Magnetism and superconductivity in EuFe₂As₂ synthesized under high pressure

We have synthesized polycrystalline samples of EuFe₂As₂ by melting the stoichiometric mixture of constituent components at a high pressure (HP) of 8GPa and measured the properties of HP-synthesized samples at ambient and high pressure up to 6GPa. SDW-type antiferromagnetism appears below 150K and Eu-related antiferromagnetism appears below 16K in all HP synthesized samples. Also, all HP-synthesized samples exhibit superconductivity with T_c=20-30K at ambient pressure, though a zero-resistance state has not been observed. This superconductivity is very robust against application of magnetic field (dH_c2/dT=-3.2TK^-1) and pressure (a tiny signal of superconductivity is still detectable below 2.5K at 5.8GPa). SDW antiferromagnetism is suppressed under pressure and is not observed above 1.7GPa and below 60K. The temperature of the magnetic ordering of Eu²⁺ moments increases under pressure. Specific heat measurements in a magnetic field reveal an increase of this temperature in field, therefore indicating field-induced ferromagnetism in the Eu sublattice.