Direct measurement of magnetocaloric effect (MCE) in frustrated Gd-based molecular complexes

Abstract

Generation of low temperatures below 1 K has been required for applications and fundamental research, given this, development of new materials utilized for demagnetization cooling has extensively been performed in recent years. Here, we studied two polynuclear Gd³⁺-based molecular compounds of Gd_0.33[Gd₄(OH)₄(OAc)₃][Rh₄Zn₄(L-cys)₁₂]·32H₂O (1_Gd) and Gd_0.33[Gd₄(OH)₄(OAc)₃][Ir₄Zn₄(L-cys)₁₂]·28H₂O (2_Gd) (L-cys = L-cysteinate) which show paramagnetic even at low temperatures due to their frustrated arrangement of Gd³⁺ ions. We discuss the magnitude of the magnetocaloric effect (MCE) in them inferred from the isothermal magnetic entropy change (\({\Delta S}_{\text{M}}\)) from isothermal magnetization data. The − \(\Delta S_{{\text{M}}}^{{{\text{max}}}}\) of 1_Gd and 2_Gd are 15.15 J kg⁻¹ K⁻¹ and 17.49 J kg⁻¹ K⁻¹ occur at 2.0 K under an applied field from 0 to 7 T, respectively. We also discussed the results of heat capacity measurement under magnetic fields to confirm the validity of the entropy change for 1_Gd. Furthermore, with an aim of detecting their MCE directly, we have developed a new non-magnetic and metal-free magnetocaloric measurement cell. The adiabatic temperature change (\(\Delta T_{{{\text{ad}}}}\)) occurs in a small amount of sample on an order of 10²-microgram with the application and removal of various magnitude magnetic fields starting from several initial temperatures were detected directly, to evaluate the potential of them to be a refrigerant for an adiabatic demagnetization refrigerator. The instrumental design for direct measurements of MCE is described along with the construction details.