The structural, morphological, and magnetic properties of MnxNi1-xO/NiO thin film bilayers

The exchange bias (EB) effect, especially in nanomaterials, is highly promising for use in antiferromagnet-based spintronics applications. NiO is a well-known antiferromagnetic material with a high Néel temperature (525 K) that can exhibit ferromagnetism/ferrimagnetism by addition of other magnetic transition elements. Our previous work has shown that the antiferromagnetic (AFM) characteristics of conventional NiO nanostructured material can be altered to have substantial ferrimagnetic (FiM) characteristics by doping with Mn or Co. Pulsed laser deposition (PLD) was used to grow heterostructures comprised of a nanostructured thin NiO film deposited on the surface of MgO (100) and Al₂O₃ (0001) substrates, followed by the deposition of a Mn_xNi_1−xO thin film layer on top of a NiO layer. X-ray diffraction (XRD), scanning electron microscopy (SEM), and physical property measurement system (PPMS) magnetometry were used to study the structural, morphological, and magnetic properties, respectively, of the thin film heterostructures. XRD and SEM characterization show that the Mn_xNi_1−xO/NiO bilayers were grown quasi-epitaxially on the MgO (100) and Al₂O₃ (0001) substrates. Although both heterostructures were found to be crystalline with nominal disorder characteristics, the Mn_xNi_1−xO/NiO/Al₂O₃(0001) thin film heterostructure was found to have more extensive crystallinity. The Mn_xNi_1−xO/NiO thin film bilayer exhibits FiM/AFM interface characteristics at 5 K, including the exchange bias effect, regardless of the nature of the substrate that the films were grown on. However, only the Mn_xNi_1−xO/NiO thin film bilayer grown on the Al₂O₃ (0001) substrate was found to have a persistent exchange bias field at room temperature.