⇦Chinese Physics Letters, 2022, Vol. 39, No. 12, Article code 128501Express Letter Large Room-Temperature Magnetoresistance in van der Waals Ferromagnet/Semiconductor Junctions Wenkai Zhu1,2†, Shihong Xie1,3†, Hailong Lin1,2†, Gaojie Zhang4,5, Hao Wu4,5, Tiangui Hu1,2, Ziao Wang1,2, Xiaomin Zhang1,2, Jiahan Xu1, Yujing Wang1,2, Yuanhui Zheng1, Faguang Yan1, Jing Zhang1, Lixia Zhao1,6, Amalia Patanè3, Jia Zhang5,7, Haixin Chang4,5*, and Kaiyou Wang1,2* Affiliations 1State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China 2Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China 3School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, United Kingdom 4Center for Joining and Electronic Packaging, State Key Laboratory of Material Processing and Die & Mold Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China 5Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan 430074, China 6School of Electrical and Electronic Engineering, Tiangong University, Tianjin 300387, China 7School of Physics, Huazhong University of Science and Technology, Wuhan 430074, China Received 14 November 2022; accepted manuscript online 17 November 2022; published online 18 November 2022 †These authors contributed equally to this work.
*Corresponding authors. Email: hxchang@hust.edu.cn; kywang@semi.ac.cn Citation Text: Zhu W, Xie S, Lin H et al. 2022 Chin. Phys. Lett. 39 128501    Abstract A magnetic tunnel junction (MTJ) is the core component in memory technologies, such as the magnetic random-access memory, magnetic sensors and programmable logic devices. In particular, MTJs based on two-dimensional van der Waals (vdW) heterostructures offer unprecedented opportunities for low power consumption and miniaturization of spintronic devices. However, their operation at room temperature remains a challenge. Here, we report a large tunnel magnetoresistance (TMR) of up to 85% at room temperature ($T = 300$ K) in vdW MTJs based on a thin ($ < 10$ nm) semiconductor spacer WSe$_{2}$ layer embedded between two Fe$_{3}$GaTe$_{2}$ electrodes with intrinsic above-room-temperature ferromagnetism. The TMR in the MTJ increases with decreasing temperature up to 164% at $T = 10$ K. The demonstration of TMR in ultra-thin MTJs at room temperature opens a realistic and promising route for next-generation spintronic applications beyond the current state of the art.
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 DOI:10.1088/0256-307X/39/12/128501 © 2022 Chinese Physics Society Article Text Magnetic tunnel junctions (MTJs) with large tunnel magnetoresistance (TMR) play an important role in many technologies ranging from magnetic-field sensing and non-volatile magnetic random-access memories (MRAM) to programmable spin logic devices.[1-4] Traditional MTJs normally consist of three-dimensionally bonded ferromagnetic metals and non-magnetic wide-gap oxides, such as Fe/MgO/Fe[5-7] and NiFe/TaO/Al$_{2}$O$_{3}$/Co.[8] Different from covalently bonded multilayer systems, van der Waals (vdW) heterostructures with atomically sharp and clean interfaces offer opportunities to build high-quality junctions without the requirement of lattice matching.[9,10] Specifically, the recent discovery of magnetic two-dimensional (2D) vdW materials such as Cr$_{2}$Ge$_{2}$Te$_{6}$,[11] CrI$_{3}$,[12,13] Fe$_{3}$GeTe$_{2}$,[14,15] and Fe$_{5}$GeTe$_{2}$[16] not only provides platforms for studies of novel physics phenomena, but also offers promising application prospects for 2D electronic devices where the spin is used as an extra degree of freedom.[17-19] Amongst the catalogue of magnetic 2D vdW materials, ferromagnet Fe$_{3}$GeTe$_{2}$ has attracted extensive attention from worldwide researchers owing to its metallic nature and strong perpendicular magnetic anisotropy (PMA).[20-29] At low temperatures, TMRs of 41% (10 K) and 300% (4.2 K) were reported in Fe$_{3}$GeTe$_{2}$/InSe/Fe$_{3}$GeTe$_{2}$ MTJs[26] and Fe$_{3}$GeTe$_{2}$/hBN/Fe$_{3}$GeTe$_{2}$ MTJs,[30] respectively. As the temperature approaches the Curie temperature of Fe$_{3}$GeTe$_{2}$ ($T_{\rm C}$, 220 K in bulk and 130 K in monolayer), the TMR of the Fe$_{3}$GeTe$_{2}$-based MTJs quickly decreases and vanishes.[25,27,30,31] Although the $T_{\rm C}$ of 2D magnets can be tuned to room temperature by electrostatic gating,[15] high-pressure[32] or interlayer magnetic coupling,[33] the room-temperature operation of vertical MTJs under these conditions remains a challenge, hindering prospects for future applications.[34] On the other hand, there is the ambition for all-2D room-temperature MTJs to be manufactured for production within the next decade.