Critical scaling in a pure organic ferromagnet

Pure organic ferromagnets, i.e. materials which contain no metallic centers, are very rare. We report here results of very low temperature AC susceptibility and DC magnetization measurements made on one of these exotic ferromagnets (formula C 13H 17N 2O 3) based on the spin S= 1 2 free radical NIT (nitronyl nitroxide). A very weak hysteresis has been observed with a coercive field H c=0.15 Oe at 0.1 K. ( T c⋍0.44 mK) The single crystal sample appears to be perfectly isotropic, so that measurements along the three crystal axis allow for a self-consistent way to correct for demagnetization effects. Scaling of the data allows us to estimate values for critical exponents γ=1.35 and β=0.33, and a very large critical region is observed, extending up to nearly 10 times T c.