RESUMO
A plethora of temperature-induced phase transitions have been observed in (CH3NH3)[M(HCOO)3] compounds, where M is Co(II) or Ni(II). Among them, the nickel compound exhibits a combination of magnetic and nuclear incommensurability below Néel temperature. Despite the fact that the zero-field behavior has been previously addressed, here we study in depth the macroscopic magnetic behavior of this compound to unveil the origin of the atypical magnetic response found in it and in its parent family of formate perovskites. In particular, they show a puzzling magnetization reversal in the curves measured starting from low temperatures, after cooling under zero field. The first atypical phenomenon is the impossibility of reaching zero magnetization, even by nullifying the applied external field and even compensating it for the influence of the Earth's magnetic field. Relatively large magnetic fields are needed to switch the magnetization from negative to positive values or vice versa, which is compatible with a soft ferromagnetic system. The atypical path found in its first magnetization curve and hysteresis loop at low temperatures is the most noticeable feature. The magnetization curve switches from more than 1200 Oe from the first magnetization loop to the subsequent magnetization loops. A feature that cannot be explained using a model based on unbalanced pair of domains. As a result, we decipher this behavior in light of the incommensurate structure of this material. We propose, in particular, that the applied magnetic field induces a magnetic phase transition from a magnetically incommensurate structure to a magnetically modulated collinear structure.
RESUMO
In the dense metal-organic framework Na[Mn(HCOO)_{3}], Mn^{2+} ions (S=5/2) occupy the nodes of a "trillium" net. We show that the system is strongly magnetically frustrated: the Néel transition is suppressed well below the characteristic magnetic interaction strength; short-range magnetic order persists far above the Néel temperature; and the magnetic susceptibility exhibits a pseudo-plateau at 1/3-saturation magnetization. A simple model of nearest-neighbor Heisenberg antiferromagnetic and dipolar interactions accounts quantitatively for all observations, including an unusual 2-k magnetic ground state. We show that the relative strength of dipolar interactions is crucial to selecting this particular ground state. Geometric frustration within the classical spin liquid regime gives rise to a large magnetocaloric response at low applied fields that is degraded in powder samples as a consequence of the anisotropy of dipolar interactions.
RESUMO
The perovskite azido compound [(CH3 )4 N][Mn(N3 )3 ], which undergoes a first-order phase change at Tt =310â K with an associated magnetic bistability, was revisited in the search for additional ferroic orders. The driving force for such structural transition is multifold and involves a peculiar cooperative rotation of the [MnN6 ] octahedral as well as order/disorder and off-center shifts of the [(CH3 )4 N](+) cations and bridging azide ligands, which also bend and change their coordination mode. According to DFT calculations the latter two give rise to the appearance of electric dipoles in the low-temperature (LT) polymorph, the polarization of which nevertheless cancels out due to their antiparallel alignment in the crystal. The conversion of this antiferroelectric phase to the paraelectric phase could be responsible for the experimental dielectric anomaly detected at 310â K. Additionally, the structural change involves a ferroelastic phase transition, whereby the LT polymorph exhibits an unusual and anisotropic thermal behavior. Hence, [(CH3 )4 N][Mn(N3 )3 ] is a singular material in which three ferroic orders coexist even above room temperature.
RESUMO
We present the first example of magnetic ordering-induced multiferroic behavior in a metal-organic framework magnet. This compound is [CH3NH3][Co(HCOO)3] with a perovskite-like structure. The A-site [CH3NH3](+) cation strongly distorts the framework, allowing anisotropic magnetic and electric behavior and coupling between them to occur. This material is a spin canted antiferromagnet below 15.9 K with a weak ferromagnetic component attributable to Dzyaloshinskii-Moriya (DM) interactions and experiences a discontinuous hysteretic magnetic-field-induced switching along [010] and a more continuous hysteresis along [101]. Coupling between the magnetic and electric order is resolved when the field is applied along this [101]: a spin rearrangement occurs at a critical magnetic field in the ac plane that induces a change in the electric polarization along [101] and [10-1]. The electric polarization exhibits an unusual memory effect, as it remembers the direction of the previous two magnetic-field pulses applied. The data are consistent with an inverse-DM mechanism for multiferroic behavior.
