Unified molecular field theory for collinear and noncollinear Heisenberg antiferromagnets

Journal Article · Fri Feb 27 00:00:00 EST 2015 · Physical Review. B, Condensed Matter and Materials Physics

DOI:https://doi.org/10.1103/PhysRevB.91.064427· OSTI ID:1227216

Johnston, David C. ^[1]

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

In this study, a unified molecular field theory (MFT) is presented that applies to both collinear and planar noncollinear Heisenberg antiferromagnets (AFs) on the same footing. The spins in the system are assumed to be identical and crystallographically equivalent. This formulation allows calculations of the anisotropic magnetic susceptibility χ versus temperature T below the AF ordering temperature T_N to be carried out for arbitrary Heisenberg exchange interactions J_ij between arbitrary neighbors j of a given spin i without recourse to magnetic sublattices. The Weiss temperature θ_p in the Curie-Weiss law is written in terms of the J_ij values and T_N in terms of the J_ij values and an assumed AF structure. Other magnetic and thermal properties are then expressed in terms of quantities easily accessible from experiment as laws of corresponding states for a given spin S. For collinear ordering these properties are the reduced temperature t=T/T_N, the ratio f = θ_p/T_N, and S. For planar noncollinear helical or cycloidal ordering, an additional parameter is the wave vector of the helix or cycloid. The MFT is also applicable to AFs with other AF structures. The MFT predicts that χ(T ≤ T_N) of noncollinear 120° spin structures on triangular lattices is isotropic and independent of S and T and thus clarifies the origin of this universally observed behavior. The high-field magnetization and heat capacity for fields applied perpendicular to the ordering axis (collinear AFs) and ordering plane (planar noncollinear AFs) are also calculated and expressed for both types of AF structures as laws of corresponding states for a given S, and the reduced perpendicular field versus reduced temperature phase diagram is constructed.

• Research Organization: Ames Laboratory (AMES), Ames, IA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES)
• Grant/Contract Number: AC02-07CH11358
• OSTI ID: 1227216
• Alternate ID(s): OSTI ID: 1180925
• Report Number(s): IS-J-8629; PRBMDO
• Journal Information: Physical Review. B, Condensed Matter and Materials Physics, Vol. 91, Issue 6; ISSN 1098-0121
• Publisher: American Physical Society (APS)
• Country of Publication: United States
• Language: English

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36 MATERIALS SCIENCE