Competing Misfit Relaxation Mechanisms in Epitaxial Correlated Oxides

Strain engineering of functional properties in epitaxial thin films of strongly correlated oxides exhibiting octahedral-framework structures is hindered by the lack of adequate misfit relaxation models. Here we present unreported experimental evidence of a four-stage hierarchical development of octahedral-framework perturbations resulting from a progressive imbalance between electronic, elastic, and octahedral tilting energies in ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ epitaxial thin films grown on ${\mathrm{SrTiO}}_3$ substrates. Electronic softening of the Mn-O bonds near the substrate leads to the formation of an interfacial layer clamped to the substrate with strongly degraded magnetotransport properties, i.e., the so-called dead layer, while rigid octahedral tilts become relevant at advanced growth stages without significant effects on charge transport and magnetic ordering.

Figure 1

(a) Thickness dependence of the in-plane and out-of-plane lattice parameters. For $c$-axis parameters, open symbols were obtained by fitting 002 rocking profiles, black closed symbols from $1/2$ $1/2$ $3/2$ superstructure reflections, and red symbols (gray) by averaging over 17 different $h/2$ $k/2$ $l/2$ ($h$, $k$, $l=1$, 3, 5) superstructure reflections [19]. (b) Shift in the XPS binding energies of the Mn $2p_{3/2}$ peak for LSMO films with different thicknesses, relative to the 61 nm thick sample. Circles and squares correspond to measurements performed with Escalab and Phoibos equipments, respectively, [19]. (c) Thickness dependence of the Poisson’s ratio. (d) Thickness dependence of the pseudo cubic unit cell volume. (e) Thickness dependence of the in-plane (${\chi }^{\parallel }$) and out-of-plane (${\chi }^{\perp }$) shear angles. The shadowed area indicates the nontwinned thickness range.

Figure 2

OC-EBS images corresponding to 1.9 nm (a) and 3.3 nm (b) thick films. Right panels show the corresponding $HK$ RSM’s of the 200 and $1/2$ $1/2$ $3/2$ reflections. (c) Schematics showing the effect of (100) and (010) twin planes on diffraction.

Figure 3

(a) Resistivity versus temperature curves for films having different thickness above and below the shear transition thickness, as indicated. (b) Thickness dependence of the magnetic moment for $h<10\mathrm{nm}$. Lower inset shows the dependence in an expanded thickness range. Upper inset: Temperature dependence of the magnetization at low field ($H=5\mathrm{kOe}$). (c) Thickness dependence of the Curie temperature, $T_C$.

Figure 4

Mechanisms of lattice distortions through regimes I,II, III, and IV. Regime II bridging the monoclinic and rhombohedral states, is not labeled in the diagram for clarity. The orbital character and its relation with octahedral strain in the monoclinic ($M$) and rhombohedral ($R$) states is indicated. The transition between the two states is mediated by a decrease of the ${\mathrm{Mn}}^{3+}/{\mathrm{Mn}}^{4+}$ ratio (blue line). Lower panels illustrate the combined octahedral tilting mechanism releasing the elastic energy stored in regime III.