Competing magnetic phases and itinerant magnetic frustration in SrCo$_{2}$As$_{2}$

Whereas magnetic frustration is typically associated with local-moment magnets in special geometric arrangements, here we show that SrCo$_{2}$As$_{2}$ is a candidate for frustrated itinerant magnetism. Using inelastic neutron scattering (INS), we find that antiferromagnetic (AF) spin fluctuations develop in the square Co layers of SrCo$_{2}$As$_{2}$ below $T\approx100$ K centered at the stripe-type AF propagation vector of $(\frac{1}{2},~\frac{1}{2})$, and that their development is concomitant with a suppression of the uniform magnetic susceptibility determined via magnetization measurements. We interpret this switch in spectral weight as signaling a temperature-induced crossover from an instability towards FM ordering to an instability towards stripe-type AF ordering on cooling, and show results from Monte-Carlo simulations for a $J_{1}$-$J_{2}$ Heisenberg model that illustrate how the crossover develops as a function of the frustration ratio $-J_1/(2J_2)$. By putting our INS data on an absolute scale, we quantitatively compare them and our magnetization data to exact-diagonalization calculations for the $J_{1}$-$J_{2}$ model [N. Shannon et al., Eur. Phys. J. B 38, 599 (2004)], and show that the calculations predict a lower level of magnetic frustration than indicated by experiment. We trace this discrepancy to the large energy scale of the fluctuations ($J_{\text{avg}}\gtrsim75$ meV), which, in addition to the steep dispersion, is more characteristic of itinerant magnetism.