Size and Crystallinity Dependence of Magnetism in Nanoscale Iron Boride, α-FeB

A nanoscale boride, alpha-FeB, with grains of variable size and crystallinity was synthesized by precipitation from solution followed by heat treatment (450 degrees C, 550 degrees C, 750 degrees C, 1050 degrees C). Analysis of transmission electron micrographs, electron diffraction, and magnetic measurements suggests superparamagnetism at room temperature for the smaller, more disordered particles of FeB, while the larger, more crystalline particles of alpha-FeB, with a particle size of approximately 20 nm, display open :magnetic hysteresis loops and blocking. In contrast to the soft ferromagnetism of bulk beta-FeB, which was synthesized by conventional solid state reaction at 1500 degrees C, the sample of alpha-FeB annealed at 1050 degrees C is a harder ferromagnet, possibly due to stacking faults that pin the magnetic domains; these stacking faults are apparent in the high resolution transmission electron micrographs. The changes in magnetic behavior are visible from the varying blocking temperatures (63 K, 94 K, 150 K, and >320 K from the smallest to the largest particles) and correlate with the transformations from amorphous to alpha-FeB and from alpha-FeB to beta-FeB.