WEAK FERROMAGNETISM, METAL-TO-NONMETAL TRANSITION, AND NEGATIVE DIFFERENTIAL RESISTIVITY IN SINGLE-CRYSTAL SR2IRO4

Sr2IrO4 has the K2NiF4 structure and its Ir4+ ion (5d(5)) is in a low spin configuration t(2g)(5) with S=1/2. In this paper, we report resul ts of our study on single-crystal Sr2IrO4. Magnetic susceptibility and isothermal magnetization display weak ferromagnetism below 240 K with an easy axis along the a axis and a spin reorientation transition in low magnetic fields. Both the effective paramagnetic moment mu(eff) (= 0.5 mu(B)) and the saturation moment mu(s)(=0.14 mu(B)) are found to b e quite small, but the ratio of mu(eff)/mu(s) (=3.5) qualitatively fit s the Rhodes-Wohlfarth plot. Resistivity, rho(T), along two principal crystallographic directions is strongly anisotropic and shows a metall ic behavior below 120 K. Sr2IrO4 exhibits strikingly nonlinear conduct ivity, i.e., a current-controlled negative differential resistivity fo r 2 less than or equal to T < 300 K. The origin of the nonohmic behavi or may be associated with charge-density-wave depinning. Unlike other systems such as ruthenates Sr2IrO4 appears to show no correlation betw een conductivity and magnetism.