Counter-intuitive Ferroelectric Property And Non-negligible Orbital Magnetic Moment In Cr/Cu Based Perovskite Metal-Organic Frameworks

TL;DR

This paper investigates the electric and magnetic properties of Cr/Cu-based perovskite MOFs, revealing counter-intuitive ferroelectric behavior and significant orbital magnetic moments, with insights from microscopic and macroscopic analyses.

Contribution

It uncovers a hybrid mode-induced electronic polarization mechanism and models the orbital magnetic moment in these MOFs, advancing understanding of their multiferroic properties.

Findings

Electronic polarization induced by non-polar hybrid modes.

Counter-intuitive enhancement of polarization by polar mode inversion.

Orbital magnetic moment in Cu-based MOF comparable to spin contribution.

Abstract

Metal-organic frameworks (MOFs) possess a hybrid nature, combining the inorganic properties from the metal ions and the organic properties from the molecular linkers. Stroppa, \textit{et al.}, showed that the perovskite-type MOF [C(NH$_2$)$_3$]M[(HCOO)$_3$] (M = Cr, Cu) exhibits the magneto-electric coupled multiferroicity [Angew. Chem. Int. Ed. 50, 5847 (2011) and Adv. Mater. 25, 2284 (2013)]. Moreover, their ferroelectricity arises from the hybrid improper mechanism, which also explains the magneto-electric coupling. In this work, we further examine the electric and magnetic properties of [C(NH$_2$)$_3$]M[(HCOO)$_3$]. We find that the hybrid mode composed of non-polar modes induces purely electronic polarization even without the polar mode. The polar mode compensates for the purely electronic polarization. It leads to a counter-intuitive argument that the inversion of the polar mode rather enhances the polarization. We provide microscopic origin and macroscopic analysis for this polarization property. In addition, we find that the orbital magnetic moment is comparable to the spin contribution in the Cu-based MOF. Finally, we establish the model for the orbital magnetic moment based on the perturbation theory.