D. Venus et al., Domains in perpendicularly magnetized ultrathin films studied using the magnetic susceptibility, PHYS REV B, 60(13), 1999, pp. 9607-9615
Measurements of the complex, low-frequency ac magnetic susceptibility X-Z o
f Fe/2 ML Ni/W(110) films, using a small field applied normal to the film s
urface, were used to characterize magnetic domain formation and motion in a
perpendicularly magnetized ultrathin-film system. Analysis of the real par
t of X shows that a broad peak in the susceptibility roughly divides low- a
nd high-temperature regimes, where the domains are pinned and move freely,
respectively. At high temperature, the domain density increases exponential
ly with temperature, producing an exponential decrease of the susceptibilit
y with decay constant K approximate to 0.05 K-1, consistent with theoretica
l expectations. At low temperature, domain-wall motion is thermally activat
ed, with the activation energy increasing from 2X10(3) K for 3.0 ML Fe to 9
X10(3) K for 1.5 ML Fe. The systematic variation of the activation energy i
ndicates an increasing sensitivity to monolayer steps for thinner films, an
d yields an average separation of pinning sites of 200+/-30 nearest-neighbo
r distances. This is consistent with a 0.2 degrees-0.3 degrees miscut of th
e substrate crystal. Films with Fe thickness less than or equal to 2 ML exh
ibit an exponential decrease in X-Z up to, and through, the transition to p
aramagnetism with no marker for the Curie temperature. Films with Fe thickn
ess >2 ML undergo a reorientation of the magnetization from perpendicular t
o in plane as the temperature is increased. However, the reorientation prod
uces no peak in the susceptibility, giving evidence that it is a discontinu
ous transition for these films.