Elastic relaxation behavior, magnetoelastic coupling, and order-disorder processes in multiferroic metal-organic frameworks

Resonant ultrasound spectroscopy has been used to analyze magnetic and ferroelectric phase transitions in two multiferroic metal-organic frameworks (MOFs) with perovskite-like structures [(CH3)(2)NH2]M(HCOO)(3)(DMA[M] F, M = Co, Mn). Elastic and anelastic anomalies are evident at both the magnetic ordering temperature and above the higher temperature ferroelectric transition. Broadening of peaks above the ferroelectric transition implies the diminishing presence of a dynamic process and is caused by an ordering of the central DMA ([(CH3)(2)NH2](+)) cation which ultimately causes a change in the hydrogen bond conformation and provides the driving mechanism for ferroelectricity. This is unlike traditional mechanisms for ferroelectricity in perovskites which typically involve ionic displacements. A comparison of these mechanisms is made by drawing on examples from the literature. Small elastic stiffening at low temperatures suggests weak magnetoelastic coupling in these materials. This behavior is consistent with other magnetic systems studied, although there is no change in Q(-1) associated with magnetic order-disorder, and is the first evidence of magnetoelastic coupling in MOFs. This could help lead to the tailoring of MOFs with a larger coupling leading to magnetoelectric coupling via a common strain mechanism. DOI: 10.1103/PhysRevB.86.214304