We report the measurements of anisotropic magnetization and magnetoresistance
on single crystals of EuFe2​As2​, a parent compound of ferro-arsenide
high-temperature superconductor. Apart from the antiferromagnetic (AFM)
spin-density-wave 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
transition under magnetic fields. Upon applying magnetic field with H∥c at 2 K, the magnetization increases linearly to 7.0 μB​/f.u. at
μ0​H=1.7 T, then keeps at this value of saturated Eu2+ moments under
higher fields. In the case of H∥ab, the magnetization increases
step-like to 6.6 μB​/f.u. with small magnetic hysteresis. A metamagnetic
phase was identified with the saturated moments of 4.4 μB​/f.u. The
metamagnetic transition accompanies with negative in-plane magnetoresistance,
reflecting the influence of Eu2+ moments ordering on the electrical
conduction of FeAs layers. The 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.Comment: 10 pages, 8 figures. accepted for publication in New Journal of
Physics as a special issue articl