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Nickel ferrite (NiFe2O4) nanomaterials with different morphologies, including nano-spheres (10–25 nm in diameter), nano-rods (50–60 nm in diameter and ~1 μm in length), and nano-octahedrons (side length ~150 nm), have been synthesized by a single mild hydrothermal method at 160 °C without any surfactant. The crystal structures have been investigated by TEM, HRTEM, and studied by simulations using the program Materials Studio. The variations in the morphology, as well as the preferential crystal growth directions, depend only on the pH value of the reaction solution. A phase formation mechanism is thus proposed. Magnetization measurements at T = 300 K indicate that the NiFe2O4 nano-spheres of 10–25 nm diameter are superparamagnetic with non-saturating magnetization at H = 7 kOe. The saturation magnetization, MS, of the nanorods is 40 emu g−1, less than the bulk value, MS = 46.7 to 55 emu g−1. The coercivity is HC = 40 Oe, reduced from the bulk value of 100 Oe. On the other hand, the nano-octahedrons have a saturation magnetization of MS = 50 emu g−1, the same as the bulk value. However the coercivity, HC = 50 Oe is also much reduced from the bulk value. [ABSTRACT FROM AUTHOR]

Magnetoresistance (MR) of a nanostructured material, monodisperse Fe3O4 nanospheres of about 200 nm coated with thin SiO2 and compactly cold pressed and sintered, has been investigated. A high MR, up to 10.8% at 1 T and 17% at 8 T, has been observed at 100 K. This enhanced MR is likely arising from the spin-polarized tunneling of conducting electrons through the insulating SiO2 boundaries. The decrease of the MR with the temperature increasing was attributed to the existence of the local spins in the grain boundaries. In addition, the MR ratio decreases with the SiO2 thickness increasing and sintering temperature decreasing. It suggests that the magnetic property of the insulating barrier is crucial for enhancing the low-field MR at a high temperature. [ABSTRACT FROM AUTHOR]

Using a one-step solution phase approach, the authors have synthesized uniform jingle bell-shaped cobalt mesopheres (550–750 nm) and assembled the mesospheres into long magnetic chains (20–30 μm). All of the cobalt spheres are hollow with ∼40 nm thick shells but each contains an ∼200 nm diameter solid ball. The nano- to mesoscale structures were realized via reaction of CoCl2·6H2O and N2H4·H2O in the presence of polyvinylpyrrolidone (PVP) in an ethylene glycol solution. Magnetic measurements show a coercivity of about 75 Oe with a remnance of 9.6 emu/g at 300 K. We propose a possible mechanism for the formation of the nanoto mesoscale structures. [ABSTRACT FROM AUTHOR]