Magnetic and structural studies of some mixed metal oxides
Powder neutron diffraction and magnetic susceptibility measurements of the antiferromagnetic phases of MnxNi1-xO,MnxCo1-xO, (MnxFe1-x)zO and (CoxFe1-x)zO reveal that the magnetic moments of unlike ions are always effectively collinear, despite the presence of competing anisotropies. The magnetic moments of MnxNi1-xO (x = 0.24,0.48 and 0.77) at 5K are confined to (111) planes by dipole-dipole forces, and the small trigonal exchangestrictions are the products of opposed antiferromagnetic Mn2+-Mn2+ and ferromagnetic Ni2+-Mn2+ nearest neighbour interactions. In MnxCo1-xO (x = 0.05, 0.10, 0.25,0.36) at 5K, the orbital degener- acy of Co2+ is removed by both Jahn-Teller (J.T) and spin-orbit coupling (S.O) mechanisms, leading to orthorhombic or triclinic symmetries and partially quenched Co2+ moments. Monoclinic symmetry is observed for 0.512+ moments are evident. The cobalt concentration is too small to support a cooperative J.T stabilisation in Mn0.83Co0.17O, where a magnetostriction (c/a < 1) is observed (in agreement with previous AFMR results). Previous results for CoxNi1-xO, and those for MnxCo1-xO, (MnxFe1-x)zO (x = 0.05, 0.1, 0.12, 0.23, 0.36,0.56,0.66,0.89) and (CoxFe1-x)zO (x = 0.04,0.12,0.50,0.63,0.81) at 5K indicate that the anisotropy order for the iron group monoxides is CoO>MnO»FezO≈NiO. The weak trigonal anisotropy of Fe2+ correlates with the near-cubic symmetries of (CoxFe1-x)z( (x = 0.04,0.12) and (MnxFe1-x)zO (0.11) magnetostriction of Fe2+ is observed in (CoxFe1-x)zO with x > 0.5. Measurements of vacancy-ferric interstitial ratios for (MnxFe1-x)zO and (CoxFe1-x)zO suggest that non-stoichiometry is accommodated by 6:2 or 8:3 defect clusters in the former, and by larger units in the latter. The observed magnetic moments of defective samples are normally larger than those calculated according to a previous model for FezO, and require the postulation of partial antiferromagnetic order around the clusters. The room temperature Mossbauer effect parameters of (MnxFe1-x)zO (02+ and Mn2+ ions are randomly distributed over octahedral sites, ferric ions are localised around defect clusters.