Long-range Ni/Mn structural order in epitaxial double perovskite ${\text{La}}_2{\text{NiMnO}}_6$ thin films

We report and compare the structural, the magnetic, and the optical properties of ordered ${\text{La}}_2{\text{NiMnO}}_6$ thin films and its disordered ${\text{LaNi}}_{0.5}{\text{Mn}}_{0.5}{\text{O}}_3$ counterpart. An x-ray diffraction (XRD) study reveals that the $B$-site Ni/Mn ordering induces additional XRD reflections as the crystal symmetry is transformed from a pseudocubic perovskite unit cell in the disordered phase to a monoclinic form with larger lattice parameters for the ordered phase. Polarized Raman-spectroscopy studies reveal that the ordered samples are characterized by additional phonon excitations that are absent in the disordered phase. The appearance of these additional phonon excitations is interpreted as the clearest signature of Brillouin-zone folding as a result of the long-range Ni/Mn ordering in ${\text{La}}_2{\text{NiMnO}}_6$. Both ordered and disordered materials display a single ferromagnetic-to-paramagnetic transition. The ordered films display also a saturation magnetization close to $4.8{\mu }_{\text{B}}/\text{f}\text{.u}\text{.}$ and a transition temperature $\left(\text{FM-}{T}_{\text{c}}\right)$ around 270 K while the disordered ones have only a $3.7{\mu }_{\text{B}}/\text{f}\text{.u}\text{.}$ saturation magnetization and a $\text{FM-}{T}_{\text{c}}$ around 138 K. The differences in their magnetic behaviors are understood based on the distinct local electronic configurations of their Ni/Mn cations.

Figure 1

Typical XRD patterns of LNMO films grown on STO (111) at (a) $500°\text{C}$ and 300 mTorr ${\text{O}}_2$ and (b) at $800°\text{C}$ and 800 mTorr. The inset in (a) presents the out-of-plane lattice parameter of the films grown on STO (001) at $800°\text{C}$ as a function of the growth pressure. The inset in (b) shows a rocking curve recorded on the (202) reflections.

Figure 2

(Color online) The phase-stability diagram of ordered ${\text{La}}_2{\text{NiMnO}}_6$ (elliptical shaded area) and disordered ${\text{LaNi}}_{0.5}{\text{Mn}}_{0.5}{\text{O}}_3$ (rectangular shaded area). The symbols position the growth conditions of our LNMO and some found in the literature: solid circles for the present work; solid squares from Ref. 19; and stars from Refs. 16, 18). It is important to note that both the ordered and disordered phases can be independently stabilized only in a very narrow parameter window. (b) Typical polarization configuration dependence of the Raman spectra for ordered LNMO films at 298 K.

Figure 3

(Color online) Raman spectra for the (a) disordered and (b) well-ordered films measured in 10–300 K temperature range under $XY$ configuration. The corresponding insets show fits (shaded curves with varying contrast) to the 10 K experimental spectra around $650{\text{cm}}^{-1}$ confirming the presence of two and four stretching modes in the disordered and ordered films, respectively.

Figure 4

(Color online) Schematics of $A_2{B}^{\prime }{B}^{″}{\text{O}}_6$ crystal structures displaying (a) a random distribution of $B^{\prime }/B^{″}$ in the pseudocubic $AB{\text{O}}_3$ with a fully disordered unit cell, (b) alternatively arranged $B^{\prime }/B^{″}$ in ideal $AB{\text{O}}_3$ unit cell leading to a long-range-ordered double perovskite. The long-range cation ordering redefines the lattice parameter and crystal structure as shown in (c). Red (dark) spheres: $B^{\prime }$ cations; yellow (gray) spheres: $B^{″}$ cations, large spheres: $A$ cations; and small blue spheres: oxygen atoms.

Figure 5

(Color online) $M\text{−}H$ loops for (a) a disordered film and (b) an ordered film at 10 K. The corresponding insets show the respective $M\text{−}T$ curves displaying the distinct single ferromagnetic-to-paramagnetic phase transitions.