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

Abstract

Resonant ultrasound spectroscopy has been used to analyze magnetic and ferroelectric phase transitions in two multiferroic metal-organic frameworks (MOFs) with perovskite-like structures [(CH 3 ) 2 NH 2 ]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 ([(CH 3 ) 2 NH 2 ] + ) 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.

Phase transition and cationic motion in the perovskite formate framework [(CH3)2NH2][Mg(HCOO)3]

“…Especially the compounds with magnetic metal ions are of particular current interest as a candidate of multiferroic materials which combine two ordered forms among magnetism, ferroelectricity and/or ferroelasticity and have a potential for applications in microwave devices, sensors, transducer, and memory devices [2,3]. The organic cation in these compounds is located in the cage formed by the M(HCOO) 3 À lattice and the degree of motional freedom of the organic cation can be used to develop http://dx.doi.org/10.1016/j.molstruc.2014.08.037 0022-2860/Ó 2014 Elsevier B.V. All rights reserved.…”

Multiferroic (ferroelastic/ferromagnetic/ferrimagnetic) aspects of phase transitions in RCo2Laves phases

“…A convenient baseline has the form of equation (.12), which has been fitted to f 2 data for ErCo 2 between 173 and 291 K and extrapolated to 0 K ( figure 11(a)). This gives essentially the same description as the equation of Varshni [67] for elastic moduli at low temperatures [68,69]. To estimate a baseline for NdCo 2 , which has f 2 almost independent of temperature above ∼500 K, it was simpler to use low temperature data for the shear modulus of YCo 2 , which does not display magnetic ordering, taken from Klimker et al [18].…”

First-order structural transition in the multiferroic perovskite-like formate [(CH3)2NH2][Mn(HCOO)3]

“…[15][16][17] Out of all [(CH 3 ) 2 NH 2 ][M(HCOO) 3 ] compounds the Mn-formate -which shows a magnetic transition at T N ≈ 8.5 K, [ 7 ,11,12] a dielectric transition at T t ≈ 190 K 7 ,10 and according to a very recent report a new magnetoelectric coupling in the paramagnetic state near the ferroelectric transition 17 -is so far the best characterized one, 11,12 even if a number of questions remain open, for example, if the structural transition is a first or second order transition.…”