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 186K associated with Fe moments, the compound undergoes another magnetic phase transition at 19K 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 parallel to c at 2 K, the magnetization increases linearly to 7.0 mu(B) f.u.(-1) at mu H-0 = 1.7 T and then remains at this value for saturated Eu2+ moments under higher fields. In the case of H parallel to ab, the magnetization increases step-like to 6.6 mu(B) f.u(-1) with small magnetic hysteresis. An MM phase was identified with the saturated moments of 4.4 mu(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.