Asymmetric hysteresis loops in amorphous ferromagnetic alloys

Kyung-Ho Shin, University of Pennsylvania


Amorphous ferromagnetic alloys annealed in a small magnetic field develop a reproducible, asymmetric hysteresis loop. If the direction of the field during annealing is regarded as +, the magnetization reversal from + to $-$ occurs in a single irreversible Barkhausen jump, but the reversal from $-$ to + is smooth and reversible, with its slope determined by the demagnetizing field of the sample. We call this phenomenon the asymmetric magnetization reversal (AMR). The asymmetric loop develops easily and readily in amorphous alloys near the zero magnetostriction composition (Co$\sb{0.94}$Fe$\sb{0.06}$)$\sb{\rm x}$(SiB)$\sb{\rm 1-x}$, but is not limited to these compositions. AMR can be produced in amorphous alloys by various combinations of time, temperature, and magnetic field applied during the anneal. However, the shape of the hysteresis loop depends sensitively on the conditions during the anneal. Oxidizing conditions are not necessary during the anneal. However, they make it easier to produce AMR and act to stabilize the effect. A typical AMR having a single Barkhausen jump can be produced by annealing a zero magnetostrictive amorphous ribbon for several hours at a temperature of 350 to 400$\sp\circ$C in air, in an applied magnetic field of about 100 mOe directed parallel to the ribbon axis. AMR can be destroyed partially or totally by light chemical or physical etching of the surface, which means that the origin of AMR resides in the surface of the sample. AMR can also be destroyed partially or totally by magnetizing the sample in a sufficiently high ac or dc magnetic field. This strongly suggests that the annealing treatments create a metastable domain structure, which can be destroyed by the application of a sufficiently high field. We propose the annealed-in anisotropy in domain walls combined with exchange anistropy between a surface layer and the bulk ribbon as the origin of AMR. The anisotropy in the domain walls is induced by the local magnetization during annealing, and the exchange anisotropy results from direct exchange coupling between a higher anisotropy surface layer (formed just below the outer oxide layer) and the bulk material.

Subject Area

Materials science

Recommended Citation

Shin, Kyung-Ho, "Asymmetric hysteresis loops in amorphous ferromagnetic alloys" (1992). Dissertations available from ProQuest. AAI9235205.