Hysteresis, critical fields and superferromagnetism of the film with perpendicular anisotropy

• The transition from the multidomain SFM to homogeneous SFM state occurs critically.
• The value of the critical field depends on the direction of the magnetizing field.
• Critical transition field depends on the anisotropy of the interparticle exchange.
• Dependence of Hc(θH) differs from expected one for an ensemble of the particles.
• Magnetization reversal occurs by turning the particle's moments in domain borders.

This paper is focused on the analysis of hysteresis and critical phenomena of magnetization reversal of superferromagnetic (SFM) state in nanogranular (NG) Co/Al2O3 film with perpendicular anisotropy. It was demonstrated that the transition from the multidomain SFM state to the homogeneous SFM state, during the magnetization process, occurs critically. The value of the field of critical transition to the homogeneous state depends on the demagnetization field, granular anisotropy and interparticle exchange anisotropy. It turned out that the temperature dependence of the coercive force of the film, despite its SFM state, accords with the Neel–Brown formula for anisotropic single-domain ferromagnetic particles, but has an anomalous angular dependence. It was concluded that domain wall motion affects these features of the coercive field. The domain wall movement may occur due to the overturn of magnetic moments of particles in the boundaries between the superdomains. At the same time, the main factors influencing the coercivity are the anisotropy of the particles, which blocks their magnetic moment reorientation, and demagnetizing factor of the film. Together they lead to the anomalous angular dependence of the coercive field.