Abstract
The paper reports evidence that the multi-spin-axis magnetic structure proposed in 1964 by van Laar is realized in antiferromagnetic CoO. This tetragonal spin arrangement produces both the strong tetragonal and the weaker monoclinic distortion experimentally observed in this material. The monoclinic distortion is proposed to be a “monoclinic-like” distortion of the array of the oxygen atoms, comparable with the rhombohedral-like distortion of the oxygen atoms recently proposed to be present in NiO and MnO. The monoclinic-like distortion has no influence on the tetragonal magnetic structure, which is generated by a special nonadiabatic atomic-like motion of the electrons near the Fermi level. It is argued that it is this atomic-like motion that qualifies CoO to be a Mott insulator.
Highlights
Cobalt monoxide is antiferromagnetic with the Néel temperature TN = 289 K
In the present paper we show that, just as in MnO and in NiO, the non-adiabatic atomic-like motion defined within the non-adiabatic Heisenberg model (NHM) is responsible for both the magnetic structure and the Mott insulation in CoO
Under the assumption that the non-collinear multi-spin-axis magnetic structure as proposed by van Laar is realized in antiferromagnetic CoO, essential features of CoO can be understood: 6.1
Summary
Just as the other isomorphic transition-metal monoxides MnO, FeO, and NiO, it is a Mott insulator in both the paramagnetic and the antiferromagnetic phase. While above TN, all the transition-metal monoxides possess the fcc structure Fm3m = Γcf Oh5 (225) (in parentheses we always give the number in the International Tables for Crystallography), CoO occupies a special position in the magnetic phase: the magnetic structures of MnO, FeO and NiO are known to be monoclinic base-centered [1,2,3,4], but the magnetic structure of CoO is not fully understood. The models can hardly be distinguished by neutron diffraction data or by reverse Monte Carlo refinements of these data [6]. They are considered as alternative structures or as structures coexisting in antiferromagnetic CoO [7]. The noncollinear structure is suggested by the marked tetragonal distortion of CoO accompanying the antiferromagnetic state, and a collinear structure could be associated with the additional small monoclinic deformation unambiguously detected in the antiferromagnetic phase of CoO [8]
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