Localized and itinerant dichotomy of electrons in Ba(Fe,Co)2As2

TL;DR

This study investigates the dual nature of 3d electrons in Ba(Fe,Co)2As2, revealing a coexistence of localized magnetic moments and itinerant electrons through high-field magnetotransport measurements.

Contribution

It provides experimental evidence for the localized and itinerant dichotomy of electrons in Ba(Fe,Co)2As2, advancing understanding of its magnetic and transport properties.

Findings

Magnetic transition is robust against magnetic field, indicating local moments.

Magnetoresistance and Hall resistivity show distinct behaviors in magnetic vs. paramagnetic states.

Electron carriers increase under magnetic field, partially compensating carrier loss at T_M.

Abstract

Variant approaches, either based on the Fermi surface nesting or started from the proximity to a Mott-insulator, were proposed to elucidate the physics in iron pnictides, but no consensus has been reached. A fundamental problem concerns the nature of their 3d electrons. Here we report the magnetoresistivity (\rho_xx) and the Hall resistivity (\rho_xy) of Ba(Fe1-xCox)2As2 (x=0 and 0.05) in a magnetic field of up to 55T. The magnetic transition is extremely robust against magnetic field, giving strong evidence that the magnetic ordering is formed by local moments. The magnetic state is featured with a huge magnetoresistance and a distinguished Hall resistivity, \rho_xy(H), which shows a pronounced parabolic field dependence, while the paramagnetic state shows little magnetoresistance and follows a simple linear magnetic field dependence on the Hall resistivity. Analyses of our data, based on a two-carrier model, demonstrate that the electron carriers in the magnetic state rapidly increase upon applying a magnetic field, partially compensating the loss of electron carriers at T_M. We argue that the 3d-electrons in Ba(Fe1-xCox)2As2 are divided into those who are close to forming localized moments controlling the magnetic transition and the others giving rise to complex transport properties through their interaction with the former.