[1] This could be facilitated the recent discovery of the vdW crystal Fe$_{3}$GaTe$_{2}$ that has a high saturation magnetic moment, robust large PMA, and Curie temperature $T_{\rm C}$ of up to 380 K.[35] In this Letter, we use Fe$_{3}$GaTe$_{2}$ as the ferromagnetic electrodes in an all-vdW MTJ. The tunnel barrier of the MTJ is the transition-metal dichalcogenide (TMDC) semiconductor WSe$_{2}$. Due to its tunable bandgap and long spin diffusion length, WSe$_{2}$ is suitable for constructions of 2D magnetic devices, such as MTJs[27,30] and spin field-effect transistors (FETs).[36] Moreover, the magnetoresistance of MTJs can be tuned by thickness of the WSe$_{2}$ spacer layer through the spin filtering effect.[27] The Fe$_{3}$GaTe$_{2}$/WSe$_{2}$/Fe$_{3}$GaTe$_{2}$ MTJ exhibits a TMR of 85% at room temperature ($T = 300$ K). Together with this significant achievement, we find that the operating temperature of the MTJ is up to 380 K, a record-high operating temperature amongst vdW MTJs, opening disruptive improvement towards practical applications for next-generation spintronic devices.
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Fig. 1. (a) Side (left) and top views (right) of the crystal structure of Fe$_{3}$GaTe$_{2}$. (b) The optical image of a Fe$_{3}$GaTe$_{2}$ Hall-bar device capped by hBN. Scale bar: 10 µm. The direction of the current and the connection of the voltmeter during Hall measurements are shown in red. (c) Longitudinal resistance $R_{xx}$ versus temperature from 10 to 380 K. Upper-left insets: the AFM image of Fe$_{3}$GaTe$_{2}$ and the height profile along the red dashed line. Scale bar: 10 µm. (d) Hall resistance $R_{xy}$ versus perpendicular magnetic field ($B$) of a $\sim $10-nm-thick Fe$_{3}$GaTe$_{2}$ at different temperatures ranging from 10 to 380 K. The bias current $I_{xx}$ is fixed at 10 µA.
High-quality vdW ferromagnetic crystal Fe$_{3}$GaTe$_{2}$ is grown by the self-flux method and has a hexagonal structure of space group $P6_{3}/mmc$ with lattice parameters $a$ = $b$ = 3.9860 Å and $c$ = 16.2290 Å ($\alpha$ = $\beta$ = 90$^{\circ}$, $\gamma$ = 120$^{\circ}$).[35] In the Fe$_{3}$GaTe$_{2}$ crystal, the Fe$_{3}$Ga heterometallic slab is sandwiched between two Te layers, and the vdW gap is between two adjacent Te atoms [Fig. 1(a)]. Two inequivalent Fe sites are distinguished by brown and blue balls, respectively. The slabs of Fe$_{3}$GaTe$_{2}$ are stacked along the $c$-axis with an interlayer spacing of $\sim $0.78 nm.[35] As the magnetic moment of Ga and Te atoms in Fe$_{3}$GaTe$_{2}$ are negligible, the ferromagnetism of Fe$_{3}$GaTe$_{2}$ mainly comes from the $3d$-orbital electrons in Fe atoms.[35] An exfoliated $\sim $10-nm-thick Fe$_{3}$GaTe$_{2}$ flake was used to fabricate a Hall-bar [Fig. 1(b)] and to investigate the magnetic properties of Fe$_{3}$GaTe$_{2}$ (see fabrication details in Supplementary Note 1). The thickness of Fe$_{3}$GaTe$_{2}$ is measured by atomic force microscopy (AFM) [Fig. 1(c) upper-left inset]. In the anomalous Hall effect (AHE) measurements, the longitudinal resistance $R_{xx}$ increases monotonically with increasing temperature, as expected for a metallic layer [Fig. 1(c)]. Figure 1(d) depicts the temperature-dependent AHE signal $R_{xy}$ (transverse resistance) as the perpendicular applied magnetic field $B$ sweeps between $-$0.6 and $+$0.6 T. As the temperature increases from 10 to 380 K, the $R_{xy}$–$B$ curves display almost perfectly square-shaped hysteresis loops, indicating that the Fe$_{3}$GaTe$_{2}$ flake has strong PMA. Especially, the AHE signal of the Fe$_{3}$GaTe$_{2}$ Hall-bar is still measurable at room temperature. Due to the enhancement of the thermal fluctuations close to $T_{\rm C}$, the coercivity also decreases monotonically with increasing temperature and nearly vanishes at 380 K. Another thicker Fe$_{3}$GaTe$_{2}$ ($\sim $40 nm) flake shows the same above-room-temperature $T_{\rm C}$ and a smaller coercivity (Fig. S1 in the Supplementary Materials). The high $T_{\rm C}$, large coercivity, and robust PMA make the Fe$_{3}$GaTe$_{2}$ flake an ideal metallic ferromagnetic electrode for MTJs working at room temperature.[35]
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Fig. 2. (a) Schematic of Fe$_{3}$GaTe$_{2}$/WSe$_{2}$/Fe$_{3}$GaTe$_{2}$ MTJs. (b) Optical image and AFM image (upper-right inset) of a representative Fe$_{3}$GaTe$_{2}$/WSe$_{2}$/Fe$_{3}$GaTe$_{2}$ heterojunction device encapsulated by a top hBN layer. Scale bar: 10 µm. The lower-left inset shows the height profile of the spacer layer WSe$_{2}$. (c) $I$–$V$ characteristics of the device at $T = 300$ K. (d) Room-temperature resistance ($R$) and TMR versus $B$ of the device at a constant bias voltage $V = 50$ mV.
