A core-shell magnet Mn70Bi30grown 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-3d⁵spins (a 4.23μ_Bmoment) align parallel via an orbital moment 0.27μ_Bof Bi-5d¹⁰6s²p³in a crystal lattice. Thus, using a surplus Mn (over Bi) in a Mn₇₀Bi₃₀type 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 Mn₇₀Bi₃₀can grow into small crystals of hexagonal (h) plates at seeds as annealed in magnetic fields (in H₂gas). 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,M_s→ 70.8 emu g^-1, with duly promoted coercivityH_c→ 10.810 kOe (15.910 kOe at 350 K), energy-product 14.8 MGOe, and the crystal-field-anisotropy,K₁→ 7.6 × 10⁷erg cm^-3, reported at room temperature. Otherwise,M_sshould decline at any surplus 3d⁵-Mn spins order antiparallel at the antisites. Enhanced Curie point 658.1 K (628 K at Mn₅₀Bi₅₀alloy) 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.

Keywords: composites; core–shells; exchange coupled magnets; high-energy magnets; magnetic properties; magnetics; rare-earth free alloys.