Interfacial control of domain structure and magnetic anisotropy in ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ manganite heterostructures

Structural domains in ferroic materials can be manipulated to achieve novel properties and functionalities of devices with multiple tunabilities. Herein, we report a study of tunable domain structures in ferroelastic ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ (LSMO) films via interfacial engineering. Distinct domain structures are formed in rhombohedral LSMO films depending on the crystallographic orientations of the orthorhombic ${\mathrm{NdGaO}}_3$ (NGO) substrates. A unidirectional lattice modulation is observed in LSMO films grown on $\mathrm{NGO}{\left(110\right)}_{\mathrm{Or}}$ substrates, whereas a unidirectional twinned-domain structure is generated by $\mathrm{NGO}{\left(001\right)}_{\mathrm{Or}}$ substrates (where Or in the subscript denotes orthorhombic notation). The orientation-dependent domain structures in LSMO films are controlled by anisotropic strain as well as interfacial oxygen octahedral coupling at the heterointerface between LSMO and NGO. The orbital occupancy and in-plane magnetic anisotropy are markedly affected by strain relief induced by the unidirectional structural domains in LSMO films. In addition, the structural results observed in LSMO/$\mathrm{NGO}{\left(001\right)}_{\mathrm{Or}}$ films further demonstrate that anisotropic strain is capable to disturb octahedral connectivity in epitaxial films. Our findings provide future directions to understand and control the domain structures in ferroelastic heterostructures of perovskite materials and open a pathway towards tailoring the desirable physical properties.

Figure 1

Schematic diagram of orthorhombic NGO single crystal. (a) Strain states of the ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ film received from the ${\mathrm{NdGaO}}_3$ substrate at various orientations, and the relationship between the orthorhombic coordinate system and pseudocubic coordinate system. (b) Schematic of the orthorhombic $Pbnm$ structure of NGO with $a^{-}{a}^{-}{c}^{+}$ rotation pattern. Schematics of the oxygen octahedral rotation of the (c) ${\left(110\right)}_{\mathrm{Or}}$- and (d) ${\left(001\right)}_{\mathrm{Or}}$-oriented NGO substrates viewed along different in-plane axes (as denoted).

Figure 2

RSMs of LSMO films. RSMs measured from the LSMO films grown on (a) $\mathrm{NGO}{\left(110\right)}_{\mathrm{Or}}$ and (b) $\mathrm{NGO}{\left(001\right)}_{\mathrm{Or}}$ substrates with the thickness of 44 nm. Totally distinct diffraction patterns are observed in the RSMs of the two oriented LSMO films. For the ${\left(110\right)}_{\mathrm{Or}}$-oriented film, two additional satellite reflections at the same $Q_X$ are shown for ${\left(031\right)}_{\mathrm{PC}}$ and ${\left(03\overline{1}\right)}_{\mathrm{PC}}$ reflections but absent for ${\left(130\right)}_{\mathrm{PC}}$ and ${\left(\overline{1}30\right)}_{\mathrm{PC}}$ reflections, as indicated by the red arrows. For the ${\left(001\right)}_{\mathrm{Or}}$-oriented film, the ${\left(113\right)}_{\mathrm{PC}}$ and ${\left(\overline{1}\overline{1}3\right)}_{\mathrm{PC}}$ reflections split into two peaks, while the peak splitting disappears for the ${\left(1\overline{1}3\right)}_{\mathrm{PC}}$ and ${\left(\overline{1}13\right)}_{\mathrm{PC}}$ reflections, indicated by the orange and green arrows. Note that horizontal shifts of the ${\left(113\right)}_{\mathrm{PC}}$ and ${\left(\overline{1}\overline{1}3\right)}_{\mathrm{PC}}$ peaks are also observed as highlighted by the orange and green plus signs.

Figure 3

Domain structures in LSMO films. (a) Schematic drawing of the unidirectional lattice modulation in LSMO/$\mathrm{NGO}{\left(110\right)}_{\mathrm{Or}}$ films. (I) The perspective view and side views along (II) ${\left[001\right]}_{\mathrm{PC}}$ and (III) ${\left[100\right]}_{\mathrm{PC}}$ of the lattice modulation. (b) Schematic illustrations of the unidirectional twinned domain in LSMO/$\mathrm{NGO}{\left(001\right)}_{\mathrm{Or}}$ films, including the side views along (I) ${\left[1\overline{1}0\right]}_{\mathrm{PC}}$ and (II) ${\left[110\right]}_{\mathrm{PC}}$ axes, as well as (III) the perspective view. The inset of (III) shows the zoomed-in view of the interface between the LSMO film and NGO substrate. Note that the ${\left(001\right)}_{\mathrm{PC}}$ plane of LSMO film tilts away from the ${\left(001\right)}_{\mathrm{PC}}$ plane of the $\mathrm{NGO}{\left(001\right)}_{\mathrm{Or}}$ substrate with a tilt angle of η.

