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

TL;DR

This study uses polarized neutron diffraction to analyze the field-induced magnetization in Ba(Fe1-xCox)2As2, revealing details about spin-singlet pairing, the van Vleck susceptibility, and the distribution of magnetization in the material.

Contribution

It provides the first detailed mapping of induced magnetization density in the superconducting state of Ba(Fe1-xCox)2As2 using polarized neutron diffraction, linking experimental results with band structure calculations.

Findings

Magnetization follows the Yosida function in the superconducting state.

A significant zero-field susceptibility component is identified, likely due to van Vleck susceptibility.

Magnetization is localized on Fe atoms and matches band structure predictions.

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.