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The potential benefits of magnetic nanostructuring is investigated for fabrication of optimized magnetocaloric materials with application for room temperature refrigeration. In particular, the prospective advantageous magnetocaloric properties of artificial antiferromagnets for near room-temperature refrigeration are explored. The effects of intra-plane and inter-plane exchange interactions on the magnetic phase diagram of Ising-type model systems are revisited using mean-field considerations. Special emphasis lies on tailoring microscopic parameters for optimized macroscopic magnetocaloric properties. Magnetic thin film heterostructures are experimentally realized by Molecular Beam Epitaxy (MBE) and Pulsed Laser Deposition (PLD). Co/Cr and Fe/Cr superlattices were fabricated using mean-field theoretical concepts as guiding principles.
The Maxwell relation provides a major tool for the characterization of the magnetocaloric properties from magnetometry data. It is shown that the Maxwell relation incorporates contributions from the spin degrees of freedom and potential lattice degrees of freedom into the isothermal entropy change. To this end, a minimalist model involving pairs of exchange coupled, vibrating Ising spins is investigated. It is explicitly shown that lattice degrees of freedom can be activated via applied magnetic fields and the integrated Maxwell relation contains this lattice contribution. A debate about this fact gave rise to some confusing statements in the literature and has now been settled. A simple and intuitive analytic expression for the isothermal entropy change in the presence of field-activated lattice degrees of freedom is provided. |