Effects of Compression and Magnetic Dilution on the First and Second Order Phase Transitions in Mn(HCOO)₂·2H₂O

Abstract

Effects of compression and magnetic dilution on the first and second order phase transitions have been examined with Mn(HCOO)₂·2H₂O, which exhibits both of the spin axis reorientation at T_R=1.70 K and the antiferromagnetic transition at T_N=3.69 K. The application of hydrostatic pressure (p) makes a clear shift of T_N(p) (dT_N(p)⁄dp=+0.022T_N(p₀) K·kbar⁻¹; p₀\simeq0 kbar) preserving a sharp heat capacity peak. However, the transition at T_R(p₀)=1.70 K becomes quite obscure in the pressure with a broadening of the heat capacity peak around T_R(p)(dT_R(p)⁄dp=+0.078T_R(p₀) K·kbar⁻¹). These phenomena are compared with the phase transitions in the magnetically diluted system with Zn²⁺ ions.