Department of Physics and Astronomy

NovoMag

Research

  • In collaboration the experimental groups of with Prof. David Sellmyer and Prof. Xiaoshan Xu from University of Nebraska-Lincoln, we have performed theoretical calculations to determine the details of the atomic structures and the magnetic properties of Fe3Co3X2 (X=Ti, Nb) alloys. The structure prediction was performed using adaptive genetic algorithm method developed from our group, based on the X-Ray diffraction spectra from the experimental group at University of Nebraska-Lincoln. The magnetic properties (including the magnetic moment and magnetocrystalline anisotropy energy (MAE)) are calculated using the ab initio VASP code.  The effect of the Fe and Co occupation disorder on the structure and magnetic properties is also investigated.

  • Among the RE-free magnets, the metastable tetragonal α''-Fe16N2 phase of iron nitrides have attracted considerable experimental and theoretical attentions due to low cost of Fe and high magnetization in α''-Fe16N2 thin films. Using adaptive genetic algorithm and first principles calculations, we have explored the structures and magnetic properties of Fe16-xCoxN2 alloys.  We show that substituting Fe by Co in Fe16N2 with Co/Fe ratio ≤ 1 can greatly improve the magnetic anisotropy of the material. The magnetocrystalline anisotropy energy from first-principles calculations reaches 3.18 MJ/m3 (245.6 μeV per metal atom) for Fe12Co4N2, much larger than that of Fe16N2 and is one of the largest among the reported rare-earth free magnets.

  • Materials synthesis under far-from equilibrium conditions has become an important route for materials design and discovery which can drive systems toward metastable configurations which could possess otherwise unattainable functionality.  Through crystal structure searches using adaptive genetic algorithm, we have explored metastable structures of binary and Fe-substituted cobalt nitrides for possible candidates for rare-earth free permanent magnets. New structures of ConN (n = 3…8) are found to have lower energies than those previously discovered by experiments. Some structures exhibit large magnetic anisotropy energy, reaching as high as 200 μeV per Co atom (or 2.45 MJ/m3) based on first-principles density functional calculation as shown in the figure.

  • We performed adaptive genetic algorithm (AGA) search for low-energy atomic structures of the rare earth free permanent magnet material Fe3S. A number of structures with energy lower than that of the experimentally reported Pnma structure have been obtained from our AGA search as shown in the figure. These low-energy structures can be classified as layer motif structures and column motif structures.  In the column motif structures, Fe atoms are self-assemble into bcc-like rods separated by the holes formed by S atoms as shown in figure (b). In the layer motif structure, the bulk bcc Fe is braked into slabs of several layers of thickness by S atoms as shown in figure (c). Magnetic calculations showed that the column motif structures exhibit reasonably high uniaxial magnetic anisotropy.

  • PARSEC

    PARSEC is a computer code that solves the Kohn-Sham equations by expressing electron wave-functions directly in real space, without the use of explicit basis sets. It uses norm-conserving pseudopotentials (Troullier-Martins and other varieties). It is designed for ab initio quantum-mechanical calculations of the electronic structure of matter, within density-functional theory.

    PARSEC is optimized for massively parallel computing environment, but it is also compatible with serial machines. A finite-difference approach is used for the calculation of spatial derivatives. Owing to the sparsity of the Hamiltonian matrix, the Kohn-Sham equations are solved by direct diagonalization, with the use of extremely efficient sparse-matrix eigensolvers. Some of its features are:

  • Structures, magnetic moments and also magnetocrystalline anisotropy energies of binary and ternary M3C (M = Fe and/or Co) materials are systematically investigated using a combination of adaptive genetic algorithm crystal structure predictions and first-principles calculations.  Besides reproducing the known cementite (Pnma) structure of Fe3C, the AGA searches also capture several new meta-stable phases which can be stable within the room-temperature range. In particular, a bainite (P6322) structure exhibits largest magnetic moment among all the structures in the Fe3C pool, yet its energy is only 4 meV higher than the cementite (pnma) phase.