ABSTRACT
Alloyed spherical FeNi-polymer nanocomposites were prepared via wet chemical method using hydrazine as a reducing agent and polymers (PVP and PEG) as reducing and stabilizing agent. Structural studies performed using XRD and TEM shows uniform dispersion of fine FeNi nanocrystallites in nanocomposite particles. The size and thermal stability of FeNi-polymer nanocomposite particles prepared under same reaction condition was found to be dependent on the type and the molecular weight of the polymer used. However, the magnetic properties of nanocomposite particles were not influenced by the polymers. The study highlights subtle differences in using polymers during the synthesis of alloyed nanocomposite particles.
ABSTRACT
Defect induced magnetic properties of CoO nanoparticles produced via mechanical ball milling have been assessed by detailed magnetic measurements. A progressive decrease in the particle size and a concomitant increase in the induced strain have been observed with the milling times. The mechanically milled nanoparticles of CoO exhibit anomalous magnetic properties such as FM hysteresis when compared with the unmilled CoO sample. The presence of weak ferromagnetism, with a highest value of magnetization of 0.532 emu/g at 10 K in the 100 h milled sample, is attributed to the uncompensated surface spins resulting from induced surface defects via mechanical milling. The ZFC coercive force, measured at 10 K, increases with milling time reaching a maximum value of 1066 Oe for the 100 h milled sample. The temperature dependent field-cooled (FC) and zero-field-cooled (ZFC) magnetic measurements indicate a presence of an exchange bias field arising from uncompensated moments generated by mechanical strain and the antiferromagnetic (AFM) core. The exchange bias field measured at 10 K reaches a value 210 Oe for the 50 h milled sample and decreases upon prolonged milling. The exchange bias field vanishes at a temperature approximately 200 K, a temperature much lower than the Neel temperature of CoO (TN approximately 291 K). The observed anomalous magnetic behavior of CoO could be interpreted in terms of the exchanged bias FM-AFM model.
Subject(s)
Cobalt/chemistry , Magnetics , Nanotechnology , Oxides/chemistry , Microscopy, Atomic Force , Particle Size , X-Ray DiffractionABSTRACT
Nanocomposite of Co-SiO2, a soft magnetic material, with Co weight fraction x = 0.3 and 0.7 was prepared via mechanical milling. The magnetic properties of these samples, both zero-field-cooled (ZFC) and field-cooled (FC), have been measured as a function of x, milling time, and temperature. The structural assessment of the composite indicates a presence of only ferromagnetic (FM) hcp-Co phase in the composite. However, reported magnetic properties of these composites appear to be dependent on the presence of antiferromagnetic (AFM) phases of cobalt oxide as well. The observed enhancement in ZFC coercivity and a reduction in saturation magnetization with the milling time are due to an increase in defect density upon milling. The ZFC coercivity for the x = 0.3 samples has been found to be much higher than the x = 0.7 samples for all milling times. The coercivity above 50 K depends on temperature according to the law corresponding to isotropic uniaxial superparamagnetic particles. Below 50 K the presence of an AFM phase Co3O4 (TN approximately 33 K) and increased interparticle interactions bring in a departure from that law. The saturation magnetization is found to be temperature dependent for the x = 0.3 samples and temperature independent for the x = 0.7 samples, which further provides evidence of the presence of higher AFM phase fraction in the composite with a low metal volume fraction. The FC magnetic measurements show a presence of an exchange bias field and an enhanced coercivity which are higher than the ZFC measurements. All magnetic measurements indicate that the overall magnetic properties of the composite are dictated by the presence of a trace amount of cobalt oxides.
