Volume 91, Issue 10, 15 May 2002
Index of content:
- MAGNETISM AND SUPERCONDUCTIVITY (PACS 74-76)
Nuclear magnetic resonance monitoring of capillary imbibition and diffusion of water into hardened white cement paste91(2002); http://dx.doi.org/10.1063/1.1467631View Description Hide Description
Nuclear magnetic resonance(NMR) experiments monitoring the imbibition (sorption) and diffusion of water into white cement paste are reported. The sample was a 1.3 cm long cylinder (6 mm o.d.) of hardened ordinary white cement paste, with a water/cement ratio of 0.42 containing 0.5% and 2% NaCl. Water proton magnetization and values were obtained as functions of time. Imbibition of and diffusion of and were monitored with NMR at 26 and 30 MHz. The countercurrent water imbibition experiments revealed a two-stage process. A rapid uptake of water, involving about 85% of the total, took place in about 45 min. Maximum saturation was reached in about 2 days. Both stages of the process were well described by a nonlinear diffusion-like equation. Diffusion of both and was characterized by a single diffusion coefficient. The diffusion coefficient for and derived by fitting the data to the diffusion equation, is well predicted by
91(2002); http://dx.doi.org/10.1063/1.1470254View Description Hide Description
Magnetic media using materials with high uniaxial magneto-crystalline anisotropy, combined with a thermal assist to overcome write field limitations have been proposed as one of the potential technologies to extend the areal density of magnetic disk recording beyond the limitations of current technology. Here we present an investigation on structural and temperature dependent magnetic properties of chemically ordered epitaxial thin films. Increasing Ni additions result in a steady reduction of magneto-crystalline anisotropy, saturation magnetization, and Curie temperature. The ability to control thermomagnetic properties over a wide range makes and similar FePt-based pseudo-binary alloys attractive base materials for media applications in thermally assisted magnetic recording.
91(2002); http://dx.doi.org/10.1063/1.1471391View Description Hide Description
Amorphous or 2) powders with a wide supercooled liquid region were prepared by high energy ball milling of rapidly quenched ribbons and subsequently hot pressed in the viscous state to receive bulk glasses. Almost complete coercivityrelaxation of the ball milled powders was achieved by annealing. The coercivity of as-milled ribbons drops drastically from more than 2200 Am−1 after 1 h of milling to 14.6 Am−1 after optimum annealing. The coercivity of this powder and that of the as-quenched ribbons (7 Am−1) differs only by the different contributions of surface irregularities to the total coercivity Drastic coercivityrelaxation was also achieved by consolidating the as-milled powders. A minimum coercivity of 30 Am−1 was found for the bulk sample after compaction of 1 h ball milled powders. It is of the same order of magnitude as of cast amorphous rods. The milling-induced coercivity increase as well as the coercivityrelaxation are discussed in terms of the change of magnetic anisotropy and the creation and elimination of stress and shear bands in the amorphous phase.
91(2002); http://dx.doi.org/10.1063/1.1473671View Description Hide Description
A ferromagnetic phase, characterized by electron carriers and a high temperature colossal magnetoresistance (HTCMR) dependent on the magnetic moment, and a semiconducting phase, characterized by hole carriers and a low temperature CMR (LTCMR), are observed in thin films by the van der Pauw method. The LTCMR is much more sensitive to the magnetic field than the HTCMR. In the ferromagnetic phase for films with anisotropic moments in two dimensions, a remnant resistivity of the order of Ω m is observed up to 100 K and increases exponentially with both a temperature up to and a magnetic field above 1 T (a positive magnetoresistivity). We found that the ferromagnetic phase below is in a polaronic state with a polaronic mobile conduction, and the carrier density dips near For resistances measured by the four-probe method with line electrodes, low temperature information of the HTCMR is not revealed. The van der Pauw method is more effective for the resistance measurement of a magnetic material than the four-probe method.
91(2002); http://dx.doi.org/10.1063/1.1471387View Description Hide Description
We investigated the structural, the magnetic, the magnetotransport, and the tunnelingproperties of junctions with an artificial ferrimagnet as a pinning layer for different Al thickness and oxidation time after isochronal annealing up to 500 °C. The main purpose of these experiments is to find relations between the structural changes upon annealing and the modifications of the physical magnetotransport and barrier properties, which could also be important for the further application of such tunneling elements in spinelectronics. The tunnelingmagnetoresistance(TMR) shows a strong increase up to 37% after annealing at 300 °C accompanied by an improvement of the dielectric stability and the voltage dependence of the TMR. At higher temperature, the TMR starts to decrease. The dielectric stability remains good up to annealing temperatures of 500 °C, indicating an excellent thermal stability of the barrier. All results can be related to thermally induced structural changes of the microstructure which have been determined by Auger depth profiling and complementary methods.