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We study the current-voltage (I-V) characteristics in a quasi-two-dimensional, -d coupled organic conductor -(BEDT-TSF) 2 FeCl 4 which undergoes a metal-to-insulator transition associated with an antiferromagnetic ordering at T MI ϭ8.3 K. Drastic nonlinear transport phenomena are observed in the I-V curve, revealing a so-called negative resistance effect at an applied electric field or a current density higher than threshold E T or J T . The overall I-V curves are quite well explained phenomenologically by a nonlinear equation for a positive feedback system that the nonlinear conductivity proportional to J n with nϭ1.4(Ϯ0.1). This indicates that some carrier decondensation mechanism works in the present antiferromagnetic insulator. Discussions are presented by following quite similar nonlinear phenomena observed in a variety of quasi-one-dimensional, mixed-stack, and segregated-stack, organic insulators.

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