Superconductivity and Field-Induced Magnetism in SrFe1.75Co0.25As2

Using muon-spin rotation, we studied the in-plane (lambda(ab)) and the out of plane (lambda(c)) magnetic field penetration depth in SrFe1.75Co0.25As2 (T-c similar or equal to 13.3 K). The penetration depth anisotropy gamma(lambda)=lambda(c)/lambda(ab) increases from gamma(lambda)similar or equal to 2.1 at T-c to 2.7 at 1.6 K. The mean internal field in the superconducting state increases with decreasing temperature, just opposite to the diamagnetic response seen in magnetization experiments. This unusual behavior suggests that the external field induces a magnetic order which is maintained throughout the whole sample volume.