Magnetic phase diagram and correlation between metamagnetism and superconductivity in Ru0.9Sr2YCu2.1O7.9

The magnetic superconductor Ru0.9Sr2YCu2.1O7.9 (Ru-1212Y) has been investigated using neutron diffraction under variable temperature and magnetic field. With the complementary information from magnetization measurements, we propose a magnetic phase diagram T-H for the Ru-1212 system. Uniaxial antiferromagnetic (AFM) order of 1.2μ B/Ru atoms with moments parallel to the c-axis is found below the magnetic transition temperature at ~140 K in the absence of magnetic field. In addition, ferromagnetism (FM) in the ab-plane develops below ~120 K, but is suppressed at lower temperature by superconducting correlations. Externally applied magnetic fields cause Ru-moments to realign from the c-axis to the ab-plane, i.e. along the {1.1.0} direction, and induce ferromagnetism in the plane with ~1μ Bat 60 kOe. These observations of the weak ferromagnetism suppressed by superconductivity and the field-induced metamagnetic transition between AFM and FM demonstrate not only competing orders of superconductivity and magnetism, but also suggest a certain vortex dynamics contributing to these magnetic transitions.