Weak ferromagnetism, metal-to-nonmetal transition, and negative differential resistivity in single-crystal Sr 2 IrO 4

${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ has the ${\mathrm{K}}_{2}{\mathrm{NiF}}_{4}$ structure and its ${\mathrm{Ir}}^{4+}$ ion ${(5d}^{5})$ is in a low spin configuration ${t}_{2g}^{5}$ with $S$=$\frac{1}{2}$. In this paper, we report results of our study on single-crystal ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}.$ 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 ${\ensuremath{\mu}}_{\mathrm{eff}}$ $(=0.5{\ensuremath{\mu}}_{B})$ and the saturation moment ${\ensuremath{\mu}}_{S}(=0.14{\ensuremath{\mu}}_{B})$ are found to be quite small, but the ratio of ${\ensuremath{\mu}}_{\mathrm{eff}}/{\ensuremath{\mu}}_{S}$ (=3.5) qualitatively fits the Rhodes-Wohlfarth plot. Resistivity, $\ensuremath{\rho}(T),$ along two principal crystallographic directions is strongly anisotropic and shows a metallic behavior below 120 K. ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ exhibits strikingly nonlinear conductivity, i.e., a current-controlled negative differential resistivity for $2<~T<300$ K. The origin of the nonohmic behavior may be associated with charge-density-wave depinning. Unlike other systems such as ruthenates ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ appears to show no correlation between conductivity and magnetism.