  A precision radio-frequency (RF) transverse modulation technique based on a
resonant tunnel diode oscillator (TDO) has been used to study the dynamic
susceptibility in novel ring-shaped single domain Fe nanoparticles synthesized
using reverse-micelle chemistry. Hysteresis loops measured using a SQUID
magnetometer indicate a large coercivity (Hc = 400Oe) below the blocking
temperature (TB ~52K). The dynamic susceptibility shows remarkable consistency
with the M-H data and exhibits peaks at characteristic anisotropy fields.
Overall, these RF experiments are well suited to probe the magnetic anisotropy
in nanophase materials.
