Metamagnetic transition in EuFe₂As₂ single crystals

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 μ_B 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 μ_B f.u.⁻¹ with small magnetic hysteresis. An MM phase was identified with the saturated moments of 4.4 μ_B 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.