Structure and magnetic properties of the MnBi low temperature phase

Abstract

High purity MnBi low temperature phase has been prepared and analyzed using magnetic measurements and neutron diffraction. The low-temperature phase of the MnBi alloy has a coercivity μ0iHc of 2.0 T at 400 K, and exhibits a positive temperature coefficient from 0 to at least 400 K. The neutron data refinement indicated that the Mn atom changes its spin direction from c axis above room temperature to nearly perpendicular to the c axis at 50 K. A canted magnetic structure has been observed below 200 K. The anisotropy field increases with increasing temperature which gives rise to a high coercivity at the higher temperatures. The anisotropic bonded magnets have maximum energy products (BH)max of 7.7 and 4.6 MGOe at room temperature and 400 K, respectively.

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

“…They are in reasonable agreement with powder diffraction reports as well [7]. Note that assignment of nominal oxidation states would give Mn 3+ Bi 3− , and trivalent Mn (3d 4 ) would be expected to have a moment of gS = 4 µ B .…”

“…3. Previously reported neutron diffraction results are limited to powder diffraction [7,9,24]. The single crystal data allow us to not only determine with greater reliability the intrinsic nature of the spin reorientation transition, but also to extract information about the lattice vibrations through analysis of the temperature dependence of the atomic displacement parameters.…”

“…The NMR results from powders [25] and magnetization measurements on single crystals [12] show the rotation away from the c-axis upon cooling begins near 140 K, increases gradually upon further cooing to T SR = 90 K, and then abruptly completes, with the moment flopping discontinuously into the ab-plane. However, neutron diffraction from MnBi powder suggests the moments are not fully in-plane below 90 K [7,9].…”

“…In the 1950's, this material was considered as a candidate to replace permanent magnets containing cobalt and nickel, and an energy product of 4.3 MGOe was achieved in the resulting "Bismanol" magnets [5,6]. Subsequently, energy products as high as 7.7 MGOe have been reported [7]. MnBi has several promising aspects as a candidate replacement for rare earth magnets.…”

“…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].…”