Electronic phase behaviors in spin-orbit coupled magnets at the localized and itinerant limits

Abstract

The magnetic interaction in materials generally can be categorized into two extremes: localized and itinerant. This work will focus on the electronic and magnetic properties of two prototypical magnetic compounds, which fall into the opposite extremes, i:e:, the spin-orbit coupled Mott insulator Sr₂IrO₄ (Sr214) described by the localized Heisenberg model and the itinerant helical (nearly-ferromagnetic) metal MnSi pictured with band or Stoner magnetism. The single layered cuprate analogue Sr₂IrO₄ has attracted considerable attentions in recent years, due to its unusual electronic and magnetic properties and the potential to access superconducting states. The exotic jeff = 1/2 ground state for the Ir⁴⁺ (5d⁵) ions results from the delicate balance of competing/cooperating energy scales, such as the stronger spin-orbit coupling (SOC) in 5d materials as compared to 3d transition metal oxides (TMOs), crystal electric field (CEF) splitting and electron-electron correlations. Superconducting states are theoretically predicted to be achievable if sufficient carriers are introduced into this spin-orbit assisted compound, which later triggers tremendous experimental works toward the realization of superconductivity. Here in this work a combined study of various probes, such as transport, magnetization, X-ray and neutron scattering measurements, focusing on the electronic and magnetic properties, is presented in the perturbed spin-orbit coupled Mott (SOM) state. Specifically in electron doped (Sr₁₋ₓLaₓ)₂IrO₄, a detailed mapping of magnetism with respect to electron doping is presented, demonstrating the gradual transition from long range magnetic order in parent state, to intermediate short range order, and eventually into the incommensurate (IC) spin density wave (SDW) state with increasing electron doping. Our picture supports the conjecture that the quenched Mott phases in electron-doped Sr₂IrO₄ and hole doped La₂CuO₄ share common competing electronic phases. On the other hand, the prototypical itinerant metal MnSi is examined by inelastic neutron scattering (INS). Our experimental data directly demonstrate the collapse of linear spin wave theory for localized Heisenberg magnets in the large energy limit, although the low energy dispersion is still described by the ferromagnetic spin wave theory. Most importantly, our observations display the chimney-like dispersion spectrum up to the energy scale of at least 240 meV, which is more than one order of magnitude larger than the Heisenberg interaction energy scale. For the first time, solid characterizations of Stoner excitations in itinerant helimagnet (nearly ferromagnetic) have been demonstrated up to an exceedingly large energy scale. Our intriguing results will greatly promote further understanding and exploration of Stoner excitations in itinerant magnets.