Large and tunable magnetoresistance in van der Waals ferromagnet/semiconductor junctions

Article https://doi.org/10.1038/s41467-023-41077-0 Large and tunable magnetoresistance in van der Waals ferromagnet/semiconductor junctions Received: 1 September 2022 & Check for updates 1234567890():,; 1234567890():,; Accepted: 23 August 2023 Wenkai Zhu 1,2,9, Yingmei Zhu3,9, Tong Zhou 4, Xianpeng Zhang5, Hailong Lin1,2, Qirui Cui3, Faguang Yan1, Ziao Wang1,2, Yongcheng Deng 1, Hongxin Yang3 , Lixia Zhao1,6 , Igor Žutić 4 , Kirill D. Belashchenko 7 Kaiyou Wang 1,2,8 Magnetic tunnel junctions (MTJs) with conventional bulk ferromagnets separated by a nonmagnetic insulating layer are key building blocks in spintronics for magnetic sensors and memory. A radically different approach of using atomically-thin van der Waals (vdW) materials in MTJs is expected to boost their figure of merit, the tunneling magnetoresistance (TMR), while relaxing the lattice-matching requirements from the epitaxial growth and supporting high-quality integration of dissimilar materials with atomically-sharp interfaces. We report TMR up to 192% at 10 K in all-vdW Fe3GeTe2/GaSe/Fe3GeTe2 MTJs. Remarkably, instead of the usual insulating spacer, this large TMR is realized with a vdW semiconductor GaSe. Integration of semiconductors into the MTJs offers energy-band-tunability, bias dependence, magnetic proximity effects, and spin-dependent optical-selection rules. We demonstrate that not only the magnitude of the TMR is tuned by the semiconductor thickness but also the TMR sign can be reversed by varying the bias voltages, enabling modulation of highly spin-polarized carriers in vdW semiconductors. The traditional path to enhance the TMR1,2 relies on carefully choosing insulators and common ferromagnets, such as MgO with Fe and Co3,4. As the MTJ size scales down, this approach poses many obstacles, from materials nonuniformity and deteriorating quality to enhanced energy consumption and reduced stability5. The breakthroughs in vdW materials and the discovery of two-dimensional (2D) ferromagnets6,7 suggest important opportunities to overcome these problems in allvdW MTJs, where realizing a large TMR ~200% could revolutionize magnetic random-access memories (MRAM)5. MTJs with both conventional or vdW ferromagnets typically include an insulating spacer layer instead of the semiconducting barrier layer, owing to an extensive research on insulators such as Al2O3, MgO, and hBN. However, a realization of tunable spin-polarized transport in semiconductors is desirable for many emerging applications1. Conventional materials, such as δ-doped Fe/GaAs junctions, already provide a degree of tunability with bias-dependent sign reversal of interfacial spin polarization and TMR8–12. Because the observed spindependent signals in such systems are only modest, switching to 2D 1 State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, 100083 Beijing, China. 2Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, 100049 Beijing, China. 3National Laboratory of Solid State Microstructures, School of Physics, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, 210093 Nanjing, China. 4Department of Physics, University at Buffalo, State University of New York, Buffalo, NY 14260, USA. 5Department of Physics, University of Basel, Basel, Basel-Stadt CH-4056, Switzerland. 6Tiangong University, 300387 Tianjin, China. 7Department of Physics and Astronomy, Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, Lincoln, NE 68588, USA. 8Beijing Academy of Quantum Information Sciences, 100193 Beijing, China. 9These authors contributed equally: e-mail: ; ; ; ; Wenkai Zhu, Yingmei Zhu. Nature Communications | (2023)14:5371 1 Article https://doi.org/10.1038/s41467-023-41077-0 for semiconductor spintronics that are unavailable to MTJs with insulators32,33, including applications in artificial neural networks34,35 and spin lasers36. vdW materials could offer significant advantages by: (i) simultaneously increasing the TMR13–15 and supporting highly spin-polarized carriers, and (ii) expanding the tunability of spin-dependent properties, as demonstrated, for example, through barrier-thickness controlled spin polarization16,17 and gate-tunable magnetic proximity effects in hybridized 2D material/ferromagnet interface for spin valves18–23, and gate-tunable spin galvanic effect in van der Waals heterostructures of graphene with a semimetal or topological insulator24,25. Recently, the all-vdW Fe3GeTe2(FGT)-based MTJs have been widely studied, among which a large TMR of ~300% (4.2 K), ~50% (10 K), and ~110% (4.2 K) have been observed in devices with insulating spacer hBN and devices with semiconductor InSe and WSe2, respectively26–28. Compared with an insulator, the advantage of a semiconductor tunnel barrier is that its Fermi level (EF) can be adjusted by doping to make the EF close to the valence band or closer to the conduction band, which plays an important role in enhancing spin-filtering effect13. In addition, a large room-temperature TMR of 85% was observed in Fe3GaTe2/ WSe2/Fe3GaTe2 MTJs29, which confirms the great potential for semiconductor-based MTJs. 2D gallium selenide (GaSe) crystal is a typical layered metal monochalcogenide with an indirect bandgap energy of ~2 eV in the bulk30, which can serve as a perfect tunnel barrier31. Furthermore, it was predicted that the giant magnetoresistance can be obtained by using the semiconductor barrier due to the spin-filtering effect13,15. However, the magnetoresistance properties of all-vdW MTJs with GaSe barriers have not been reported yet. In this work, we not only find that the magnitude of TMR increases first and then decreases with increasing the thickness of the semiconductor spacer GaSe in Fe3GeTe2/GaSe/Fe3GeTe2 MTJs, but also find the magnitude and sign of the TMR can be tuned by the bias voltage. The maximum TMR of up to 192% is obtained with 9 layers of a GaSe spacer. This realization greatly expands materials design opportunities Results The typical optical image of the core structure of the MTJ devices is plotted in Fig. 1a, where the two FGT electrodes are separated by a GaSe layer. To avoid oxidation, we covered the core structure of the device with a hBN flake. The schematic diagram of the device and magnetotransport setup is shown in the inset of Fig. 1a, where an outof-plane magnetic field B controls the magnetization alignment of the FGT electrodes. Our previous works confirmed that GaSe and FGT have high-quality crystal structure28,37–40. The photoluminescence spectrum measurement shows that GaSe has a bandgap of ~2 eV (Supplementary Fig. 1). The MTJ devices (A, B, C, D, E, F, and G) with different GaSe-layer thicknesses were fabricated using mechanical exfoliation and dry transfer method (see “Methods”), where the GaSe-layer thicknesses were determined by atomic force microscope (AFM) for device A, B, C, D, (...truncated)