Experimental specific heat of iron, cobalt, and nickel clusters studied ina molecular beam

Specific heat measurements on free iron, cobalt and nickel clusters in diff erent size ranges from 130 to 400 atoms are presented and the experimental method is discussed in detail. These measurements are achieved in a Stem-Ge rlach experiment where the magnetization of transition metal clusters refle cts their vibrational temperature. Hence, it can be used as a thermometer a fter 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 Ni200-240 clusters is abroad peak centered at T=340 K which adds to an approximately constant ba seline 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 Co200-240 clusters does not show any prom inent feature within the temperature range 80-900 K except a steady increas e from 5.5 cal/(mol K) at T=300 K to 15 cal/(mol K) at T=900 K. In iron clu sters, the specific heat exhibits a peak which is poorly described by a Wei ss mean field theory. Furthermore, the specific heat value:of Fe250-290 clu sters at room temperature is up to 50% lower than the Dulong-Petit value. W e discuss the possibility that iron clusters undergo a-magnetic transition between a high moment to a low moment state, which have different lattice p arameters.