Motivated by the potential of realizing new topological states, such as topological Mott insulators, Weyl semimetals, axion insulators, and topological crystalline insulators, scientists of condensed matter physics are paying great attention to pyrochlore iridates A₂Ir₂O₇.

They hope to access these correlation regimes by tuning the A-site elements.  However, to realize novel topological states in pyrochlore iridates, it is essential to understand their magnetic properties as their electronic structures are strongly coupled with the magnetic ground states. There are at least three major magnetic interactions (i.e., the Heisenberg-type antiferromagnetic coupling, the Dzyaloshinskii-Moriya interaction, and the single-ion anisotropy) within the magnetic Ir⁴⁺ ions network. The competitions among them lead to various magnetic configurations. When the A³⁺ ion is magnetic, the possible f-d exchange interaction between the Ir⁴⁺ and A³⁺ can cause even more complex magnetic structures.

To clarify the fundamental magnetic properties associated with the iridium network, Assistant Prof. Shixiong ZHANG and his group from Indiana University chose relative simple system, i. e., the nonmagnetic A-site ions pyrochlore Y₂Ir₂O₇ for investigation as it is predicted to be a Weyl semimetal when its magnetic ground sate is the peculiar all-in/all-out antiferromagnetic (AFM) phase. They synthesized Y₂Ir₂O₇ and hole-doped compounds, and performed magnetic, electron spin resonance, electrical transport, and x-ray photoelectron spectroscopy (XPS) measurements on these samples. They demonstrated the both samples contain Ir⁵⁺, and the hole doping by calcium enhances both the ferromagnetism and the electrical conductivity.  After analysis and discussions thoroughly, they concluded the possible origins of the weak ferromagnetism associated with the formation of Ir⁵⁺. They also observed a vertical shift in the M-H curves after field cooling, and indicated that may result from a strong coupling between the ferromagnetic phase and the antiferromagnetic background.

This work involved SQUID (partially) and ESR facilities in the High Magnetic Field Laboratory, Chinese Academy of Sciences(CHMFL). The results were published on Physical Review B, August 2014.

M-T curves show two transition temperatures, and FC M-H curves show vertical shifts. ZFC and FC ESR spectra and calculated local effective field versus temperatures demonstrate the appearance and evolution of weak ferromagnetism

Link to the article: http://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.054419.