Electronic structure and exchange interactions in MnGeP₂ in the quasiparticle-self-consistent GW approach

Controlling the magnetic moment and carrier concentration of room-temperature magnetic semiconductors offers potentially great enhancements to the field of spintronics but so far has remained elusive. Possible routes proposed in the past were to dope ZnGeP₂ or CdGeP₂ with a magnetic transition metal like Mn or to synthesize pure MnGeP₂ by epitaxial growth or gradient freeze method. While these materials were recently found to exhibit ferromagnetic order, previous computational studies within density functional theory found them to be antiferromagnetic. Thus, the origin of ferromagnetism remains unclear.

Here we use the quasiparticle-self-consistent GW method to calculate the electronic band structure, optical dielectric function, and exchange interactions in chalcopyrite, I-42d, structure MnGeP₂. The material is found to be an antiferromagnetic semiconductor with a direct band gap of 2.25 eV and an indirect gap of 1.71 eV. The exchange interactions are calculated using a linear response approach. The antiferromagnetic exchange-interaction between nearest neighbors in the primitive unit cell is dominating (-1.02 mRy) and found to be slightly decreasing upon carrier doping (studied by adding carriers in a rigid band model with a homogeneous compensating charge density) but not sufficiently to change the interaction to become ferromagnetic. Supercells are studied at the GW level to explore the dominant exchange interactions in the presence of Mn_Ge antisites. We find that the exchange interaction of the antisite Mn with nearby Mn on regular lattice sites is ferromagnetic (1.31 mRy) and could be responsible for the observations of ferromagnetism. In one model with a 16 atom cell, this results in a layer of MnP while in a larger supercell of 64 atoms, the antisites are more isolated. We conclude that both a sufficient density of such antisites and MnP clusters could explain the ferromagnetism.

Further, the bare (non-interacting) and enhanced magnetic susceptibilities are calculated using the GW infrastructure, and the spin-wave spectra are found along the reciprocal space. The Néel temperature is calculated using the mean-field and Tyablikov (RPA) estimations. The dielectric function is calculated using the Bethe Salpeter equation and shows significant excitonic effects.