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

Effects of compression and magnetic dilution on the first and second order phase transitions have been examined with Mn(HCOO) 2 ·2H 2 O, 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 ) (d T N ( p )/d p = +0.022 T N ( p 0 ) K·kbar -1 ; p 0 ≃0 kbar) preserving a sharp heat capacity peak. However, the transition at T R ( p 0 ) = 1.70 K becomes quite obscure in the pressure with a broadening of the heat capacity peak around T R ( p )(d T R ( p )/d p = +0.078 T R ( p 0 ) K·kbar -1 ). These phenomena are compared with the phase transitions in the magnetically diluted system with Zn 2+ ions.