Giant magnetovolume effect in a cubic perovskite Sr$_{1-x}$Ba$_x$CoO$_3$ with competing magnetic orders*

Perovskite oxides with unusually high valence transition metal ions have been of great interest because of the unique magnetism inherent to the strong p-d hybridization. This is exemplified by the unusual helimagnetism in SrFeO$_3$ and room-temperature ferromagnetism in SrCoO$_3$. However, search for novel functions in these oxides has been hampered by the difficulty in growing crystals. In this work, we synthesized single crystals of a cubic perovskite Sr$_{1-x}$Ba$_x$CoO$_3$ by high-pressure and chemical oxidation technique to study the effect of controlling the p-d hybridization on magnetism. By magnetization measurements, we established the phase diagram [H. Sakai et al.], where a novel helimagnetic phase appears at around x = 0.35. To discuss this result from the viewpoint of the lattice change, we applied pressure on the heilmagnetic compound with x = 0.4 and found the helimagnetic to ferromagnetic transition. This novel pressure effect can be regarded as a giant magnetovolume effect, the origin of which will be discussed with showingthe first-principles calculations[S. Yokoyama et al.].