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 to 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 (Hc), 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 Hc, as much as 11.110 kOe (16.370 kOe at 350 K), with a fairly large magnetization, 76.5 emu/g, 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.

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

“…Traditionally, it is believed to form and exist of a line compound Mn 50 Bi 50 of crystallites at equal Mn/Bi atoms [9][10][11][12][13][14][15][16][17]. Then, a question arises how a Mn/Bi ratio keeps as the atoms converge at surfaces at NAs in small clusters [19,22]. The clusters control the average magnetic viscosity and in turn the emerging properties [22,29].…”

“…Then, a question arises how a Mn/Bi ratio keeps as the atoms converge at surfaces at NAs in small clusters [19,22]. The clusters control the average magnetic viscosity and in turn the emerging properties [22,29]. In this case, a part of Mn can move to the Bi sites (vice versa) to adjust the surface charge (spin) density in surface stabilized NAs of small 'core-shells' [22,29].…”

“…The clusters control the average magnetic viscosity and in turn the emerging properties [22,29]. In this case, a part of Mn can move to the Bi sites (vice versa) to adjust the surface charge (spin) density in surface stabilized NAs of small 'core-shells' [22,29]. Collective spins at small core-shells can interplay a vital role in determining the crystal field anisotropy K 1 , phase stability, united spin dynamics, and other functional properties.…”

“…As a result, a renewed interest to examine how surplus Mn-3d 5 spins can tailor the crystallite structure, surface topology and in turn the engineered properties in the form of a hybrid FM-AFM nanocomposite of small clusters. Recently, our group developed a strategy in α-MnBi can grow in small anisotropic h-plates (SMDs) via local Mn/Bi reactions at small α-MnBi seeds [19,22]. Ultimately, an inbuilt shell, thickness t 2.0 nm, turns up in a core-shell structure that promotes H c and energy-product (BH) max at a steady PTC well up to close to the T C point [22].…”

“…Recently, our group developed a strategy in α-MnBi can grow in small anisotropic h-plates (SMDs) via local Mn/Bi reactions at small α-MnBi seeds [19,22]. Ultimately, an inbuilt shell, thickness t 2.0 nm, turns up in a core-shell structure that promotes H c and energy-product (BH) max at a steady PTC well up to close to the T C point [22]. An inbuilt shell interlocks surface spins at the FM surfaces a way to facilitating H c at integrated core-shell spins [37,38].…”