Fe-doping-induced evolution of charge-orbital ordering in a bicritical-state manganite

Impurity effects on the stability of a ferromagnetic-metallic state in a bicritical-state manganite, ${\left({\mathrm{La}}_{0.7}{\mathrm{Pr}}_{0.3}\right)}_{0.65}{\mathrm{Ca}}_{0.35}\mathrm{Mn}{\mathrm{O}}_3$, on the verge of metal-insulator transition have been investigated by substituting a variety of transition-metal atoms for Mn ones. Among them, Fe doping exhibits the exceptional ability to dramatically decrease the ferromagnetic transition temperature. Systematic studies on the magnetotransport properties and x-ray diffraction for the Fe-doped crystals have revealed that charge-orbital ordering evolves down to low temperatures, which strongly suppresses the ferromagnetic-metallic state. The observed glassy magnetic and transport properties as well as diffuse phase transition can be attributed to the phase-separated state where short-range charge-orbital-ordered clusters are embedded in the ferromagnetic-metallic matrix. Such a behavior in the Fe-doped manganites form a marked contrast to the Cr-doping effects on charge-orbital-ordered manganites known as the impurity-induced collapse of charge-orbital ordering.

Figure 1

(Color online) Temperature profiles of (a) resistivity at ${\mu }_{0}H=0\mathrm{T}$ and (b) magnetization at ${\mu }_{0}H=0.5\mathrm{T}$ for crystals of ${\left({\mathrm{La}}_{1-x}{\mathrm{Pr}}_x\right)}_{0.65}{\mathrm{Ca}}_{0.35}\mathrm{Mn}{\mathrm{O}}_3$ $(0.2⩽x⩽0.8)$. Solid and dashed lines indicate cooling and warming runs, respectively.

Figure 2

(Color online) Phase diagram for ${\left({\mathrm{La}}_{1-x}{\mathrm{Pr}}_x\right)}_{0.65}{\mathrm{Ca}}_{0.35}\mathrm{Mn}{\mathrm{O}}_3$. The ferromagnetic metal and charge-orbital-ordered insulator are denoted by FM and CO/OOI, respectively. $T_C$ and $T_{\mathrm{CO}}$, which were determined in cooling runs, are denoted with closed circles and closed squares, respectively. An arrow shows the parent compound $(x=0.3)$ for the impurity doping experiment described in this paper. The data for $x=0$ and 1 are from Ref. 33 (polycrystal) and Ref. 32 (single crystal), respectively.

Figure 3

(Color online) Temperature profiles of resistivity at ${\mu }_{0}H=0\mathrm{T}$ for undoped, 5% $M$-doped ${\left({\mathrm{La}}_{0.7}{\mathrm{Pr}}_{0.3}\right)}_{0.65}{\mathrm{Ca}}_{0.35}{\mathrm{Mn}}_{0.95}{M}_{0.05}{\mathrm{O}}_3$ ($M=\mathrm{Fe}$, Cr, and Ga), and 2% Ru-doped ${\left({\mathrm{La}}_{0.7}{\mathrm{Pr}}_{0.3}\right)}_{0.65}{\mathrm{Ca}}_{0.35}{\mathrm{Mn}}_{0.98}{\mathrm{Ru}}_{0.02}{\mathrm{O}}_3$ manganites. The peaks, corresponding to $T_C$, are denoted with closed triangles. Inset: $T_C$ versus the doping level for the above crystals.

Figure 4

(Color online) Temperature profiles of (a) resistivity at ${\mu }_{0}H=0\mathrm{T}$ and (b) magnetization at ${\mu }_{0}H=0.5\mathrm{T}$ for crystals of ${\left({\mathrm{La}}_{0.7}{\mathrm{Pr}}_{0.3}\right)}_{0.65}{\mathrm{Ca}}_{0.35}{\mathrm{Mn}}_{1-y}{\mathrm{Fe}}_y{\mathrm{O}}_3$ $(0⩽y⩽0.1)$. Measurements were performed in both cooling and warming runs. The magnetization data measured after the field-cooling (FC) and zero-field-cooling (ZFC) processes are denoted with the closed circles and the open ones, respectively.

Figure 5

(Color online) (a) Temperature dependence of x-ray diffraction profiles along the [110] direction around the (200) Bragg peak for ${\left({\mathrm{La}}_{0.7}{\mathrm{Pr}}_{0.3}\right)}_{0.65}{\mathrm{Ca}}_{0.35}{\mathrm{Mn}}_{1-y}{\mathrm{Fe}}_y{\mathrm{O}}_3$ ($y=0$, 0.05, and 0.06) crystals. The closed circles represent the data points and the dashed lines indicate the estimated backgrounds including the intensity of the diffuse scattering. (b) Temperature profiles of the integrated intensity of the x-ray superlattice reflection related to the charge-orbital ordering around a wave vector of $\left(\frac{1}{4}\frac{1}{4}0\right)$ for the $y=0$, $y=0.05$, and $y=0.06$ crystals. The dashed lines represent the corresponding temperature profiles of resistivity. These measurements were performed in a warming run.

Figure 6

(Color online) Phase diagrams for ${\left({\mathrm{La}}_{1-x}{\mathrm{Pr}}_x\right)}_{0.65}{\mathrm{Ca}}_{0.35}\mathrm{Mn}{\mathrm{O}}_3$ and ${\left({\mathrm{La}}_{0.7}{\mathrm{Pr}}_{0.3}\right)}_{0.65}{\mathrm{Ca}}_{0.35}{\mathrm{Mn}}_{1-y}{\mathrm{Fe}}_y{\mathrm{O}}_3$. The phase-separated (PS) state, or cluster glass (CG), where the ferromagnetic-metallic (FM) clusters and the short-range charge-orbital-ordered (SR CO/OO) clusters coexist, appears around $y\sim 0.06$. With further increasing $y$, it changes to the homogeneous spin-glass (SG) with atomic-scale correlation. The transition temperature to the FM, CO/OO, and SG (CG) phases is denoted with closed circles, squares, and triangles, respectively. The onset temperature of SR CO/OO, determined by the integrated intensity of x-ray superlattice reflection, is denoted with open squares.

Figure 7

(Color online) (a) Temperature profiles of resistivity at various magnetic fields for the ${\left({\mathrm{La}}_{0.7}{\mathrm{Pr}}_{0.3}\right)}_{0.65}{\mathrm{Ca}}_{0.35}{\mathrm{Mn}}_{1-y}{\mathrm{Fe}}_y{\mathrm{O}}_3$ $(y=0.07)$ crystal. Magnetic-field dependence of (b) resistivity and (c) magnetization at $2\mathrm{K}$ after the magnetic-field annealing procedures for the $y=0.07$ crystal. The measurements were performed once the magnetic fields were applied at room temperature and the crystal was cooled down to $2\mathrm{K}$ at the respective annealing fields, as depicted in the inset.