Magnetic, Thermal and Ferroelectric Properties of Mofs (Mhym, M = Fe, Mn) Close To Phase Transitions

Abstract

Magnetic, thermal and ferroelectric properties of MOFs (metal organic frameworks), in particular, the compounds CH3NH2NH2M(HCOO)(3) (MHyM) with M = Fe and Mn, are studied close to phase transitions. For the magnetic properties, the molecular field theory at low temperatures and the power-law formula within the framework of an Ising pseudospin-phonon coupled model close to TC are performed by using the observed magnetization data for MHyFe and MHyMn from the literature. For the thermal properties, particularly, the heat capacity CP, Ising compressible model with the power-law formula is introduced to analyze the experimental data by considering the two anomalies in CP at the critical temperatures (TC1 and TC2) for MHyFe and MHyMn. Within these two anomalies, the ferroelectric properties of MHyMn are also studied by analyzing the observed data for the spontaneous polarization PS using the power-law formula. Our calculations show that the molecular field theory is satisfactory for the temperature dependence of the magnetization at low temperatures well below TC at constant fields, with the M(T) at 0.1 kOe close to TC in MHyFe. Values of the critical exponent beta for the magnetization from our analysis using the power-law formula close to T-C(=T-m), are acceptable for both compounds (MHyFe and MHyMn). Regarding the thermal properties, an Ising compressible model is adequate to describe the observed behaviour of the heat capacity C-P with the two anomalies (TC1 and TC2) for MHyFe and MHyMn. Also, for the ferroelectric properties of those compounds the temperature dependence of the spontaneous polarization P-S at TC1 and TC2 (MHyMn) is studied by the power-law formula from an Ising compressible model which describes the observed behaviour of P-S in this compound.