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Eu₂⁺ spin relaxation and magnetic fluctuations in EuFe_2-xCo_xAs₂

Fernando Garcia (MPI-Dresden - Alemanha)

The discovery of superconductivity in iron-based pnictides sparked a tremendous surge of interest and research activities. This class of materials displays a high structural and chemical flexibility comprising several closely related structure types based on common layered Fe-pnictogen building blocks. The layered compound EuFe₂As₂ is an interesting model system where layers of Fe₂As₂ alternate with Eu layers along the c-axis. They offer an important basis to investigate the effects of well defined local Eu₂+ 4f states on the itinerant electronic and magnetic properties of the Fe₂As₂ layers. To address this subject, we combined Eu₂+  electron spin resonance (ESR), dc-susceptibility measurements and band structure calculations of EuFe_2-xCo_xAs₂ (0.1< x <0.75) in an effort to understand the nature of the Eu₂+ spin relaxation and its relation with the electronic and magnetic properties of the itinerant subsystem. The ESR experiments were carried out at three resonance fields by using three frequencies (L-, X-, and Q-band). We found that the electronic spin relaxation is well understood in terms of the exchange coupling among the local spins. In turn, the Korringa process has only a marginal role in the Eu₂+ spin relaxation. This talk starts with a "crash course" on the field of the iron arsenide superconductors and ESR measurements. Afterwards, we discuss our experiments showing that the Eu₂+ spin relaxation is field dependent and under direct influence of the magnetic fluctuations at the Fe_2-xCo_xAs₂ layers. Our results present the Eu₂+ ESR to be suitable to probe the spin dynamics of the itinerant subsystem