Ultrashort magnetic field pulses and the elementary process of magnetization reversal

Ultrashort magnetic field pulses can be generated in the final focus test beam facility at the Stanford Linear Accelerator. The 50 GeV electron bunches carrying currents up to 1000 A are squeezed through a focus smaller than 1 μm². The resulting magnetic field lines are equivalent to those of a straight current carrying wire with amplitudes up to 20 T and duration of 2–5 ps. These unique magnetic field pulses have been used to study magnetization reversal in thin films with in-plane and perpendicular easy magnetization directions. For perpendicular magnetized samples we observe ring domains with Kerr microscopy, which are reminiscent of the field contour during exposure. Their radii represent switching fields in quantitative agreement with the coherent rotation model. The broadening of the transition between oppositely magnetized domains can be attributed to the existence of static and dynamic fluctuations of the magnetic anisotropy. For films with an uniaxial anisotropy in the plane of the film we observe that smaller fields are sufficient to reverse the magnetization, provided the field is orthogonal to the magnetization. We show that it is the demagnetizing field that completes the reversal after the external field ceases to exist.