Magnetic Studies of Fine Maghemite Nanoparticles Prepared by Microwave Plasma Synthesis

Full text: Magnetic properties of maghemite nanoparticles (5nm) (prepared by microwave plasma synthesis) have been investigated by SQUID-magnetometry. Fine maghemite nanoparticles consist of magnetically aligned core and of disordered surface spins. Disordered and frustrated surface spins give pinning effects and spin-glass freezing at low temperatures. Zero field cooled / field cooled (ZFC/FC) magnetic measurements show magnetic blocking at 75 K. A model for noninteracting nanoparticles have been simulated to reproduce the experimental ZFC/FC curves and to calculate effective anisotropy. Effective anisotropy calculated from the model is larger than the bulk value due to increased surface anisotropy. Frequency dependent AC susceptibility measurements in the frequency range (0.1- 1000 Hz) have shown an increase of blocking temperature with increasing frequency. A model of AC susceptibility for non-interacting nanoparticles has been developed to investigate the shift of blocking temperature within a decade of frequency. Simulation of the AC susceptibility model produce large shift in blocking temperature in the frequency range (0.1-1000 Hz). The large value of effective anisotropy from ZFC/ FC model and discrepancy in the AC susceptibility experiment and simulation signify the existence of freezing effects at low temperature on the surface of maghemite nanoparticles. Spin glass freezing on the surface of nanoparticles is a possible mechanism to overcome the superparamagnetic limit which commonly erases the magnetic memory of nanomagnets. (author)