Competing magnetic phases and itinerant magnetic frustration in SrCo2As2

Whereas magnetic frustration is typically associated with local-moment magnets in special geometric arrangements, here we show that SrCo2As2 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 SrCo2As2 below T≈100 K centered at the stripe-type AF propagation vector of (12,12), 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 J1−J2 Heisenberg model that illustrates how the crossover develops as a function of the frustration ratio −J1/(2J2). By putting our INS data on an absolute scale, we quantitatively compare them and our magnetization data to exact-diagonalization calculations for the J1−J2 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 (Javg≳75 meV), which, in addition to the steep dispersion, is more characteristic of itinerant magnetism.