Field-induced magnetic phase transitions in Ti-doped Ca3Ru2O7 bilayer ruthenates

Bilayer ruthenate Ca$_{\mathrm{3}}$Ru$_{\mathrm{2}}$O$_{\mathrm{7}}$ shows strong magnetic instability that depends sensitively on chemical doping and magnetic fields. Previously we have shown that [1] Ti doping induces Mott insulating ground state with a G-type antiferromagnetic (AFM) structure where nearest-neighbor spins align antiferromagnetically, a feature dramatically distinct from the metallic ground state with an AFM-b structure where the ferromagnetically aligned spins (pointing along the $b$-axis) within the bilayer are coupled antiferromagnetically along the $c$-axis. Here we report magnetic phases of the Ti-doped Ca$_{\mathrm{3}}$Ru$_{\mathrm{2}}$O$_{\mathrm{7}}$ in a magnetic field revealed via neutron diffraction study. In sharp contrast to pure Ca$_{\mathrm{3}}$Ru$_{\mathrm{2}}$O$_{\mathrm{7}}$ [2], below the metal-insulator transition we find a field-induced magnetic phase transition from G-type AFM to AFM-a with spins projected along the $a$-axis. Concomitantly, a sharp change in lattice parameters is observed, suggesting strong magnetoelastic coupling. The effect of such a field-induced phase transition on the magnetotransport property in the Ti-doped Ca$_{\mathrm{3}}$Ru$_{\mathrm{2}}$O$_{\mathrm{7}}$ will be discussed as well. [1] X. Ke et al., Phys. Rev. B \textbf{84}, 201102 (R) (2011). [2] W. Bao et al., Phys. Rev. Lett. \textbf{100}, 247203 (2008).