Thermodynamics and phase diagrams of layered superconductor/ferromagnet nanostructures

We study the thermodynamics of clean, layered superconductor/ferromagnet nanostructures using fully self-consistent methods to solve the microscopic Bogoliubov-deGennes equations. From these self-consistent solutions the condensation free energies are obtained. The trilayer superconductor/ ferromagnet/superconductor junction is studied in particular detail: first-order transitions between 0 and π states as a function of the temperature T are located by finding where the free energies of the two phases cross. The occurrence of these transitions is mapped as a function of the thickness dF of the F layer and of the Fermi wave-vector mismatch parameter Λ. Similar first-order transitions are found for systems with a larger number of layers: examples are given in the seven-layer (three-junction) case. The latent heats associated with these phase transitions are evaluated and found to be experimentally accessible. The transition temperature to the normal state is calculated from the linearized Bogoliubov-deGennes equations and found to be in good agreement with experiment. Thus, the whole three-dimensional phase diagram in T, dF, and Λ space can be found. The first-order transitions are associated with dips in the transition temperature Tc to the nonsuperconducting state, which should facilitate locating them. Results are also given for the magnetic moment and the local density of states at the first-order transition.