  We have measured the thermal expansion of Ni nanowires electrodeposited into
self-organized nanoporous amorphous aluminum oxide (AAO) membranes without Al
substrate using X-ray diffraction between 110K and 350K. The results indicate
an average thermal expansion of the Ni nanowires -- along the wire axis -- of
$\langle\alpha\rangle_{\rm NiNW}=-(1.6 \pm 1.5)\times 10^{-6}$/K. Assuming a
bulk-like thermal expansion of the isolated Ni nanowires, this result indicates
that AAO has also a negative thermal expansion. We estimate the thermal
expansion of nanoporous AAO to be $\langle\alpha\rangle_{\rm AAO}=-(5 \pm 1)
\times 10^{-6}$/K. We show that data obtained previously on the thermal
expansion of metallic nanowires grown in the nanoporous AAO may be interpreted
as originated in a negative thermal expansion of the matrix.
