Influence of Ni/Mn cation order on the spin-phonon coupling in multifunctional ${\text{La}}_2{\text{NiMnO}}_6$ epitaxial films by polarized Raman spectroscopy

We report the influence of Ni/Mn ordering on the spin-phonon coupling in multifunctional ${\text{La}}_2{\text{NiMnO}}_6$. Three types of films with different levels of structural order, including long-range Ni/Mn cation order, cation disorder, and an admixture of the ordered and disordered phases are compared by polarized micro-Raman spectroscopy and magnetometry. Each film displays a strong dependence on the polarization configuration and a unique set of Raman-active phonon excitations. Long-range cation ordering results in the splitting of Raman-active phonon peaks because of Brillouin-zone folding and lowering symmetry. Phonon mode softening begins clearly at a distinct temperature for each sample revealing a strong spin-lattice interaction. It follows closely the magnetization curve in ordered films. Unlike the admixture and the ordered films, softening behavior is strongly suppressed in the cation-disordered films. These differences may be understood based on the variation in amplitude of the spin-spin correlation functions due to the local Ni/Mn cation ordering.

Figure 1

(Color online) (a) Typical $M\text{−}T$ curves of ordered (top curve from 10 K side), disordered (bottom curve from 10 K side), and admixture (middle curve from 10 K side) LNMO films. Arrows point out the magnetic transition temperatures for each type of films. The inset shows a typical $M\text{−}H$ loop of an ordered film at 10 K. (b) A typical polarization configuration dependence of the Raman spectra for ordered LNMO films at 298 K. The bottom inset shows a magnified view of the low-frequency spectral features. The top inset shows the polarization scattering configuration used to measure the Raman spectra. Spectra have been shifted vertically for clarity.

Figure 2

(Color online) Raman spectra of ordered (top), admixture (middle), and disordered (bottom) films measured in the (a) XX and (b) XY polarization configurations at 10 K. Respective insets show the magnified view of the low-frequency regime for the ordered (top), the admixture (middle), and the disordered (bottom) films. These spectra have been shifted vertically for clarity. Arrows show the selected excitations which evolve as a function of Ni/Mn ordering.

Figure 3

(Color online) Temperature dependence of the Raman spectra for an ordered film in the (a) XX and (b) XY configurations. The spectra from top to bottom are, respectively, measured at 10, 100, 150, 200, 250, and 298 K. The spectra are shifted vertically. In (a), the dotted vertical line at a fixed frequency is a guide to the eye to show the shift with temperature of the Raman excitation.

Figure 4

(Color online) Temperature dependence of the Raman spectra for a disordered film in the (a) XX and (b) XY configurations. The spectra from top to bottom are, respectively, measured at 10, 100, 150, 200, 250, and 298 K. The spectra are also vertically shifted. In (a), the dotted vertical line at a fixed frequency is a guide to the eye to show the shift with temperature of the Raman excitation.

Figure 5

(Color online) Temperature dependence of the Raman spectra for an admixture film in the (a) XX and (b) XY configurations. The spectra from top to bottom are, respectively, measured at 10, 100, 150, 200, 250, and 298 K. The spectra are shifted vertically. In (a), the dotted vertical line at a fixed frequency is a guide to the eye to show the shift with temperature of the Raman excitation.

Figure 6

(Color online) A schematic illustration of (a) a random distribution of $B^{\prime }/B^{″}$ cations; (b) well-ordered $B^{\prime }/B^{″}$ cations at the $B$ sites in an ideal pseudocubic $AB{\text{O}}_3$ unit cell. Schematics of (c) an ideal $A_2{B}^{\prime }{B}^{″}{\text{O}}_6$ double-perovskite unit cell; and (d) stacking of the $B\prime {\text{O}}_6$ and $B″{\text{O}}_6$ octahedra in long-range ordered $A_2{B}^{\prime }{B}^{″}{\text{O}}_6$. Red (dark) and yellow (gray) spheres represent $B^{\prime }$ and $B^{″}$ cations, respectively, big spheres represent La and small spheres represent oxygen. $B\prime {\text{O}}_6$ octahedra are represented by red (dark) and $B″{\text{O}}_6$ by yellow (gray). In (d), La and O atoms have been removed for clarity.

Figure 7

(Color online) Softening in the stretching $667{\text{cm}}^{-1}$ phonon excitation as a function of temperature for (a) disordered, (b) admixture, and (c) ordered LNMO films. Arrows point to the observed magnetic transition temperatures in these films. It is important to note that the highest $\text{FM-}{T}_{\text{c}}$ is close to 295 K in the admixture film which is close to the limit in temperature of our Raman spectrometer.