The magnetic behaviors of the metamagnetic and ferromagnetic phases of [Fe(C5Me5)2][TCNQ] (TCNQ = 7,7,8,8-tetracyano-p-quinodimethane). Determination of the phase diagram for the metamagnetic phase

Abstract

The detailed magnetic behaviors of the ferro- (1FO) and metamagnetic (1MM) phases of [FeCp*₂][TCNE] (Cp* = pentamethylcyclopentadienide; TCNE = tetracyanoethylene) aligned parallel to the applied magnetic field, H, were obtained using eicosane (E). The T_c for 1FO is 3.1 K from the maximum in the frequency independent χ′(T) data and 3.0 K from the maximum in the C_p(T) data, and exhibits a hysteresis with a coercive field, H_cr, ∼50 Oe, a remanent magnetization of 1900 emu Oe mol⁻¹ at 2 K, and saturation magnetization of 16 740 emu Oe mol⁻¹. Significant differences were observed between aligned 1MM (1MM + E) and unaligned samples. Metamagnetic 1MM + E saturates to 15 900 emu Oe mol⁻¹, and has a 1300 Oe critical field at 2 K that decreases with increasing temperature. The 2 K M(H) of 1MM + E displays a small bump between |3000 and 4000| Oe that is not observed in 1MM. The T_c for 1MM + E is 2.5 ± 1 K from the maximum in the frequency independent χ′(T) data, and peak maximum in C_p(T) data. The lack of a χ″(T) response for H = 0 is in accord with 1MM + E having an antiferromagnetic ground state. Upon application of an applied field a χ″(T) signal appears and increases in intensity until 1500 Oe in accord with 1MM + E going from an antiferromagnetic to a ferromagnetic-like state. For H > 1500 Oe the χ″(T) signal decreases. In contrast to 1MM, between 2.2 and 2.8 K χ′(H) and χ″(H) exhibit peaks between 3000 and 4000 Oe. The temperature at which the peak maximum in C_p(T) occurs (2.4 K) for 1MM and is unexpectedly independent of H. The peak maxima observed in χ′(T,H) at 100 and 1000 Hz were use to construct a magnetic phase diagram, H(T), for metamagnet 1MM + E, which has three different magnetic phases: paramagnetic, antiferromagnetic, and an intermediate phase. The detailed magnetic characterization of the structurally similar 1MM and 1FO phases will provide a basis for theorists to understand the subtle spin exchange interactions that lead not only to magnetic ordering, but the type of the ordering.