Anisotropy and Ising-like transition of the S=5/2 two-dimensional Heisenberg antiferromagnet Mn-formate di-Urea

TL;DR

This paper models the thermodynamic behavior of Mn-formate di-Urea as an easy-axis 2D Heisenberg antiferromagnet, revealing a crossover from 2D-Heisenberg to 2D-Ising behavior and estimating the anisotropy driving the phase transition.

Contribution

It introduces a theoretical model using PQSCHA to describe the compound's thermodynamics and identifies a crossover and phase transition in the magnetic behavior.

Findings

Crossover from 2D-Heisenberg to 2D-Ising behavior with decreasing temperature.

Estimation of the easy-axis anisotropy from the phase transition location.

Specific heat follows a T^3-law below T* indicating a dimensional crossover.

Abstract

Recently reported measurements of specific heat on the compound Mn-formate di-Urea (Mn-f-2U) by Takeda et al. [Phys. Rev. B 63, 024425 (2001)] are considered. As a model to describe the overall thermodynamic behavior of such compound, the easy-axis two-dimensional Heisenberg antiferromagnet is proposed and studied by means of the 'pure quantum self-consistent harmonic approximation' (PQSCHA). In particular it is shown that, when the temperature decreases, the compound exhibits a crossover from 2D-Heisenberg to 2D-Ising behavior, followed by a 2D-Ising-like phase transition, whose location allows to get a reliable estimate of the easy-axis anisotropy driving the transition itself. Below the critical temperature T_N=3.77 K, the specific heat is well described by the two-dimensional easy-axis model down to a temperature T*=1.47 K where a T^3-law sets in, possibly marking a low-temperature crossover of magnetic fluctuations from two to three dimensions.