Spin semimetallic behavior and sublattice spin crossover in the fully compensated ferrimagnetic half-Heusler compound (Co0.5Mn0.5)MnAl

We experimentally demonstrate rarely observed fully compensated ferrimagnetic behavior in the half-Heusler compound $({\mathrm{Co}}_0.5{\mathrm{Mn}}_0.5)\mathrm{MnAl}$. A comprehensive structural investigation at room temperature was carried out using x-ray and neutron diffraction techniques, which indicated that the sample exhibited a $C{1}_b$ structure with B2-type disorder. The magnetic measurement on this compound reveals that the sample has a completely compensated ferrimagnetic behavior with a very low saturation magnetization of $0.09 {\ensuremath{\mu}}_{\mathrm{B}}/\text{f.u.}$ and a Curie temperature of 634 K. This finding was further verified by neutron diffraction techniques. Around 200 K, a change in the sign of the magnetic moment and anomalous Hall resistivity was observed, resulting from sublattice spin crossing. The resistivity measurement indicates an unusual temperature dependence with a negative temperature coefficient, revealing the possibility of alternative mechanisms for electronic transport observed in materials such as semimetals or gapless/spin-gapless semiconductors. The compensated ferrimagnetic behavior is analyzed and substantiated by carrying out fixed spin moment calculations within the framework of density functional theory. The electronic structure analysis predicts a spin semimetallic quantum phase for this half-Heusler compound.