Ru-doping-induced ferromagnetism in charge-ordered ${\text{La}}_{0.4}{\text{Ca}}_{0.6}{\text{MnO}}_3$

Impurity effects on the stability of a charge-ordered antiferromagnetic state in electron-doped manganite ${\text{La}}_{0.4}{\text{Ca}}_{0.6}{\text{MnO}}_3$ are investigated by doping Ru for Mn. Our results show that Ru doping at a tiny level of $\sim 2\%$ is sufficient to suppress significantly the charge-ordered insulated state and generate a ferromagnetic metallic one. The blocked metastable state and the first-order metal-insulator transitions observed in the slightly doped samples can be attributed to the phase-separated state where ferromagnetic domains are embedded in the charge-ordered matrix. The doping effect is further investigated theoretically based on the two-orbital model. The calculation suggests that two factors are vitally important for the Ru-doping driven strong ferromagnetic tendency: (1) the ferromagnetic coupling between Ru and Mn spins and (2) the enhancement of $e_g$ electron density while the topological defects always contribute to the instability of charge-ordered state.

Figure 1

(Color online) (a) X-ray diffraction $\theta \text{−}2\theta$ spectra at room temperature for a series of samples of variance $y$ (from bottom to top): (A) $y=0$; (B) $y=0.02$; (C) $y=0.03$; (D) $y=0.05$; and (E) $y=0.07$. (b) Rietveld refinements for sample $y=0.07$. The cross points and solid line represent the experimental data points and the Rietveld refined result, respectively. The lower rows of markers indicate the positions of structure reflections. The difference is shown on the bottom as solid line.

Figure 2

(Color online) Temperature profiles of resistivity at various external magnetic fields for samples of (a) $y=0$; (b) $y=0.02$; (c) $y=0.05$; and (d) $y=0.07$, respectively. The inset of (b) shows $T_{\text{MI}}$ of sample $y=0.02$ for both cooling (open circle) and warming (solid circle) cases as a function of magnetic field $H$. The inset of (d) shows $T_{\text{MI}}$ of the cooling case as a function of $y$, the black line is guide for the eyes.

Figure 3

(Color online) Temperature profiles of magnetization at $H=100\text{Oe}$ for sample of (a) $y=0$; (b) $y=0.02$; (c) $y=0.05$; and (d) $y=0.07$, respectively. Measurements were performed in both cooling and warming runs.

Figure 4

(Color online) Measured magnetization as a function of $H$ at $T=5\text{K}$ for samples of (a) $y=0.02$ and $y=0.05$; and (b) $y=0.07$, respectively. The arrows indicate the cycle of $H$ varying during measurements. The inset of (b) presents the $y$ dependence of magnetization at $T=5\text{K}$ and $H=3\text{T}$.

Figure 5

(Color online) Magnetization (close circle) and resistivity (open circle) as a function of magnetic field $H$ of sample $y=0.02$ at $T=5\text{K}$.

Figure 6

(Color online) Typical Monte Carlo snapshots of the spin configuration. (a) $y=0.031$ and (b) $y=0.063$. The CE-type spin orders are indicated using dot squares in (a).