Pressure Induced Enhancement of Tc in Weak-Ferromagnetic Radical Crystal 1,3,5-Triphenyl-6-Oxoverdazyl

Study of pressure effect of organic weak-ferromagnet 1,3,5-triphenyl-6-oxoverdazyl (TOV) with the transition temperature T_c(P₀) = 5.0 K (P₀; ambient pressure) has been motivated by the experimental finding that slight dilution of TOV with the non-magnetic derivative changed the Weiss temperature (Θ) from negative to positive, i.e. from antiferromagnetic (AF) to ferromagnetic (F). We understood that above change of Θ would originate from the slight change of the crystal structure by an internal stress. Expecting the pressurization would bring about the same result as the dilution effect, we measured the ac magnetic susceptibility of pure TOV under the pressure up to P = 10.9 kbar. The pressure effect of pure TOV, however, revealed no transition from AF to F, but did the enhancement of T_C(P). The pressure dependence of T_C(P) is characterized with T_c(P) = T_c(P₀) (1 + 0.086 P), and T_c(P) exceeds 10.0 K at P = 10.9 kbar.