders ball milled for 2 and 10 h. The coercivity is found to increase with the increase in temperature reaching a maximum of 25.8 kOe at 280 °C and then decrease as the temperature is increased further. We also found that a peak in coercivity is observed for the samples milled for 10 h. MnBi shows a first-order transition to a paramagnetic phase at 360 °C. In an attempt to increase this transition temperature, an alloy of composition Mn0.75Ni0.25Bi0.5Sb0.5 was made by induction melting. The transition temperature increases from 360 °C for MnBi to 400 °C for Mn0.75Ni0.25Bi0.5Sb0.5.

Symmetry-lowering lattice distortion at the spin reorientation in MnBi single crystals

“…For permanent magnet applications, fine-grained, polycrystalline samples are usually desired. These have been produced from melt-spun and/or mechanically milled material [20,21], and by magnetic separation of MnBi from excess Bi in samples produced by powder-metallurgical routes [7].…”

Magnetic and Structural Properties of MnBi1-xTixAlloys

“…2(a), with the increase of temperature, magnetization of MnBi alloy decreases slightly, reaches minimum value at around 497 K, and then increases gradually up to 590 K. Increase of magnetization with temperature is considered to be due to increasing the volume fraction of LTP MnBi, which possesses a higher magnetization value [8]. The increase of magnetization above 500 K is due to the formation of the LTP of MnBi [14]. At 620 K, the value of magnetization drops dramatically to nearly zero, which is due to the magnetic phase transition of MnBi from LTP to paramagnetic high-temperature phase (HTP) [15].…”

“…It has attracted research interest, mainly due to its unusually large magnetic anisotropy of the low-temperature phase (LTP) [1,2], and the excellent magneto-optical properties of the quenched high-temperature phase (QHTP) [3]. It is remarkable that the coercivity of the LTP exhibits a positive temperature coefficient, and is much larger than that of Nd-Fe-B magnets at high temperature [4][5][6]. Therefore, MnBi has good potential to be used in high temperature circumstances [7].…”

MnBi nanoparticles with perpendicular magnetic anisotropy

“…Epitaxial growth or subsequent deposition of each element, which have been generally used for aligning the c-axis, the easy magnetization axis, in MnBi thin films, are not allowed in a melt-spinning process [4,5]. This is the reason why no caxis preferred orientation has been found in ribbons fabricated by melt-spinning [6,7].…”