Amorf ferromanyetik tellerde miknatıslanma süreçleri.

Hysteresis loops in a ferromagnetic amorphous wire longer than the critical length are studied. Wires with a deformation in the middle are considered. Both theoretical and experimental studies using DC vibrating sample magnetometer were performed. Amorphous wires with compositions Fe-Si-B and with diameter 125μm of positive magnetostriction coefficient are used in the experiments. Samples have been deformed at the middle by pressing with a hard steel needle of 0.75mm diameter, perpendicular to the axis of the wire. Another group of measurements was carried out of the samples cut at the middle. A third group of experiments was intended to be a bridge to the hysteresis model proposed. 1.5mm coils were wound up at the middle of the wire under investigation to apply a local magnetic bias. An oppositely directed pair of local fields is applied with a pair of identical coils reversely connected is employed to investigate its effect on the hysteresis loop. The DC hysteresis loops were measured with a home made vibrating sample magnetometer. The sample vibrates sinusoidally along the axis of a long solenoid at the frequency of 23 Hz. The pick-up coils have been particularly designed to measure long samples. Magnetic field has been changed with a step of 0.3 A/m, and measured with a sensitivity of 0.01 A/m. In the first group of experiments, although the length of the wire is greater than the critical length for bistable behavior, after a threshold value of increasing, the hysteresis loop starts to narrow keeping its rectangular shape. Furthermore deformations more than 20 μm lead to two large Barkhausen jumps separated by a staircase relaxation. The only difference of the hysteresis loops yielded by the second group of experiments from the first is that the slope of the inclined parts is bigger than those in the first group. The unidirectional magnetic bias gave rise to a staircase relaxation, followed by a long Barkhausen jump in the descending branch of the loop. In the ascending branch, the domain wall completes its almost entire motion in a negative external field and the magnetization approaches saturation with staircase movements. As the current through the reversely connected pair of coils is increased, the hysteresis loop starts developing nearly horizontal tails first at the up right, and then at the bottom left corners. The model proposed in this work is based on the calculation of the magnetic moment distribution in the core of the wire by means of computer simulations. To simulate the magnetization process of the deformed wire, the total energy is taken as a function of five variables. Three of these correspond to the nucleation at two ends and at the middle of the wire while the remaining two represent the domain wall locations in the two regions. The simulation traces the gradient in the six dimensional energy landscape to find the set of coordinates, which, for a given value of external field, minimizes the total energy. Magnetization makes a jump to another stable position when the total energy loses its local minimum in the landscape. Since in general there are more than one energy minima along the entire process, a magnetic hysteresis occures. The total energy of the system is taken as the summation of mutual magnetostatic interactions along all the domains, Zeeman energy, and the anisotropy energies. Exchange interactions are taken into accound by assuming the domain wall motions along the wire. When the deformation created in the middle of the wire is represented by just an anisotropy field, but no nucleation allowed, the upper and lower halves of the calculated loop is shifted symmetrically to the right and to the left respectively, with horizontal stretch lines in the two directions. The horizontal lines result from the energy barrier created by the local anisotropy field in the middle. Although the great simplicity of this structure, it reveals the response mechanism of the wire beyond a certain threshold of deformation; no change in the hysteresis occurs before the threshold is reached. When the wall motion starts at a point around the deformation, and nucleation mechanism occurring at the deformed part and at the ends is taken into account, the same staircase character in the hysteresis loops is obtained as the experimental ones. The ends and the deformed part of the wire act as pinning sites. [ABSTRACT FROM AUTHOR]