Electronic structure and magnetism in some transition metal nitrides: Manganese-doped scandium nitride, dilute magnetic semiconductor and chromium nitride, Mott insulator

The thesis presented here deal with calculations of electronic and magnetic properties of transition metal based materials.; Electronic structure of Mn-doped ScN: a possible new magnetic semiconductor. We performed fully relaxed full-potential linear muffin-tin orbital method calculations of Mn-doped ScN using a supercell approach. We found that a t2g like defect level exists in the gap and gives rise to a magnetic moment between 2 an 3 muB. Calculations for 64 atom cells with two Mn in 1st-4th neighbor positions indicated a preference for ferromagnetic coupling. By mapping the energy differences on a Heisenberg Hamiltonian and assuming interactions with distant atoms except those in the adjacent unit cells are zero, we extracted the exchange interactions, which were found to be rather large and indicate a Curie temperature above room temperature even for only 3% Mn. Calculations of the miscibility indicated only 1% equilibrium miscibility at typical growth temperatures. However, non-equilibrium growth techniques have shown that in practice mixed alloys up to 26% Mn can be grown. We also studied the effect of n-type doping. Unexpectedly, Mn defects in the negative charge state still have an even larger magnetic moment with an increase in the eg state contribution. Subsequently, we carried out further calculations of the exchange interactions using non-collinear magnetic configurations in which the spin is slowly rotated. It was found that the previous calculations overestimate the J 0 = sumi J0i, i.e. the sum of all interactions connected to a given site, by about 30%. Further studies using the Liechtenstein linear response approach show that the latter is a sum over many long range interactions extending significantly beyond the range of the cells we had used. In this approach the long range interactions are obtained by Fourier transform of the Jij (k) for a mesh of k-points in the supercell. The nearest neighbor interactions are found to be an order of magnitude smaller than the first direct calculations indicated. Calculations for special quasirandom structures with different concentrations of Mn in large supercells of 256-432 atoms furthermore showed that randomness has a strong effect in suppressing the exchange interactions even further and in turning many longer-range interactions negative. This ultimately indicates that Mn-doped ScN is a spin-glass rather than a ferromagnet.; Electronic structure of CrN: a borderline Mott insulator. Our LSDA+U calculations following the rotationally invariant approach of Liechtenstein et al. provide support for the idea that CrN may be close to a Mott-insulator transition. With reasonable values of U in the range 3-5 eV, estimated from the excited atom model we find that the density of states near the Fermi level is strongly depleted by the spin separation of the Cr d states and an actual gap of order a few 0.1 eV opens in the known AFM-[110] 2 ground state. From a critical examination of the experimental data, it also appears that CrN is at best a nearly zero gap semiconductor with a gap Eg < 0.1 eV but may have optical threshold for direct transition of about 0.5 eV. Nitrogen vacancies play a significant role in doping the material and the transport data strongly suggest that disorder induced localization phenomena occur above the antiferromagnetic phase transition.