Chemical pressure Tuning of Magnetocaloric effect in Frustrated Breathing Pyrochlore System CuAlCr4Se8

Author

Rupali Mangotra, Kennesaw State UniversityFollow

Date of Award

Spring 5-7-2025

Track

Chemistry

Degree Type

Thesis

Degree Name

Master of Science in Chemical Sciences (MSCB)

Department

Chemistry

Committee Chair/First Advisor

Madalynn Marshall

Committee Member

Carl Saint-Louis

Committee Member

Chetan Dhital

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, large MCEs 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 exhibit pyrochlore lattice, when two different cations occupy the A site, breathing pyrochlore latticeis fromed in AA'Cr₄X₈-type materials. This leads to two inequivalent tetrahedra and tunable exchange interactions (J, J'). In this work, we synthesized polycrystalline Cu₀.₉Al₀.₉Cr₃.₈Se₈ and induced frustration through careful selection of anion and A-site variation. Magnetic measurements indicate a competition between ferromagnetic and antiferromagnetic interactions, and the susceptibility deviates from Curie-Weiss behavior. AC susceptibility reveals a spin-glass-like feature, suggesting frozen, disordered spin states. MCE was also evaluated using heat capacity data, showing a suppressed magnetic entropy change, likely due to the absence of long-range magnetic order.

These results confirm that chemical pressure tuning via 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.