Competing magnetic fluctuations and orders in a multiorbital model of doped SrCo$_2$As$_2$

TL;DR

This study investigates the magnetic fluctuations and potential ordering in doped SrCo$_2$As$_2$ using a multiorbital Hubbard-Hund model, revealing doping-dependent magnetic instabilities and itinerant frustration effects consistent with experiments.

Contribution

It provides a detailed theoretical analysis of magnetic instabilities and frustration in SrCo$_2$As$_2$ considering doping and Hund's coupling, aligning with experimental observations.

Findings

Ferromagnetic instability is dominant in the model.

Electron doping lowers the critical interaction for magnetic order.

Itinerant frustration is quantified and linked to experimental neutron scattering data.

Abstract

We revisit the intriguing magnetic behavior of the paradigmatic itinerant frustrated magnet $\rm{Sr}\rm{Co}_2\rm{As}_2$, which shows strong and competing magnetic fluctuations yet does not develop long-range magnetic order. By calculating the static spin susceptibility $\chi(\mathbf{q})$ within a realistic sixteen orbital Hubbard-Hund model, we determine the leading instability to be ferromagnetic (FM). We then explore the effect of doping and calculate the critical Hubbard interaction strength $U_c$ that is required for the development of magnetic order. We find that $U_c$ decreases under electron doping and with increasing Hund's coupling $J$, but increases rapidly under hole doping. This suggests that magnetic order could possibly emerge under electron doping but not under hole doping, which agrees with experimental findings. We map out the leading magnetic instability as a function of doping and Hund's coupling and find several antiferromagnetic phases in addition to FM. We also quantify the degree of itinerant frustration in the model and resolve the contributions of different orbitals to the magnetic susceptibility. Finally, we discuss the dynamic spin susceptibility, $\chi(\mathbf{q}, \omega)$, at finite frequencies, where we recover the anisotropy of the peaks at $\mathbf{Q}_\pi = (\pi, 0)$ and $(0, \pi)$ observed by inelastic neutron scattering that is associated with the phenomenon of itinerant magnetic frustration. By comparing results between theory and experiment, we conclude that the essential experimental features of doped SrCo$_2$As$_2$ are well captured by a Hubbard-Hund multiorbital model if one considers a small shift of the chemical potential towards hole doping.