Emergent magnetic order and correlated disorder in formate metal-organic frameworks

Abstract

<jats:p> Magnetic materials with strong local interactions but lacking long-range order have long been a curiosity of physicists. Probing their magnetic interactions is crucial for understanding the unique properties they can exhibit. Metal-organic frameworks have recently gathered more attention as they can produce more exotic structures, allowing for controlled design of magnetic properties not found in conventional metal-oxide materials. Historically, magnetic diffuse scattering in such materials has been overlooked but has attracted greater attention recently, with advances in techniques. In this study, we investigate the magnetic structure of metal-organic formate frameworks, using heat capacity, magnetic susceptibility and neutron diffraction. In Tb(DCO <jats:sub>2</jats:sub> ) <jats:sub>3</jats:sub> , we observe emergent magnetic order at temperatures below 1.2 K, consisting of two <jats:bold>k</jats:bold> -vectors. Ho(DCO <jats:sub>2</jats:sub> ) <jats:sub>3</jats:sub> shows diffuse scattering above 1.6 K, consistent with ferromagnetic chains packed in a frustrated antiferromagnetic triangular lattice, also observed in Tb(DCO <jats:sub>2</jats:sub> ) <jats:sub>3</jats:sub> above 1.2 K. The other lanthanides show no short- or long-range order down to 1.6 K. The results suggest an Ising-like one-dimensional magnetic order associated with frustration is responsible for the magnetocaloric properties, of some members in this family, improving at higher temperatures. </jats:p> <jats:p>This article is part of the theme issue ‘Mineralomimesis: natural and synthetic frameworks in science and technology’.</jats:p>

Ferromagnetic Ising chains in frustrated LnODCO3: the influence of magnetic structure in magnetocaloric frameworks

“…[19][20][21] Similarly recent neutron scattering studies of Tb(HCO 2 ) 3 and Ho(HCO 2 ) 3 suggests that their magnetocaloric properties, also optimised for use above 4 K in less than 2 T applied fields, are linked to the presence of 1D ferromagnetic chains packed in a frustrated antiferromagnetic triangular lattice. 17,22,23 A subsequent study finds ferromagnetic chains improve magnetocaloric performance, over a similar temperature range, also in oxide materials. 24 This emphasises the case for using neutron diffraction to understand the way in which the atomic-level magnetic interactions in these materials influence their macroscopic properties.…”

“…As previously suggested for Tb(HCO 2 ) 3 and Ho(HCO 2 ) 3 the ferromagnetic Ising chains allow for high entropy changes in small applied magnetic fields as ferromagnetic units are more readily aligned with the applied magnetic field. 17,23 The competing weaker antiferromagnetic interchain interactions help to suppress long-range order, required for paramagnetic magnetocalorics, but are weak enough to require only small fields to be overcome to lead to a ferromagnetic field-induced state. This dominant ferromagnetic intrachain coupling allows the moments to be more easily aligned with the applied field.…”

In situ observation of the magnetocaloric effect through neutron diffraction in the Tb(DCO2)3 and TbODCO3 frameworks

“…8,33 We have previously established the magnetic states that form in both Tb(HCO 2 ) 3 and TbOHCO 3 in the absence of applied magnetic fields. 9,35 Tb(HCO 2 ) 3 , which adopts R3m rhombohedral symmetry at all measured temperatures, 36 shows strong diffuse scattering indicative of short range order below 20 K, where its magnetocaloric effect begins to be significant. This short range order is formed of ferromagnetically coupled linear chains of Isinglike moments aligned along the c-axis, antiferromagnetically coupled in a triangular array within the ab plane.…”

“…8,33 This is consistent with the rapid increases in their magnetisation under modest fields, 8,33 contributing to a high efficiency as a magnetocaloric. 36 The mechanism for the magnetocaloric cycle in these materials, has not been experimentally observed. Neutron diffraction is the obvious method for probing the magnetic structure of these materials due to its sensitivity to magnetic moments.…”

“…8,33 Below 1.6 K it displays long range 1D order along the c-axis and the chain direction, and finite correlations in the ab plane, extending B50 Å. 8,36 Long range order appears to be prevented by the frustrated interactions between chains leading to the emergence of a triangular Ising antiferromagnetic (TIA) state. TbODCO 3 , which adopts P2 1 2 1 2 1 orthorhombic symmetry, also shows strong diffuse scattering below 20 K attributed to the formation of ferromagnetic correlations between the Ising-like cations in the buckled ferromagnetic chains along the b-axis, which are packed into an antiferromagnetic triangular like lattice.…”