Muon spin rotation study of magnetism and superconductivity in Ba(Fe1−xCox)2

Abstract

Publication date: 2012 Document VersionPublisher's PDF, also known as Version of record Link back to DTU Orbit Citation (APA): Bernhard, C., Wang, C. N., Nuccio, L., Schulz, L., Zaharko, O., Larsen, J., ... Niedermayer, C. (2012). Muon spin rotation study of magnetism and superconductivity in Using muon spin rotation (μSR) we investigated the magnetic and superconducting properties of a series of Ba(Fe 1−x Co x ) 2 As 2 single crystals with 0 x 0.15. Our study details how the antiferromagnetic order is suppressed upon Co substitution and how it coexists with superconductivity. In the nonsuperconducting samples at 0 < x < 0.04 the antiferromagnetic order parameter is only moderately suppressed. With the onset of superconductivity this suppression becomes faster and it is most rapid between x = 0.045 and 0.05. As was previously demonstrated by μSR at x = 0.055 [P. Marsik et al., Phys. Rev. Lett. 105, 57001 (2010)], the strongly weakened antiferromagnetic order is still a bulk phenomenon that 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. A different kind of magnetic order that was also previously identified [C. Bernhard et al., New J. Phys. 11, 055050 (2009)] occurs at 0.055 < x < 0.075 where T c approaches the maximum value. 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 T c . 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 T c . These findings point toward a drastic change in the relationship between the magnetic and superconducting orders from a competitive one in the strongly underdoped regime to a constructive one in near-optimally and overdoped samples.

Antiferromagnetic order and spin dynamics in iron-based superconductors

“…In addition, the AF Néel temperature of the system saturates to a value near or slightly above T c before vanishing in a first order fashion with increasing Co-doping [ Fig. 34(b)] (Bernhard et al, 2012). This region of Co-doping is consistent with the appearance of the transverse incommensurate AF order seen in neutron diffraction experiments .…”

“…34(a)] (Bernhard et al, 2012). For Codoped samples in the underdoped regime (x ≤ 0.045), the µSR results find full volume AF ordered phase coexisting and competing with superconductivity.…”

“…This region of Co-doping is consistent with the appearance of the transverse incommensurate AF order seen in neutron diffraction experiments . However, instead of a uniform incommensurate spin-density-wave ordered phase, the µSR data indicates a spatially inhomogeneous magnetic state for which the volume fraction of the ordered phase decreases with increasing Co-doping (Bernhard et al, 2012). The NMR measurements on Ba(Fe 1−x Co x ) 2 As 2 samples near optimal superconductivity reveal that the system is in the spin-glass state which competes with superconductivity (Dioguardi et al, 2013).…”

“…32(d)]. Since the measurable extinction release at temperatures well above The open symbols show the magnetic properties in the spatially inhomogeneous magnetic state near optimum doping (Bernhard et al, 2012).…”

“…5(b)] (Bernhard et al, 2012;Dioguardi et al, 2013;Lu et al, 2013Lu et al, , 2014bLuo et al, 2012b). There is no evidence for a conventional magnetic quantum critical point near optimal superconductivity, and the AF order disappears in the superconducting phase with increasing Co/Ni-doping in a first order fashion (Bernhard et al, 2012;Dioguardi et al, 2013;Lu et al, 2013Lu et al, , 2014bLuo et al, 2012b).…”