Ab initio study on the possible magnetic topological semimetallic state in MnMg2O4.

Abstract

The search for magnetic topological semimetals has been one of the most intriguing issues in the field of spintronics because they exhibit anomalous transport properties that can be utilized for novel spintronic devices. Quite recently, it was theoretically predicted that an inverse spinel compound VMg2O4 is a good magnetic Weyl semimetal [1,2]. The Weyl points are found just above the Fermi energy in the eg-band formed by the 3d-orbitals of the tetravalent V on the diamond lattice.Stimulated by these previous works, here we study the electronic states and transport properties of another spinel compound MnMg2O4, in which V is totally substituted for Mn with a smaller ionic radius. The significant difference between V and Mn is that, if they maintain the nominal tetravalent configuration, V has one electron in the eg-orbital while Mn also has one hole in the eg-orbital in the low-spin state or one electron in the t2g orbital in the high-spin state. Here we focus on how the topological properties found in VMg2O4 can be affected by the orbital symmetry by first-principles calculations using WIEN2k code.The band structure and the density of states of MnMg2O4 are shown in Figure 1 (a) and (b), respectively. We find that the Fermi energy lies at the t2g band and high-spin state is realized in the 3d electrons of Mn. The resulting band structure is half-metallic with the minority band gap is found to be 3eV. We note that, unlike the eg-band lies around 1eV below the Fermi level, there is no obvious linear crossing in the t2g band.We also compute the intrinsic anomalous Hall conductivity (AHC) within the linear-response theory in the clean limit, and the energy dependence of the AHC is shown in Fig. 2. As expected from the band structure in Fig. 1(a), the AHC is very small at the Fermi energy, shown with black line. We also examine the effects of the compressive strain on the electronic structures and AHC of the systems. Lifting the degeneracy of the t2g band, the Weyl points appears slightly above the Fermi energy, resulting in a large AHC ( -144 S/cm ).  Fig.1 (a) The band structure and (b) the density of states of MnMg2O4.  Fig.2 The energy dependences of the anomalous Hall conductivity with several compressive strains.