Phase transitions in coupled double-layer systems

Magnetic systems consisting of a capping layer with uniaxial anisotropy and a recording layer with vertical anisotropy are studied by means of a variational method for a continuum model covering the exchange, anisotropy, and Zeeman energies. The phase transitions between different alignments of magnetization are observed when the thickness of the capping layer and/or the temperature is varied. For the capping layer with in-plane anisotropy, the phase transitions are in second order and the critical behaviors are characterized by a critical exponent 1/2. For the capping layer with vertical anisotropy, the phase transitions are in first order. The analytic expressions of the critical points are presented, in terms of the relevant magnetic constants.