Magnetic properties of a magnetoactive elastomer (MAE) filled with µm-sized soft-magnetic iron particles have been experimentally studied in the temperature range between 150 K and 310 K. By changing the temperature, the elastic modulus of the elastomer matrix was modified and it was possible to obtain magnetization curves for an invariable arrangement of particles in the sample as well as in the case when the particles were able to change their position within the MAE under the influence of magnetic forces. At low (less than 220 K) temperatures, when the matrix becomes rigid, the magnetization of the MAE does not show a hysteresis behavior and it is characterized by a negative value of the Rayleigh constant. At room temperature, when the polymer matrix is compliant, a magnetic hysteresis exists and exhibits local maxima of the field dependence of the differential magnetic susceptibility. The appearance of these maxima is explained by the elastic resistance of the matrix to the displacement of particles under the action of magnetic forces.

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Magnetic properties of nano-granular Co0.55–(Al2O3)0.45 films have been studied by vibrating sample magnetometry, ferromagnetic resonance, and magnetic force microscopy (MFM). The films possess a growth-induced perpendicular anisotropy related to the shape anisotropy of the granules. Being unpercolated physically, the films demonstrated a clear magnetic percolation behavior in the temperature dependence of the coercivity and MFM scans. The temperature dependence of the coercivity measured along the easy magnetization axis of the granules indicates the presence of a collective long-range magnetic state sustained by a ferromagnetic interparticle interaction. This interaction is supposed to form a homogeneously magnetized superferromagnetic state, however, the perpendicular anisotropy and the competition between the dipole-dipole and ferromagnetic interactions lead to the emergence of an inhomogeneous labyrinth-like magnetic stripe-domain structure with the averaged domain width being about two orders of magnitude larger than the averaged granule size. The temperature evolution of the spontaneous magnetization inside the stripe domains has been measured by the MFM up to the temperature of superferromagnetic ordering (420 K). The formerly developed mean-field-based model of the coercivity of an interacted ensemble has been tested and found to yield adequate quantitative predictions.

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