Magnetic ordering in a high-spin Fe19 magnetic nanomagnet

We report evidence of antiferromagnetic (AF) transition in Fe19metheidi, a molecular nanomagnet with a total spin S = 33/2. The temperature (T) dependence of the specific heat (C) shows a λ 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 ∼190 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/R ∼4.2×10-5T-2 term mainly due to the protons closest to iron.