Rotating Magnetocaloric Effect in an Anisotropic Two-Dimensional Cu[W(CN)] Molecular Magnet with Topological Phase Transition: Experiment and Theory.

Conventional (MCE) and rotating (RMCE) magnetocaloric effects have been explored in the two-dimensional (2D) coordination polymer {(tetren)H)Cu[W(CN)]·7.2HO} (WCu-t; tetren = tetraethylenepentamine). The unusual magnetostructural properties were exploited, including the bilayered Prussian Blue like coordination skeleton and the XY easy-plane magnetic anisotropy based on the in-plane correlation between W and Cu spins of /, underlying the Berezinskii-Kosterlitz-Thouless (BKT) topological phase transition to the long-range-ordered state at T = 33 K. The magnetic properties were studied on single crystals along the H∥ac easy plane and H∥b hard axis. The maximal entropy change for MCE for easy-plane geometry at 38.0 K and the magnetic field change μΔH = 7.0 T reached ∼4.01 J K kg. The strong magnetic anisotropy was used to study the RMCE in which the maximal entropy change was observed at 35.5 K for 7.0 T, attaining 1.81 J K kg. Moreover, easy-plane anisotropy introduces the inverse magnetocaloric effect for H∥b, which enhances the RMCE by up to 47%. This observation was confirmed by a theoretical investigation considering the XY model using a molecular field and cluster variational method in the pair approximation approach, dedicated to the bilayered systems with the adequate nearest neighbor number z = 5 and spin S = /.