Phonon metamorphosis in ferromagnetic manganite films: Probing the evolution of an inhomogeneous state

The analysis of phonon anomalies provides valuable information about the cooperative dynamics of lattice, spin, and charge degrees of freedom. Significant is the anomalous temperature dependence of the external modes observed in ${\mathrm{La}}_{2∕3}{\mathrm{Sr}}_{1∕3}\mathrm{Mn}{\mathrm{O}}_3$ (LSMO) films. The two external modes merge close to the ferromagnetic to paramagnetic transition at $T_C$ and, moreover, two new modes evolve in this temperature range with strong resonances at slightly higher frequencies. We propose that such a phonon metamorphosis probes the inhomogeneous Jahn-Teller distortion, manifest on the temperature scale $T_C$. The analysis is based on the first observation of all eight phonon modes in the metallic phase of LSMO and on susceptibility measurements which identify a Griffiths-like phase.

Figure 1

(Color online.) Rocking curves of the (111) reflex of LSMO No. 1, LSMO No. 2 grown on LSAT. The small FWHM value of both films confirms the good quality.

Figure 2

(Color online.) $3d$ plot of the (211) reflex of LSAT (substrate) and the LSMO No. 2 film. The bottom plane shows a contour plot of the $2\theta \text{−}\chi$ space.

Figure 3

Temperature-dependent optical conductivity $\left(\sigma \right)$ spectra of LSMO No. 2 (solid lines), with a spectral resolution $\sim 1{\mathrm{cm}}^{-1}$ on a logarithmic frequency scale. We found similar spectra for LSMO No. 1 (open circles at $333\mathrm{K}$ and right inset), except for a reduction of the respective absolute values and a slightly reduced temperature scale for the evolution of the external modes. The vertical lines connect the maxima of the external modes. Left inset: At $6\mathrm{K}$ the splitting of the external modes is strongest.

Figure 4

Metamorphosis of phonon modes: optical conductivity $\sigma$ of the external modes $({\nu }_1,{\nu }_2)$ and the new modes $({\nu }_3,{\nu }_4)$. The latter modes are absent at low temperatures. With increasing temperature, spectral weight is transferred from the low-energy external group to the new modes (see Fig. 6).

Figure 5

Left panel: Splitting $\Delta \nu ={\nu }_2-{\nu }_1$ between the eigenfrequencies of the two external modes at various temperatures (full circles). The solid line is the temperature dependent magnetization of LSMO No. 2. Right panel: Temperature dependence of the frequency of the new mode ${\nu }_3$. At $T_C$ the direction of the shift is reversed. Beyond $T_{\mathrm{MIT}}$ the phonon spectrum is invariant with respect to $T$.

Figure 6

Temperature dependence of the optical weight of the external modes for LSMO No. 2 and LSMO No. 1. $({\nu }_1+{\nu }_2)$ refers to the weight of the original external modes while $({\nu }_3+{\nu }_4)$ applies to the new modes. The inset displays the ratio of the optical weight of the new modes to that of the original modes.

Figure 7

Temperature dependence of the low-field inverse magnetic susceptibility for three samples (LSMO (sc): single crystal, LSMO No. 2 and LSMO No. 1). Arrows indicate $T_C$ from magnetization measurements. The effective magnetic moment $p_{\mathrm{eff}}$ in the paramagnetic phase slightly above $T^{*}$ is approximately $5{\mu }_B$ for all samples (Ref. 20).