From mean-field localized magnetism to itinerant spin fluctuations in the "nonmetallic metal" FeCrAs

FeCrAs displays an unusual electrical response that is neither metallic in character nor divergent at low temperatures, as expected for an insulating response, and therefore it has been termed a "nonmetal metal." The anomalous resistivity occurs for temperatures below similar to 900 K. We have carried out neutron scattering experiments on powder and single crystal samples to study themagnetic dynamics and critical fluctuations in FeCrAs. Magnetic neutron diffraction measurements find Cr3+ magnetic order setting in at T-N = 115 K similar to 10 meV with a mean-field critical exponent. Using neutron spectroscopy we observe gapless, high velocity, magnetic fluctuations emanating from magnetic positions with propagation wave vector (q) over right arrow (0) = (1/3, 1/3), which persists up to at least 80 meV similar to 927 K, an energy scale much larger than TN. Despite the mean-field magnetic order at low temperatures, the magnetism in FeCrAs therefore displays a response which resembles that of itinerant magnets at high energy transfers. We suggest that the presence of stiff high-energy spin fluctuations extending up to a temperature scale of similar to 900 K is the origin of the unusual temperature dependence of the resistivity.