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

Journal Article · Fri Mar 17 00:00:00 EDT 2017 · Physical Review B

DOI:https://doi.org/10.1103/PhysRevB.95.094421· OSTI ID:1355898

Johnston, David C. ^[1]

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1. Ames Lab. and Iowa State Univ., Ames, IA (United States)

Here, the influence of uniaxial single-ion anisotropy –DS²_z 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), 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 field H_z = 0 are calculated versus D and temperature T, including the ordered moment μ, the Néel temperature T_N, the magnetic entropy, internal energy, heat capacity, and the anisotropic magnetic susceptibilities χ_∥ and χ_⊥ in the paramagnetic (PM) and AFM states. The high-field average magnetization per spin μ_z(H_z,D,T) is found, and the critical field H_c(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 T_N(D) and μ(D,T). The high-field μ_z(H_z,D,T) is determined, together with the associated spin-flop field H_SF(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 H_z–T phase diagrams are constructed. For f_J =–1 and –0.75, where f_J = θ_pJ/T_NJ and θ_pJ and T_NJ are the Weiss temperature in the Curie-Weiss law and the Néel temperature due to exchange interactions alone, respectively, phase diagrams in the H_z–T plane similar to previous results are obtained. However, for f_J = 0 we find a topologically different phase diagram where a spin-flop bubble with PM and AFM boundaries occurs at finite H_z and T. Also calculated are properties arising from a perpendicular magnetic field, including the perpendicular susceptibility χ_⊥(D,T), the associated effective torque at low fields arising from the –DS²_z term in the Hamiltonian, the high-field perpendicular magnetization μ_⊥, and the perpendicular critical field H_c⊥ at which the second-order AFM to PM phase transition occurs. In addition to the above results for D > 0, the T_N(D) and ordered moment μ(T,D) for collinear AFM ordering along the x axis with D < 0 are determined. In order to compare the properties of the above spin systems with those of noninteracting systems with –DS²_z uniaxial anisotropy with either sign of D, Supplemental Material is provided in which results for the thermal and magnetic properties of such noninteracting spin systems are given.

• Research Organization: Ames Lab., Ames, IA (United States)
• Sponsoring Organization: USDOE
• Grant/Contract Number: AC02-07CH11358
• OSTI ID: 1355898
• Alternate ID(s): OSTI ID: 1347263
• Report Number(s): IS-J-9319; PRBMDO; TRN: US1702607
• Journal Information: Physical Review B, Vol. 95, Issue 9; ISSN 2469-9950
• Publisher: American Physical Society (APS)
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS