Small core–shell Mn_0.5Bi_0.5−Bi (⩽3 at%) magnets, the anisotropic growth of crystallite nanoplates, interface-bridging, and tailored magnetic properties

Abstract

The binary alloy Mn0.5+xBi0.5−x, x ⩽ 0.05, is a promising rare-earth-free magnetic material, with high-energy-density (a critical characteristic for electric motors and power electronics), low cost, and significant magnetic properties for multiple uses at room temperature. In this article, we report how a free Bi, when precipitated over Mn0.5+xBi0.5−x (x ⩽ 0.05) of small crystallites, diffuses back into a stable Mn0.5+xBi0.5−x, x → 0, via a peritectic reaction, which facilitates preferential growth of small core–shell crystallites with multiple facets, having the potential for tailored magnetic properties. This growth travels slowly in the anisotropic channels of vacancies on annealing the reactive nanopowder at a critical 573 K temperature in Ar gas. Thus, an initial crystallite size of D ∼ 27 nm grows to only 38 nm in a reaction extended over a period of 96 h. A transient phase, x > 0, which has Bi vacancies, primarily grows in the (101) and (110) facets, filling the vacancies over a 6.41% larger crystal density. If any excess Mn is present, it segregates over a saturated phase, combines with free Bi, and ultimately forms a stable alloy phase. The small crystallites contain an inbuilt surface Bi-layer (shell), with a 1–2 nm thickness, in a core–shell of nanoplates (20–60 nm width), as shown in the high resolution transmission electron microscope images. In the proposed microscopic model, with hybridized Mn-d5 and Bi-p3 electrons (also spins), the magnetic properties are readily controlled. Thus, at 300 K, a maximum coercivity Hc = 9.850 kOe (14.435 kOe at 350 K) develops (Hc = 5.010 kOe in the initial) in critical single domains (D ∼ 33 nm). A net 72.5 emu g−1 magnetization occurs, with an enhanced TC = 641.5 K (600.5 K at x ∼ 0.05) on an order of enhanced anisotropy constant K1, demonstrating the significant effects of this core–shell structure of small crystallites.

Spin-up conversion, exchange-interactions, and tailored magnetic properties in core-shell La2NiMnO6 of small crystallites

“…A similar μ ℓ =(−) 0.27 μ B/ Bi favors K 1 in a uniaxial Mn 0.5 Bi 0.5 magnet [32][33][34]. A micromagnetic model in figure 9(b) describes how a La 3+ −5d°shell keeps Ni 3+ and Mn 3+ spins aligned via O 2in a stable FM phase over its μ ℓ value in a tripod.…”

“…A micromagnetic model in figure 9(b) describes how a La 3+ −5d°shell keeps Ni 3+ and Mn 3+ spins aligned via O 2in a stable FM phase over its μ ℓ value in a tripod. Adversely, an antisite Mn 4+ (or Ni 2+ ) spin setups oppositely to its adjacent sites in a lattice, so as the net μ sℓ drops in an FM−AFM composite [33,34], but it favours H c [4,7] and ferroics [12] in thin films. In figure 9(c), a model describes the spins setup in support of a DM field of La 3+ and O 2− species.…”

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

“…In the present work, we study the effects of a rather larger Mn, x → 20 at%, in which the M s is shown to decline merely up to 5.5%, and that is it is in a refined nano-powder. Small NAs have an adverse effect in the net M s declines on the spin canting (as a magnetic dead layer) at the surfaces and superparamagnetic (SPM) behaviors [9,19]. So, it stays well beyond a value M s expected at an AFM order of the 3d 5 -Mn spins at x → 20.…”

“…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].…”

“…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].…”