We now focus on the fabrication and properties of the MTJ. Figures 2(a) and 2(b) show the schematic and optical image of the two-terminal MTJ. Here, two metallic Fe$_{3}$GaTe$_{2}$ ferromagnetic electrodes are separated by a non-magnetic WSe$_{2}$ spacer layer. The whole structure is capped with an hBN layer (typically 10–30 nm) to prevent degradation or oxidation once the MTJ is exposed to air (see details of the fabrication in Supplementary Note 1). The semiconductor 2H-WSe$_{2}$ is a typical TMDC material in a hexagonal structure with space group $P6_{3}/mmc$, consisting of van der Waals bonded Se–W–Se layers.[37] The bulk WSe$_{2}$ is an indirect-gap semiconductor with a bandgap of 1.20 eV, while WSe$_{2}$ has a direct bandgap of 1.65 eV in its monolayer form.[38] The AFM study of the MTJ indicates that the thickness of the spacer WSe$_{2}$ layer is $\sim $7 nm [inset of Fig. 2(b)]. To distinguish the coercivities of the top and bottom ferromagnetic electrodes in the MTJs, we selected Fe$_{3}$GaTe$_{2}$ flakes with different thicknesses ($\sim $11 nm bottom Fe$_{3}$GaTe$_{2}$ and $\sim $17 nm top Fe$_{3}$GaTe$_{2}$). The different switching fields of the top and bottom ferromagnetic layers enable the parallel and antiparallel configurations under different specific magnetic fields, corresponding to the low and high resistance states of the MTJ.[39] Raman spectroscopy was used to characterize the Fe$_{3}$GaTe$_{2}$ and WSe$_{2}$ layers. The WSe$_{2}$ layer has a room-temperature photoluminescence (PL) emission centered at 1.38 eV (Fig. S2), in good agreement with previous experimental results.[35,40] The preliminary studies indicate a high-quality WSe$_{2}$ spacer layer, as required to preserve the polarization of the electron as it tunnels in the MTJ.[17] Figure 2(c) shows a symmetric, nonlinear $I$–$V$ curve of the device, which points to typical tunneling behavior.[26,31] By applying a constant bias voltage ($V = 50$ mV) and sweeping the perpendicular magnetic field ($B$), we observed a sharp jump of the resistance from low ($R_{\rm P}$) to high-resistance state ($R_{\rm AP}$) at $B = 0.20$ T [Fig. 2(d)]. With further increasing $B$ to 0.31 T, the resistance jumps back to the low-resistance state. Symmetric resistance jumps are also observed for sweeping $B$ from positive to negative values. The distinct low- and high-resistance states correspond to the magnetization of the two Fe$_{3}$GaTe$_{2}$ layers in the parallel ($\downarrow \downarrow$ or $\uparrow \uparrow$) and antiparallel ($\downarrow \uparrow$ or $\uparrow \downarrow$) magnetization configurations, respectively. In the parallel configuration, spin-dependent tunneling occurs between the same spin states, representing $R_{\rm P}$ with the product of the DOS of the majority ($D_{\uparrow}$) and minority ($D_{\downarrow}$) spins as $1/R_{\rm P} \propto (D_{\uparrow} \times D_{\uparrow})+(D_{\downarrow}\times D_{\downarrow})$.[30] In the antiparallel configuration, $R_{\rm AP}$ is conversely determined by the cross product of the DOS with different spin states as $1/R_{\rm AP} \propto 2 D_{\uparrow} \times D_{\downarrow}$, leading to a larger $R_{\rm AP}$ for MTJs under low bias voltages.[30] At $V = 50$ mV and $T = 300$ K, the $R_{\rm P}$ and $R_{\rm AP}$ of the junction are 43.3 M$\Omega$ and 80.4 M$\Omega$, respectively. The corresponding TMR = ($R_{\rm AP}- R_{\rm P}$)/$R_{\rm P}$ is 85%, even higher than that of some reported conventional MTJs working at room temperature, such as Fe/Al$_{2}$O$_{3}$/Fe (TMR = 18%[41]) and Fe/MgO/Fe (TMR = 30%[42]). Similar results were obtained in another device displaying a TMR of 53% at room temperature (Fig. S3). Thus, the room-temperature TMR in our devices is an important achievement, filling a gap in the current literature on TMR in all-2D vdW MTJs.
