Competing paramagnetic 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 ${\mathrm{SrCo}}_{2}{\mathrm{As}}_{2}$ 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 ${\mathrm{SrCo}}_{2}{\mathrm{As}}_{2}$ below $T\ensuremath{\approx}100$ K centered at the stripe-type AF propagation vector of $(\frac{1}{2},\phantom{\rule{0.16em}{0ex}}\frac{1}{2})$, 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 toward ferromagnetism ordering to an instability toward stripe-type AF ordering on cooling, and show results from Monte-Carlo simulations for a ${J}_{1}\ensuremath{-}{J}_{2}$ Heisenberg model that illustrates how the crossover develops as a function of the frustration ratio $\ensuremath{-}{J}_{1}/(2{J}_{2})$. By putting our INS data on an absolute scale, we quantitatively compare them and our magnetization data to exact-diagonalization calculations for the ${J}_{1}\ensuremath{-}{J}_{2}$ 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 (${J}_{\text{avg}}\ensuremath{\gtrsim}75$ meV), which, in addition to the steep dispersion, is more characteristic of itinerant magnetism.