Unconventional temperature enhanced magnetism in Fe_1.1Te

Our inelastic neutron scattering study of spin excitations in iron telluride reveals remarkable thermal evolution of the collective magnetism. In the temperature range relevant for the superconductivity in FeTe _1-xSe_x materials, where the local-moment behavior is dominated by liquidlike correlations of emergent spin plaquettes, we observe unusual, marked increase of magnetic fluctuations upon heating. The effective spin per Fe at T10K, in the phase with weak antiferromagnetic order, corresponds to S1, consistent with the recent analyses that emphasize importance of Hund's coupling. However, it grows to S3/2 in the high-T disordered phase, suggestive of the Kondo-type behavior, where local magnetic moments are entangled with the itinerant electrons.