Specific heat measurements on free iron, cobalt and nickel clusters in different size ranges from 130 to 400 atoms are presented and the experimental method is discussed in detail. These measurements are achieved in a Stern-Gerlach experiment where the magnetization of transition metal clusters reflects their vibrational temperature. Hence, it can be used as a thermometer after a calibration procedure. The specific heat is measured by heating the clusters in flight with a laser. In the temperature range of the experiment (80–600 K), the main feature of the specific heat of ${\mathrm{Ni}}_{200-240}$ clusters is a broad peak centered at $T=340\mathrm{}$ K which adds to an approximately constant baseline of 6 cal/(mol K). We attribute this peak to a ferro- to paramagnetic transition since its shape and area are well described by the Weiss mean field model. The specific heat of ${\mathrm{Co}}_{200-240}$ clusters does not show any prominent feature within the temperature range 80–900 K except a steady increase from 5.5 cal/(mol K) at $T=300\mathrm{}$ K to 15 cal/(mol K) at $T=900\mathrm{}$ K. In iron clusters, the specific heat exhibits a peak which is poorly described by a Weiss mean field theory. Furthermore, the specific heat value of ${\mathrm{Fe}}_{250-290}$ clusters at room temperature is up to 50% lower than the Dulong-Petit value. We discuss the possibility that iron clusters undergo a magnetic transition between a high moment to a low moment state, which have different lattice parameters.

• Received 3 May 1999

DOI:https://doi.org/10.1103/PhysRevB.62.7491