Tunable Second-Order Structural Transition in As-Deficient MnAs

TL;DR

This study investigates how arsenic deficiency in MnAs affects its magnetic and structural phase transitions, revealing a tunable second-order structural transition and altered thermal expansion behavior compared to stoichiometric MnAs.

Contribution

It demonstrates that As-deficiency in MnAs shifts the structural transition temperature and modifies thermal expansion, highlighting the role of structural anisotropy in phase transition tuning.

Findings

Structural transition temperature is lowered by ~50 K in As-deficient MnAs.

Thermal expansion shows negative values perpendicular to the basal plane between $T_C$ and 315 K.

Correlation between lattice parameter ratio $c/a$ and $T_S$ suggests structural anisotropy influences transition temperature.

Abstract

We report measurements of magnetization, specific heat, and thermal expansion performed on As-deficient MnAs single crystals (MnAs$_{0.968}$). Ferromagnetic order is observed near $T_C \simeq$ 306 K on warming and $T_C \simeq$ 302 K on cooling, which is consistent with previously-reported values for stoichiometric MnAs samples. In contrast, the second-order structural phase transition is observed at $T_S \simeq$ 353 K, which is nearly 50 K lower than in the stoichiometric compound. We observe differences in the thermal expansion of our samples when compared to reports of stoichiometric MnAs including: (1) the $\sim$1.5% volume decrease at $T_C$ is smaller than the expected value of 1.9%, (2) the lattice parameters perpendicular to the basal plane exhibit a discontinuous jump of $\sim$1.1% at $T_C$ instead of being continuous across $T_C$, and (3) thermal expansion perpendicular to the basal plane for $T_C \le T \le$ 315 K is negative rather than positive. We also observe a correlation between the ratio of hexagonal lattice parameters, $c/a$, and $T_S$, strongly suggesting that the degree of structural anisotropy in MnAs could play an important role in tuning $T_S$.