Phase transitions in uniaxial ferromagnetic film covered by superconducting layers

The phase diagram and the single-domain uniform state for a uniaxial ferrom agnetic film with the superconducting layers covering one or both sides of a ferromagnet are investigated. The superconductor is supposed to be a seco nd-order one and the interaction between the magnetic sub-system and with t he conductivity electrons in a superconductor is purely electromagnetic and the vortices in a superconductor are pinned. The critical thickness of the magnetic film for which the uniform state becomes absolutely stable is cal culated when the external magnetic field is supposed to be in-plane of the film. It is shown that the critical thickness of the film from the magnetic material with the quality factor Q > 1 monotonically decreases as the magn etic field increases in the range from zero value to the value of the trans ition field where the collinear phase transforms into the angular (canted) phase. Further the critical thickness increases with the increase of the fi eld. The quasi-single-domain magnetic film states were considered when the film thickness was close to the critical one. It is shown that for a thin i solated magnetic film the domain period exponentially increases with the de crease of the film thickness. Such dependence, however fbr the film with do uble-side superconducting cover and close to the transition into the single domain state becomes logarithmic and for the him covered by superconductor only on the one side varies as the power series. The single-domain state e xistence and the asymptotic behaviour of the domain structure is explained by the features of the asymptotic behaviour of the domain walls within the system. As for isolated magnetic film and for a film with the superconducto r cover layers the transition from the collinear phase to the inhomogeneous state is the second-order phase transition and the transition from the uni form angular phase to the inhomogeneous phase is the first-order transition . (C) 1999 Elsevier Science B.V. All rights reserved.