Influence of classical anisotropy fields on the properties of Heisenberg antiferromagnets within unified molecular field theory

A comprehensive study of the influence of classical anisotropy fields on the magnetic properties of Heisenberg antiferromagnets within unified molecular field theory versus temperature T , magnetic field H , and anisotropy field parameter h A 1 is presented for systems comprised of identical crystallographicallyequivalent local moments. The anisotropy field for collinear z -axis antiferromagnetic (AFM) ordering is constructed so that it is aligned in the direction of each ordered and/or field-induced thermal-average moment with a magnitude proportional to the moment, whereas that for XY anisotropy is defined to be in the direction of the projection of the moment onto the x y plane, again with a magnitude proportional to the moment. Properties studied include the zero-field Néel temperature T N , ordered moment, heat capacity, and anisotropic magnetic susceptibility of the AFM phase versus T with moments aligned either along the z axis or in the x y plane. Also determined are the high-field magnetization perpendicular to the axis or plane of collinear or planar noncollinear AFM ordering, the high-field magnetization along the z axis of a collinear z axis AFM, spin-flop (SF), and paramagnetic (PM) phases, and the free energies of these phases versus T , H , and h A 1 . Phase diagrams at T = 0 in the H z − h A 1 plane and at T > 0 in the H z − T plane are constructed for spins S = 1 / 2 . For h A 1 = 0 , the SF phase is stable at low field and the PM phase at high field with no AFM phase present. As h A 1 increases, the phase diagram contains the AFM, SF, and PM phases. Further increases in h A 1 lead to the disappearance of the SF phase and the appearance of a tricritical point on the AFM-PM transition curve. Applications of the theory to extract h A 1 from experimental low-field magnetic susceptibility data and high-field magnetization versus field isotherms for single crystals of AFMs are discussed. Disciplines Condensed Matter Physics This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/ameslab_manuscripts/89 PHYSICAL REVIEW B 96, 224428 (2017) Influence of classical anisotropy fields on the properties of Heisenberg antiferromagnets within unified molecular field theory David C. Johnston Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA (Received 9 October 2017; revised manuscript received 25 November 2017; published 26 December 2017) A comprehensive study of the influence of classical anisotropy fields on the magnetic properties of Heisenberg antiferromagnets within unified molecular field theory versus temperature T , magnetic field H , and anisotropy field parameterhA1 is presented for systems comprised of identical crystallographically-equivalent local moments. The anisotropy field for collinear z-axis antiferromagnetic (AFM) ordering is constructed so that it is aligned in the direction of each ordered and/or field-induced thermal-average moment with a magnitude proportional to the moment, whereas that for XY anisotropy is defined to be in the direction of the projection of the moment onto the xy plane, again with a magnitude proportional to the moment. Properties studied include the zero-field Néel temperature TN, ordered moment, heat capacity, and anisotropic magnetic susceptibility of the AFM phase versus T with moments aligned either along the z axis or in the xy plane. Also determined are the high-field magnetization perpendicular to the axis or plane of collinear or planar noncollinear AFM ordering, the high-field magnetization along the z axis of a collinear z-axis AFM, spin-flop (SF), and paramagnetic (PM) phases, and the free energies of these phases versus T ,H , and hA1. Phase diagrams at T = 0 in the Hz-hA1 plane and at T > 0 in the Hz-T plane are constructed for spins S = 1/2. For hA1 = 0, the SF phase is stable at low field and the PM phase at high field with no AFM phase present. As hA1 increases, the phase diagram contains the AFM, SF, and PM phases. Further increases in hA1 lead to the disappearance of the SF phase and the appearance of a tricritical point on the AFM-PM transition curve. Applications of the theory to extract hA1 from experimental low-field magnetic susceptibility data and high-field magnetization versus field isotherms for single crystals of AFMs are discussed. DOI: 10.1103/PhysRevB.96.224428