Horizontal Bloch line motion in magnetic bubble materials

[摘要] The purpose of this work is to extend experimental and theoretical understanding of horizontal Bloch line (HBL) motion in magneticbubble materials. The present theory of HBL motion is reviewed, andthen extended to include transient effects in which the internaldomain wall structure changes with time. This is accomplished bynumerically solving the equations of motion for the internal azimuthalangle ɸ and the wall position q as functions of z, the coordinateperpendicular to the thin-film material, and time. Theeffects of HBL's on domain wall motion are investigated by comparingresults from wall oscillation experiments with those from the theory.In these experiments, a bias field pulse is used to make a stepchange in equilibrium position of either bubble or stripe domainwalls, and the wall response is measured by using transient photography. During the initial response, the dynamic wall structureclosely resembles the initial static structure. The wall acceleratesto a relatively high velocity (≈20 m/sec), resulting in a short(≈22 nsec ) section of initial rapid motion. An HBL gradually formsnear one of the film surfaces as a result of local dynamic properties,and moves along the wall surface toward the film center. The presenceof this structure produces low-frequency, triangular-shapedoscillations in which the experimental wall velocity is nearly constant, v_s≈ 5-8 m/sec. If the HBL reaches the opposite surface,i.e., if the average internal angle reaches an integer multiple ofπ, the momentum stored in the HBL is lost, and the wall chiralityis reversed. This results in abrupt transitions to overdamped motionand changes in wall chirality, which are observed as a function ofbias pulse amplitude. The pulse amplitude at which the n^th punch-through occurs just as the wall reaches equilibrium is given within0.2 0e by H_n = (2v_sH'/γ)^1/2 • (nπ)^1/2 + H_sv), where H' is the effective field gradient from the surrounding domains, and H_sv is a small(less than 0.03 0e), effective drag field. Observations of wall oscillationin the presence of in-plane fields parallel to the wall show that HBLformation is suppressed by fields greater than about 40 0e (≈2πM_s),resulting in the high-frequency, sinusoidal oscillations associatedwith a simple internal wall structure.

[发布机构] University:California Institute of Technology;Department:Engineering and Applied Science

[关键词] Applied Physics