Van der Waals junction spin valves without spacer layer
The fundamental principle of a spin valve is that the resistance is dependent on the parallel or antiparallel configurations of the two ferromagnetic electrodes, thus associating the magnetoresistance (MR) effect, whose basic structure consists of two ferromagnetic metals decoupled by the insertion of a non-magnetic spacer. The MR effect in such a sandwiched structure is the cornerstone of magnetic-sensing, data-storage, and processing technologies, which is best represented by the development of the giant magnetoresistance (GMR) and tunneling magnetoresistance (TMR) information industry over the past two decades.
The physical mechanism underlying the GMR and TMR effects is due to electron transport dominated either by spin-dependent scattering or by spin-tunneling probability, respectively. To produce appreciable MR effect, the spin moment of the electrons must be maintained across the spacer layer and the interfaces, which is the key issue for spintronics. Thus, tremendous efforts have been devoted to optimizing the spacer layer and to pursuing high-quality electronic interfaces between the ferromagnetic layers and the spacer layer.
In this context, two-dimensional (2-D) van der Waals (vdW) layered materials—especially emerging 2-D magnetic materials—have provided researchers with another versatile way to tackle such obstacles in traditional magnetic multilayer systems. In particular, homo- or hetero-junctions incorporating these vdW materials without direct chemical bonding, avoiding the associated intermixing effect and defect-induced gap states, may show performance exceeding that of covalently bonded magnetic multilayers.
A research group led by Prof. Kaiyou Wang from State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, collaborating with Prof. Kai Chang and Prof. Zhongming Wei, has recently reported the fabrication of spin valves without spacer layers using vdW homo-junctions in which exfoliated Fe3GeTe2 nanoflakes act as ferromagnetic electrodes and/or interlayers. They demonstrated the textbook behavior of two-state and three-state MR for devices with two and three Fe3GeTe2 nanoflakes having different coercive fields, respectively. Interestingly, the all-metallic spin valves exhibit small resistance-area products (~10-4 Ω* cm2) and low operating current densities (down to 5 nA), and they possess vertical two-terminal setups, all of which are properties of major interest for future spintronics applications. This work demonstrates that two ferromagnetic layers without a spacer layer are sufficient to obtain the classical spin-valve effect, and it demonstrates the superiority of vdW interfaces.