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Nickel ferrite nanoparticles dispersed in SiO2 matrix have been synthesized by sol-gel method. Structural analysis has been performed by using x-ray diffraction and transmission electron microscopy. Magnetic properties have been investigated by using superconducting quantum interference device magnetometry. In addition to the average blocking temperature peak at TB=120 K measured by a zero field cooled temperature scan of the dc susceptibility, an additional hump near 15 K is observed. Temperature dependent out-of-phase ac susceptibility shows the same features: one broad peak at high temperature and a second narrow peak at low temperature. The high temperature peak corresponds to magnetic blocking of individual nanoparticles, while the low temperature peak is attributed to surface spin-glass freezing which becomes dominant for decreasing particle diameter. To prove the dynamics of the spin (dis)order in both regimes of freezing and blocking, the frequency dependent ac susceptibility is investigated under a biasing dc field. The frequency shift in the “frozen” low-temperature ac susceptibility peak is fitted to a dynamic scaling law with a critical exponent zv=7.5, which indicates a spin-glass phase. Exchange bias is turned on at low temperature which signifies the existence of a strong core-shell interaction. Aging and memory effects are further unique fingerprints of a spin-glass freezing on the surface of isolated magnetic nanoparticles.

Magnetic properties of 6 nm maghemite nanoparticles (prepared by microwave plasma synthesis) have been studied by ac and dc magnetic measurements. Structural characterization includes x-ray diffraction and transmission electron microscopy. The temperature scans of zero field cooled/field cooled (ZFC/FC) magnetization measurements show a maximum at 75 K. The ZFC/FC data are fitted to the Brown-Néel relaxation model using uniaxial anisotropy and a log-normal size-distribution function to figure out the effective anisotropy constant K eff . K eff turns out to be larger than the anisotropy constant of bulk maghemite. Fitting of the ac susceptibility to an activated relaxation process according to the Arrhenius law provides unphysical values of the spin-flip time and activation energy. A power-law scaling shows a satisfactory fit to the ac susceptibility data and the dynamic critical exponent (zv % 10) takes value between 4 and 12 which is typical for the spin-glass systems. The temperature dependence of coercivity and exchange bias shows a sharp increase toward low temperatures which is due to enhanced surface anisotropy. The source of this enhanced magnetic anisotropy comes from the disordered surface spins which get frozen at low temperatures. Memory effects and thermoremanent magnetization experiments also support the existence of spin-glass behaviour. All these magnetic measurements signify either magnetic blocking or surface spin-glass freezing at high and low temperatures, respectively. V C 2012 American Institute of Physics. [http://dx.

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The magnetic properties of CaCo2As2 single crystal was systematically studied by using dc magnetization and magnetic torque measurements. A paramagnetic to antiferromagnetic transition occurs at TN = 74 K with Co spins being aligned parallel to the c axis. For H c, a field-induced spin-flop transition was observed below TN and a magnetic transition from antiferromagnetic to paramagnetic was inferred from the detailed analysis of magnetization and magnetic torque. Finally, we summarize the magnetic phase diagram of CaCo2As2 based on our results in the H-T plane.

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Pristine carbon nanotube (CNT)/elastomer composites were fabricated using pristine multi-walled carbon nanotubes and a thermoplastic elastomer. These composites exhibited a unique phenomenon of two electrical percolation thresholds that invoked very high dielectric values for the resulting composites. The first percolation was associated with a relatively low dielectric constant value of about 100, while in the vicinity of the second percolation threshold a very high dielectric constant value of 8,000 was achieved. The presence of two percolation thresholds was attributed to the unique distribution patterns of CNTs that ensued in a CNT/elastomer composite system with unique electrical properties.