Structural ordering at magnetic seeds with twins at boundaries of a core–shell alloy Mn60Bi40 and tailored magnetic properties

Abstract

A spin Mn3d5-rich Mn60Bi40 alloy reveals a model system in order to tailor profound magnetic properties at unpaired 3d5 spins in such alloys of a core–shell structure. As annealed (at a critical temperature 573 K in H2 gas), a refined powder (in glycine) grows on α-MnBi seeds (crystallites) present in it at Mn/Bi atoms order over topological layers, preferentially along (110) planes, at a self-confined structure at seeds of an anisotropic shape of hexagonal (h) plates (25–85 nm widths). In terms of the HRTEM images, the atoms turn down at edges (at the plates grow up) in a spiral layer, ≤ 2.1 nm thickness, of small core–shells. A spin model is proposed to delineate a way at the spins can pin down at the edges, form single magnetic domains, and raise coercivity (H c), with no much loss of net magnetic moment. The X-ray diffraction and HRTEM images corroborate the results of topological pacing of atoms at the h-plates at anneals. A novelty is that a core–shell leads to tailor a superb H c, as much as 11.110 kOe (16.370 kOe at 350 K), with a fairly large magnetization, 76.5 emu g−1, at near 300 K. An enhanced Curie point 650.1 K (628 K at Mn50Bi50 alloy) confers a surplus 3d5-Mn spin sensitively tunes α-MnBi stoichiometry and so its final magnetic structure. A refined alloy powder so made is useful to make powerful magnets and devices in the forms of films and bonded magnets in different shapes for uses as small tools, tweezers, and other devices.