A 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 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.

A neutron scattering study of the interplay between structure and magnetism in Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$

Single crystal neutron diffraction is used to investigate the magnetic and structural phase diagram of the electron doped superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. Heat capacity and resistivity measurements have demonstrated that Co doping this system splits the combined antiferromagnetic and structural transition present in BaFe$_2$As$_2$ into two distinct transitions. For $x$=0.025, we find that the upper transition is between the high-temperature tetragonal and low-temperature orthorhombic structures with ($T_{mathrm{TO}}=99 pm 0.5$ K) and the antiferromagnetic transition occurs at $T_{mathrm{AF}}=93 pm 0.5$ K. We find that doping rapidly suppresses the antiferromagnetism, with antiferromagnetic order disappearing at $x approx 0.055$. However, there is a region of co-existence of antiferromagnetism and superconductivity. The effect of the antiferromagnetic transition can be seen in the temperature dependence of the structural Bragg peaks from both neutron scattering and x-ray diffraction. We infer from this that there is strong coupling between the antiferromagnetism and the crystal lattice.