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 T C are performed by using the observed magnetization data for MHyFe and MHyMn from the literature. For the thermal properties, particularly, the heat capacity C P , Ising compressible model with the power-law formula is introduced to analyze the experimental data by considering the two anomalies in C P at the critical temperatures ( T C 1 and T C 2 ) 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 P S 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 T C at constant fields, with the M(T) at 0.1 kOe close to T C in MHyFe. Values of the critical exponent β 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 ( T C 1 and T C 2 ) for MHyFe and MHyMn. Also, for the ferroelectric properties of those compounds the temperature dependence of the spontaneous polarization P S at T C 1 and T C 2 (MHyMn) is studied by the power-law formula from an Ising compressible model which describes the observed behaviour of P S in this compound.