  Transition metal oxides (TMOs) exhibit many emergent phenomena ranging from
high-temperature superconductivity and giant magnetoresistance to magnetism and
ferroelectricity. In addition, when TMOs are interfaced with each other, new
functionalities can arise, which are absent in individual components. Here, we
report results from first-principles calculations on the magnetism at the
BiFeO3/YBa2Cu3O7 interfaces. By comparing the total energy for various magnetic
spin configurations inside BiFeO3, we are able to show that a metallic
ferromagnetism is induced near the interface. We further develop an interface
exchange-coupling model and place the extracted exchange coupling interaction
strengths, from the first-principles calculations, into a resultant generic
phase diagram. Our conclusion of interfacial ferromagnetism is confirmed by the
presence of a hysteresis loop in field-dependent magnetization data. The
emergence of interfacial ferromagnetism should have implications to electronic
and transport properties.
