A nanoscale boride, α-FeB,
with grains of variable size and
crystallinity was synthesized by precipitation from solution followed
by heat treatment (450 °C, 550 °C, 750 °C, 1050 °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 α-FeB, with a particle size of
approximately 20 nm, display open magnetic hysteresis loops and blocking.
In contrast to the soft ferromagnetism of bulk β-FeB, which
was synthesized by conventional solid state reaction at 1500 °C,
the sample of α-FeB annealed at 1050 °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 α-FeB and from α-FeB to β-FeB.