Abstract
The influence of uniaxial single-ion anisotropy 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 (2015)], where thermodynamic variables are expressed in terms of directly measurable parameters. The properties of collinear AFMs with ordering along the axis () in applied field are calculated versus and temperature , including the ordered moment , the Néel temperature , the magnetic entropy, internal energy, heat capacity, and the anisotropic magnetic susceptibilities and in the paramagnetic (PM) and AFM states. The high-field average magnetization per spin is found, and the critical field 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 and . The high-field is determined, together with the associated spin-flop field 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 phase diagrams are constructed. For and , where and and are the Weiss temperature in the Curie-Weiss law and the Néel temperature due to exchange interactions alone, respectively, phase diagrams in the plane similar to previous results are obtained. However, for we find a topologically different phase diagram where a spin-flop bubble with PM and AFM boundaries occurs at finite and . Also calculated are properties arising from a perpendicular magnetic field, including the perpendicular susceptibility , the associated effective torque at low fields arising from the term in the Hamiltonian, the high-field perpendicular magnetization , and the perpendicular critical field at which the second-order AFM to PM phase transition occurs. In addition to the above results for , the and ordered moment for collinear AFM ordering along the axis with are determined. In order to compare the properties of the above spin systems with those of noninteracting systems with uniaxial anisotropy with either sign of , Supplemental Material is provided in which results for the thermal and magnetic properties of such noninteracting spin systems are given.
24 More- Received 19 January 2017
DOI:https://doi.org/10.1103/PhysRevB.95.094421
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