Anomalous lattice tilting across a magnetic oxide heterostructure

It is well known that the conventional lattice flexibility of an epitaxial layer is restricted by its growth template and bulk counterpart. Here, we report that the interface-engineered ${(\mathrm{La},\mathrm{Ca})\mathrm{MnO}}_3$ layer can exhibit an anomalous lattice-tilting pattern, featured by the interaxis angle $\alpha$ that exceeds the range of the lattice flexibility mentioned earlier. By increasing the adjacent ${\mathrm{CaRuO}}_3$ layer thickness, the ${(\mathrm{La},\mathrm{Ca})\mathrm{MnO}}_3$ layer shows a decreasing $\alpha$ down to 89.25 °, which is out of the flexible range between 93.82 ° (from the ${\mathrm{CaRuO}}_3$ template) and 89.86 ° [from the ${(\mathrm{La},\mathrm{Ca})\mathrm{MnO}}_3$ bulk]. The resulting antiparallel lattice tilting makes the ${(\mathrm{La},\mathrm{Ca})\mathrm{MnO}}_3/{\mathrm{CaRuO}}_3$ interface similar to a crystal twinning plane to lower the interfacial energy raised by the structural discontinuity. Also, a monotonic reduction of magnetic coercivity (from 205 to 70 Oe) is observed on decreasing $\alpha$ (from 90 ° to 89.25 °) in ${(\mathrm{La},\mathrm{Ca})\mathrm{MnO}}_3$ layers, providing an additional approach to tunable magnetic properties without changing the epitaxial strain. Our results not only present a new lattice-engineering strategy of using the interface similar to a crystal-twinning plane in designing heterostructures, but also they reveal the application of such a strategy for tunable magnetic properties beyond the epitaxial strain.

Figure 1

The sketches for conventional and unconventional lattice flexibility. Under a tensile strain, the sketch of the layer lattice length $a$ with the conventional flexibility of $a_S\ge a\ge a_B$ is plotted in (a), and unconventional $a<a_B$ in (b). For lattice tilting, the sketch of the layer interaxis angle $\alpha$ with the conventional flexibility of ${\alpha }_S\ge \alpha \ge {\alpha }_B$ is plotted in (c), and unconventional $\alpha <{\alpha }_B$ in (d). The dashed rectangles represent the bulk counterpart's lattice for the layer.

Figure 2

Conventional lattice tilting in thick LCMO layers. (a) The STEM image for CRO/LCMO/CRO//NGO. The layer thickness is 32 u.c. for the CRO buffer layer, 15 u.c. for LCMO, and 16 u.c. for CRO capping. (b) The modified STEM image, which is horizontally stretched by three times compared to the original area highlighted in (a). (c) RSMs around $(0\pm 13)$ reflections of LCMO/CRO//NGO. The asymmetric reflections for CRO and LCMO are indicated by orange and blue arrows, respectively. Also, the corresponding $L$ scans (gray open circles) and fitting curves (red lines) are compared at both sides of (c). Here, $Q_y$ and $Q_z$ stand for $\lambda /2b$ and $3\lambda /2c$, where $\lambda$ is the wavelength of X-ray (1.5406 Å) [39, 44]. (d) Relationship between the asymmetric $(0\pm 13)$ reflections and lattice tilting.

Figure 3

Unconventional lattice tilting in ferromagnetic 8-u.c. LCMO layers. (a) The sketches for $x$-dependent monoclinic-like lattice tilting of trilayers. (b) and (c) RSMs around $(0\pm 13)$ reflections for CRO/LCMO/CRO//NGO with different $x$. The asymmetric reflections of LCMO and CRO are indicated by blue and orange arrows, respectively. (d) The evolution of the LCMO $(0\pm 13)$ reflections on $x$ (from 12 to 32 u.c.). (e) $M\text{−}H$ hysteresis loops measured at 150 K for trilayers with $x$ from 12 to 32 u.c.; $H_C$ and $H_S$ are denoted by blue and red circles, respectively.

Figure 4

OOR-related lattice tilting. The monoclinic-like lattice tilting can be induced from a nonrotated oxygen octahedral network in (a) by applying the out-of-phase “–” rotation along the $c$-axis in (b), and then the “–” rotation along the $b$-axis in (c). The HIXRD scans around (1 1/2 3/2) and (1/2 3/2 5/2) are plotted for trilayers with $x=32\mathrm{u}.\mathrm{c}.$ in (d) and $x=12\mathrm{u}.\mathrm{c}.$ in (e). Insets indicate the corresponding monoclinic-like lattice tilting in the heterostructure.

Figure 5

The monotonic reduction of magnetic coercivity on decreasing $\alpha$. (a) The sketches for the monoclinic-like lattice tilting in trilayers with different $y$. (b) $M\text{−}H$ loops measured at 150 K for four samples: (24, 6), (24, 16), (32, 6) and (32, 16). (c) $H_C$ and $H_S$ plotted as a function of $\alpha$ obtained from different trilayers. The 80-u.c. LCMO//NGO sample with bulk-like $\alpha$ has been added for reference.