Magnetic domain observations in a FeCo-based nanocrystalline alloy by Lorentz microscopy

Lorentz microscopy was used to investigate the domain structure in an Fe0.5Co0.5 80Nb4B13Ge2Cu1 alloy annealed at 500, 550, and 610 °C, respectively, for 1 h. The average grain size d, observed by conventional transmission electron microscopy, is 10 nm for the alloy annealed at 500 °C, 18 nm for 550 °C, and 90 nm for 610 °C. The domain structure of sample annealed at 500 °C exhibits few pinning sites and large domains, which correlate with the low coercivity and good soft magnetic properties. More pinning sites and smaller domains are observed in the sample annealed at 550 °C. In the sample annealed at 610 °C, the domain walls are observed to be localized at the grain boundaries and the domains include several grains. It therefore appears that the grain boundaries act as effective pinning sites in this sample. As expected, domain walls in the 610 °C sample are less mobile in small applied fields than in the 500 and 550 °C samples. This result is consistent with coercivity measurement and the random anisotropy model commonly used to rationalize the coercivity dependence on average grain size in these alloys. © 2007 American Institute of Physics. DOI: 10.1063/1.2714309