Molecular-field-theory fits to magnetic susceptibilities of antiferromagnetic GdCu2Si2, CuO, LiCrO2, and alpha-CaCr2O4 single crystals below their Neel temperatures

TL;DR

This paper applies a molecular field theory to fit magnetic susceptibility data of various antiferromagnetic single crystals below their Neel temperatures, highlighting the theory's strengths and limitations for different magnetic structures.

Contribution

The study extends molecular field theory fits to anisotropic susceptibility data of specific collinear and noncollinear antiferromagnets, demonstrating its effectiveness and limitations.

Findings

Good fit for GdCu2Si2, LiCrO2, alpha-CaCr2O4

Poor fit for CuO due to quantum fluctuations

MFT can fit heat capacity data of GdNiGe3

Abstract

A recently-developed molecular field theory (MFT) has been used to fit single-crystal magnetic susceptibility chi versus temperature T data below the respective antiferromagnetic ordering temperatures TN for a variety of collinear and coplanar noncollinear Heisenberg antiferromagnets. The spins in the system are assumed to interact by Heisenberg exchange and to be identical and crystallographically equivalent. The fitting parameters for chi(T) of collinear antiferromagnets are measurable quantities: the Weiss temperature theta_p in the Curie-Weiss law, TN, chi(TN), and the spin S. For coplanar noncollinear helix and cycloid structures, an additional fitting parameter is the turn angle between layers of ferromagnetically-aligned spins. Here MFT fits to anisotropic chi(T) data from the literature for single crystals of the collinear antiferromagnets GdCu2Si2 and CuO and the noncollinear antiferromagnets LiCr2As2 with a 120 degree cycloidal structure and alpha-CaCr2O4 with a 120 degree helical structure below their respective Neel temperatures are presented. The MFT fit to the anisotropic chi(T < TN) data for CuO is poor, whereas the fits to the data for GdCu2Si2, LiCrO2, and alpha-CaCr2O4 are quite good. The poor fit for CuO is attributed to the influence of strong quantum fluctuations associated with the small Cu spin and the quasi-one-dimensional magnetism that are not taken into account by the MFT. The magnetic contribution to the zero-field heat capacity of the collinear antiferromagnet GdNiGe3 at T < TN is also fitted by the MFT.