Reversible interlayer exchange coupling regulation induced by phase change of atomically thin VO2

Abstract

Regulation of electronic properties in spintronic interfaces (spinterfaces) can give rise to the optimization and even emergence of abundant spintronic effects. However, a proof-of-concept demonstration of such a strategy has rarely been achieved. Here, we experimentally demonstrate a reversible switching from antiferromagnetic coupling through insulating spinterface to ferromagnetic coupling through metallic spinterface in [Pt/Co]2/VO2/[Co/Pt]2 heterostructure, where atomically VO2 is considered as a controllable spinterface via its reversible metal-to-insulator transition. Furthermore, we attribute such an evolution to two distinct coupling mechanisms of spin-dependent tunneling and Rudermann-Kittel-Kasuya- Yosida interaction determined by the electronic states of VO2. The effect of VO2-tailored interlayer exchange coupling highlights the great potential of spinterface as a magic building block in beyond-CMOS electronic devices.1.Preparation and characterization of atomically thin VO2 and heterostructuresBoth atomically thin VO2 and [Pt/Co]2/VO2/[Co/Pt]2 heterostructure are deposited by sputtering. Ultrathin VO2 is chosen as an electrically-controllable spacer to regulate the interlayer exchange coupling via its metal-insulating transition around 340K1. First, the phase change of atomically thin VO2 lies at the heart of whole works. XPS result in Fig. 1a is displayed to check the valence state of vanadium of VO2 thin film from which we can see that the proportion of VO2 reaches 97%. Meanwhile electrical transport with a junction device of Au/VO2/ Au is also performed to confirm the phase change of VO2. From It is clearly in Fig. 1b that the I-V curve changes from tunneling (300K) to linear (340K) suggesting a transition from insulating state to metallic state of VO2. We also show the dependence of resistance ratio of ultrathin VO2 (2nm) on temperature comparing with a bulk sample (40 nm) (Fig. 1c). Based on such an atomically thin VO2, we prepare the heterostructure of [Pt/Co]2/VO2/[Co/Pt]2 (Fig. 1d). To prove the good quality and continuity of heterostructure, high-resolution transmission electron microscopy measurement is required profiled in Fig. 1e. In addition, PMA of heterostructure is also measure by p-MOKE in Fig. 1f.2.Interlayer exchange coupling switching via phase changeThe change on electronic state of spinterface induced by phase change will arise the interlayer exchange coupling switching. In order to investigate the coupling change, we measure the hysteresis loops of heterostructure with various thicknesses of VO2 as temperature rises to trigger the phase change. The loops change from double spin flips to single flip (Fig. 2a) that indicates a transition from antiferromagnetic coupling at low temperature to ferromagnetic coupling at high temperature. Besides such a dynamic process of coupling switching, we extract the exchange coupling field of different samples with temperature increasing (Fig. 2b left). To further verify the reliability of the experimental data, we compare the coupling strength value calculated by experimental data with that calculated by theoretical fitting at room temperature with various thicknesses of VO2 (Fig. 2b right). As for theoretical fitting, due to different electronic state of VO2, spin-dependent tunneling model2 is applied at insulating VO2 while RKKY interaction model3 is used at metallic VO2.In the final, to confirm reversibility of such a promising interlayer exchange coupling regulation, we also show the hysteresis loop after the temperature decrease back to the low temperature (Fig. 2c). **