Nickel-silica nanocomposite: Variation of the blocking temperature with magnetic field and measuring frequency

The variations of the blocking temperature TB with measuring frequency fm and applied field H are reported for Ni nanoparticles (NPs) embedded in SiO2 matrix with the nominal composition Ni∕SiO2 (15∕85). Transmission electron microscopy of the sol-gel prepared NPs yielded a log-normal distribution with diameter D=3.8(0.2)nm whereas x-ray diffraction data yielded D≃3.3nm. The TB versus fm variation for fm=0.1–1000Hz fits the equation TB=Ta∕ln(f0∕fm) with f0=2.6×109Hz and Ta=270K whereas TB versus H data follows the equation TB(H)=33(1−H∕H0)m with H0=3200Oe and m=2(3∕2) for lower (higher) H. These results are compared with the predictions of the Néel-Brown model of magnetic relaxation in noninteracting NPs.