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Microstructure, magnetic and transport properties of magnetoresistive CuFeNi ( x = 5, 10 and 15 at.%) melt-spun ribbons were studied for the as-spun and annealed for 2 h at 400 to 600 °C samples. The microstructure was characterised by means of X-ray diffraction, transmission electron microscopy, energy filtered transmission electron microscopy and Mössbauer spectrometry (MS). Magnetic and transport measurements such as magnetization, zero field cooled/field cooled and giant magnetoresistance (GMR) measurements at 5 and 300 K were performed. The aim of this work is to study the influence of the microstructure of as-spun and annealed ribbons on magnetic and transport properties at 5 and 300 K. We have precisely determined the nature of precipitation of nanoparticles, the relative proportion and the magnetic properties of each phase in such a system using MS at both temperatures. We report the conclusive observation of the magnetic behaviour of each phase which was correlated to the structural data to explain the GMR properties that are observed. The influence of magnetic interactions between nanoparticles on magnetic and magnetoresistive properties of these ribbons was determined. We have clearly shown the existence and the nature of the magnetic interactions that reduce the GMR. [ABSTRACT FROM AUTHOR]

Magnetite nanoparticles having mean diameter of about 8 nm have been prepared by a thermo-chemical route. Different amounts (5 and 10% wt) of a stable dispersion of magnetite nanoparticles in n-hexane were added to polyethylene glycol diacrylate (PEGDA-600) oligomer containing 2% wt of radicalic photoinitiator. The homogenized mixture was poured on a silica glass substrate and the resulting film was photoreticulated in N atmosphere using a UV lamp. As a result, a polymer-based magnetic nanocomposite was obtained, where the magnetic nanoparticles are dispersed in the diamagnetic matrix, as checked by SEM. Morphology, composition, and size of as-prepared nanoparticles were checked by SEM and X-ray diffraction. The magnetic properties of magnetite nanoparticles prior to and after inclusion in the polymeric matrix have been studied by means of an alternating-gradient magnetometer (T interval: 10-300 K, H: 18 kOe). FC-ZFC curves were obtained in the same temperature interval. The results show that the nanocomposites cannot be simply described as containing superparamagnetic particles undergoing an anisotropy-driven blocking and that collective magnetic interactions play a non-negligible role. Low-temperature hysteretic properties indicate that the polymeric matrix affects the effective anisotropy of magnetite nanoparticles. Dispersion of magnetite NPs in PEGDA has non-trivial consequences on their magnetic properties. [ABSTRACT FROM AUTHOR]

Array of dots have been designed by assembling a monolayer of polystyrene nanospheres (PN) on sputtered thin films having NiFe and Co composition with different thickness, ranging in the interval 20 ÷ 80 nm. Subsequently the films are nanopatterned using the nanospheres as a mask during sputter etching with Ar ions. A Reactive Ion Etching (RIE) process before sputter etching is used to control the final diameter of the magnetic dots that thus can be tailored as desired (typically ranging in the interval 250 ÷ 400 nm depending on the PN starting diameter). In addition, electron beam lithography has been exploited to obtain arrays of dots in NiFe thin films having approximately the same mean size and dot distance as in self-assembled samples. All films have been routinely characterized by SEM and AFM microscopy to evaluate the microstructure. Magnetic domain patterns at magnetic remanence and in the demagnetised state have been imaged by MFM microscopy technique. Room-temperature hysteresis properties have been measured by an alternating gradient force magnetometer. In general, the magnetization process in all patterned films has been observed to have features typical of a vortex whose nucleation field depends on sample thickness and mean dot dimension. A comparison between magnetic arrays of NiFe dots prepared by self-assembling of polystyrene nanospheres and electron beam lithography is presented to rule out the role of microstructure (i.e., order, size, and mutual distance of the magnetic dots) on magnetic properties. [ABSTRACT FROM AUTHOR]