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Theory of the low-temperature phases in boracites: latent antiferromagnetism, weak ferromagnetism, and improper magnetostructural couplings

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Published in Physical Review. B, Condensed Matter. 1985, vol. 32, no. 9, p. 6006-6038
Abstract A phenomenological theory of the magnetic and magnetoelectric properties of boracites is developed, using the Landau-Dzialoshinskii approach. An interpretation is proposed for the main distinctive features characterizing 11 members of this family of compounds. Three subclasses of materials are distinguished corresponding to 2 essentially different phys. situations. In Ni-I boracite the simultaneous ferromagnetic-ferroelectric-ferroelastic transition results from a nonlinear (improper) coupling of the magnetization, polarization, and strain components, to the primary order parameter identified as a "latent" antiferromagnetic ordering. Monoclinic Ni-I can be viewed as the 1st exptl. example of a new class of magnetic materials, which should be recognized macroscopically by an M .apprx. (Tc - T)3/2 variation law, the existence of a spontaneous structural ordering occurring simultaneously with the magnetic ordering, and a weak value of the magnetization despite its exchange origin. In the 2 other classes of trigonal and orthorhombic boracites, the transitions are purely magnetic, the order parameter having a 1- or 2-dimensional antiferromagnetic order, bilinearly coupled to a weak magnetization of relativistic origin. The dielectric and elastic anomalies, and the magnetoelectric properties in these 2 groups of boracites are explained by secondary couplings of the order parameter with the relevant nonspontaneous polar tensors. The complementarity of the 2 additional Landau-type approaches of structural and magnetic transitions is illustrated.
Keywords Structure magnetic boracite phys propertyMagnetoelectric magnetism boracite theory
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Structures
Project FNS: 1984
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TOLEDANO, Pierre et al. Theory of the low-temperature phases in boracites: latent antiferromagnetism, weak ferromagnetism, and improper magnetostructural couplings. In: Physical Review. B, Condensed Matter, 1985, vol. 32, n° 9, p. 6006-6038. https://archive-ouverte.unige.ch/unige:33130

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