Influence of uniaxial single-ion anisotropy on the magnetic and thermal properties of Heisenberg antiferromagnets within unified molecular field theory

Abstract

The influence of uniaxial single-ion anisotropy -DSz^2 on the magnetic and thermal properties of Heisenberg antiferromagnets (AFMs) is investigated. The uniaxial anisotropy is treated exactly and the Heisenberg interactions are treated within unified molecular field theory (MFT) [Phys. Rev. B 91, 064427 (1915)], where thermodynamic variables are expressed in terms of directly measurable parameters. The properties of collinear AFMs with ordering along the z axis (D > 0) in applied fields Hz = 0 are calculated versus D and temperature T. The high-field average magnetization per spin muz(Hz,D,T) is found, and the critical field Hc(D,T) is derived at which the second-order AFM to PM phase transition occurs. The magnetic properties of the spin-flop (SF) phase are calculated, including the zero-field properties TN(D) and mu(D,T). The high-field muz(Hz,D,T) is determined, together with the associated spin-flop field HSF(D,T) at which a second-order SF to PM phase transition occurs. The free energies of the AFM, SF and PM phases are derived from which Hz-T phase diagrams are constructed. For a certain combination of parameters we find a topologically distinct phase diagram where a spin-flop bubble occurs at finite Hz and T. Also calculated are properties arising from a perpendicular magnetic field. In addition to the above results for D > 0, some properties with D < 0 are determined. In order to compare the properties of the above spin systems with those of noninteracting systems with DSz^2 uniaxial anisotropy with either sign of D, an Appendix is included in which results for the thermal and magnetic properties of such noninteracting spin systems are provided.

41 pages, 41 figures

References in corpus (1)

• Unified Molecular Field Theory for Collinear and Noncollinear Heisenberg Antiferromagnets

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