Effect of antiferromagnetic spin correlations on lattice distortion and charge ordering in Pr0.5Ca1.5MnO4

Abstract

We use neutron scattering to study the lattice and magnetic structure of the layered half-doped manganite Pr(0.5)Ca(1.5)MnO(4). On cooling from high temperature, the system first becomes charge-and orbital-ordered (CO/OO) near T(CO) = 300 K and then develops checkerboard-like antiferromagnetic (AF) order below T(N) = 130 K. At temperatures above T(N) but below T(CO) (T(N)<T<T(CO)), the appearance of short-range AF spin correlations suppresses the CO/OO-induced orthorhombic strain, contrasting with other half-doped manganites, where AF order has no observable effect on the lattice distortion. These results suggest that a strong spin-lattice coupling and the competition between AF exchange and CO/OO ordering ultimately determines the low-temperature properties of the system.

Fig. 1.

Structural scatterings and their temperature dependence. (a) Schematic view of the CE-type AF ordering in the MnO₂-plane. The black dashed line represents the periodicity of the unit cell for the Mn³⁺ sublattice, and the blue dashed line shows that of the Mn⁴⁺ sublattice. Possible spin arrangements in the c/2 stacking layers are marked by red arrows. The directions of Mn³⁺ orbitals form zigzag ferromagnetic chains (red line) that order antiferromagnetically. (b) The observed nuclear peaks (black open circles), CO/OO-induced superlattice peaks (green open circles) and magnetic ordering (solid circles) in reciprocal space. The dotted open circles represent the observed weak nuclear peaks that are disallowed by orthorhombic symmetry, indicating that the symmetry is lower than orthorhombic. Temperature dependence of the AF peak intensity from (1/4, 1/4, 3/2) (c) and (1/2, 0, 1/2) (e) and temperature dependence of CO/OO peak intensity from (3/2, 3/2, 0) (d) and (3/4, 5/4, 0) (f ) are shown.

Fig. 2.

The magnetic structure determination of PCMO. (a) Two possible spin arrangements for the Mn³⁺ sublattice as obtained from Rietveld analysis of the HRNPD data and fits to single crystal integrated intensities at different positions. (b) The geometrical relationship between the Mn³⁺ spin and the MnO₂ plane. (c) Scattering data along q = (1/4, 1/4, L) at T = 50 and 300 K, respectively. (d–f) The θ − 2θ scans for q = (3/4, 3/4, L), (1/2, 0, L), and (3/2, 0, L) that are projected to the [0, 0, L] direction. The intensities of observed magnetic peaks are fit to the generic magnetic form factor for Mn³⁺ (g) and Mn⁴⁺ (h) ions.

Fig. 3.

Temperature dependence of lattice parameters and unit cell volume. The dashed line near 300 K marks the CO/OO transition temperature T_CO. Whereas the in-plane a and b lattice parameters show clear anomalies around T_CO and T_N, the c-axis lattice parameter changes smoothly across both transitions. The dash-dotted lines in a are guides to the eye.

Fig. 4.

Strong spin-lattice coupling near the magnetic transition temperature. (a–c) Mesh-scans around the nuclear Bragg peak (2,0,0)_O (in orthorhombic notation) at T = 30, 160, and 320 K. (d and e) The corresponding mesh-scans around CO/OO-induced superlattice peak (2,1/2,0)_O at 30 and 160 K. (f) Wave vector scans of the same CO/OO peak at selected temperatures. (g) Temperature dependence of the peak intensity from powder monitored at 2θ = 36.61°, which corresponds to the (1,1,2)_t structural peak in tetragonal notation. (Inset) The splitting of the (1,1,2)_t peak [the actual (0,2,2)_O and (2,0,2)_O in orthorhombic symmetry] becomes much more prominent at 160 K and recovers back to one peak at low temperature. (h) Temperature dependence of the obliqueness, the separation between the center of the split peaks in reciprocal space, for (2,0,0)_O and (2,1/2,0)_O.

Fig. 5.

Crossover from two-dimensional AF fluctuations to three-dimensional AF order. Wave vector scans of AF scattering from the Mn³⁺ sublattice near q = (1/4, 1/4, 1/2) within Mn-O plane (a) and along the interplane (c) direction. Similar scans from the Mn⁴⁺ sublattice near (1/2, 0, 1/2) are presented in b and d. (Insets) The evolution of magnetic correlation lengths above the long range AF order temperature T_N = 130 K. (a and f) Temperature profiles of short-range magnetic scattering measured at q = (0.28, 0.28, 3/2) (e) and q = (0.535, 0, 1/2) (f). Those wave vectors have been chosen to avoid the contamination from the magnetic Bragg peaks.