Magnetic Polarons Enable Exceptional Magnetocaloric Response

TL;DR

This paper demonstrates that magnetic polarons in EuB₆ significantly enhance magnetocaloric effects, offering a promising approach for cryogenic cooling in the 10-40 K range.

Contribution

It introduces magnetic polarons as a novel intermediate regime that improves magnetocaloric response, providing a new design principle for cooling materials.

Findings

EuB₆ exhibits a giant magnetocaloric response near its Curie temperature.

Nanoscale ferromagnetic clusters enhance entropy change and temperature shift.

Magnetic polarons serve as an effective route for optimizing magnetocaloric materials.

Abstract

Magnetocaloric materials are typically limited by a trade-off between magnetic entropy and field responsiveness. Here we show that magnetic polarons provide an intermediate regime that mitigates this constraint and enables an exceptional magnetocaloric response. Using EuB$_6$ as a model system, we combine thermodynamic and magnetic measurements to demonstrate that nanoscale ferromagnetic clusters emerging near the Curie temperature strongly enhance the field-induced entropy collapse. These clusters possess large effective moments that respond efficiently to applied fields while retaining substantial entropy due to their small size and dynamic fluctuations. As a result, EuB$_6$ exhibits a giant cryogenic magnetocaloric response, with both large isothermal entropy change and adiabatic temperature change in the technologically important 10-40 K range. Our results identify magnetic polarons as an underexplored route for optimizing magnetocaloric performance and establish an intermediate magnetic length scale as a design principle for high-performance cryogenic cooling materials.