Comparative Study of Magnetocaloric Properties for Gd^{3+} Compounds with Different Frustrated Lattice Geometries

Abstract

As materials with suppressed ordering temperatures and enhanced ground-state entropies, frustrated magnetic oxides are ideal candidates for cryogenic magnetocaloric refrigeration. While previous material design focused on tuning the magnetic moments, their interactions, and the density of moments on the lattice, relatively little attention has been paid to frustrated lattices. Prior theoretical work has shown that the magnetocaloric cooling rate at the saturation field is proportional to a macroscopic number of soft mode excitations that arise due to the classical ground-state degeneracy. The number of these modes is directly determined by the geometry of the frustrated lattice. For corner-sharing geometries, the pyrochlore lattice has 50% more modes than the garnet and kagome lattices, whereas the edge-sharing fcc lattice has only a subextensive number of soft modes. Here we study the role of soft modes in the magnetocaloric effect of four large-spin Gd^{3+} (L=0, J=S=7/2) Heisenberg antiferromagnets on a kagome, garnet, pyrochlore, and fcc lattice down to T=2 K. By comparing measurements of the magnetic entropy change ΔS_{m} of these materials at fields up to 9 T with predictions obtained with use of mean-field theory and Monte Carlo simulations, we are able to understand the relative importance of spin correlations and quantization effects. We observe that tuning the value of the nearest-neighbor coupling has a more significant contribution to the magnetocaloric entropy change in the liquid-He cooling regime (2–20 K) than tuning the number of soft mode excitations. Our results provide a base for future refrigerant-material design in terms of dimensionality, degree of magnetic frustration, and lattice geometry.