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# A polarized neutron diffraction study of the field-induced magnetization in the normal and superconducting states of Ba(Fe1-xCox)2As2 (x=0.65)

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### Abstract

We use polarised neutron diffraction to study the induced magnetization density of near optimally doped Ba(Fe0.935Co0.065)2As2 (T_C=24 K) as a function of magnetic field (1<H<9 T) and temperature (2<T<300 K). The T-dependence of the induced moment in the superconducting state is consistent with the Yosida function, characteristic of spin-singlet pairing. The induced moment is proportional to applied field for H < 9 T ~ Hc2/6. In addition to the Yosida spin-susceptibility, our results reveal a large zero-field contribution M (H=>0,T=>0)/H ~ 2/3 \chi_{normal} which does not scale with the field or number of vortices and is most likely due to the van Vleck susceptibility. Magnetic structure factors derived from the polarization dependence of 15 Bragg reflections were used to make a maximum entropy reconstruction of the induced magnetization distribution in real space. The magnetization is confined to the Fe atoms and the measured density distribution is in good agreement with LAPW band structure calculations which suggest that the relevant bands near the Fermi energy are of the d_{xz/yz} and d_{xy} type.

### Most cited references17

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### Iron-based layered superconductor La[O(1-x)F(x)]FeAs (x = 0.05-0.12) with T(c) = 26 K.

(2008)
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### Superconductivity in the iron selenideKxFe2Se2(0≤x≤1.0)

(2010)
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### Determination of the phase diagram of the electron doped superconductor Ba(Fe$$_{1-x}$$Co$$_x$$)$$_2$$As$$_2$$

(2008)
Systematic measurements of the resistivity, heat capacity, susceptibility and Hall coefficient are presented for single crystal samples of the electron-doped superconductor Ba(Fe$$_{1-x}$$Co$$_x$$)$$_2$$As$$_2$$. These data delineate an $$x-T$$ phase diagram in which the single magnetic/structural phase transition that is observed for undoped BaFe$$_2$$As$$_2$$ at 134 K apparently splits into two distinct phase transitions, both of which are rapidly suppressed with increasing Co concentration. Superconductivity emerges for Co concentrations above $$x \sim 0.025$$, and appears to coexist with the broken symmetry state for an appreciable range of doping, up to $$x \sim 0.06$$. The optimal superconducting transition temperature appears to coincide with the Co concentration at which the magnetic/structural phase transitions are totally suppressed, at least within the resolution provided by the finite step size between crystals prepared with different doping levels. Superconductivity is observed for a further range of Co concentrations, before being completely suppressed for $$x \sim 0.018$$ and above. The form of this $$x-T$$ phase diagram is suggestive of an association between superconductivity and a quantum critical point arising from suppression of the magnetic and/or structural phase transitions.
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### Author and article information

###### Journal
22 June 2011
1106.4506 10.1103/PhysRevB.84.134514