Anisotropy Configuration Research Articles

Temperature effect on the asymmetric giant magnetoimpedance in amorphous materials

The asymmetric giant magnetoimpedance (GMI) curve has been measured under the rotating field as a function of temperature in a Co-based amorphous ribbon (Co/sub 66/Fe/sub 4/B/sub 15/Si/sub 15/). The magnetic softness in the amorphous phase is enhanced at temperature between 370 and 415 K. The bias-field caused by the hard magnetic phase is nearly constant up to 350 K and then rapidly decreases with temperature. The field angle of 95/spl deg/ from the ribbon axis for maximum impedance increases to 97/spl deg/ at a temperature of 440 K, indicating little change of anisotropy configurations with temperature.

Interplay between the vortex lattice and arrays of submicrometric pinning centers

Ordered arrays of submicrometric magnetic pinning centers have been fabricated in superconducting Nb films by electron beam lithography. Periodic pinning effects, observed when the vortex lattice matches the array of dots, have been studied as a function of temperature, current and array geometry in order to analyze the physics of the interaction between the vortex lattice and the periodic array of microscopic pinning centers. These results have allowed pointing to the magnetic origin of the relevant pinning mechanism in this system and to dynamic ordering effects of the vortex lattices. The effect of the relative direction of the driving force with respect to the pinning potential is dependent on the anisotropy configuration of the artificial submicrometric array.

Damage evolution and acoustic emission of wood at tension perpendicular to fiber

This paper presents the tracing of damage evolution in spruce loaded in tension perpendicular to fiber direction by means of acoustic emission (AE) analysis. The 2 dimensional burst source location in cross-sectional slabs of boards utilized full wave form recording of AE signals monitored by six simultaneously triggered multiple resonant longitudinal (p-)wave sensors and fast transient recorder PC cards. The location algorithm was based on the minimization of the run time differences between experimental and theoretical signal travel times. The latter accounted for the global polar material anisotropy leading to considerably variable off-axis wave velocities along the wave propagation paths, assumed to be straight; the employed orthotropic on- and off-axis p-wave velocities were obtained experimentally from ultrasound pulse measurements. Below 50% of the ultimate load generally very few burst events were recorded. At further increased load, burst agglomerations were predominantly associated to local areas of elevated stresses here resulting from the specific configuration of anisotropy and loading. For all specimens a distinct burst localisation was obtained in the range of 80 to 90% of the ultimate load, coinciding well with the theoretically highest stressed areas of the fracture plane at brittle failure. The correlation of AE event rates with global strain allowed a tracing of the damage evolution especially when events in confined areas are regarded. Distinct damage localisation was then denoted by a clear AE rate increase, however, not accompanied by a global stiffness change. The investigations revealed that failure of spruce - and most probably of similar softwoods - in tension perpendicular to grain, although being very brittle from a macroscopic perspective, is preceded by progressive and localizing damage evolution on the micro-level which can be traced accurately in space and time by acoustic emission burst source analysis.

The effects of elastic anisotropy and dislocation configuration on the superdislocation dissociation widths in TiAl

Theoretical calculation of the dissociation widths of\(\langle 10\bar 1]\) and\(1/2\langle 11\bar 2]\) superdislocations with different orientations and configurations have been carried out under the equilibrium condition that the total elastic interaction force acting on partial dislocations is balanced by the fault surface tension acting in the opposite direction. The results show that the superdislocation dissociation widths depended not only on stacking fault energies and dislocation characteristics but also on elastic anisotropy, superdislocation types and dissociation modes. Under the elastic anisotropy, the dissociation width of screw\(1/2\langle 11\bar 2]\) superdislocation is larger than that of screw\(\langle 10\bar 1]\) superdislocation, and the dissociation width of edged\(1/2\langle 11\bar 2]\) superdislocation is smaller than that of edged\(\langle 10\bar 1]\) superdislocation with the same stacking fault energy. The dissociation widths under the twofold, threefold and fourfold dissociations are also evaluated with anisotropy. The present results help to determine the stacking fault energies and evaluate the mobility of superdislocation in TiAl.

Noise in NiFeCo/Cu spin valve sensors

Noise properties of FeMn/NiFeCo/Cu/NiFeCo spin valve sensors have been investigated for various sensor widths. The spin valve thin films were patterned into stripes of varying widths from 2 μm to 20 μm. The width-to-length ratio was kept constant giving a fixed resistance of about 840 Ω. The spin valve sensors have been configured to allow signal and noise measurements to be performed using a bridge system. Noise measurement in spin valves with parallel anisotropy axes and cross anisotropy axes show field dependent noise near the high sensitivity region around zero bias field. In spin valves with parallel anisotropy axes, the noise increases with decreasing sensor width but for the cross anisotropy configuration, the noise decreases with decreasing sensor width. As the applied alternating field is increased, the noise peaks splits into two centred at zero bias. This can be explained in terms of a degenerate energy state.

Domain-wall patterns in magnetostatically coupled bilayers

Magnetic domain-wall patterns in magnetostatically coupled bilayers separated by a non-magnetic interlayer have been studied for different anisotropy configurations. A numerical treatment is presented for the case of a parallel anisotropy axis between magnetic layers for samples with thicknesses of 1000Åfor the magnetic layers and 100Åfor the interlayer. For these thicknesses coupled Néel walls appear with a similar structure to that found in previous works on thinner films except for the much higher magnetic poles found at the interlayer boundary surfaces. Samples of Fe 20Ni 80/Cu/Fe 20Ni 80 of 500/200/500Åthickness have been grown whilst uniaxial anisotropies were induced in the magnetic layers with the perpendicular easy axis directions between them. Bitter observations show that coupling between domain walls also exists in samples with the non-parallel magnetic anisotropy axis in both layers.

Domain-wall-pinning simulations for different anisotropy modulations.

A dynamic simulation has been applied to study the interaction of a moving domain wall with different anisotropy modulation to understand the mechanism of domain-wall pinning. The application of this method to some well-known anisotropy configurations provides coherent results. In particular the domain-wall-stabilization phenomena have been studied, the results being in good agreement with the experimental ones. © 1993 The American Physical Society.

Chaos and coherence in classical one-dimensional magnets

Numerical studies are reported for one-dimensional classical ferromagnetic chains and single spins with easy-plane anisotropy and various applied magnetic fields and damping terms. The possibility of temporal chaos is very sensitive to the configuration of anisotropy and magnetic fields. In the chain case, it is found that large amplitude spatial structures can dominate long-time attractors and compete with temporally chaotic tendencies.