Powder neutron diffraction and magnetic susceptibility measurements of the antiferromagnetic phases of Mn<sub>x</sub>Ni<sub>1-x</sub>O,Mn<sub>x</sub>Co<sub>1-x</sub>O, (Mn<sub>x</sub>Fe<sub>1-x</sub>)<sub>z</sub>O and (Co<sub>x</sub>Fe<sub>1-x</sub>)<sub>z</sub>O reveal that the magnetic moments of unlike ions are always effectively collinear, despite the presence of competing anisotropies. The magnetic moments of Mn<sub>x</sub>Ni<sub>1-x</sub>O (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 Mn<sup>2+</sup>-Mn<sup>2+</sup> and ferromagnetic Ni<sup>2+</sup>-Mn<sup>2+</sup> nearest neighbour interactions. In Mn<sub>x</sub>Co<sub>1-x</sub>O (x = 0.05, 0.10, 0.25,0.36) at 5K, the orbital degener- acy of Co<sup>2+</sup> is removed by both Jahn-Teller (J.T) and spin-orbit coupling (S.O) mechanisms, leading to orthorhombic or triclinic symmetries and partially quenched Co<sup>2+</sup> moments. Monoclinic symmetry is observed for 0.51<x<0.67, and Jahn-Teller stabilisations and spin-only Co<sup>2+</sup> moments are evident. The cobalt concentration is too small to support a cooperative J.T stabilisation in Mn<sub>0.83</sub>Co<sub>0.17</sub>O, where a magnetostriction (c/a < 1) is observed (in agreement with previous AFMR results). Previous results for Co<sub>x</sub>Ni<sub>1-x</sub>O, and those for Mn<sub>x</sub>Co<sub>1-x</sub>O, (Mn<sub>x</sub>Fe<sub>1-x</sub>)<sub>z</sub>O (x = 0.05, 0.1, 0.12, 0.23, 0.36,0.56,0.66,0.89) and (Co<sub>x</sub>Fe<sub>1-x</sub>)<sub>z</sub>O (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»Fe<sub>z</sub>O≈NiO. The weak trigonal anisotropy of Fe<sup>2+</sup> correlates with the near-cubic symmetries of (Co<sub>x</sub>Fe<sub>1-x</sub>)<sub>z</sub>( (x = 0.04,0.12) and (Mn<sub>x</sub>Fe<sub>1-x</sub>)<sub>z</sub>O (0.1<x<0.66), and a tetragonal (c/a>1) magnetostriction of Fe<sup>2+</sup> is observed in (Co<sub>x</sub>Fe<sub>1-x</sub>)<sub>z</sub>O with x > 0.5. Measurements of vacancy-ferric interstitial ratios for (Mn<sub>x</sub>Fe<sub>1-x</sub>)<sub>z</sub>O and (Co<sub>x</sub>Fe<sub>1-x</sub>)<sub>z</sub>O 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 Fe<sub>z</sub>O, and require the postulation of partial antiferromagnetic order around the clusters. The room temperature Mossbauer effect parameters of (Mn<sub>x</sub>Fe<sub>1-x</sub>)<sub>z</sub>O (0<x<0.975,0.910<z<1.0) indicate that while Fe<sup>2+</sup> and Mn<sup>2+</sup> ions are randomly distributed over octahedral sites, ferric ions are localised around defect clusters.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:331110 |
| Date | January 1981 |
| Creators | Hope, D. A. O. |
| Contributors | Cheetham, A. K. : Gehring, Gillian : Cheetham, A. K. : Gehring, G. A. |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://ora.ox.ac.uk/objects/uuid:7c1899c4-c495-49e9-9ee5-185b1f8d9e3e |
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