Tuning the Magnetic Properties of CrI3 Using Ni Thin Film Deposition for Applications in Spintronic Devices

Abstract

Chromium triiodide (CrI₃), a van der Waals magnet, has recently been shown to host Ising ferromagnetism down to the monolayer limit. It is a potentially important material in 2D magnet-based applications, such as magnetic sensors and spintronic devices. Prior studies have revealed the coexistence of two different types of interlayer magnetic coupling, the antiferromagnetic (AFM) coupling near the surface and ferromagnetic (FM) coupling in the deep bulk layers, in pristine CrI₃ crystals below the Curie temperature. In this study, we used Ni thin film deposition to tune the surface magnetic states in bulk CrI₃. A nanometer thickness (4 nm thick) of Ni was deposited on the surface of CrI₃ using electron-beam evaporation to form a Ni/CrI₃ heterostructure. The deposited Ni thin layer forms nanoclusters that completely cover the CrI₃ surface. Magnetic states of CrI₃ before and after Ni deposition are probed by ultralow-frequency magneto-Raman spectroscopy. Instead of seeing three spin wave branches in Raman scattering below 2 T, as in pristine bulk CrI₃, in Ni/CrI₃, we observe only a single spin wave branch, which is softened compared to that in pristine CrI₃ and displays a Zeeman shift under an out-of-plane magnetic field up to 7 T. This observation reveals that the AFM layers on pristine CrI₃ crystal surfaces transit into FM layers, so the entire CrI₃ crystal is in the FM state after Ni deposition. First-principles calculations show that Ni atoms tend to diffuse into the CrI₃ lattice, and the FM interlayer coupling has a much lower energy than AFM coupling in the presence of Ni atoms. Our studies show that the magnetic state of CrI₃ can be modified through deposition of a thin metal layer on the surface, offering a route for controlling the spin degree of freedom in van der Waals magnets.