Interfacial reconstruction effects in insulating double perovskite Nd$_2$NiMnO$_6$/SrTiO$_3$ and Nd$_2$NiMnO$_6$/NdGaO$_3$ thin films

TL;DR

This study investigates how interfacial reconstruction affects the structural and magnetic properties of ultrathin Nd$_2$NiMnO$_6$ films on different substrates, revealing polarity and symmetry-driven modifications crucial for spintronics applications.

Contribution

It provides detailed insights into interfacial effects, including octahedral domain formation and lattice modifications, in FMI double perovskite films grown on different substrates.

Findings

Interfacial polarity compensation modifies the out-of-plane lattice parameter.

Structural symmetry mismatch leads to multiple octahedral rotational domains.

Orbital symmetry and magnetism are influenced by interfacial reconstructions.

Abstract

Ferromagnetic insulating (FMI) double perovskite oxides (DPOs) $A_2BB'$O$_6$ with near-room-temperature Curie temperatures are promising candidates for ambient-temperature spintronics applications. To realize their potential, epitaxial stabilization of DPO films and understanding the effect of multiple broken symmetries across the film/substrate interface are crucial. This study investigates ultrathin films of the FMI Nd$_2$NiMnO$_6$ (NNMO) grown on SrTiO$_3$ (STO) and NdGaO$_3$ (NGO) substrates. By comparing growth on these substrates, we examine the influence of polarity and structural symmetry mismatches, which are absent in the NGO system. The interface exhibits immeasurable resistance in both cases. Using synchrotron X-ray diffraction, we show that films have three octahedral rotational domains because of the structural symmetry mismatch with the STO substrate. Furthermore, our coherent Bragg rod analysis of specular X-ray diffraction reveals a significant modification of the out-of-plane lattice parameter within a few unit cells at the film/substrate interface and the surface. This arises from polarity compensation and surface symmetry breaking, respectively. These structural alterations influence the Mn orbital symmetry, a dependence that we further confirm through X-ray linear dichroism measurements. Since the ferromagnetism in insulating DPOs is mediated by orbital-dependent superexchange interactions [Phys. Rev. Lett. 100, 186402 (2008)], our study provides a framework for understanding the evolution of magnetism in ultrathin geometry.