Melting of magnetic correlations in charge-orbital ordered ${\text{La}}_{1/2}{\text{Sr}}_{3/2}{\text{MnO}}_4$: Competition of ferromagnetic and antiferromagnetic states

The magnetic correlations in the charge and orbital ordered manganite ${\text{La}}_{1/2}{\text{Sr}}_{3/2}{\text{MnO}}_4$ have been studied by elastic and inelastic neutron scattering techniques. Out of the well-defined charge exchange (CE)-type magnetic structure with the corresponding magnons, a competition between CE-type and ferromagnetic fluctuations develops. Whereas ferromagnetic correlations are fully suppressed by the static CE-type order at low temperature, elastic and inelastic CE-type correlations disappear with the melting of the charge-orbital order at high temperature. In its charge-orbital disordered phase, ${\text{La}}_{1/2}{\text{Sr}}_{3/2}{\text{MnO}}_4$ exhibits a dispersion of ferromagnetic correlations, which remarkably resembles the magnon dispersion in ferromagnetically ordered metallic perovskite manganites.

Figure 1

(Color online) Schematic representation of the charge, orbital, and spin ordering in the CE-type arrangement, as proposed by Goodenough (Ref. 12). The orbital and the magnetic lattices of the ${\text{Mn}}^{3+}$ subsystem are indicated by the dotted and dashed lines, respectively. Notice that the zigzag chains run along the [110] direction.

Figure 2

(a) Temperature dependence of the in-plane electric resistivity ${\rho }_{ab}$, (b) the specific heat $c_p$, and (c) the macroscopic magnetization for a field $H=1\text{T}$ applied parallel and perpendicular to the $ab$ planes. Vertical gray lines mark $T_{\text{CO}}$ and $T_N$ as determined in the neutron scattering experiments.

Figure 3

(Color online) Intensity mappings of the elastic magnetic scattering around the magnetic CE-type position ${\mathit{Q}}_{\text{mag}}=\left(0.750.250\right)$ and around the two-dimensional FM zone center (1 0 0) at various temperatures (a) above the charge-orbital ordering at $T=250\text{K}$, [(b) and (c)] below $T_{\text{CO}}$ but above the Néel transition at $T=200\text{K}$ and $T=150\text{K}$, and (d) below the AFM transition at $T=100\text{K}$. All maps were calculated from a grid of $41\times 41$ data points with $\Delta q_h=\Delta q_k=0.0125$. The two arrows in (c) denote the directions of the scans investigated in more detail; scan (1) senses the stacking of zigzag fragments in the CE phase and scan (2) their length.

Figure 4

(Color online) Temperature dependence of the elastic magnetic intensity along scan 1, which is perpendicular to the zigzag chains. (a) Contour plot derived from a grid of data with $\Delta T=10\text{K}$ and $\Delta q_k=0.005\frac{2\pi }{a}$. (b) Representative raw-data scans underlying the contour plot. For clarity, the data are successively shifted vertically by 300 counts. The inset gives the profile of the magnetic Bragg reflection at low temperatures, $T=2.5\text{K}$. Lines correspond to fits as described in the text. In all data, a minor contamination by second harmonic neutrons centered at $\mathit{Q}=\left(100\right)$ is subtracted. The different scatterings of the two CE-type reflections arise from the Mn form-factor and geometry conditions.

Figure 5

(Color online) Temperature dependence of the elastic magnetic intensity along scan 2, which is parallel to the zigzag chains, at selected temperatures (a) above and (b) below the Néel transition at $T_N=110\text{K}$. Note that for the scans at 150 and 130 K, the scale on the ordinate is the same in both panels. In all data, a common background is subtracted. Lines correspond to fits with either Lorentzians (folded with the resolution) or resolution-limited Gaussians as discussed in the text.

Figure 6

(Color online) (a) Raw-data scans along the line $\left(0.250.25q_l\right)$ for various temperatures below $T_{\text{CO}}$. Gray-shaded areas mark spurious contributions by the scattering from aluminum. (b) Raw-data scans along [0 0 1] centered around $q_l=2.5$ for various temperatures close to $T_N$. Lines denote fits with Lorentzians.

Figure 7

Summary of the results of the analysis of the elastic magnetic scattering showing the temperature dependence of (a) the intensity observed at the FM position $\mathit{Q}=\left(100\right)$ and [(b) and (c)] at the AFM position $\mathit{Q}=\left(0.750.250\right)$ on a linear and logarithmic scale (d) of the determined correlation length $\xi$ in a direction parallel to the chains ${\xi }_{\parallel }$, perpendicular to the chains, and within the $ab$ plane, ${\xi }_{\perp }$, and along the tetragonal axis ${\xi }_c$, and (e) of the position of the AFM signal along the line $\mathit{Q}=\left(1\mp \epsilon \pm \epsilon 0\right)$.

Figure 8

(Color online) (a) Energy scan at the AFM zone center $\mathit{Q}=\left(0.750.250\right)$ at $T=4\text{K}$ and (b) at different positions perpendicular to the propagation of the chains at $\mathit{Q}=\left(0.75+q_{h}0.75-q_{h}0\right)$. (c) $q_l$ dependence of the magnon signal taken with a different experimental setup. Lines denote fits to the data. See text for details.

Figure 9

(Color online) (Upper panel) $\mathit{Q}$ scans at a finite energy $E=2.75\text{meV}$ for different temperatures across the CE position (0.75–0.75 0) in a direction (a) parallel to the zigzag chain [1 1 0] and (b) perpendicular to the chains along $\left[1\overline{1}0\right]$. (Lower panel) Raw-data scans tracking (c) the temperature dependence of the magnetic fluctuations around the FM position ${\mathit{Q}}_{\text{FM}}=\left(100\right)$ with $E=2.75\text{meV}$ and (d) the $\mathit{q}$ dependence of the FM fluctuations for $T=250\text{K}$. All scans are corrected for the different Bose contributions after the subtraction of a linear background. For clarity, subsequent scans are shifted by a constant amount on the abscissa. Lines correspond to fits with Gaussians.

Figure 10

(Color online) $\mathit{q}$ dependence of the FM fluctuations in the disordered phase at $T=250\text{K}$ for $\mathit{q}=\left(q_{h}00\right)$ and $\mathit{q}=\left(q_h{q}_{h}0\right)$. Solid lines denote a fit to the data using an isotropic dispersion relation according to Eq. (1); dotted lines give the magnon dispersion in the FM metallic state $\left(T=10\text{K}\right)$ of the perovskite ${\text{La}}_{0.7}{\text{Pb}}_{0.3}{\text{MnO}}_3$ taken from Ref. 48.

Figure 11

(Color online) Real-space sketches of the magnetic correlations in the ${\text{MnO}}_2$ layers for various temperatures (a) in the long-range ordered phase below $T_N$, [(b) and (c)] in the paramagnetic, but orbital ordered state $T_N<T<T_{\text{CO}}$, and (d) in the disordered regime above $T_{\text{CO}}$. Qualitatively, the four sketches are correlated with the intensity mappings presented in Fig. 3, as indicated by the temperatures associated with each sketch.