Pressure-dependent enhanced Tc and magnetic behavior of the metamagnetic and ferromagnetic polymorphs of [Fe~(III)Cp _2]?~+[TCNQ]?~- (Cp* = Pentamethylcyclopentadienide; TCNQ = 7,7,8,8-tetracyano-p-quinodimethane)

摘要

The magnetic behaviors of the metamagnetic and ferromagnetic polymorphs of [Fe~(III)Cp_2]?~+[TCNQ]?~- (Cp* = pentamethylcyclopentadienide; TCNQ = 7,7,8,8-tetracyano-p- quinodimethane) were studied as a function of hydrostatic pressure. Both polymorphs exhibit a reversible enhancement of magnetic properties with increasing pressure. The T_c for the ferromagnetic polymorph increased by 70% from 2.95 to 5.01 K at 10.3 kbar at a rate of 0.21 K/kbar, which is similar to the 0.22 K/kbar reported for [FeCp_2]~+[TCNE] ~-. The coercive field and remnant magnetization exhibit exponential-like growth upon application of external pressure, increasing from zero at ambient pressure to 550 Oe and 8880 emu·Oe/mol at 10.3 kbar, respectively. The T_c for the metamagnetic polymorph was determined to be 2.10 K from the maximum in the Fisher specific heat data, that is, d(χT)/dT, and it increases by 38% to 2.90 K at 2.9 kbar at a rate of 0.28 K/kbar, before vanishing, in accord with a transition to a paramagnetic state. The metamagnetic critical field, Hc, determined from dM/dH increases linearly from 1300 Oe at ambient pressure to 1800 Oe at 2.9 kbar, but is not evident at and above 3.9 kbar, also in accord with a transition to a paramagnetic state.