Magnetic hysteresis in Fe/SiO2 and Fe/BN granular solids

The magnetic hysteresis behavior of ceramic Fe granular solids is strongly dependent on the type of ceramic matrix and the preparation conditions used. Hysteresis studies in Fe/SiO2 and Fe/BN granular films showed large differences in the coercivity values, with lower values in the BN matrix. In an attempt to understand the differences in coercivity detailed structural, microstructural, and magnetic studies have been done in samples prepared by the tandem deposition: method and deposition from a composite target both with and without titanium sublimation. Typical granular structure was observed, with grain size in the range of 2-20 nm with the lower size observed in composite films. The lower coercivities in Fe/BN are due to the formation of amorphous Fe-B particles in BN which have a lower anisotropy. In Fe/SiO2 magnetic properties showed that by varying the deposition method and some of the sputtering parameters (sputtering rate, argon flow, film thickness, and substrate temperature), it is possible to switch from a relatively magnetically hard sample (H-c similar to 700 Oe) to a soft sample (H-c similar to 20 Oe). A dramatic increase in H, has been observed at cryogenic temperatures in samples prepared with the tandem technique, whereas a small effect has been observed in the composite samples. Mossbauer data suggest a shell/core granule morphology in films made by the tandem technique with an Fe and/or (Fe-Si) core surrounded by Fe2SiO4 The data in composite films show pure Fe(Fe-Si) particles free of oxides. The Mossbauer data can explain the large temperature dependence in coercivity in tandem-made samples through exchange coupling at the Fe/Fe2SiO4 interface which leads to pinning of surface/interface spins.