Search

The basic mechanism of Bloch wall motion associated with very-fast-rise-time hard-axis pulses appears to be the same for wall streaming or worm motion, creep, and a new uniform motion. Creep under these conditions as distinguished from creep excited by relatively low-frequency sinusoidal excitation must involve important gyromagnetic effects and exhibits the same threshold behavior as wall streaming and the new uniform motion. Creep may be associated with either an applied easy-axis bias or with an equivalent stray-field bias. The uniform motion is more easily produced in thicker films (1500 Å) with very small or no bias and consists of a "parade" of tip domains, first nucleated at the film edge and then moving in the hard-axis direction until annihilated at the edge. The threshold field for wall motion in all of these processes depends on the rate of change of the pulse field, increasing with increasing rise time. The threshold field decreases with easy-axis bias and is related to a characteristic coercive force. The movement per pulse increases with increasing magnitude of drive field to a maximum and then decreases. Several mechanisms are suggested for this behavior.

Dynamic and nearly static magnetization reversal mechanisms in thin permalloy films are investigated experimentally using pulse techniques and vector locus configurations. At least for the driving field strength used, easy-axis switching waveforms indicate that wall motion is predominant when no transverse field is applied. At a given transverse bias field, the simultaneous pick-up signals from aligned and crossed loops show that the voltage-time integral at zero crossing time of the transverse signal becomes dominant for increasing drive field. The complicated irreversible magnetization phenomena on the astroid are illustrated experimentally on the coordinate system by the vector locus for a 10 kc/s sinusoidal driving field and pulse field having 0.5 ns rise-time. Wall motion and rotation during flux reversal are clearly distinguished on these configurations for various combinations of externally applied fields. The critical angle for coherent rotation is in good agreement with that derived from the Stoner-Wohlfarth model at a 10 kc/s sinusoidal field. However, for excess driving pulse fields, the dynamic vector locus suggests that until the walls nucleate and start to move, the coherent rotation continues over the critical angle suggested by the astroid. This gives a clear answer as to the cause of the nonlinearity on the plots of the inverse reversal time vs. driving field with the transverse bias field as a parameter.

A very high resolution Bitter pattern technique is used to study low-frequency creep in Bloch-wall Permalloy films. The basic wall motion consists of a sudden displacement when the hard-axis exciting field passes through two critical transition values. A net displacement per half-cycle or slow pulse occurs in the presence of an easy-axis bias. The motion appears to be gyro-magnetically induced by the precession of the magnetization in the wall about a demagnetizing field perpendicular to the wall plane. The demagnetizing field may be produced by an instability in the relation between the hard-axis field and the magnetization in the domains on sides of the wall rather than in the wall.

When a laser beam is focused to a spot on a MnBi single crystal or a polycrystalline MnBi film, a domain wall is nucleated which propagates from the heated spot by various mechanisms dependent on the direction of magnetization, the process of the crystal growth, and the effect of the adjacent crystal. The mechanism of the propagation can be interpreted by strip and bubble-domain theory. It is expected that the crystallographic and magnetic properties of films or crystals can be microscopically investigated by this heating method. The condition for stable writing according to bubble theory indicate that a stable spot can be written in a homogeneous and oriented MnBi film of thickness between 200 and 500 Å. This condition was experimentally confirmed.

Domain-wall velocity was measured for vacuum-deposited permalloy films in the thickness range of from 310 to 3000 Å. The dependence of the velocity on drive field amplitude was non-linear for all of the forty-two films. The ν-H characteristics were clearly separated into two regions (low and high field), except for the thick films ( > 2500 Å). The high field regions were fairly linear, and the interpolated lines intersected theHaxis at much lower fields than the coercive force and often at the origin. The wall mobilities that were determined from the slope of the high field regions were found to be about one third of the mobilities reported by other authors. These were compared with the theoretical value and found to be in fair agreement. As one of the characteristics of wall motion under fields lower than the coercive force, the mean-free-path of a wall was measured as a function of the easy axis field.