Structure and magnetism in the polymorphous MnFeAs

Abstract

Two crystallographic phases of MnFeAs were stabilised: the tetragonal Fe 2As-type structure or the hexagonal Fe 2P-type one, if synthesis is performed under normal conditions, or high pressure and temperature. The structural, magnetic and electronic properties of both MnFeAs polytypes are investigated using magnetisation, neutron diffraction measurements as well as band structure calculations by the Korringa–Kohn–Rostoker method. The tetragonal phase is antiferromagnetic ( T N=470 K) with the magnetic cell doubled along c-axis, with respect to the chemical one. The local magnetic moment on Mn(2c) refined from neutron study is 3.36 μ B, while no significant magnetic moment is observed on the Fe(2a) site. The hexagonal phase is ferromagnetic with T C near 190 K. The magnetic moments determined by neutron diffraction are 3.14 and 1.54 μ B on Mn(3g) and Fe(3f), respectively. Upon applying a magnetic field as large as 10 T the saturation magnetisation of 4.5 μ B/f.u. is measured at 5 K. From non-polarised KKR calculations we deduce, that the Fermi level falls into a deep minimum of density of states (DOS) in the tetragonal as well as in the hexagonal variants of MnFeAs, which may tentatively explain the structural stability of both phases. The spin-polarised band structure study of the tetragonal MnFeAs (AFM) shows DOS splitting exclusively on the Mn(2c) sites, resulting in the local magnetic moment of 3.42 μ B, unlike the hexagonal MnFeAs (FM) where the magnetic moments of 3.13 and 1.10 μ B are found on Mn(3g) and Fe(3f), respectively. Moreover, the influence of lattice parameter variation on electronic and magnetic properties is discussed based on total energy KKR calculations.