Phase Transition of Uniaxial Ferromagnet Cu(NH₄)₂Br₄·2H₂O

Abstract

Magnetic properties of Cu(NH₄)₂Br₄2H₂O have investigated. Particular attention is drawn to the role of anisotropy at the phase transition. The critical exponents of the susceptibility along the easy axis and the spontaneous magnetization have been obtained as γ=1.30±0.03 and β=0.34±0.02 respectively.
The susceptibility perpendicular to the easy axis does not diverge at the Curie temperature. The second order phase boundary in a perpendicular field has been obtained as {T_c(0)−T(H_⊥^c)}⁄T_c(0)=(H_⊥^c⁄H_⊥⁰)^ω with ω=2.63±0.1 and H_⊥⁰=(270±10)Oe, where H_⊥^c is the critical field at temperature T.
The observed properties are well explained, except for the values of the exponents, by a molecular field model with anisotropic exchange interaction. The uniaxial character of the salt has been established.
There is some evidence that the Curie temperature of powder specimen may differ from that of single crystals.