Field-induced electronic phase separation in the high-temperature superconductor La_1.94Sr_0.06CuO_4+y

We present a combined neutron diffraction and high field muon spin rotation (μSR) study of the magnetically ordered and superconducting phases of the high-temperature superconductor La_1.94Sr_0.06CuO_4+y (T_ c = 37.5(2) K) in a magnetic field applied perpendicular to the CuO_2 planes. We observe a linear field-dependence of the intensity of the neutron diffraction peak that reflects the modulated antiferromagnetic stripe order. The magnetic volume fraction extracted from μSR data likewise increases linearly with applied magnetic field. The combination of these two observations allows us to unambiguously conclude that stripe-ordered regions grow in an applied field, whereas the stripe-ordered magnetic moment itself is field-independent. This contrasts with earlier suggestions that the field-induced neutron diffraction intensity in La-based cuprates is due to an increase in the ordered moment. We discuss a microscopic picture that is capable of reconciling these conflicting viewpoints.