Signature of magnetic phase separation in the ground state of ${\text{Pr}}_{1-x}{\text{Ca}}_x{\text{MnO}}_3$

Neutron scattering has been used to investigate the evolution of long- and short-range charge-ordered (CO), ferromagnetic, and antiferromagnetic (AF) correlations in single crystals of ${\text{Pr}}_{1-x}{\text{Ca}}_x{\text{MnO}}_3$. The existence and population of spin clusters as reflected by short-range correlations are found to drastically depend on the doping and temperature. Concentrated spin clusters coexist with long-range canted AF order in a wide temperature range in the $x=0.3$ while clusters do not appear in the $x=0.4$ crystal. In contrast, both CO and AF order parameters in the $x=0.35$ crystal show a precipitous decrease below $\sim 35\text{K}$ where spin clusters form. These results provide direct evidence of magnetic phase separation and demonstrate a critical doping $x_c$ (close to $x=0.35$) that divides the inhomogeneous from homogeneous CO ground state.

Figure 1

(Color online) (a) Generic phase diagram of ${\text{Pr}}_{1-x}{\text{Ca}}_x{\text{MnO}}_3$ (Ref. 6). (b) Schematic $ab$-plane view of the CE-type structure with both orbital $\left(d_{3x^2-r^2}/d_{3y^2-r^2}\right)$ and spin orderings. The bigger square denotes the ${\text{Mn}}^{3+}$ orbital/magnetic unit cell while the smaller square shows the ${\text{Mn}}^{4+}$ magnetic unit cell. (c) The probed superlattice peak positions and the scan directions (marked by arrows) in reciprocal space. (d) Normalized $q$-scan profiles of the scattering near (1, 0, 0) at $T=10\text{K}$ for PCMO30, PCMO35, and PCMO40. The solid lines are guides to the eye.

Figure 2

(Color online) $T$ dependence of the FM diffuse component measured at (0.96, 0, 0) $\left(\Delta q=0.04\text{rlu}\right)$ for different $x$’s of ${\text{Pr}}_{1-x}{\text{Ca}}_x{\text{MnO}}_3$ on cooling (solid symbols) and warming (open symbols). The solid curves are guides to the eye. Arrows mark the Curie and Néel temperatures, as well as the onset of spin phase separation for PCMO35. The insets present the $T$ dependence of (a) the FM order parameter (Bragg-peak intensity) and (b) the FM diffuse component at selected $q$ positions from (1, 0, 0) for PCMO30.

Figure 3

(Color online) $T$ dependence of the intensity and extracted short-range correlation length of (a) FM at (1, 0, 0) and (b) ${\text{Mn}}^{4+}$ AF at (0.5, 0, 0) in PCMO30. The solid lines are guides to the eye. The solid symbols are for cooling while open symbols for warming. The insets show the $T$ dependence of (c) FM short-range correlation length in PCMO35 with large error bars due to the weak diffuse scattering profiles and (d) $q$ scans of ${\text{Mn}}^{4+}$ AF at (0.5, 0, 0) in PCMO30.

Figure 4

(Color online) $T$ dependence of order parameters for (a) CO structure measured at (2.25, 0.25, 0), (b) ${\text{Mn}}^{3+}$ AF ordering at (0.75, 0.25, 0), and (c) ${\text{Mn}}^{4+}$ AF ordering at (0.5, 0, 0) for the three doping $x$’s of ${\text{Pr}}_{1-x}{\text{Ca}}_x{\text{MnO}}_3$. The ${\text{Mn}}^{4+}$ AF order parameters for both $x=0.3$ and 0.4 (open dots) are plotted together in the bottom of panel (c) for comparison. Arrows mark the onset of phase separation for PCMO35. Column (d) illustrates the evolution from an inhomogeneous to a homogeneous CO ground state for corresponding doping levels. A $T$-induced change from homogeneous to inhomogeneous phase occurs in PCMO35.