Spectroscopic study of the field- and pressure-induced phases of the bilayered ruthenate Ca3 Ru2 O7

We present a magnetic-field- and pressure-dependent Raman scattering study of the complex orbital, magnetic, and conducting phases of Ca3 Ru2 O7, which result from a rich interplay between the orbital, spin, and electronic degrees of freedom. The Raman-active phonon and magnon excitations in Ca3 Ru2 O7 convey sufficient information to map out the orbital, magnetic, and conducting (H,T) and (P,T) phase diagrams of this material. We find that quasihydrostatic pressure causes a linear suppression of the orbital-ordering temperature (TOO =48 K at P=0), up to a T=0 critical point near P* ∼55 kbar, above which the material is in a metallic, orbital-degenerate phase. We associate this pressure-induced collapse of the antiferromagnetic orbital-ordered phase with a suppression of the Ru O6 octahedral distortions that are responsible for orbital-ordering. We also find that an applied magnetic field at low temperatures induces a change from an orbital-ordered to orbital-degenerate phase for fields aligned along the in-plane b -axis (H hard axis), but induces a reentrant orbital-ordered to orbital-disordered to orbital-ordered phase change for fields aligned along the in-plane a -axis (H easy axis). This complex magnetic field dependence betrays the importance of spin-orbit coupling in this system, which makes the field-induced phase behavior highly sensitive to both the applied magnetic-field magnitude and direction. It is further shown that rapid field-induced changes in the structure and orbital populations are responsible for the highly field-tunable conducting properties of Ca3 Ru2 O7, and that the most dramatic magnetoconductivities are associated with an "orbital disordered" phase regime in which there is a random mixture of a - and b -axis oriented Ru moments and d -orbital populations on the Ru ions.