Figure 4

OOR signs network in perovskite structure. (a) The definition of the rotation signs of an individual octahedron in a perovskite unit cell with clockwise $(+)$ and anticlockwise $(-)$. (b) Two oxygen octahedral rotation signs networks of the orthorhombic structure with the $a^{-}{a}^{-}{a}^{+}$ pattern, ${\left[+--\right]}_{\mathrm{Or}}$ and ${\left[---\right]}_{\mathrm{Or}}$ corresponding to $\gamma >0$ and $\gamma <0$, respectively. (c) Four OOR signs networks of the rhombohedral structure with the $a^{-}{a}^{-}{a}^{-}$ pattern, ${\left[-++\right]}_{\mathrm{Rh}}, {\left[+-+\right]}_{\mathrm{Rh}}, {\left[+++\right]}_{\mathrm{Rh}}$, and ${\left[--+\right]}_{\mathrm{Rh}}$. In these four OOR networks, the out-of-plane axes tilt along in-plane ${\left[1\overline{1}0\right]}_{\mathrm{PC}}, {\left[\overline{1}10\right]}_{\mathrm{PC}}, {\left[\overline{1}\overline{1}0\right]}_{\mathrm{PC}}$, and ${\left[110\right]}_{\mathrm{PC}}$ directions, respectively, as indicated by the solid arrows.

Figure 5

Interfacial OOR- and strain-dependent domain structure in LSMO films on $\mathrm{NGO}{\left(110\right)}_{\mathrm{Or}}$ and $\mathrm{NGO}{\left(001\right)}_{\mathrm{Or}}$ substrates. (a) Rotation signs networks across LSMO/$\mathrm{NGO}{\left(110\right)}_{\mathrm{Or}}$ interfaces. The left and right panels show the ${\left[+--\right]}_{\mathrm{Or}}/{\left[+--\right]}_{\mathrm{Or}}$ and ${\left[---\right]}_{\mathrm{Or}}/{\left[+--\right]}_{\mathrm{Or}}$ configurations, respectively. To ensure the octahedral connectivity at the interface, the left configuration is energetically favored. (b) Rotation signs networks across LSMO/$\mathrm{NGO}{\left(001\right)}_{\mathrm{Or}}$ interfaces, including ${\left[-++\right]}_{\mathrm{Rh}}/{\left[+--\right]}_{\mathrm{Or}}, {\left[+-+\right]}_{\mathrm{Rh}}/{\left[+--\right]}_{\mathrm{Or}}, {\left[+++\right]}_{\mathrm{Rh}}/{\left[+--\right]}_{\mathrm{Or}}$, and ${\left[--+\right]}_{\mathrm{Rh}}/{\left[+--\right]}_{\mathrm{Or}}$. (c) Strain states in LSMO/$\mathrm{NGO}{\left(001\right)}_{\mathrm{Or}}$ films with different rotation signs networks. (d) Structural domains corresponding to the four rotation signs networks shown in (b). From the perspective of interfacial octahedral connectivity, only the ${\left[-++\right]}_{\mathrm{Rh}}/{\left[+--\right]}_{\mathrm{Or}}$ and ${\left[+--\right]}_{\mathrm{Rh}}/{\left[+--\right]}_{\mathrm{Or}}$ interfacial octahedral connectivities are allowed, forming the structural domains in the top panel, while from the perspective of epitaxial strain, the ${\left[+++\right]}_{\mathrm{Rh}}/{\left[+--\right]}_{\mathrm{Or}}$ and ${\left[--+\right]}_{\mathrm{Rh}}/{\left[+--\right]}_{\mathrm{Or}}$ interfacial octahedral connectivities are energetically favored, forming the structural domains in the bottom panel.

Figure 6

Magnetic anisotropy of LSMO/NGO films with different domain structures. (a), (d) Schematic illustrations of the LSMO films grown on $\mathrm{NGO}{\left(110\right)}_{\mathrm{Or}}$ and $\mathrm{NGO}{\left(001\right)}_{\mathrm{Or}}$ substrates with the in-plane axes denoted. (b), (e) Magnetic hysteresis loops ($M\text{−}H$ curves) measured at 100 K from the above LSMO/$\mathrm{NGO}{\left(110\right)}_{\mathrm{Or}}$ and LSMO/$\mathrm{NGO}{\left(001\right)}_{\mathrm{Or}}$ films with the applied field along the in-plane axes which are perpendicular with respect to each other, as indicated by the arrows in (a), (d). The strong paramagnetic background from the NGO substrates is subtracted. (c), (f) The corresponding magnetic anisotropy energy extracted from the $M\text{−}H$ curves.

Figure 7

Orbital occupancy probed by x-ray absorption spectroscopy. (a), (b) X-ray linear dichroism of Mn $L_{2,3}$ edge of LSMO films grown on $\mathrm{NGO}{\left(110\right)}_{\mathrm{Or}}$ and $\mathrm{NGO}{\left(001\right)}_{\mathrm{Or}}$ substrates, respectively. The integrating range for ALD is colored in gray. All XLDs were measured at 300 K. (c), (d) Integrated linear dichroism from 649–659 eV normalized by the $\left(I_{\mathrm{in}}+I_{\mathrm{out}}\right)$ for LSMO films, $\mathrm{ALD}=\left(I_{\mathrm{in}}-I_{\mathrm{out}}\right)/\left(I_{\mathrm{in}}+I_{\mathrm{out}}\right)\times 100\%$. The corresponding MAE are also included for reference.