| Summary: | <p>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.</p> <p>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.</p> <p>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="" co<sup="" jahn-teller="" spin-only="" stabilisations="">2+</x<0.67,></p> 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). <p>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), (c="" a="" and="" tetragonal="">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.</x<0.66),></p> <p>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.</p> <p>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) fe<sup="" indicate="" that="" while="">2+</x<0.975,0.910<z<1.0)></p> and Mn<sup>2+</sup> ions are randomly distributed over octahedral sites, ferric ions are localised around defect clusters.
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