Synopsis: Squeezing in magnetism

A slightly denser version of a transition-metal oxide, with the same chemical composition, has radically different magnetic properties.
Synopsis figure
Credit: Y. Shirako et al., Phys. Rev. B (2011)

Strong Coulomb interactions and anisotropic crystal structure play an important role in the many forms of order that are seen in the transition-metal oxides. Small changes in the composition or structure of a material can lead to dramatic changes in its properties. A particular example is CaRuO3. In its natural form, this material crystallizes into a perovskite structure and does not show any form of long-range magnetic order. However, adding a tiny amount of disorder makes CaRuO3 ferromagnetic, implying that this phase lies close to a magnetic critical point.

In a paper appearing in Physical Review B, Yuichi Shirako of Gakushuin University and colleagues at several institutions in Japan have synthesized CaRuO3 with a different crystal structure from the natural one, without changing its chemical composition. Shirako et al. prepared CaRuO3 in the so-called postperovskite structure by applying high pressure and high temperature and then quenching the material to retain that structure at room temperature. This new crystal is only 1.7% denser than the perovskite structure but has a significantly more anisotropic crystal structure. This form of CaRuO3 appears to be a quasi-one-dimensional magnet, implying long-range antiferromagnetic correlations between the spins on the ruthenium (Ru) sites. If so, it would be the first example of a S=1 antiferromagnetic integer spin chain involving the 4d orbitals of the metal ions in a correlated oxide. This dramatic effect from a small change is likely to spur interest in further studies of isocompositional versions of other oxides and the effects of carrier doping on them. – Sarma Kancharla


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