Pressure effects on an organic radical ferromagnet: 2,5-difluorophenyl-α-nitronyl nitroxide

Raising a transition temperature (formula presented) in organic radical ferromagnets is a desire for material scientists. We investigated the pressure effects on an organic radical ferromagnet 2,5-difluorophenyl-α-nitronyl nitroxide (2,5-DFPNN), which has a ferromagnetic transition at 0.45 K. The hydrostatic pressure effects were investigated through measurements of ac magnetic susceptibility (χ) up to (formula presented) heat capacity (formula presented) up to (formula presented) and powder x-ray diffraction up to (formula presented) Furthermore, ac magnetic susceptibility under nonhydrostatic pressure was also measured in the pressure region up to 10.0 GPa. As for 2,5-DFPNN, we observed the pressure-induced enhancement of (formula presented) as (formula presented) (formula presented) while other prototypes, the β phase of p-NPNN and (formula presented) show the negative pressure effects. The results for the (formula presented) and the crystal structural analysis suggest that the magnetic dimension of the short-range order developing above (formula presented) transforms from one dimension (a axis) to two dimensions (ac plane) under high pressure. This increase of the magnetic dimension probably promotes to increase (formula presented) The ferromagnetic signal of χ, however, decreases with increasing pressure, and finally disappears for (formula presented) The decrease seems to originate from the decrease of the ferromagnetic interaction along the b axis. Similar instability of organic ferromagnetic long range order against pressure has been observed for the β phase of p-NPNN and (formula presented)