J. Losby

Magnetic behavior of mechanically Milled FeNi-CoO nanocomposites

The effects of crystallite size and ball-mill-induced defects on the magnetic properties of ferromagnetic (FM) FeNi-antiferromagnetic (AFM) CoO (Neel Temperature=290 K) nanocomposites are assessed by magnetic measurements. Asymmetric zero-field-cooled (ZFC) hysteresis loops were observed for samples milled for 30h and longer. The field-cooled (FC) hysteresis loops were asymmetric and shifted in the direction opposite to the cooling field. The enhancement of coercivity and squareness ratio (M/sub R//M/sub S/), along with a presence of a loop shift after field cooling indicates the presence of an exchange bias effect between the FM and AFM phases of the composite. The exchange bias field extracted from the 5 K FC measurement shows continuous enhancement with milling times up to 30 hours and a reduction upon prolonged milling. The temperature dependent exchange bias field measurement shows that the exchange bias field reduces to zero at temperature T<200 K<T/sub N/ (290 K) of CoO. Thus, in agreement with thin film systems, exchange bias properties can be obtained in FM-AFM fine powder nanocomposites upon mechanical milling. Further, the present study has the potential implication of improving magnetic properties of hard magnetic materials upon milling with antiferromagnetic materials.

Thermal effects on ESR signal evolution in nano and bulk CuO powder

Abstract In order to understand the effects of low dimension on the magnetic properties of CuO, a systematic electron spin resonance study is carried out on CuO nano and bulk powders. Sol–gel produced CuO nanopowder was calcined at temperatures from 200 to 1000 °C to produce nanoparticles of varying size. A broad electron spin resonance (ESR) signal was obtained for the CuO nanoparticles, representative of antiferromagnetic ordering. This ordering persists even at higher calcination temperatures. On the contrary, CuO bulk powder shows two separate ESR signals which merge into one at higher temperatures. These data indicate that the antiferromagnetic ordering is preserved up to 800 °C in CuO nanomaterial. In bulk CuO powder, the ESR signal breaks from antiferromagnetic ordering at lower temperatures. This is a result of the high latent heat of bulk material, which leads to early decomposition of the powder. With evidence from detailed thermal analysis, the observed differences in the ESR data for bulk and nano CuO as a function of calcination temperature are explained on the basis of particle size difference.

Magnetic properties of iron nitride-silica nanocomposite materials prepared by high-energy ball milling.

Powder mixtures of (FexN)y and (SiO2)1-y, with x between 3 and 4 and y equal to 0.2 or 0.6, were ball-milled for 4, 8, 16, 32, and 64 h. X-ray diffraction, thermal analysis, and magnetization measurements allowed an investigation of structural and magnetic properties to be carried out. The samples consist of nanostructured Fe3N and Fe4N particles in a SiO2 matrix. As the milling time increases, the Fe4N phase is eliminated from the particles in favor of Fe3N. Coercive fields as high as 270 and 84 Oe are obtained for (FexN)0.2(SiO2)0.8 at 5 and 300 K, respectively. This higher coercive field, upon cooling, indicates the presence of small superparamagnetic particles. The coercive field also increases with milling time, which is due to the reduced particle size and induced stain. The saturation magnetization decreases with increased milling time as a consequence of an increase in the superparamagnetic fraction and increased strain. Hard and soft magnetic properties are observed for y = 0.2 and y = 0.6 samples, respectively.

Magneto-Transport Properties of Compression Molded CrO2-Polymer Composite

We report detailed electrical and magnetotransport properties of compression molded CrO2—Polyimide polymer composites. The electrical transport data of the composites in the temperature range of 15—300 K were analyzed using a two-channel conductivity model based on spin dependent tunneling and multi-step hopping. The low temperature conductance is essentially dominated by spin dependent tunneling. The contribution from the multi-hopping conductivity increases with temperature increase. The presence of significant magnetoresistance in this composite is attributed partially to the opening of new conduction channels at the CrO2—polymer interface.

Transport Properties of Half-metal-Polymer Composite

We report the detail transport properties of compression molded CrO2-polymer composites. The transport measurements of the composites in the temperature range of 10-300 K were modeled using a two channel conductivity model based on spin dependent tunneling and multi-step hopping. The conductance behavior of the composite is dependent on the nature of insulating barrier which changes the nature of the grain boundaries at which spin polarized tunneling takes place.

Half-metal-polymer magnetoresistive composite

The conductivity and magneto transport properties of compression molded half-metallic CrO/sub 2//polyimide composites over a range of different metallic concentrations have been studied. The conductivity measurements on these composites show negative slope of resistance versus temperature. The magnetoresistance measurement indicates obvious enhancement at low temperatures. The maximum in magnetoresistance is found to be temperature and metal volume fraction dependent. Significant differences in high and low temperature magnetoresistive behavior in the composite have been observed. Thus, it is found that the polymer barrier can contribute to enhancing magnetoresistive properties of the composite.

Magnetic properties of magnetically soft nanocomposite Co-SiO/sub 2/ prepared via mechanical milling

Nanocomposite of Co-SiO/sub 2/, 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 hcp/fcc-Co phase in the composite. However, reported magnetic properties of these composites appear to be dependent on the presence of antiferromagnetic phases of cobalt oxide as well. 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 which remain undetected in XRD.

Magnetotransport Properties of Compression Molded CrO 2 -Polyimide Composite

The conductivity and magnetotransport properties of compression molded half-metallic CrO 2 /Polyimide composites over a range of different metallic concentrations have been studied. The conductivity measurements on these composites show negative slope of resistance versus temperature. The magnetoresistance measurement indicates obvious enhancement at low temperatures. The maximum in magnetoresistance (MR) is found to be temperature and metal volume fraction dependent. Significant differences in high and low temperature magnetoresistive behavior in the composite have been observed. The high field, 15 T MR measurements show 23% and 19% MR enhancement at 5K and 75 K, respectively. Thus, it is found that the polymer barrier can contribute to enhancing magnetoresistive properties of the composite.