Realization of Spin Gapless Semiconductors: The Heusler Compound ${\mathrm{Mn}}_2\mathrm{CoAl}$

Recent studies have reported an interesting class of semiconductor materials that bridge the gap between semiconductors and half-metallic ferromagnets. These materials, called spin gapless semiconductors, exhibit a band gap in one of the spin channels and a zero band gap in the other and thus allow for tunable spin transport. Here, we report the first experimental verification of the spin gapless magnetic semiconductor ${\mathrm{Mn}}_2\mathrm{CoAl}$, an inverse Heusler compound with a Curie temperature of 720 K and a magnetic moment of $2{\mu }_B$. Below 300 K, the compound exhibits nearly temperature-independent conductivity, very low, temperature-independent carrier concentration, and a vanishing Seebeck coefficient. The anomalous Hall effect is comparatively low, which is explained by the symmetry properties of the Berry curvature. ${\mathrm{Mn}}_2\mathrm{CoAl}$ is not only suitable material for room temperature semiconductor spintronics, the robust spin polarization of the spin gapless semiconductors makes it very promising material for spintronics in general.

Figure 1

Density of states schemes. The schematic density of states $n(E)$ as a function of energy $E$ is shown for (a) a half-metallic ferromagnet and (b) a spin gapless semiconductor. The occupied states are indicated by filled areas. Arrows indicate the majority ($\uparrow$) and minority ($\downarrow$) states.

Figure 2

Band structure of ${\mathrm{Mn}}_2\mathrm{CoAl}$.

Figure 3

Temperature dependence of conductivity ${\sigma }_{xx}(T)$, Seebeck coefficient $S(T)$, and carrier concentration $n(T)$ of ${\mathrm{Mn}}_2\mathrm{CoAl}$.

Figure 4

Magnetoresistance of ${\mathrm{Mn}}_2\mathrm{CoAl}$ measured at different temperatures.

Figure 5

Magnetic properties and anomalous Hall effect (AHE) of ${\mathrm{Mn}}_2\mathrm{CoAl}$. The Hall conductivity ${\sigma }_{xy}$ is shown as a function of the applied magnetic field. The anomalous Hall effect is defined by the difference between ${\sigma }_{xy}$ values in zero and saturation fields ($\approx 0.3\mathrm{T}$). Inset (a) shows the hysteresis $m(H)$ measured at 2 K. Inset (b) shows the temperature dependence of the magnetization $m(T)$ measured in a field of 1 T.

Figure 6

Berry curvature in the $k_z=0.25$ plane (a) and $k_z=-0.25$ plane (b) for ${\mathrm{Mn}}_2\mathrm{CoAl}$.