Hall effect near the metamagnetic high-field transition in Uranium Mononitride

Compounds containing uranium cover a wide range of electronic behavior. This diversity is based on the similar energy scales of the $5f$-electron bandwidth, $f-f$ Coulomb, spin-orbit, and exchange interaction. In particular, the 5f-electron correlation effects are a topical issue for uranium compounds. Uranium mononitride (UN) is one example of an intensively studied compound with a rather simple crystal structure (face centered cubic), but not fully understood electronic and magnetic structure. There are indications for a dual nature of the uranium $5f$ electrons with both local and itinerant character. Recent experiments revealed a magnetic-field-induced spin-flop transition at a critical field of about $H_\mathrm{SF} \approx 60$\,T, supporting the local picture [1,2]. We report on the electronic and thermodynamic properties of UN near the metamagnetic transition studied by means of magnetotransport, Hall-effect, and magnetic-torque measurements in pulsed magnetic fields. The experiments were conducted on microstructures fabricated by focused-ion-beam assisted patterning from single crystals. We confirm the evolution of the high-field transition from a broad and complex behavior to a sharp first-order-like step, associated with the spin flop at low temperature. In the high-field state, the magnetic contribution to the temperature dependence of the resistivity is suppressed completely. It evolves into an almost quadratic dependence at low temperatures indicative of a metallic character. Our detailed investigation of the Hall effect provides evidence for a prominent Fermi-surface reconstruction as the system is pushed into the high-field state [3].