Heterometallic Gd-Dy Formate Frameworks for Enhanced Magnetocaloric Properties

Lanthanide-based metal-organic frameworks (MOFs) have great potential as magnetic refrigerants under cryogenic conditions and are comparable to conventional alloys and magnetic nanoparticles. In particular, MOFs with Gd³⁺ ions behave as excellent magnetic refrigerants because of their large spin ground states. However, the major drawback of Gd³⁺-based MOFs is that they are not affected by the ligand material owing to the excessively large spin-only magnetic moment; therefore, their application is limited to the cryogenic region in the magnetic cooling field. In this study, we report the magnetic properties and magnetocaloric effect (MCE) resulting from heterogenized MOFs obtained from the reaction of Gd³⁺ and Dy³⁺ ions and their varied molar composition with the formate ligand. For GdDy_1--(HCOO)₃, where 0 ≤ x ≤ 1, the isothermal magnetic entropy change (ΔS_m) increased with the increase in the fraction of Gd in the heterogenized MOFs. Meanwhile, with increasing Dy contents, the maximum peak temperature of ΔS_m is shifted to a higher temperature while preserving a relatively high ΔS_m value of 22.35 J·kg^-1 K^-1 at T = 7 K for an applied field change (ΔH) of 7 T despite the anisotropy and crystalline electric field effects. Furthermore, it was confirmed that the samples with a Dy content of 75% or more maintained the ΔS_m operating temperature longer. Therefore, the current approach of including Dy³⁺ ions in lanthanide compounds provides the possibility of further extending the operating temperature of magnetic cooling materials from cryogenic temperatures.