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

We use polarized neutron diffraction to study the induced magnetization density of near optimally doped Ba(Fe0.935Co0.065)(2)As-2 (T-C = 24 K) as a function of magnetic field (1< mu H-0< 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 mu H-0 <= 9 T approximate to mu H-0(c2)/6. In addition to the Yosida spin-susceptibility, our results reveal a large zero-field contribution M(H -> 0, T -> 0)/H approximate to 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.