Volume 87, Issue 7, 01 April 2000
Index of content:
- MAGNETISM AND SUPERCONDUCTIVITY (PACS 74-76)
Magnetic properties and microstructure of mechanically milled -based powders with Hf, Nb, V, Ti, Cr, Cu and Fe87(2000); http://dx.doi.org/10.1063/1.372359View Description Hide Description
The structure, microstructure, and magnetic properties of nanostructured -based powders synthesized by mechanically milling and subsequent annealing have been systematically studied. It has been found that a nanoscale 2:17 phase with an average grain size of about 30 nm is developed within the powders, which have an average particle size of about 5 μm. Optimum magnetic properties of emu/g, emu/g, kOe, and MGOe have been obtained in stoichiometric powders milled for 6 h and annealed at for 30 min. The observed magnetic hardening is believed to arise from the high anisotropy of the phase and its nanoscale grain size. A small amount of Zr substitution for Co significantly increases the coercivity by increasing the anisotropy field of the phase. Cu substitution in Zr-contained samples further increases the coercivity by introducing a nanoscale 1:5 phase which forms a uniform mixture with the 2:17 nanograins. The highest coercivity of 20.6 kOe has been obtained in the powders. Fe substitution enters the Co lattice sites of the 2:17 structure, leading to an increase of the magnetization but a decrease of the coercivity. An optimum maximum energy product of 14.0 MGOe is obtained in the powders.
Quantitative interpretation of the magnetoresistive response (amplitude and shape) of spin valves with synthetic antiferromagnetic pinned layers87(2000); http://dx.doi.org/10.1063/1.372360View Description Hide Description
We present an analytical calculation of the shape of the magnetoresistive response of spin-valve structures with synthetic antiferromagnetic pinned layer, i.e., of the form buffer/ferromagnet1/Cu/ferromagnet2/Ru/ferromagnet3/antiferromagnet. The magnetization reversal in the three magnetic layers is assumed to occur via coherent rotation. An analytical expression of the whole hysteresis loop is given as a function of the characteristic parameters of the system (coupling strength through the Ru spacer, ferromagnet3/antiferromagnet pinning energy). We also extended a code based on the Boltzmann equation of transport to calculate the giant magnetoresistance (GMR) amplitude in these structures from the microscopic transport parameters. In order to explain the relatively high GMR amplitude experimentally observed in such spin valves, it is shown that some degree of specular reflection must be introduced at the ferromagnet2/Ru interface. Good agreement with both the shape and amplitude of the experimental magnetoresistance curves can be obtained.
87(2000); http://dx.doi.org/10.1063/1.372361View Description Hide Description
A series of (volume fraction) nanogranular films were prepared by ion-beamsputtering at room temperature. A tunnelingmagnetoresistance(TMR) of about 4% at room temperature was observed. With applied magnetic field of about Oe parallel to the plane of substrates during fabrication, the TMR ratio of the sample increased up to about 5% at room temperature. The results of ferromagnetic resonance showed that the isolated Co particles embedded in the matrix were pancake-like in shape. Thus the discrepancy of TMR between the applied magnetic field parallel and perpendicular to the plane of the sample may be due to shape-induced anisotropy.
87(2000); http://dx.doi.org/10.1063/1.372362View Description Hide Description
Fine powders have been synthesized by mechanochemical processing, involving high-energy ball milling of and and subsequent heat treatment. Single-phase particles, dispersed in a NaCl matrix, were obtained for heat-treatment temperatures of or above, with the crystallite size increasing with from about 20 nm at 750 °C to about 1 μm at 1200 °C. Magnetic measurements showed that as the crystallite size decreases, the saturation magnetization at low temperature decreases from the theoretical value for full polarization of Mn spins, 3.7 per Mn ion, while increases from 250 up to 278 K for the crystallite size of ∼20 nm; and that there exists a narrow temperature range near in which the magnetization for a larger crystallite size is smaller in low fields but becomes larger in high fields than for a smaller crystallite size. These observations are discussed in terms of the ferromagnetic transition unique to perovskite manganites, involving the formation and growth of magnetic polarons above
87(2000); http://dx.doi.org/10.1063/1.372363View Description Hide Description
Iron–carbon nanocompositethin films with iron concentrations ranging from 12 to 74 at. % were grown by ion-beamsputtering codeposition at different substrate temperatures. The microstructure of the films was characterized by transmission electron microscopy, atomic force microscopy, extended x-ray absorption fine structure, and grazing incidence small-angle x-ray scattering. A granular morphology consisting of body-centered-cubic iron-rich nanoparticles (2–5 nm in diameter and 3–8 nm in height with a relatively sharp size distribution) regularly distributed within a more or less graphitic matrix was obtained. Structuralproperties as well as magnetic ones were found to depend strongly on composition, substrate temperature, and postdeposition treatments (annealing or irradiation). The temperature dependence of the susceptibility exhibited a superparamagnetic response with blocking temperature in the range of 13–180 K. The maximum low-temperature (5 K) coercivity value obtained in this study was 850 Oe for the film with the lowest iron content The saturation magnetization was found to be reduced compared to the corresponding bulk value for pure α-Fe, and increased with increasing substrate temperature. Otherwise, when increasing the iron content, a decrease in coercivity was observed in correlation with an increase in remanent magnetization.