Magnetic phase diagram in Eu$_{1-x}$La$_x$Fe$_2$As$_2$ single crystals

TL;DR

This study investigates the magnetic phase transitions in Eu$_{1-x}$La$_x$Fe$_2$As$_2$ single crystals, revealing a field-induced transition from antiferromagnetism to ferromagnetism and strong coupling between magnetic moments and charge carriers.

Contribution

It provides detailed magnetic phase diagrams and proposes a magnetic structure, advancing understanding of magnetic behavior in Eu-based FeAs superconductors.

Findings

Metamagnetic transition from antiferromagnetism to ferromagnetism at a critical field.

Suppression and shift of specific heat jump with magnetic field.

Strong coupling between Eu$^{2+}$ moments and Fe-As charge carriers.

Abstract

We have systematically measured resistivity, susceptibility and specific heat under different magnetic fields (H) in Eu$_{1-x}$La$_x$Fe$_2$As$_2$ single crystals. It is found that a metamagnetic transition from A-type antiferromagnetism to ferromagnetism occurs at a critical field for magnetic sublattice of $Eu^{2+}$. The jump of specific heat is suppressed and shifts to low temperature with increasing H up to the critical value, then shifts to high temperature with further increasing H. Such behavior supports the metamagnetic transition. Detailed H-T phase diagrams for x=0 and 0.15 crystals are given, and possible magnetic structure is proposed. Magnetoresistance measurements indicate that there exists a strong coupling between local moment of $Eu^{2+}$ and charge in Fe-As layer. These results are very significant to understand the underlying physics of FeAs superconductors.