Simulation of Bloch Wall Motion
Experimenta1 and computer simulation results for Bloch wall motion are compared. While good agreement is obtained for wall displacement under low drive fields, and for the critical fields for thin samples and/or long drive pulses, discrepancies arise for short pulses and thicker samples. To investigate the latter, simulations were performed for a two-wall configuration, using Slonczewski's equations of wall motion. The results, which reflected the effect of the demagnetizing field, were better than for single-wall simulations, but discrepancies still remain for short pulses and high drive fields; this may be partly due to the assumption of a constant wall width.
Theory of domain-wall motion in magnetic films and platelets
We calculate dynamical properties of plane and cylindrical magnetic domain walls in a uniaxial film or platelet whose plane is perpendicular to the easy axis. First, we calculate the internal structure of a freely moving wall to determine the nonlinear velocity‐momentum relation. Using a Bloch‐line approximation to the kinetic wall energy, we find that stray magnetic fields emanating from surface poles destabilize the structure at a critical velocity Vp, above which uniform motion is not possible. For a plane wall, we have Vp=24γA/hK1/2, where γ is the gyromagnetic ratio, A is the exchange stiffness, h is the film thickness, and K is the anisotropy. This velocity is usually much less than the critical velocity for bulk wall motion derived by Walker. Combination of the velocity‐momentum relation with the conservation of momentum provides a simple method of calculating the time‐dependent velocity in the presence of a small drive field and small viscous damping. For very small drive, the terminal velocity eq...
rPET detectors design and data processing
Small animal PET systems based on rotating planar detectors posses some interesting advantages for high sensitivity, high resolution imaging. We have designed the rPET detectors based on MLS crystals assembled on a 30times30 matrix optically coupled to a flat-panel PS-PMT. Weighted position readout circuits pre-process the 64 signals from the 8times8 anodes matrix, which are digitized using a charge-integrating converter. The amplification electronics, including the trigger output for coincidence detection, and the high voltage supply are integrated in a three PCBs stack that forms the base attached to the back of the PMT. The whole assembly is enclosed in a light tight, lead (Pb) shielded aluminum box. The detectors are mounted on a rotating gantry with more than 180 degrees rotation span. The digitized events are screened and histogramed, and a modified center of gravity algorithm removes from the position calculation those signals with poor signal to noise ratio. Apparent mean crystal size on the 511 keV field-flood images is 0.6 mm, mean peak-to-valley ratio is better than 8, and intrinsic resolution is 1.5 mm at the central row, with the energy window wide open. Sensitivity (CPS) for a pair of these detectors set in coincidence at 160 mm distance is 1%