  Pyrochlore iridates have recently attracted growing interest in condensed
matter physics because of their potential for realizing new topological states.
In order to achieve such quantum states, it is essential to understand
themagnetic properties of these compounds, as their electronic structures are
strongly coupled with theirmagnetic ground states. In this work, we report a
systematic study of the magnetic properties of pyrochlore Y2Ir2O7 and its
hole-doped compounds by performing magnetic, electron spin resonance,
electrical transport, and x-ray photoelectron spectroscopy (XPS) measurements.
We demonstrate the existence of weak ferromagnetism on top of a large
antiferromagnetic background in the undoped compound. Hole doping by calcium
was found to enhance both the ferromagnetism and the electrical conductivity.
The XPS characterization shows the coexistence of Ir4+ and Ir5+ in the undoped
compound, and the amount of Ir5+ increases with Ca doping, which highlights the
possible origins of the weak ferromagnetism associated with the formation of
Ir5+.We also observe a vertical shift in the M-H curves after field cooling,
which may arise from a strong coupling between the ferromagnetic phase and the
antiferromagnetic background.
