In the last years considerable attention has been devoted to materials composed of fine magnetic particles because of their novel properties (superparamagnetism) and technological applications. A well known method for preparation of alloys in the form of fine particles is the chemical reduction of aqueous solution of metal salts, with an alkali metal borohydride as the reducing agent. By changing the composition and synthesis conditions it is possible to control the morphology (i. e. size and shape) of these nanoparticles. It is known that magnetic particles smaller than 100 nm could exhibit superparamagnetism, and we suppose that such particles embedded in MgB2 superconductor (Tc39 K) could enhance the critical current density. At this stage of research we have tried to synthesize amorphous nanosized particles with well defined dimensions and with narrow and reproducible size distribution. Two series os samples were prepared and analyzed, namely NiCoB without and NiCoB with SiO2 coating. We suppose that particles with SiO2 coating could be more appropriate for embedding in MgB2 superconductor because the SiO2 coating could prevent a direct contact among NiCoB particles and decrease the magnetic interaction among the particles. The resulting microstructures were characterized by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray analysis (EDX). For magnetic measurements superconducting quantum interference device (SQUID) magnetometer was used. The analysis of FE-SEM micrographs of NiCoB/NiCoB-SiO2 nanoparticles showed that the particles were nearly uniaxial, with sizes from 12 to 24 nm for NiCoB sample and from 12 to 32 nm for the SiO2 coated NiCoB sample, showing relatively narrow log normal distribution. X-ray diffraction patterns showed that the samples were amorphous. EDX results confirmed the existence of expected elements (Ni, Co, B, Si and O). Also the preliminary magnetic measurements for both samples confirmed the superparamagnetic behaviour.

Physics