Magnetic force microscopy of avalanche dynamics in magnetic media

The slow dynamics of the micromagnetic structure in a CoPtCr thin film were imaged in an applied field with a magnetic force microscope (MFM). Images taken over time scales ranging from minutes to weeks revealed slow micromagnetic avalanches along the direction of the applied field. The avalanche sizes followed a power law distribution in a manner consistent with Bak, Tang, and Weisenfeld's concept [Phys. Rev. Lett. 59, 381 (1987); Phys. Rev. A 38, 364 (1988)] of "self- organized criticality.'' The avalanches appeared to be the result of both random (thermal) excitations and micromagnetic perturbations from the tip of the MFM. We extended the sandpile model of Bak, Tang, and Weisenfeld to include a periodically rastering drop site (analogous to raster scanning a MFM tip over a sample). Although the sequence of avalanche events in the sandpile model depended on the manner in which the grains were dropped, the underlying distribution of avalanche sizes remained the same for both random and periodic grain-dropping methods. This suggests that the distribution of the magnetic avalanches reflects an underlying structure, independent of how the switching is initiated. (C) 1998 American Institute of Physics. [S0021-8979(98)07920-1].