Exploring disorder in the spin gapless semiconductor Mn₂CoAl

Abstract

Since the prediction of spin-gapless semiconducting behaviour in the Heusler compound Mn2CoAl, 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 Mn2CoAl, we have undertaken an experimental study of the structural, magnetotransport and optical properties from the far infrared to the UV, on DC magnetron sputtered Mn2CoAl 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 µΩ.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 Mn2CoAl 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 Mn2CoAl films are best described as a disordered metal, rather than a spin gapless semiconductor.

Two-channel anomalous Hall effect originating from the intermixing in Mn2CoAl/Pd thin films

“…From our previous work, the ferromagnetic Mn 2 CoAl layer shows a positive AHE from 300 K to 5 K [34]. Recently, Basha et al, directly observed that there is an intermixing CoPd alloy in Co 2 MnSi/Pd ultrathin films similar to our system [35].…”

“…With increasing the thickness of the Pd layer, there is more CoPd formed with a stronger negative AHE and μ 0 H CoPd increases so that the total AHE reverses to negative with a peak. Regarding the samples with a thick Pd layer, like t Pd 3.2 nm, most of the electric current will pass through the Pd layer because the bulk resistivity of Pd, ∼11 μ cm [37], is much lower than that of Mn 2 CoAl, 200 μ cm [34], which is the so-called shunting effect. This effect leads to a reduction of the net AHE of the Mn 2 CoAl/Pd bilayer with intermixed CoPd at the interface and a decline of the peak for the samples with a thick Pd layer as shown in Fig.…”

Effects of Disorder on the Electronic Structure and Thermoelectric Properties of an Inverse Full-Heusler Mn2CoAl Alloy

“…Some studies have reported that Mn 2 CoAl is a spin-gapless semiconductor, 20,23,24 while other studies reported that rather Mn 2 CoAl is a half-metal due to its disordered crystal structure. 25−28 It was suggested that antisite defects between Mn and Al exist in thin films, 26,28 and partial substitution of Co for Mn and substitution of Mn for Al are found in bulk samples. 27 It should be noted that in these studies, the measured samples have nonstoichiometric compositions such as Mn 1.7 Co 1.2 Al 1.1 26 and Mn 1.8 Co 1.4 Al 0.8 .…”

“…25−28 It was suggested that antisite defects between Mn and Al exist in thin films, 26,28 and partial substitution of Co for Mn and substitution of Mn for Al are found in bulk samples. 27 It should be noted that in these studies, the measured samples have nonstoichiometric compositions such as Mn 1.7 Co 1.2 Al 1.1 26 and Mn 1.8 Co 1.4 Al 0.8 . 27 Such a nonstoichiometry may be the origin of the abovementioned antisite defects.…”

Skyrmion generation and perpendicular magnetic anisotropy modification in Heusler alloy ultrathin films

“…The domain period of demagnetizing image is extracted using a two-dimensional fast Fourier transform by the Gwyddion software [110]. Here, the basic properties of thick Mn 2 CoAl (100 nm) films will be introduced, which have been fully studied by my group [68]. The Mn 2 CoAl thin films are deposited on single-crystalline MgO(001) substrates by the magnetron sputtering with a base pressure below 3×10 −8 Torr.…”