High-field electron spin resonance spectroscopy study of GdFeAsO1−xFxsuperconductors

Long- to short-range magnetic order in fluorine-doped CeFeAsO

“…The FeAs magnetic order, a hallmark also for the Ce-1111 family, is strongly affected by an increase in F content: (i) its Néel temperature decreases sharply; (ii) the AF magnetic order evolves from long-to short-ranged; (iii) superconductivity appears when the Fe 2+ magnetic moment is significantly reduced with respect to its value in the undoped case. These findings seriously question the presence of a quantum critical point in Ce-1111 and, together with previous results on Nd-, 13 Sm- 36 and Gd-1111 16 compounds, most likely suggest that the phase diagram of the 1111 family is RE independent and consists of a narrow M-SC crossover region, where nanoscopic coexistence takes place. Surprisingly, though, this coexisting region seems to be immeasurably small or totally absent in La-1111.…”

“…8 The REFeAsO parent compounds generally show a commensurate spin-density wave (SDW) magnetic order characterized by strongly reduced Fe momenta, as evidenced by standard neutron scattering studies: 9 Fe magnetic moments are comprised between 0.25 and 0.8 μ B , to be compared with 4 μ B , the spin-only value for the Fe 2+ ion. For this very reason, the determination of the temperature dependence of the magnetic order parameter via sensitive local probe techniques, such as muon-spin rotation, [10][11][12][13] Mössbauer spectroscopy, 14,15 electron spin resonance (ESR), 16 or nuclear magnetic resonance (NMR), 17 is supposed to provide a higher accuracy than that possible using standard powder neutron scattering. 18 Currently, the accepted magnetic moment value for iron at 2 K in the REFeAsO families is 0.63(1) μ B , 12,19 seemingly independent of the rare-earth ion.…”

Competing effects of Mn and Y doping on the low-energy excitations and phase diagram ofLa1−yYyFe1−xMnxMoroni1, Sanna2, Lamura3 et al. 2016Phys. Rev. BSelf Cite9370View full textAdd to dashboardCite

“…The samples were synthesized using a two-step solid-state reaction 17 . Details on sample preparation and characterization by means of powder x-ray diffraction, electron microscopy (SEM) and SQUID magnetometry, used to determine T c , have been already partially reported in Refs.…”

EPR Study of Superconductors

“…Finally, soon after the discovery of a new class of high temperature superconductors on the basis of iron pnictides in 2008 54 , the EPR method has begun to make valuable contributions to the understanding of the physics of these novel materials. One can mention a number of recent interesting works addressing the normal and superconducting properties of iron arsenides by Eu-EPR in the so-called 122-family 55,56 and by Gd-EPR in the 1111-family 57,58 . Other results of our study performed from 1972 to 1977 also should be mentioned.…”