Structural and ferromagnetic properties of an orthorhombic phase of MnBi stabilized with Rh additions

TL;DR

This study explores how small Rh additions stabilize an orthorhombic phase of MnBi, affecting its thermal stability and magnetic properties, and reveals a new ferromagnetic phase with significant anisotropy.

Contribution

It demonstrates that Rh alloying stabilizes a new orthorhombic MnBi phase with distinct magnetic and structural properties, expanding understanding of rare-earth-free magnetic materials.

Findings

Rh stabilizes an orthorhombic MnBi phase with Curie temperature 416 K.

The magnetic transition exhibits 3D Ising-like critical behavior.

The magnetic anisotropy remains large and temperature-independent.

Abstract

The article addresses the possibility of alloy elements in MnBi which may modify the thermodynamic stability of the NiAs-type structure without significantly degrading the magnetic properties. The addition of small amounts of Rh and Mn provides an improvement in the thermal stability with some degradation of the magnetic properties. The small amounts of Rh and Mn additions in MnBi stabilize an orthorhombic phase whose structural and magnetic properties are closely related to the ones of the previously reported high-temperature phase of MnBi (HT~MnBi). To date, the properties of the HT~MnBi, which is stable between $613$ and $719$~K, have not been studied in detail because of its transformation to the stable low-temperature MnBi (LT~MnBi), making measurements near and below its Curie temperature difficult. The Rh-stabilized MnBi with chemical formula Mn$_{1.0625-x}$Rh$_{x}$Bi [$x=0.02(1)$] adopts a new superstructure of the NiAs/Ni$_2$In structure family. It is ferromagnetic below a Curie temperature of $416$~K. The critical exponents of the ferromagnetic transition are not of the mean-field type but are closer to those associated with the Ising model in three dimensions. The magnetic anisotropy is uniaxial; the anisotropy energy is rather large, and it does not increase when raising the temperature, contrary to what happens in LT~MnBi. The saturation magnetization is approximately $3$~$\mu_B$/f.u. at low temperatures. While this exact composition may not be application ready, it does show that alloying is a viable route to modifying the stability of this class of rare-earth-free magnet alloys.