Local Energy Barriers Research Articles

Coercivity mechanisms in magneto-optical recording media

The sources for the nucleation coercivity and wall motion coercivity in magneto-optical recording thin films are investigated based on the Connection Machine simulations. It was assumed that the thin films consist of nanoscale patches which are magnetic structure other than columnar structure or crystal grains. The postulated inhomogeneities have not been observed directly, but we believe that magnetic structure must exist in view of the fact that the media suitable for practical applications do not have columnar structure and crystal grains, yet they exhibit coercivity phenomena which can only be caused by nanoscale inhomogeneities. The nucleation and wall motion processes were simulated in the patchy films with different types of inhomogeneities and with the average patch size ranging from 60 to 200 Å. The simulation results show that the nucleation coercivity depends sensitively on the patch size characterizing the fluctuation of the anisotropy constant, but weakly on the exchange at the patch borders and the fluctuation of easy-axis orientation. To account for the observed nucleation coercivity in magneto-optical thin films the average patch size should be on the order of 100 Å. In contrast, the wall motion coercivity is mostly caused by the fluctuation in the exchange stiffness constant and the patch-to-patch variations of the easy-axis orientation. It was found that the domain wall, when encountering a high anisotropy region, first encircles it and then reverses its magnetization due to the domain wall force. This suggests a scenario other than thermal fluctuation by which a wall can go over a local energy barrier.

Mouvement du lithium dans deux nouvelles familles d'électrolytes solides: Les boracites [formula omitted] et les halogénoborates de lithium vitreux

The boracites Li 4+x B 7O 12+ x 2 Cl and the related B 2O 3 xLi 2O yLiCl glasses have been studied by cw and pulsed NMR between 130 and 500°K. Above 160°K the 7Li spectrum is composed of two lines: a broad one due to nonmobile Li + ions and a narrow one due to diffusing Li + with a hopping frequency greater than the dipolar frequency. The activation energy deduced from spin-lattice relaxation time measurements ( T 1) is lower than that given by variation of conductivity with temperature. At low temperature T 1 disagrees with the BPP prediction ( T 1 ∼ ω 2 o). The diffusion process may be explained by the existence of a distribution of the local energy barriers.