Pressure dependence of magnetic ordering temperature for decamethylferrocenium tetracyanoethanide

Z. J. Huang, Feng Chen, Y. T. Ren, Y. Y. Xue, C. W. Chu, J. S. Miller
1993 Journal of Applied Physics  
It has been demonstrated that the linear-chain charge-transfer salt, decamethylferrocenium tetracyanoethanide (DMeFc) (TCNE), is a ferromagnet with a transition temperature of -4.8 IS. This low-temperature 3D ordering has been attributed to a strong intrachain and a weak interchain interaction. To study these interactions, we have determined the T, up to 20 kbar by measuring the ac susceptibility ,JJ at low frequency. Our results show that the T, increases with pressure at a rate of -0.22
more » ... rate of -0.22 K/kbar, while the x peak indicative of the ferromagnetic transition continues to decrease rapidly. A small peak was also detected above the main t.ransition at pressures above 3 kbar. This new peak persists even after the pressure is removed. The result from dc magnetization suggests that this corresponds to a metamagnetic state. For the first time, we have observed pressure-induced phase-transition in this material. 1.lNTI?ODUCTlON Studies of molecular and polymeric ferromagnetism are important to solid-state physics. Decamethylferrocenium tetracyanoethanide (DMeFc) (TCNE) is the first known molecular ferromagnet.' The ( DMeFc) (TCNE) consists of stiacks of alternate donors (DMeFc) * i and acceptors (TCNE) * -, each with spin S=&."*3 dc susceptibility, magnetizat.ion, neutron diffraction, and specificheat studies's"A5 show that the system becomes a 3D ferromagnet below the transition temperature T,--4.8 K. At higher temperatures ( 17-300 K) the susceptibility can be fit well by a 1D Heisenberg model6 with S=i and ferromagnetic coupling, J/kB-27 K.' Specific-heat measurements show that -4% of the entropy is involved in the 3D ordering of the spins and most of the entropy is consumed during the 1D ferromagnetic ordering at much higher temperatures." This suggests that strong intrachain coupling, with weak interchain coupling, is responsible for the observed ferromagnetic phase transition. The mechanisms that govern the ferromagnetic coupling m this class of linear-chain systems are not firmly established. However, the admi?iing of a virtual triplet excited state with the ground model, originally proposed by McConnell,7 offers an attractive explanation for ferromagnetic coupling in such a linear-chain system.s Although the ferromagnetic interaction in the chains (intrachain interaction) can be obtained qualitatively within the frame of such a model> the interchain interaction, which is very crucial for the 3D phase transition, is far more complicated. For example, the disproportionality of adjacent inregistry (DMeFc) * +'s to form S= 1 (DMeFc)'+ and S=O (DMeFc)e can lead to a ferromagnetic exchange interaction, whereas the disproportionality of adjacent inregistry (TCNE) --'s to form S=O (TCNE)'-and S=O (TCNE)" will lead an antiferromagnetic exchange interactions The competition between the ferro-and antiferro-magnetic exchange interactions determines the ground state of a specific compound. The competition might be drastically changed under high pressure because (i) organic compounds are usually rather compressible and the magnetic interaction would depend on the distance between neighboring spins; and (ii) the charge transfer between donors and acceptors depends roughly on the Madelung energy at their positions.' Therefore, high pressure is useful in the exploration of the magnetic interaction in organic compounds. We report ac susceptibility studies under high pressure at 1.2-40 K and dc magnetization measurements in (DMeFc) (TCNE). II. EXPERIMENT The preparation of polycrystalline samples of (DMe-Fc) (TCNE) is described in Ref. 2. The hydrostatic pressure environment was provided by a Be-Cu high-pressure clamp with a Teflon cell using 3M fluorinert liquid as the pressure medium. to The pressur e w as determined by a superconducting Pb-manometer. The real part of ac susceptibility was measured with an ac mutual-inductance bridge operating at 16 Hz and a constant excitation current ranging from 0.1 to 10 mA. The primary coil ( 1000 turns) was built in the pressure clamp. The secondary coils were wound on a quartz tube with 2 mm i.d., 3 mm o.d., and 9 mm long. Each of the two coils was 450 turns and about 3 mm long. A -2 mg sample of the material was put into one of the secondary coils. Then the secondary coils were placed inside the Teflon cell. The temperature was measured with a Ge thermometer in the range of 1.2-40 K. dc magnetization at different fields and temperatures was carried out in a Quantum Design superconducting quantum interference device Magnetometer. 6563
doi:10.1063/1.352564 fatcat:rludkp3zhncdzgguyku33vu4ze