Speaker: Rupali Mangotra, MSCB student
Title: Chemical pressure Tuning of Magnetocaloric effect in Frustrated Breathing Pyrochlore System CuAlCr₄Se₈
Abstract: The magnetocaloric effect (MCE) is a thermodynamic phenomenon with promising applications in magnetic refrigeration, ranging from household cooling to hydrogen liquefaction. As a greener alternative to gas compression, it has been widely studied in rare-earth intermetallic compounds. However, a large MCE can also be achieved through alternative mechanisms. Here, we explore a route toward rare-earth-free magnetocaloric materials for low-temperature applications by utilizing magnetic frustration to generate highly degenerate ground states, which can undergo large entropy changes under field polarization. In magnetically frustrated systems, competing magnetic interactions and lattice symmetry prevent the stabilization of a unique magnetic ground state. While spinels with the general formula AM₂X₄ host a pyrochlore lattice, when two different cations occupy the A site a breathing pyrochlore lattice is formed with the formula AA'Cr₄X₈. This leads to two inequivalent tetrahedra and tunable exchange interactions (J, J'). In this work, we synthesized polycrystalline Cu_0.9Al_0.9Cr_3.8Se₈ and induced frustration through careful selection of the anion and A-site variation. Magnetic measurements indicate a competition between ferromagnetic and antiferromagnetic interactions, and the susceptibility deviates from the Curie-Weiss behavior. AC susceptibility reveals a spin-glass-like feature, suggesting frozen, disordered spin states. The MCE was also evaluated using heat capacity, showing a suppressed magnetic entropy change, potentially due to the absence of long-range magnetic order. These results confirm that chemical pressure tuning via the A-site substitution and anion choice can effectively induce frustration and complex magnetism, highlighting a new pathway for designing rare-earth-free materials for low-temperature magnetic cooling applications.