Low temperature specific heat and magnetic properties of the pure organic ferromagnet Dupeyredioxyl

Abstract

The bulk magnetic properties of the genuine pure organic ferromagnet Dupeyredioxyl ( N, N′-dioxy-1,3,5,7-tetramethyl-2,6-diazaadamantane, C 12N 2O 2H 20) are presented. Magnetization below T c=1.48 K presents no measurable hysteresis, and the anisotropy seems to be due only to crystal shape demagnetization effects. The specific heat presents a nice lambda peak at 1.44 K, and a T 3/2 power law below 1 K that is expected for spin wave excitations in the low T limit. The entropy change due to the phase transition does not give the expected value for two independent spins: 1/2 per molecule. The high-temperature susceptibility presents a Curie–Weiss temperature of 7.0 K. Magnetization curves up to 9 T above T c do not follow a universal behavior with H/ T below 50 K. The results are consistent with a model where intramolecular interactions between pairs of the ‘free spins’ form a triplet state well above T c and these S=1 molecules finally order into a true three-dimensional ferromagnetic transition due to intermolecular interactions below 1.5 K.