Study of the anisotropic magnetic order of the Eu sublattice in single crystals of EuFe_2-xCo_xAs_2 (x = 0, 0.2) by means of magnetization and magnetic torque

TL;DR

This study investigates the magnetic ordering of Eu sublattice in EuFe2-xCoxAs2 crystals using magnetization and torque measurements, revealing doping-dependent changes in magnetic phases and anisotropy.

Contribution

It provides detailed magnetic phase diagrams for EuFe2As2 and EuFe1.8Co0.2As2, highlighting the effects of Co doping on Eu magnetic order and anisotropy.

Findings

EuFe2As2 exhibits antiferromagnetic order with a canted phase at low temperatures.

Co doping suppresses the low-field AFM phase, leaving only a canted AFM phase.

Magnetic anisotropy is reduced with Co doping, affecting magnetic coupling and superconductivity.

Abstract

Here, we present a combination of magnetization and magnetic torque experiments to investigate the magnetic orders in undoped EuFe2As2 and Co doped EuFe1.8Co0.2As2 single crystals. Although at low temperatures typical results for an antiferromagnetic (AFM) state in EuFe2As2 were found, our data strongly indicate the occurrence of a canted antiferromagnetic (C-AFM) order of the Eu2+ moments between 17 K and 19 K, observed even in the lowest studied magnetic fields. However, unlike in the parent compound, no low-field and low-temperature AFM state of the Eu^2+ moments was observed in the doped EuFe1.8Co0.2As2. Only a C-AFM phase is present at low fields and low temperatures, with a reduced magnetic anisotropy as compared to the undoped system. We present and discuss for both, EuFe2As2 and EuFe1.8Co0.2As2, the experimentally deduced magnetic phase diagrams of the magnetic ordering of the Eu^2+ sublattice with respect to the temperature, the applied magnetic field, and its orientation to the crystallographic axes. It is likely that the magnetic coupling of the Eu and the Fe sublattice is strongly depending on Co doping, having detrimental influence on the magnetic phase diagrams as determined in this work. Their impact on the occurrence of superconductivity with higher Co doping is discussed.