Overcoming the spin-multiplicity limit of entropy by means of lattice degrees of freedom: A minimal model

Authors

Tathagata Mukherjee, University of Nebraska-LincolnFollow
Steven A. Michalski, University of Nebraska-LincolnFollow
Ralph Skomski, University of Nebraska-LincolnFollow
David J. Sellmyer, University of Nebraska-LincolnFollow
Christian Binek, University of Nebraska-LincolnFollow

Date of this Version

2011

Citation

PHYSICAL REVIEW B 83, 214413 (2011); DOI: 10.1103/PhysRevB.83.214413

Comments

Copyright 2011 American Physical Society

Abstract

The discovery of the giant magnetocaloric effect with isothermal field-induced entropy change beyond the spin-multiplicity limit gave rise to some indistinctness in the literature regarding the applicability of fundamental thermodynamics in data analysis. Those misleading interpretations concerning, for instance, the rigorousness of phenomenological thermodynamics are clarified here. 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.