Pressure Effect of Organic Ferromagnetic Radical Crystal p-Cl-C₆H₄CH=N-TEMPO

4-(p-chlorobenzylideneamino)-2,2,6,6-tetramethylpiperidin-1-yloxyl) (p-Cl-C₆H₄CH=N-TEMPO) radical crystal is the organic bulk-ferromagnet with T_c(P₀) = 0.28 K (P₀; ambient pressure). In realizing the ferromagnetic intermolecular interaction, the spin polarization on the hydrogen of the methyl moiety is considered to play an important role. We expected that the pressurization would change the environment around the methyl moiety, and influence the bulk-ferromagnetic transition. So we measured the ac magnetic susceptibility under the pressure up to 10.9 kbar. With increasing pressure, the periodical down-and-up of T_c(P) and the pressure-induced ferro- to antiferromagnetic transition were observed. These experimental results may suggest that the pressurization destroys the bulk-ferromagnetism of P-Cl-C₆H₄CH=N-TEMPO as in β-phase p-NPNN, making the methyl moiety rotate.