Magnetic ordering in a high-spin Fe19 molecular nanomagnet -: art. no. 064408

We report evidence of antiferromagnetic (AF) transition in Fe(19)metheidi, a molecular nanomagnet with a total spin S=33/2. The temperature (T) dependence of the specific heat (C) shows a lambda anomaly at 1.19 K and at the same temperature an anomaly is also observed in the low-field (B<0.12 T) magnetization M-vs-T curves. Since the dipolar interaction between clusters is estimated to be similar to190 mK, the origin of the AF transition is probably due to superexchange. Analysis of the specific heat at very low temperature reveals an exponential T scaling of C, which we ascribe to the presence of an energy gap in the spin-wave spectra and, below 150 mK, a hyperfine C/Rsimilar to4.2x10(-5)T(-2) term mainly due to the protons closest to iron.