With perovskite materials, rapid progress in power conversion efficiency (PCE) to reach 25% has gained a significant amount of attention from the solar cell industry.

Two-dimensional (2D) materials including graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenides (TMDCs) have revolutionized electronic, optoelectronic and spintronic devices. Recent progress has been made in the knowledge of spin injection, detection, and manipulation utilizing spintronic devices based on 2D materials. However, some bottlenecks still need to be addressed to employ spintronic devices for logical applications. Here, we review the major advances and progress in vertical magnetic tunnel junctions (MTJs) made of various 2D materials as spacer layers between distinct ferromagnetic electrodes. Spin transportation characteristics depending on the magnetic field are investigated by considering the magnetoresistance (MR) and tunneling magnetoresistance (TMR) ratio in vertically stacked structures. This review examines the important features of spin transfer through the various spacer 2D materials in MTJs by carefully analyzing the temperature-dependent phenomena. The underlying physics, reliance of spin signals on temperature, quality of junction, and various other parameters are discussed in detail. Furthermore, newly discovered 2D ferromagnets introduce an entirely new type of van der Waals junction enabling effective dynamic control and spin transport across such heterojunctions. Finally, the challenges and prospects of 2D materials-based spin-valve MTJs for improving spintronic devices are discussed in detail.

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Two-dimensional (2D) layered materials and their heterostructures have opened a new avenue for next-generation spintronic applications, benefited by their unique electronic properties and high crystallinity with an atomically flat surface. Here, we report magnetoresistance of vertical magnetic spin-valve devices with multi-layer (ML) MoSe2 and its heterostructures with few-layer graphene (FLG). We employed a micro-fabrication procedure to form ultraclean ferromagnetic–non-magnetic–ferromagnetic interfaces to elucidate the intrinsic spin-transferring mechanism through both an individual material and combinations of 2D layered materials. However, it is revealed that the polarity of tunneling magnetoresistance (TMR) is independent of non-magnetic spacers whether the spin valve is composed of a single material or a hybrid structure, but it strongly depends on the interfaces between ferromagnetics (FMs) and 2D materials. We observed positive spin polarizations in ML-MoSe2 and FLG/ML-MoSe2/FLG tunnel junctions, whereas spin-valve devices comprised of FLG/ML-MoSe2 showed a reversed spin polarization and demonstrated a negative TMR. Importantly, in Co/FLG/ML-MoSe2/FLG/NiFe devices, the polarization of spin carriers in the FM/FLG interface remained conserved during tunneling through MoSe2 flakes in spin-transferring events, which is understandable by Julliere’s model. In addition, large TMR values are investigated at low temperatures, whereas at high temperatures, the TMR ratios are deteriorated. Furthermore, the large values of driving ac-current also quenched the amplitude of TMR signals. Therefore, our observations suggest that the microscopic spin-transferring mechanism between ferromagnetic metals and 2D materials played a momentous role in spin-transferring phenomena in vertical magnetic spin-valve junctions.

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Metal‐oxide spin valve junctions are the building blocks for spintronic devices and are to be utilized for miniaturized magnetic sensors. Here, the fabrication and characterization of the vertical spin valve (VSV) based on the CoFe/TiO2/CoFe structure are described. A spacer layer (TiO2) of different thicknesses in the spin valve is utilized and the effect on MR is studied. This VSV showed significant positive magnetoresistance (MR) at different temperatures from low to room temperature. The maximum value of tunneling MR is investigated to be 3.4% at 30 K and 1.03% at room temperature (300 K), and the spin polarization obtained at 30 K is 12.8%. The MR of the spin valve is investigated by changing the orientation of the device at different angles with respect to an applied magnetic field, the switching points shifted toward the higher magnetic fields and the signal became wider. Interestingly, a negative tunneling MR is observed when the thickness of the spacer layer (TiO2 = 5.5 nm) is enhanced which may be due to the spin filtering effect. The demonstrated devices identify TiO2 as favorable spacer material in spin valves and open a way to integrate high‐performance memory storage devices.

One of the most prominent and effective applications of graphene in the field of spintronics is its use as a spacer layer between ferromagnetic metals in vertical spin valve devices, which are widely used as magnetic sensors. The magnetoresistance in such devices can be enhanced by a selection of suitable spacer materials and proper fabrication procedures. Here, we report the use of dry-transferred single- and double-layer graphene, grown by chemical vapor deposition (CVD), as the spacer layer and the fabrication procedure in which no photo-resist or electron-beam resists is used. The measured maximum magnetoresistance of NiFe/CVD-Graphene/Co junction is 0.9% for the single- and 1.2% for the double-layer graphene at 30 K. The spin polarization efficiency of the ferromagnetic electrodes is about 6.7% and 8% for the single- and the double-layer graphene, respectively, at the same temperature. The bias-independent magnetoresistance rules out any contamination and oxidation of the interfaces between the ferromagnet and the graphene. The magnetoresistance measured as a function of tilted magnetic field at different angles showed no changes in the maximum value, which implies that the magnetoresistance signal is absent from anisotropic effects.

A few-layer WSe2/WS2 heterojunction diode on an h-BN substrate shows improved electronic and optoelectronic characteristics with a robust diode rectification ratio and photo responsivity compared to that on a SiO2 substrate.