Anomalous magnetic and transport behavior in the magnetic insulator ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$

We report magnetic and transport properties of single-crystal ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7,$ a bilayered orthorhombic system with Bbca symmetry. Our study reveals that ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ is a ferromagnetic insulator with a Curie temperature of $T_C=285\mathrm{}\mathrm{K}$ and a small ordered magnetic moment. The saturation moment ${\mu }_S$ is found to be less than $0.04{\mu }_B/\mathrm{Ir}$ or 4% of the expected moment for a localized spin $S=\frac{1}{2}.$ For fields larger than 1 T the magnetic susceptibility is the superposition of a Curie law (with negligible Weiss temperature) and a constant term. Surprisingly, at low temperatures (<20 K) the magnetization is opposite to the applied magnetic field when ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ is cooled in a modest magnetic field (⩽0.2 T). The resistivity $\rho \mathrm{}\left(T\right)\mathrm{}$ measured from 1.7 to 1000 K along two principal crystallographic directions exhibits insulating behavior with anomalies corresponding to the magnetic properties. Both the magnetization and the resistivity are strongly anisotropic as a consequence of the layered structure. Possible crystalline field and band scenarios are briefly discussed.