Pressure effects of a genuine organic crystalline ferromagnet dupeyredioxyl

Abstract

We have revealed that the isothermal magnetization M of the genuine organic crystalline dupeyredioxyl (N,N′-dioxy-1,3,5,7-tetramethyl-2,6-diazaadamantane; Tc(0)=1.48 K) observed below 10 K converges on the S=1 Brillouin function B1((H+λM)/kBT) with λ=2.4±0.2 or 2zJ/kB=3.6±0.3 K, where J and z are, respectively, the averaged exchange interactions and coordination numbers for the S=1 spin system. This fact suggests that S=1 is constructed within a molecule via a strong ferromagnetic coupling between two S=1/2 spins on each of the two NO moieties. The modified notation of the Rushbrooke and Wood theory, Tc=2AzJS(S+1)/kB (A=0.23±0.02 for the three-dimensional Heisenberg systems), is found to quantitatively hold not only for this S=1 spin system but also for other S=1/2 ferromagnets β-phase p-NPNN (2zJ/kB=3.6 K) and 2,5-DFPNN (2zJ/kB=2.8 K). Pressure effects of this compound have been studied under the hydrostatic pressure (P) up to 15 kbar. Tc(P) is revealed to show a negative pressure effect with the initial gradient a=d(Tc(P))/dP=−0.047 kbar−1, nearly the same value for other organic ferromagnets as β-phase p-NPNN (−0.048 kbar−1) and p-Cl-TEMPO radical (−0.03 kbar−1), in contrast to the positive pressure effect for genuine antiferromagnets such as TANOL (a=+0.15 kbar−1). Microscopically, different from the above two ferromagnets, the pressure-induced destruction of the orthogonality of molecular orbitals associated with the two NO moieties plays an effective role in reducing the intramolecular ferromagnetic interaction J0. The possible weaker intermolecular interactions other than J0 and J are also expected to be more susceptible to the stress of pressure to result in the reduction of their values perhaps even changing their sign, just as in the case of β-phase NPNN or p-Cl-TEMPO.