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Unconventional temperature enhanced magnetism in iron telluride

Highly energetic magnetic fluctuations, discovered in high-temperature superconductors (HTSC) by inelastic neutron scattering (INS), are now widely believed to be vital for the superconductivity. In two competing scenarios, they either originate from local atomic spins, or are a property of cooperative spin-density-wave (SDW) behavior of conduction electrons. Both assume clear partition into localized electrons, giving rise to local spins, and itinerant ones, occupying well-defined, rigid conduction bands. Here, by performing an INS study of spin dynamics in iron telluride, a parent material of one of the iron-based HTSC families, we have discovered that this very assumption fails, and that conduction and localized electrons are fundamentally entangled. We find that the real-space structure of magnetic correlations can be explained by a simple local-spin plaquette model. However, the effective spin implicated in such a model, appears to increase with the increasing temperature. Thus, we observe a remarkable redistribution of magnetism between the two groups of electrons, which occurs in the temperature range relevant for the superconductivity. The analysis of magnetic spectral weight shows that the effective spin per Fe at T \approx 10 K, in the antiferromagnetic phase, corresponds to S \approx 1, consistent with the recent analyses that emphasize importance of Hund's intra-atomic exchange. However, it grows to S \approx 3/2 in the disordered phase, a result that profoundly challenges the picture of rigid bands, broadly accepted for HTSC.