Abstract
We report the measurements of the anisotropic magnetization and magnetoresistance (MR) on single crystals of EuFe2As2, a parent compound of ferro-arsenide high-temperature superconductor. Apart from the antiferromagnetic (AFM) spin density wave (SDW) transition at 186 K associated with Fe moments, the compound undergoes another magnetic phase transition at 19 K due to AFM ordering of Eu2+ spins (J=S=7/2). The latter AFM state exhibits metamagnetic (MM) transition under magnetic fields. Upon applying magnetic field with H∥c at 2 K, the magnetization increases linearly to 7.0 μB f.u.−1 at μ0H=1.7 T and then remains at this value for saturated Eu2+ moments under higher fields. In the case of H∥ab, the magnetization increases step-like to 6.6 μB f.u.−1 with small magnetic hysteresis. An MM phase was identified with the saturated moments of 4.4 μB f.u.−1. The MM transition accompanies negative in-plane MR, reflecting the influence of Eu2+ moments ordering on the electrical conduction of FeAs layers. These results were explained in terms of spin-reorientation and spin-reversal based on an A-type AFM structure for Eu2+ spins. The magnetic phase diagram has been established.
Highlights
To reconcile the AFM ordering and the FM interaction, and considering the enhanced χab just above the Neel temperature, the Eu2+ spins probably align ferromagnetically within the ab planes, but antiferromagnetically along the c-axis (see the inset of figure 5)
The property of the AFM order of Eu2+ spins and the evolution of the magnetic ordering under various magnetic fields were studied by the measurements of the magnetization and MR using EuFe2As2 single crystal samples
Under external magnetic fields with H ab or H c, the Eu2+ moments undergo spin-reorientation and/or spin-reversal transition depending on the relative orientations between Eu2+ moments and the magnetic field
Summary
To reconcile the AFM ordering and the FM interaction, and considering the enhanced χab just above the Neel temperature, the Eu2+ spins probably align ferromagnetically within the ab planes, but antiferromagnetically along the c-axis (see the inset of figure 5). In the SDW state, Fe2+ moments order antiferromagnetically with a collinear stripe-like spin structure.
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