Magnetic Ordering in Mn(HCOO)2·2H2O and Related Compounds

Publisher Website
Google Scholar
Abstract
The heat capacities of the isostructural dihydrated formates of Mn2+, Fe2+, and Ni2+ have been measured between 1.4° and 20°K. Sharp peaks associated with long‐range spin ordering are found at 3.72°K for Mn(HCOO)2·2H2O, 3.74°K for Fe(HCOO)2·2H2O, and 14.5°K for Ni(HCOO)2·2H2O. An additional sharp anomaly in Cp is found at 1.72°K for the Mn+2 salt, while Cp of the Ni+2 salt exhibits a Schottky‐like maximum at 3°K. Each of these anomalies can be correlated with some feature of the observed magnetic susceptibilities. A unit cell in these salts contains two each of two inequivalent metal ion sites, types A and B. Formate groups constitute AA and AB bridges and appear to mediate the dominant superexchange couplings. A simplified molecular field treatment of Mn(HCOO)2·2H2O in which JAA = 0.65 k, JAB = −0.10 k, and JBB = 0 provides interpretations of several striking effects. In particular, it shows that below a common transition temperature, B spins order much more slowly than A spins and are nearly ``free'' at 1°K as seen in both magnetic and thermal data. Analysis of the data for the Ni+2 and Fe2+ salts suggests that the effective spins of A and B ions may differ in a given substance.