Exploring Disorder in the Spin Gapless Semiconductor Mn$_2$CoAl

TL;DR

This study investigates how atomic disorder affects the electronic and magnetic properties of Mn$_2$CoAl thin films, revealing that disorder turns the material into a disordered metal rather than a spin-gapless semiconductor.

Contribution

It provides experimental evidence that disorder in Mn$_2$CoAl thin films destroys the spin-gapless state, reclassifying the material as a disordered metal based on transport and optical measurements.

Findings

Short mean free path indicating high disorder

Resistivity and temperature dependence consistent with disordered metals

Optical and transport data support a disordered metallic state

Abstract

Since the prediction of spin-gapless semiconducting behaviour in the Heusler compound Mn$_2$CoAl, evidence of spin-gapless behaviour in thin films has typically been inferred from magnetotransport measurements. The spin gapless state is however fragile, and further, band structure calculations indicate that even a small amount of atomic disorder may destroy it. To explore the impact of disorder on the properties of Mn$_2$CoAl, we have undertaken an experimental study of the structural, magnetotransport and optical properties from the far infrared to the UV, on DC magnetron sputtered Mn$_2$CoAl thin films. A very short mean free path, of the order of a lattice spacing, is extracted from the DC transport data. A room temperature resistivity of 200 $\mu$$\Omega$cm along with a small and negative temperature coefficient of resistance between 4 and 400 K was measured. We note that parameters of this magnitude are often observed in disordered metals. We find this behaviour is well described by a weak localisation model, a result that is supported by a large Drude contribution to the optical response, where a high scattering rate is derived, which is equal to the value derived from the DC conductivity and Hall effect data. We also note the strong similarities between the magnetotransport behaviour reported for Mn$_2$CoAl films in the literature, including ours. We conclude that, based on comparisons between the experimental data, and recent band structure calculations that explicitly include disorder, as-prepared Mn$_2$CoAl films are best described as a disordered metal, rather than a spin gapless semiconductor.