Colossal Magnetoresistance in a Mott Insulator via Magnetic Field-Driven Insulator-Metal Transition

We present a new type of colossal magnetoresistance (CMR) arising from an anomalous collapse of the Mott insulating state via a modest magnetic field in a bilayer ruthenate, Ti-doped ${\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$. Such an insulator-metal transition is accompanied by changes in both lattice and magnetic structures. Our findings have important implications because a magnetic field usually stabilizes the insulating ground state in a Mott-Hubbard system, thus calling for a deeper theoretical study to reexamine the magnetic field tuning of Mott systems with magnetic and electronic instabilities and spin-lattice-charge coupling. This study further provides a model approach to search for CMR systems other than manganites, such as Mott insulators in the vicinity of the boundary between competing phases.

Figure 1

Temperature and magnetic field dependence of the resistivity. (a),(b) Temperature dependence of the resistivity, ${\rho }_c$ and ${\rho }_{ab}$, measured at $B=0\mathrm{T}$ and 9 T applied along the $b$ axis. The magnetic field was applied at $T=100\mathrm{K}$ and the measurements were taken while cooling. (c),(d) Magnetic field dependence of ${\rho }_c$ and ${\rho }_{ab}$ at $T=10\mathrm{K}$.

Figure 2

(a) Field dependence of lattice parameters. Statistical errors from the fitting procedure are smaller than the symbol size. (b) Field dependence of the integrated intensity of magnetic Bragg peaks (1 0 2) and (0 0 1), respectively, measured at $T=10\mathrm{K}$. The magnetic field was increased for both nuclear and magnetic Bragg peak scans. (c),(d) Rocking curves of the (1 0 2) and (0 0 1) magnetic Bragg peaks measured at $B=0\mathrm{T}$ and 10 T applied along the $b$ axis. Measurement temperature $T=10\mathrm{K}$.

Figure 3

Phase diagram and the schematics of the magnetic structures. (a) $T\text{−}B$ phase diagram of ${\mathrm{Ca}}_3{({\mathrm{Ru}}_{0.97}{\mathrm{Ti}}_{0.03})}_2{\mathrm{O}}_7$ in a magnetic field along the $b$ axis. The solid squares, circles, and diamonds are phase boundaries determined by neutron diffraction measurements: solid squares denote the points where (0 0 1) magnetic Bragg peak shows up, and solid circles represent where (1 0 2) magnetic Bragg peak disappears. The solid diamonds stand for the points where (0 0 1) signal disappears completely. The open diamonds are the phase boundaries determined by the resistivity measurements. (b) Schematics of spin structures of AFM-a, G-AFM, and CAFM phases.

Figure 4

Projected density of states (PDOS) of the Ru $d_{xy}$ and $d_{xz}/d_{yz}$ orbitals. (a) PDOS calculated using the low temperature crystal structure and G-AFM magnetic structure but with the on-site Coulomb interaction $\mathrm{U}=0$. (b) PDOS calculated using the low temperature crystal structure and G-AFM magnetic structure and with $\mathrm{U}=2\mathrm{eV}$. (c) PDOS calculated using high temperature crystal structure and AFM-a type magnetic structure, a state similar to the field-induced one above the critical field, and with $\mathrm{U}=2\mathrm{eV}$.