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Fig. 3. (a) and (b) TMR versus $B$ of the device at different temperatures ranging from 10 to 400 K. (c) TMR versus temperature of the device at $V = 50$ mV. The values of TMR are derived from (a) and (b). (d) Spin polarization ($P$) versus temperature of the device. The fitting curve follows the function of $P = P_{0}(1-T/T_{\rm C})^{\beta}$.
Figures 3(a) and 3(b) show the TMR curves measured at various temperatures. A TMR of 164% in the device is observed at $T = 10$ K, while another device exhibits a larger TMR of 210% at the same temperature (Fig. S4). With increasing temperature, the magnitude of the TMR decreases and vanishes above $T_{\rm C}$ [Fig. 3(c)]. The spin polarization is proportional to the magnetization and decreases with increasing temperature,[43] thus leading to a smaller TMR. Moreover, due to the reduction of perpendicular anisotropic energy and the enhancement of thermal energy, the coercivity of Fe$_{3}$GaTe$_{2}$ decreases with increasing temperature as well.[43] This is reflected by the reduction of the critical magnetic switching field of the MTJ in Figs. 3(a) and 3(b). Interestingly, some TMR–$B$ curves in Fig. 3(a) are asymmetric, indicating that the coercivity of Fe$_{3}$GaTe$_{2}$ can be different at positive and negative magnetic field. This can be attributed to the defect-related pinning effect, which is weakened at high temperatures [Fig. 3(b)].[44] The TMR can be defined as TMR = $2 P^{2}/(1 - P^{2})$, where $P = (D_{\uparrow} - D_{\downarrow})/(D_{\uparrow}+D_{\downarrow})$ denotes the spin polarization at the Fermi-level ($E_{\rm F}$) for the injector and detector Fe$_{3}$GaTe$_{2}$ electrodes.[45] A room-temperature spin polarization $P$ of 55% is calculated by the TMR of 85%. As shown in Fig. 3(d), the estimated temperature dependence of the spin polarization can be fitted well by a power law given by $P = P_{0}(1-T/T_{\rm C})^{\beta}$, where $P_{0}$ is the spin polarization at 0 K and $\beta$ is a material-dependent constant that can be fitted to $\beta = 0.137$.[46,47] Our fitting results also indicate that $T_{\rm C} = 400$ K, which is close to that obtained in Fig. 3(b). Another MTJ shown in Fig. S4 shows a similar behavior, but with a lower $T_{\rm C}$ of 357 K (see Supplementary Note 2). In summary, our results demonstrate that the vdW heterostructure Fe$_{3}$GaTe$_{2}$/WSe$_{2}$/Fe$_{3}$GaTe$_{2}$ provides an excellent platform for MTJs. It exhibits an ideal tunneling behavior with a TMR signal as large as 85% at room temperature, corresponding to a spin polarization of 55%. The large room-temperature magnetoresistance is comparable to that of state-of-the-art conventional MTJs and represents a significant advance due to the absence of room-temperature TMR in all-2D MTJs. By adjusting the thickness of the spacer layer, the TMR could be further increased. Also, we envisage that the use of vdW tunnel barriers in Fe$_{3}$GaTe$_{2}$-based MTJs, such as the wide-bandgap semiconductor GaSe or insulator hBN, larger TMRs could be achieved, offering opportunities for further advances and new possibilities for next-generation spintronic devices. Acknowledgments. This work was supported by the National Key Research and Development Program of China (Grant Nos. 2022YFA1405100 and 2022YFE0134600), the Beijing Natural Science Foundation Key Program (Grant No. Z190007), the National Natural Science Foundation of China (Grant Nos. 61774144, 62005265, and 52272152), the Key Research Program of Frontier Sciences (Grant No. QYZDY-SSW-JSC020), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant Nos. XDB44000000 and XDB28000000). 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