Abstract
Soft materials embedded with hard magnetic particles have recently gained widespread recognition as small-scale actuators due to their capacity to be a rapid and precise shape-shifting material[1, 2]. Strontium ferrite (SrFe12O19) particles have been shown to be a great candidate for such applications, since it is an inert hard magnetic material [3]. The study of magnetic properties of hard-magnetic soft materials is a critical step to better design these upcoming actuators.The preparation of such material is done by mixing the particles into the uncured elastomer (PDMS), in liquid form, and then pouring the mixture in a mold for curing. If the samples are subjected to a magnetic field during the curing process, chains of particles are formed in the direction of the applied field (see Fig 1). We expect that the individual particles rotate in the fluid such that their easy axes becomes parallel to the curing field, thus creating an easy axis in the composite in this same direction. The magnetic properties of such types of composites, or for any other soft matrix, cannot yet be found in literature.In this study, we analyzed three concentrations of strontium ferrite particles in PDMS under three field configurations, resulting in 9 different samples. The concentrations used were 1:1, 2:1 and 4:1 ratios of PDMS to strontium ferrite per weight. All three samples of each concentration were cured either in a zero magnetic field, or over the north pole of a permanent magnet, or over the side of said magnet (see Fig 2). A biaxial vibrating sample magnetometer (VSM) was used to measure torque curves and the hysteresis curves parallel and perpendicular to the curing field. The samples cured in a field show a squareness ratio of up to 0.94 while the samples cured in zero field, only close to 0.5. The samples cured in a field show a magnetic anisotropy with an easy axis parallel to the curing field. Anisotropy values are compared to the crystal anisotropy of strontium ferrite to determine the shape anisotropy contribution due to the observed chaining.  Fig. 1 PDMS/strontium ferrite composite showing alignment of particles when cured under magnetic field.  Fig. 2 Curing process of samples
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