  In an effort to understand the impact of nanostructuring on the
magnetocaloric effect, we have grown and studied gadolinium in MgO/W(50
$\textrm{\AA}$)/[Gd(400 $\textrm{\AA}$)/W(50 $\textrm{\AA}$)]$_8$
heterostructures. The entropy change associated with the second order magnetic
phase transition was determined from the isothermal magnetization for numerous
temperatures and the appropriate Maxwell relation. The entropy change peaks at
a temperature of 284 K with a value of approximately 3.4 J/kg-K for a 0-30 kOe
field change; the full width at half max of the entropy change peak is about 70
K, which is significantly wider than that of bulk Gd under similar conditions.
The relative cooling power of this nanoscale system is about 240 J/kg, somewhat
lower than that of bulk Gd (410 J/kg). An iterative Kovel-Fisher method was
used to determine the critical exponents governing the phase transition to be
$\beta=0.51$, and $\gamma=1.75$. Along with a suppressed Curie temperature
relative to the bulk, the fact that the convergent value of $\gamma$ is that
predicted by the 2-D Ising model may suggest that finite size effects play an
important role in this system. Together, these observations suggest that
nanostructuring may be a promising route to tailoring the magnetocaloric
response of materials.
