Free Rotation of Magnetic Nanoparticles in a Solid Matrix

Magnetic data for a composite material consisting of nanocrystalline γ-Fe2O3 dispersed in an alginate methanol matrix indicate the presence of rotationally free magnetic particles. The decoupling of the particles from the matrix was achieved by subjecting the sample to a magnetic field applied in alternating directions. Before field treatment, a magnetic blocking process is observed at low temperature, which is characteristic of nanoscale magnetic materials in the presence of barriers for the magnetic moment of the particles. After the field treatment, no magnetic hysteresis is observed and the magnetization follows a Curie−Weiss law. This behavior can only be interpreted as due to magnetic moments that follow the external magnetic field without delay, implying, therefore, the existence of free particles.