A core–shell magnet Mn₇₀Bi₃₀ grown at seeds in magnetic fields and its impacts on its spin-dynamics, Curie point and other tailored properties

Abstract

The MnBi alloys is a model series of rare-earth free magnets for surge of technologies of small parts of automobiles, power generators, medical tools, memory systems, and many others. The magnetics stem primarily at unpaired Mn-3d5 spins (a 4.23 B moment) align parallel via an orbital moment 0.27 B of Bi-5d106s2p3 in a crystal lattice. Thus, using a surplus Mn (over Bi) in a Mn70Bi30 type alloy designs a spin-rich system of duly tailored properties useful for magnetics and other devices. In this view, we report here a strategy of a refined alloy powder Mn70Bi30 can grow into small crystals of hexagonal (h) plates at seeds as annealed in magnetic fields (in H2 gas). So, small h-plates (30 to 50 nm widths) are grown up at (002) facets, wherein the edges are turned down in a spiral ( 2.1 nm thicknesses) in a core-shell structure. The results are described with X-ray diffraction, lattice images and magnetic properties of a powder Mn70Bi30 (milled in glycine) is annealed at 573 K for different time periods, so to the Mn/Bi order at the permeable facets (seeds). Duly annealed samples exhibit an enhanced magnetization, Ms  70.8 emu/g, with duly promoted coercivity Hc  10.810 kOe (15.910 kOe at 350 K), energy–product 14.8 MGOe, and the crystal-field-anisotropy, K1  7.6x107 erg/cm3, reported at room temperature. Otherwise, Ms should decline at any surplus 3d5-Mn spins order antiparallel at the antisites. Enhanced Curie point 658.1 K (628 K at Mn50Bi50 alloy) anticipates that a surplus Mn does favor the Mn-Bi exchange interactions. Proposed spin models well describe the spin-dynamics and lattice relaxations (on anneals) over the lattice volume (with twins) and spin clusters.