Giant Rotational Magnetocaloric Effect in Ni(en)(H 2 O) 4 ·2H 2 O: Experiment and Theory.

An experimental study of the rotational magnetocaloric effect in Ni(en)(H₂O)₄SO₄∙2H₂O (en = ethylenediamine) single crystal is presented. The study was carried out at temperatures above 2 K and was associated with adiabatic crystal rotation between the easy plane and hard axis in magnetic fields up to 7 T. The magnetocaloric properties of the studied system were investigated by isothermal magnetization measurement. The experimental observations were completed with ab initio calculations of the anisotropy parameters. A large rotational magnetic entropy change ≈12 Jkg⁻¹K⁻¹ and ≈16.9 Jkg⁻¹K⁻¹ was achieved in 5 T and 7 T, respectively. The present study suggests a possible application of this material in low-temperature refrigeration since the adiabatic rotation of the single crystal in 7 T led to a cooldown of the sample from the initial temperature of 4.2 K down to 0.34 K. Finally, theoretical calculations show that S = 1 Ni(II)-based systems with easy-plane anisotropy can have better rotational magnetocaloric properties than costly materials containing rare-earth elements in their chemical structures. [ABSTRACT FROM AUTHOR]