Spontaneous fluctuations in a ferromagnetic film

A close-up look at the hysteresis loop of a ferromagnet often reveals a series of sudden magnetization jumps. The so-called Barkhausen jumps come from domain-wall fluctuations, whose distributions in size and duration follow a power law and can be analyzed for scaling behavior and critical exponents. But the physics in those critical exponents is a complex mix of disorder, magnetic anisotropy, and dipole–dipole interactions. Andrew Balk and colleagues of NIST’s Center for Nanoscale Science and Technology (Balk is also with the University of Maryland) set out to simplify the problem. They nudged the magnetic domain walls in a platinum-cobalt- platinum multilayer into spontaneously fluctuating, even without an external magnetic field. Roughening the interfaces between the different layers with argon ion milling drove the film close to a transition that reorients the cobalt magnetization from perpendicular to parallel orientation relative to the film plane. That trick reduced the magnetic anisotropy and domain wall pinning energies enough to allow spontaneous thermal fluctuations of the domain walls. The group used Kerr microscopy, which detects the change in polarization when light reflects off a magnetic sample, to make videos of the changing magnetic domain patterns. The figure’s left panel shows one video frame, in which light and dark areas represent oppositely oriented domains; the right panel maps the number of fluctuations summed over a one-minute period—brighter colors indicate more fluctuations. As with field-driven domain-wall motion, the size distribution of the fluctuating areas showed a power-law behavior. (A. L. Balk, M. D. Stiles, J. Unguris, Phys. Rev. B 90, 184404, 2014, doi:10.1103/PhysRevB.90.184404 .)