Pressure effect of organic ferromagnetic radical crystal p-Cl-C6H4CH=N-TEMPO

4-(p-chlorobenzylideneamino)-2,2,6,6-tetramethylpiperidin-1-yloxyl) (p-Cl-C 6H4CH=N-TEMPO) radical crystal is the organic bulk-ferromagnet with T-c(P-0 )=0.28 K (P-0; ambient pressure). In realizing the ferromagnetic intermolec ular interaction, the spin polarization on the hydrogen of the methyl moiet y is considered to play an important role. We expected that the pressurizat ion would change the environment around the methyl moiety, and influence th e bulk-ferromagnetic transition. So we measured the ac magnetic susceptibil ity under the pressure up to 10.9 kbar. With increasing pressure, the perio dical down-and-up of T-c(P) and the pressure-induced ferro- to antiferromag netic transition were observed. These experimental results may suggest that the pressurization destroys the bulk-ferromagnetism of p-Cl-C6H4CH=N-TEMPO as in beta-phase p-NPNN, making the methyl moiety rotate.