Impact of $\mathrm{Co}∕\mathrm{Mn}$ cation ordering on phonon anomalies in ${\mathrm{La}}_2\mathrm{Co}\mathrm{Mn}{\mathrm{O}}_6$ double perovskites: Raman spectroscopy

Manifestation of $\mathrm{Co}∕\mathrm{Mn}$ ordering on the spin-lattice dynamics of ${\mathrm{La}}_2\mathrm{Co}\mathrm{Mn}{\mathrm{O}}_6$ (LCMO) double perovskites is studied by Raman spectroscopy. We investigate the temperature dependence of the magnetic and the optical properties for (i) microcrystals containing both the ordered and disordered phases and (ii) $\mathrm{Co}∕\mathrm{Mn}$ ordered epitaxial films. Our results show distinct onsets of phonon mode softening in the vicinity of the magnetic phase transitions, revealing a strong spin-lattice coupling in LCMO. Moreover, $\mathrm{Co}∕\mathrm{Mn}$ ordering in the thin films leads to the appearance of additional excitations that had not been observed previously. We interpret these features as a result of Brillouin zone folding due to the doubling of the unit cell in the ordered phase.

Figure 1

(Color online) Normalized $M\text{−}T$ curves of the bulk and film ${\mathrm{La}}_2\mathrm{Co}\mathrm{Mn}{\mathrm{O}}_6$. The inset shows the $\mathbf{M}\text{−}\mathbf{H}$ loop of the bulk sample recorded at $10\mathrm{K}$.

Figure 2

(Color online) Raman spectra of (a) microcrystal and (b) epitaxial film recorded at $295\mathrm{K}$ in the parallel-$XX$ and crossed scattering-$XY$ configurations. (c) Raman spectra of the epitaxial film and micro crystal recorded at $10\mathrm{K}$ in crossed scattering-$XY$ configurations.

Figure 3

(Color online) (a) Raman spectra of the epitaxial film recorded at different temperatures in the crossed scattering-$XY$ scattering configurations. (b) A typical fit to the experimental $10\mathrm{K}$ curve displaying the four peaks.

Figure 4

(Color online) Raman spectra of (a) the epitaxial film and (b) the microcrystal recorded at different temperatures in the parallel-$XX$ scattering configuration. Curves are presented in increasing temperature from top to bottom. (c) Temperature dependence relative shift in the stretching mode frequency on (c) thin film and (d) microcrystal.