Investigation of ferrimagnetism in some compounds using Polarized neutrons

Abstract

Polarized neutrons offer a very useful method of studying ferrimagnetic materials such as ferrites, in which the multiplicity of cations and their valence states prevent a unique determination of the spin structure when only polycrystalline material is available. This paper presents the first extensive use of polarized neutron powder-diffraction patterns for determining the structure of a carbide of Mn and Co and of a number of mixed ferrites having the spinel structure. The ferrimagnetic carbide with composition (Mn,Co)₄C is found to have a cubic structure of the Mn₄N type, with one of the Mn atoms at the cube corner and the face centers being occupied by one Mn and two Co atoms in a statistical distribution. The two Mn atoms are antiferromagnetically aligned with respect to each other, the corner atom having a moment of 4.0μ_B while the face center moment is 3.0μ_B. The two Co atoms have a moment of 0.8μ_B each and are aligned parallel to the corner Mn moment along the cube edges. The material has a Néel temperature of 810°K and saturates at a field of 4.5 kOe. Polarized neutrons were used to choose a structure of cubic symmetry in preference to one of tetragonal symmetry. Two mixed ferrite systems of the general formula M_xM'_1-xFe₂O₄ where M = Mg, Zn and M' = Mn, Ni have been studied for various values of x. Their Néel temperatures and cation distributions have been determined.