Engineering Unequal Antipolar Displacement in Ferromagnetic Layered Oxide Heterostructures

Abstract

Heterostructure engineering provides a versatile route for tailoring emergent functionalities that are often difficult to realize in single-phase materials. In this work, the focus is on superlattices composed of the insulating and ferromagnetic double perovskites La₂NiMnO₆ and Sm₂NiMnO₆, which constitute an intriguing model system. These layered structures are predicted to feature unequal antipolar displacements of the La and Sm ions; when combined with odd periodicity stacking, this structural motif is expected to give rise to polar behavior. The respective superlattices are grown with atomic precision and display robust ferromagnetism, as confirmed by in-house magnetometry and synchrotron measurements. Scanning transmission electron microscopy combined with first-principles calculations confirms the presence of the predicted unequal antipolar displacements, paving the way for the realization of hybrid improper ferroelectricity in such oxide heterostructures.

Keywords: antipolar diplacements; double perovskites; ferromagnetism; oxide heterostructures.

Figure 1

Illustration of a La₂NiMnO₆/RE₂NiMnO₆ (RE ≠ La) (1,1) superlattice projected along the orthorhombic a (left) and c (right) axis. The in‐plane antipolar displacement of the RE and La ions are labeled as δ₁ and δ₂, respectively. The unequal magnitude of δ₁ and δ₂ in combination with the odd superlattice periodicity is required for the emergence of a net electric polarization along b.

Figure 2

Structural characterization. a) Tracking of the RHEED intensity during the sputter deposition of a (3, 3)₁₀ superlattice on NGO(001). b) XRD for (3, 3)₁₀ superlattices grown on NGO (black) and STO (red). c) RSMs around the pseudocubic ($1/2$ 0 2)_pc, (0 $1/2$ 2)_pc, and (1 0 $5/2$)_pc positions indicate an out‐of‐plane orientation of both the NGO substrate's and the superlattice's c‐axis. d) RSMs around the orthorhombic (2 0 5)_o and (0 2 5)_o positions indicate only one in‐plane orientation of the superlattice grown on NGO. The horizontal color bar represents the intensity scale common to all RSM plots. e–g) Cross‐sectional STEM imaging of a (3, 3)₁₀ superlattice along the orthorhombic [100] direction of the NGO(001) substrate. The HAADF‐STEM data in (e) highlights the in‐plane shifts of the A‐site ions (zig‐zag pattern in the out‐of‐plane direction) corresponding to the (001) orientation of the superlattice with a ⁻ a ⁻ c ⁺ tilt pattern. The EDX data in (f,g) demonstrates the (3,3) superlattice layering.

Figure 3

Magnetic Characterization. a) M(T) for superlattices of different periodicities on STO(001) measured at an in‐plane applied magnetic field of 0.5 T. For comparison, the M(T) curves for bare 30 pc unit cells LNMO and SNMO films are included. For the high periodicity superlattices, the two individual transitions of LNMO and SNMO are conserved, whereas they collapse into a single transition for the low periodicity samples. The curves are normalized to their value at 50 K and vertically offset for better visibility. b) M(H) for a (3, 3)₁₀ superlattice on STO(001) measured at a temperature of 2 K and the magnetic field applied in the in‐plane direction. The inset shows a magnification around the origin. c) XMCD asymmetry loops for a (3, 3)₁₀ superlattice on NGO(001) recorded at the Mn L ₃ (top) and the Ni L ₃ (bottom) absorption edge at a temperature of 20 K. The red triangles (black dots) correspond to measurements acquired while increasing (decreasing) the applied magnetic field.

Figure 4

Characterization of the antipolar displacement. a) DFT‐calculated structure of a (3,3) superlattice projected along a. The La ions are shown in green, Sm in blue, Ni in grey, Mn in purple and O in red. b) Displacements of the La and Sm ions along the b axis extracted from the structure in (a). c) HAADF‐STEM image of a (3, 3)₁₀ superlattice imaged along the [100] axis. The inset illustrates how the A‐site displacements (±δ _x ) are determined. d) Layer‐averaged displacements of the Sm and La ions with respect to the position of the B‐site ions in the layers above and below. The error bars correspond to the standard error of the mean within one layer. The data is shifted horizontally such that the displacement is symmetric around 0 for the Nd ions in the substrate.