Faster data transmission rates and low-power consumption are increasingly desirable properties in today’s digital technologies. However, traditional electronics are reaching their limits due to the fundamental properties of their materials and architectures.

Spintronics, a field of physics that explores and harnesses the intrinsic angular momentum of electrons, or electron spin, is providing a promising alternative to achieving the desired high data transmission rates and low-power consumption in electronic devices. In these technologies, functionality is linked closely to the control and manipulation of electron spin, which requires unique materials and carefully designed architectures. This represents a challenge that has seen increased interest in recent years. Already spintronic devices have been shown to dissipate less heat compared to traditional electronics.

Now, a research team from the National University of Singapore, led by Assistant Professor Ahmet Avsar and Professor Barbaros Özyilmaz, has achieved a significant breakthrough in this area with the discovery of the highly anisotropic spin transport nature of two-dimensional black phosphorus. Their findings were recently published paper in Nature Materials.

Monolayers of graphene and its heterostructures have been shown to allow electron spin to be controlled in spin-based devices. Black phosphorus, an allotrope of phosphorus, is similarly a 2D material that exhibits favourable spintronics properties. In particular, it can form spin channels, which are conduits or wires capable of transporting electron spins within the material, a process known as spin transport. The confined geometry and electronic structure of such 2D materials can influence the behaviour of spin channels, to enhance spin transport and thus enable advancements in the materials information processing and transmission abilities.

Prof Barbaros Özyilmaz, a globally recognised leader in the field of 2D materials, emphasises the important role of material choice in spintronics devices: "Choosing the right material is paramount in spintronics. Highly performant and functional spin channel materials are the backbone of spintronics devices, allowing us to manipulate and control electron spins for diverse applications."

The unique and inherent puckered crystal structure of black phosphorus imparts directional characteristics to the electron spin. This means that the behaviour of electron spin is dependent on the direction of measurement. Dr Ahmet Avsar, an expert in transport phenomena within 2D materials, highlights: "Black phosphorus showcases highly anisotropic spin transport, deviating from the normal isotropic behaviour seen in conventional spin channel materials."

Anisotropic spin transport is the directional dependence of the transport properties of electron spin in a material. Understanding spin transport anisotropy is crucial to efficiently manipulate and control spin states.

Preparation of two-dimensional black phosphorus-based spin valves

In their study, a black phosphorus layer (BP) serving as the spin channel was placed on a hexagonal boron nitride (hBNB) substrate which promotes high carrier mobility by reducing scattering effects. The heterostructure was then encapsulated by an hBN layer (hBNT) to protect the spin channel from the adsorption of airborne contaminants, which can degrade channel performance. Additionally, this top hBN layer acts as a tunnel barrier to increase the efficiency of spin injection into the black phosphorus layer. Electron beam lithography was used to define contact regions, and metal contacts (Co and Au) were deposited over the top hBN layer. The structure constitutes to a four-terminal spin-valve geometry, which is widely utilised in the study of spin-dependent transport to spatially separate the paths of the charge current and the detected spin current, providing a reliable measurement of spin transport.

Characterisation of two-dimensional black phosphorus heterostructure

Spin transport anisotropy was then studied by injecting electron spins into the black phosphorus at the injector and measuring the spin signal at the detector. Measurements of the spin signals were done while applying a strong magnetic field perpendicular to the black phosphorus layer and comparing it to measurements with a weak magnetic field applied.

The team found that applying a strong magnetic field (B > 0.25 T) resulted in a large increase in the spin signal, with a five-fold enhancement in the out-of-plane spin signal compared to in-plane. The spin signal was also estimated to have a spin-lifetime anisotropy of approximately six. This interesting observation was also confirmed by standard oblique Hanle measurements. The team also observed spin anisotropy with in-plane spins. When the spins are aligned in the armchair direction, their lifetimes are found to be nearly twice as long compared to spins aligned along the zigzag direction.

Observation of such anisotropic spin transport along three orthogonal axes in a pristine material is unique to black phosphorus and its origin is related to black phosphorus’ puckered crystal structure. In such anisotropic crystals, direction dependence in the effective mass of carriers can cause anisotropy in the strength of spin-orbit coupling, which is the quantum interaction between the intrinsic spin of charge carriers and their orbital motion. This phenomenon affects the dynamics of spin transport, resulting in such anisotropic spin transport observed.

In addition, this study also uncovers the electrically tunable spin lifetimes of black phosphorus. The team observed a gate-controlled OFF/ON/OFF transition in spin transport. During spin transport, spin lifetimes can be additionally tuned by using a back-gate voltage (VBG), which is the application of external voltage to the hBN substrate.

The exceptional spin transport anisotropy observed, coupled with the ability to electrically modulate spin transport, positions black phosphorus as a unique platform for the manipulation of electron spins— utilising spin anisotropy to realise directional-controlled spintronics devices. Findings from this study not only expand the horizons of potential spintronic applications but also hold the promise of achieving high data transmission rates and other unprecedented functionalities in future spintronic devices.