Optimization of the Magnetocaloric Effect in Low Applied Magnetic Fields in LnOHCO₃ Frameworks

Abstract

This study probes the structure and magnetocaloric effect of the LnOHCO3 (Ln = Gd3+, Tb3+, Dy3+, Ho3+, and Er3+) frameworks. The combination of single crystal X-ray and neutron powder diffraction indicates that these materials solely adopt the P212121 structure under these synthetic conditions and magnetic susceptibility measurements indicate they remain paramagnetic down to 2 K. We show that the magnetocaloric effects of TbOHCO3 and DyOHCO3 have peak entropy changes of 30.99 and 33.34 J kg–1 K–1 for a 2–0 T field change, respectively, which are higher than that of the promising GdOHCO3 framework above 4 K in moderate magnetic fields. The magnetic entropy changes of TbOHCO3 and DyOHCO3 above 4 K for smaller than 2 T field changes also exceed those of Gd3Ga5O12 and Dy3Ga5O12, making them suitable magnetic cooling materials for use at liquid helium temperatures using the low applied magnetic fields accessible using permanent magnets, advantageous for efficient practical cooling devices.

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

“…The emergence of short range correlations in only these materials is an important result, as these materials show magnetocaloric properties maximised for use above 4 K for low applied magnetic field changes. 7 Only ErODCO 3 , whose magnetocaloric properties gradually increases on cooling down to 2 K and resembles GdOHCO 3 but with overall poorer performance, did not show any sign of magnetic diffuse scattering, indicating a lack of significant magnetic correlations. It was noted that the strength of magnetic diffuse scattering observed at 1.5 K decreased significantly from TbODCO 3 to HoODCO 3 to DyODCO 3 , indicating a decrease in the strength of the magnetic correlations giving rise to this, but does not correlate with bond distances.…”

“…It has previously been reported that the LnOHCO 3 frameworks do not show any indication of long range order down to 2 K, and follow Curie-Weiss behaviour, with Weiss temperatures of À5.04 K, À0.84 K, À3.83 K and À7.47 K, for Tb, Dy, Ho and Er. 7 Magnetic susceptibility measurements below 2 K of TbOHCO 3 and HoOHCO 3 show features indicative of the formation of long range order (Fig. 2) at B1.2 K and B0.8 K, respectively.…”

“…Solid state caloric refrigerants are one efficient alternative for such applications; 4 recent advances have been made in developing magnetocalorics with good performance at low temperatures under fields that can be generated using permanent magnets, required for energy efficient and practical magnetocaloric devices. [5][6][7][8][9] In recent years work on magnetocaloric materials has extended from purely ionic systems to those containing molecular building blocks and this appears a very promising route forward for developing improved materials. [10][11][12] The paramagnetic magnetocaloric effect (MCE) is an entropically driven cooling process that occurs when paramagnets are in a cycled magnetic field.…”

“…Recently, however, literature has shown that materials containing cations with strong Ising anisotropy improves the magnetocaloric effect in powders under the low applied magnetic fields that can be generated using a permanent magnet (o2 T). 5,7,17 This surprising result is an outcome of the greater ease of magnetisation of these materials under low applied fields, but the microscopic cause of this remains unknown. Uncovering how magnetic interactions in such compounds are best optimised to improve magnetocaloric performance requires an understanding of these materials at the microscopic level, which is most readily achieved using neutron scattering rather than indirect bulk property measurements.…”

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

“…[7] Recently however, magnetic studies of MOFs have emerged, highlighting the potential of magnetic metal-organic frameworks in functional magnetic materials. MOFs have already been shown to exhibit a range of magnetic properties, with uses in chemical and magnetic sensors, [8] magnetocalorics [9,10] and multiferroics that combine magnetism with ferroelectric order. [11] The ability for greater flexibility in the design of MOFs, allows for more specific topologies and magnetically isolated sheets and chains, opening up the means to low dimensional and frustrated magnetism.…”