Crystallographic Engineering of Spin Transport in Antiferromagnetic NiO Thin Films

In this work, we investigate how the crystallographic growth direction influences spin current transmission in antiferromagnetic (AF) NiO thin films. By manipulating epitaxial growth, we explored the spin transport characteristics in La_2/3Sr_1/3MnO₃/NiO/Pt heterostructures grown on top of (001)- and (111)-oriented SrTiO₃ substrates, varying the NiO barrier thickness (t_NiO). Spin currents were generated via spin pumping (SP), and detection was done by the inverse spin Hall effect (ISHE). X-ray diffraction and high-resolution electron microscopy techniques confirmed high-quality epitaxial films with nearly atomically sharp interfaces and similar dislocation distributions, irrespective of the growth direction. Nevertheless, epitaxially engineered (111) heterostructures exhibited superior spin transport properties, including lower magnetic damping (α), longer spin diffusion lengths (λ_Sd), and higher spin mixing conductance (g^↑↓). The temperature dependence of the ISHE voltage signal (V_ISHE) also showed orientation-dependent behavior: while (001)-oriented samples followed a monotonic trend, (111)-oriented samples displayed a peak that shifted to higher temperatures with increasing t_NiO, associated with the emergence of AF ordering. Moreover, (111)-oriented samples demonstrated notable spin current amplification at room temperature, peaking at t_NiO ≈ 1 nm before decaying quasi-exponentially, indicative of spin diffusion-mediated conduction. Although the spin diffusion length in (111)-oriented samples was roughly double that of their (001)-oriented counterparts, it was still too short to be explained by angular momentum transport by mobile antiferromagnons through NiO. Instead, these findings point to a mechanism involving magnetic correlations and short-range thermal magnons. The superior spin transport properties and the enhanced spin conduction in (111)-oriented samples are primarily attributed to a synergistic combination of interfacial and dynamic effects, a more favorable Néel vector alignment and distinct interface symmetry, which can enhance spin-Hall effects or enable different spin textures. Overall, this study underscores the pivotal role of the Néel vector and crystallographic orientation in AF spin transport, providing valuable insights for the design and optimization of spintronic devices.