C. Bernhard, C. N. Wang, L. Nuccio, L. Schulz, O. Zaharko, J. Larsen, C. Aristizabal, M. Willis, A. J. Drew, G. D. Varma, T. Wolf, Ch. Niedermayer
Using muon spin rotation (\muSR) we investigated the magnetic and superconducting properties of a series of Ba(Fe1-xCox)2As2 single crystals with 0 < x < 0.15. Our study details how the antiferromagnetic order parameter is suppressed upon Co substitution and identifies two distinct magnetic phases that coexist with superconductivity. The first one at 0.04 < x < 0.055 involves a bulk antiferromagnetic state that has a strongly suppressed order parameter and competes with superconductivity. The comparison with neutron diffraction data suggests that the antiferromagnetic order remains commensurate whereas the amplitude exhibits a spatial variation that is likely caused by the randomly distributed Co atoms. The second magnetic phase occurs at 0.055 < x < 0.075 where Tc approaches the maximum. The magnetic order develops here only in parts of the sample volume and it seems to cooperate with superconductivity since its onset temperature coincides with Tc. Even in the strongly overdoped regime at x = 0.11, where the static magnetic order has disappeared, we find that the low energy spin fluctuations are anomalously enhanced below Tc. These findings point toward an intricate relationship between the magnetic and superconducting orders that needs to be better understood.
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http://arxiv.org/abs/1206.7085
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