  The effect of magnetic anisotropy-axis alignment on the superparamagnetic
(SPM) and superspin glass (SSG) states in a frozen ferrofluid has been
investigated. The ferrofluid studied here consists of maghemite nanoparticles
(\gamma-Fe2O3, mean diameter = 8.6 nm) dispersed in glycerine at a volume
fraction of ~15%. In the high temperature SPM state, the magnetization of
aligned ferrofluid increased by a factor varying between 2 and 4 with respect
to that in the randomly oriented state. The negative interaction energy
obtained from the Curie-Weiss fit to the high temperature susceptibility in the
SPM states as well as the SSG phase onset temperature determined from the
linear magnetization curves were found to be rather insensitive to the
anisotropy axis alignment. The low temperature aging behaviour, explored via
"zero-field cooled magnetization" (ZFCM) relaxation measurements, however, show
distinct difference in the aging dynamics in the anisotropy-axis aligned and
randomly oriented SSG states.
