Tuning the competing phases of bilayer ruthenate $\mathrm{C}{\mathrm{a}}_3\mathrm{R}{\mathrm{u}}_2{\mathrm{O}}_7$ via dilute Mn impurities and magnetic field

We have systematically investigated the evolution of the magnetic structure of the bilayer ruthenate $\mathrm{C}{\mathrm{a}}_3{\left(\mathrm{R}{\mathrm{u}}_{1-x}\mathrm{M}{\mathrm{n}}_x\right)}_2{\mathrm{O}}_7$ induced upon Mn doping. For $0<x\le 0.03$, the materials exhibit the same spin structure as that of the parent compound at low temperature, while an incommensurate cycloidal magnetic structure emerges at $T$ slightly above the metal-insulator transition (MIT) temperature $\left(T_{\mathrm{MIT}}\right)$. In contrast, for $x\ge 0.04$ the ground state becomes a G-type antiferromagnetic Mott insulator. Furthermore, we have observed magnetic-field-induced transitions in $\mathrm{C}{\mathrm{a}}_3{\left(\mathrm{R}{\mathrm{u}}_{0.96}\mathrm{M}{\mathrm{n}}_{0.04}\right)}_2{\mathrm{O}}_7$, which is positioned at the phase boundary. Below $T_{\mathrm{MIT}}$, the magnetic transition is accompanied by a structural transition, as well as a dramatic change in the electronic properties from a Mott insulator to a localized phase. On the contrary, an incommensurate-to-commensurate spin structure transition is observed for $T_{\mathrm{MIT}}<T<T_{\mathrm{ICM}}$. Our results suggest strong competing magnetic tendencies in this bilayer ruthenate system that are very susceptible to $3d$ transition-metal substitution and magnetic field.

Figure 1

(a) Schematics of the crystal and magnetic structures of Mn-doped $\mathrm{C}{\mathrm{a}}_3\mathrm{R}{\mathrm{u}}_2{\mathrm{O}}_7$. AFM-b: ferromagnetic bilayers coupled antiparallel along the $c$ axis. The staggered magnetic moments are along the $b$ axis. G-AFM: the nearest-neighbor magnetic moments are antiferromagnetic. CAFM: canted antiferromagnetic structure consisting of an antiferromagnetic component (AFM-a) and a ferromagnetic component along the $b$ axis. (b), (c) In-plane view of one $\mathrm{Ru}{\mathrm{O}}_2$ layer of AFM-b and ICM cycloidal magnetic structures, respectively.

Figure 2

Temperature dependence of the intensity of the representative magnetic Bragg peaks (1 0 2), (0 0 1), and (δ 0 1) of ${\mathrm{Ca}}_3{\left({\mathrm{Ru}}_{1-x}{\mathrm{Mn}}_x\right)}_2{\mathrm{O}}_7$ $(x=0.03$, 0.04, and 0.05), $\delta =0.023$ and 0.026 for $x=0.03$ and 0.04, respectively.

Figure 3

(a), (b) Neutron diffraction scans along the [1 0 0] direction across the magnetic wave vector (0 0 5) at representative temperatures. Note that $H$ is in reduced lattice units. (c), (d) Neutron diffraction scans across the magnetic wave vector (0 0 1) in the ICM magnetic phase. (e), (f) Rocking curve scans across the magnetic wave vector (1 0 2) at representative temperatures.

Figure 4

Lattice parameters of ${\mathrm{Ca}}_3{\left({\mathrm{Ru}}_{1-x}{\mathrm{Mn}}_x\right)}_2{\mathrm{O}}_7$ $(x=0.03$ and 0.04) as a function of temperature.

Figure 5

(a) Temperature dependence of the in-plane resistivity ${\rho }_{\mathrm{ab}}$ at $B=0\mathrm{T}$ and 9 T, $B\parallel b$ axis. (b), (c) Field dependence of the in-plane resistivity ${\rho }_{\mathrm{ab}}$ and magnetization $M$ at $T=10$ K and 34 K, $B\left|\right|b$ axis. Measurements were done on ${\mathrm{Ca}}_3{\left({\mathrm{Ru}}_{1-x}{\mathrm{Mn}}_x\right)}_2{\mathrm{O}}_7$ with $x=0.04$.

Figure 6

(a), (b) Rocking curve scans on $\mathrm{C}{\mathrm{a}}_3{\left(\mathrm{R}{\mathrm{u}}_{1-x}\mathrm{M}{\mathrm{n}}_x\right)}_2{\mathrm{O}}_7$ $\left(x=0.04\right)$ across $\mathbf{q}=\left(102\right)$ and (0 0 1) at $B=0\mathrm{T}$ and 8 T, respectively, $T=10\mathrm{K}$. (c), (d) The intensity of $\mathbf{q}=\left(102\right)$ and (0 0 1) magnetic Bragg peaks as a function of magnetic field, $T=10\mathrm{K}$.

Figure 7

(a), (b) Neutron diffraction scan on $\mathrm{C}{\mathrm{a}}_3{\left(\mathrm{R}{\mathrm{u}}_{1-x}\mathrm{M}{\mathrm{n}}_x\right)}_2{\mathrm{O}}_7$ $\left(x=0.04\right)$ along [1 0 0] direction across the magnetic Bragg peak $\mathbf{q}=\left(001\right)$ at $B=0\mathrm{T}$ and 5 T, $T=34\mathrm{K}$. Note that $H$ is in reduced lattice units (r.l.u.). (c), (d) The intensity of $\mathbf{q}=\left(001\right)$ and (0.0245 0 1) magnetic Bragg peaks as a function of magnetic field, $T=34\mathrm{K}$.

Figure 8

Lattice constants $a$ and $c$ of ${\mathrm{Ca}}_3{\left({\mathrm{Ru}}_{1-x}{\mathrm{Mn}}_x\right)}_2{\mathrm{O}}_7$ $\left(x=0.04\right)$ as a function of magnetic field at (a) $T=10\mathrm{K}$ and (b) $T=32\mathrm{K}$.