Physics and Astronomy, Department of


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A DISSERTATION Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfilment of Requirements For the Degree of Doctor of Philosophy, Major: Physics and Astronomy, Under the Supervision of Professor Christian Binek. Lincoln, Nebraska: November, 2011

Copyright (c) 2011 Tathagata Mukherjee


Our interest in thermodynamics of magnetic thin film heterostructure began by exploring the possibility to use magnetic nanostructures in the search for optimized magnetocaloric materials for potential room temperature refrigeration. In the present thesis 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 potential of artificial antiferromagnets for near room-temperature refrigeration is explored. Magnetocaloric properties are deduced from measurements of the temperature and field dependence of the magnetization of our samples. The effects of intra-plane and inter-plane exchange interactions on the magnetic phase diagram in Ising-type model systems are revisited in mean-field considerations with special emphasis on tailoring magnetocaloric properties. The experimental results are discussed in light of our theoretical findings, and extrapolations for future improved nanostructures are provided. Further, magnetization relaxation is investigated in a structurally ordered magnetic Co/Cr superlattice. Magnetization transients are measured after exposing the heterostructure to a magnetic set-field for various waiting times. Scaling analysis reveals an asymptotic power-law behavior in accordance with a full aging scenario. The temperature dependence of the relaxation exponent shows pronounced anomalies at the equilibrium phase transitions of the antiferromagnetic superstructure and the ferromagnetic to paramagnetic transition of the Co layers. The latter leaves only weak fingerprints in the equilibrium magnetic behavior but gives rise to a prominent change in non-equilibrium properties. Our findings suggest scaling analysis of non-equilibrium data as a probe for weak equilibrium phase transitions. In addition some misleading interpretations concerning the rigorousness of phenomenological thermodynamics are clarified. Specifically, 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. A minimalist model involving pairs of exchange coupled, mobile 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 simple and intuitive analytic expression for the isothermal entropy change in the presence of field-activated lattice degrees of freedom is provided.We quantify the impact of quantum corrections in the low-temperature limit. To this end, we compare calculations which include elastic interaction with the rigid exchange model in the high-temperature limit. We find that quantum effects provide quantitative corrections in the low-temperature limit. In addition we show that the elastic contributions to the isothermal entropy change can be additive but, remarkably, it can also give rise to reduced isothermal entropy change in certain temperature regions.

Adviser: Christian Binek