Magnetic coupling in boron-rich NeFeB nanocomposites

In this article, we report types of magnetic domain patterns caused by eith er dipolar coupling or exchange coupling inside a heat-treated boron-rich N dFeB melt-spun ribbon. The average grain size of the sample was about 38 nm . Single particle domains were commonly observed in Fresnel images all thro ugh the specimen, suggesting little exchange coupling among the grains in t hese regions. Snake-shaped long interactive domains consisting of chains of grains were found in many Foucault images at the regions where exchange co upling is weak. These domains are caused by dipolar coupling along the chai ns. Wider domains consisting of many grains were also observed by Fresnel i mages, although they were much less common than the single particle domains . The grains inside a wider domain are believed to be exchange coupled with each other. The magnetic properties of such boron-rich NdFeB composites ar e influenced more by the dipolar coupling than by exchange coupling between the nanocrystalline grains. (C) 1999 American Institute of Physics. [S0021 -8979(99)47608-X].