Volume 89, Issue 4, 15 February 2001
Index of content:
- MAGNETISM AND SUPERCONDUCTIVITY (PACS 74-76)
89(2001); http://dx.doi.org/10.1063/1.1340600View Description Hide Description
A theoretical study of the magnetization curves of quasiperiodic magnetic multilayers is presented. We consider structures composed by ferromagneticfilms (Fe) with interfilm exchange coupling provided by intervening nonferromagnetic layers (Cr). The theory is based on a realistic phenomenological model, which includes the following contributions to the free magnetic energy: Zeeman, cubic anisotropy, bilinear, and biquadratic exchange energies. The experimental parameters used here are based on experimental data recently reported, which contain sufficiently strong biquadratic exchange coupling.
89(2001); http://dx.doi.org/10.1063/1.1332417View Description Hide Description
Three types of iron-based oxide nanoparticles (weight compositions and embedded in a polyethylene matrix are studied using the electron paramagnetic resonance technique. All nanoparticles are found to be multiphase. Thermal variations of electron paramagnetic resonance spectra reveal the presence of two phases in the nanoparticles. One such phase undergoes an antiferromagnetic-liketransition near 6 K. Nanoparticles of demonstrate a resonance anomaly near 125 K that could indicate the presence of a magnetic phase. Reduced magnetic anisotropy in nanoparticles may be related to either structural imperfection or particle smallness (effective diameter of less than 10 nm). Our data clearly show that low temperature experiments are desirable for the correct identification of nanoparticles by means of the electron paramagnetic resonance technique.
89(2001); http://dx.doi.org/10.1063/1.1344221View Description Hide Description
Nanocompositemagnets with compositions 0.5, 1, 1.5, and 2) have been synthesized by melt spinning using low wheel speeds in the range from 18 to 21.7 m/s. It has been found that the coercivity is significantly increased by Dy substitution. An optimum coercivity of 6.6 kOe is obtained in the magnet as compared to 4.3 kOe in the magnet. As a result, the energy product is increased from 9.1 MGOe in the magnet to 16.9 MGOe in the magnet. The reason for this is the finer and more uniform 2:14:1/α-Fe nanoscale microstructure developed in the magnets with Dy substitution. Because of the low wheel speed spinning, the majority of the nanoscale microstructure is crystallized directly out of the melt. The enhancement of magnetic properties by Dy substitution is mainly due to the microstructure refinement which leads to an enhanced exchange coupling between the and α-Fe, whereas the anisotropy increase by the Dy substitution provides only a minor contribution to the enhanced coercivity.
Spatial distribution of dopant ions in vitreous silica: A pulsed electron paramagnetic resonance spectroscopic study89(2001); http://dx.doi.org/10.1063/1.1338513View Description Hide Description
The structural aspects of clustering of ions in -doped glasses and the effect of codoping on these clusters have been investigated with pulsed electron paramagnetic resonance(EPR) techniques over a temperature range of 1.5–4 K. The concentrations in these glasses range between 700 and 2400 ppm by weight. The echo-detected EPR (EDEPR) spectra of the Al-free glasses show indications of weak exchange coupling between ions due to clustering. The EDEPR spectra also suggest that the local coordination environment of ions is affected by codoping with Al. The electronic spin-lattice relaxation rates are found to be not sensitive to the spatial distribution of ions over the entire temperature range of measurements. On the other hand, the concentration dependence of phase relaxation rates show clear evidence of clustering of ions in Al-free glasses, even at the lowest doping levels. These clusters are found to break up and homogenize with an increase in the average Nd–Nd distance on codoping with Al. Analyses of the hyperfine sublevel correlation spectrum of a Nd and Al codoped glass indicate that the homogenization of clusters is possibly a consequence of the formation of Nd–O–Al linkages.