Nontrivial Phenomena in Magnetic Nanocomposites Co/Al2O3 and Co/SiO2

Abstract

Magnetic nanocomposites (MNC), in which nanoparticles of ferromagnetic metals are distributed in wide-gap dielectric matrices (Al2O3 or SiO2), are prospective materials for electronics due to their amenability to technological control of the concentration and size of ferromagnetic nanoparticles. Co/Al2O3 and Co/SiO2 MNC layers with Co concentrations below the percolation threshold were deposited on polycor substrates using electron-beam deposition in a vacuum (EB-PVD). Scanning electron microscopy showed the presence of tightly packed Co grains of irregular shape with sizes of 5–50 nm in MNC. Low-temperature measurements of the magnetization for MNC Co/Al2O3 were made in the temperature range 4–300 K and magnetic fields up to 10 kOe. The ‘magnetic exchange bias’ has been detected, and it increases with a rise in Co concentration. By studying the electrical properties of the MNC Co/Al2O3 and Co/SiO2 in the temperature range of 77–280 K, the Mott type electron transport hopping mechanism was established. We first observed the effect of a giant positive thermoelectric power in a magnetic field in MNC Co/Al2O3 and the effect of a negative magnetothermoelectric power in a Co/SiO2.

Self-Assembly of Large Magnetic Nanoparticles in Ultrahigh Molecular Weight Linear Diblock Copolymer Films

“…The corresponding remanence ( M r ) and coercivity ( H c ) decrease from around 0.95 to 0.64 emu cm –3 and from around 90 to 46 Oe, respectively (Figure b). The observed behavior of M r and H c is caused by thermal fluctuations. − With increasing temperature, the magnetic moments fluctuate faster, causing an increase in random anisotropy of the magnetic domains. Moreover, a strong temperature dependence of the magnetic susceptibility (χ) is shown in Figure b.…”