⇦Chinese Physics Letters, 2023, Vol. 40, No. 5, Article code 058501Express Letter Giant Tunneling Magnetoresistance in Spin-Filter Magnetic Tunnel Junctions Based on van der Waals A-Type Antiferromagnet CrSBr Guibin Lan (兰贵彬)1,2†, Hongjun Xu (许洪军)1,3†, Yu Zhang (张雨)1,2, Chen Cheng (程琛)1, Bin He (何斌)1,2, Jiahui Li (李嘉辉)1,2, Congli He (何聪丽)4*, Caihua Wan (万蔡华)1,2,3, Jiafeng Feng (丰家峰)1,2, Hongxiang Wei (魏红祥)1,2, Jia Zhang (张佳)5, Xiufeng Han (韩秀峰)1,2,3, and Guoqiang Yu (于国强)1,2,3* Affiliations 1Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China 2Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China 3Songshan Lake Materials Laboratory, Dongguan 523808, China 4Institute of Advanced Materials, Beijing Normal University, Beijing 100875, China 5School of Physics and Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan 430074, China Received 12 March 2023; accepted manuscript online 3 April 2023; published online 7 April 2023 †These authors contributed equally to this work.
*Corresponding authors. Email: conglihe@bnu.edu.cn; guoqiangyu@iphy.ac.cn Citation Text: Lan G B, Xu H J, Zhang Y et al. 2023 Chin. Phys. Lett. 40 058501 Abstract Two-dimensional van der Waals magnetic materials have demonstrated great potential for new-generation high-performance and versatile spintronic devices. Among them, magnetic tunnel junctions (MTJs) based on A-type antiferromagnets, such as CrI$_{3}$, possess record-high tunneling magnetoresistance (TMR) because of the spin filter effect of each insulating unit ferromagnetic layer. However, the relatively low working temperature and the instability of the chromium halides hinder applications of this system. Using a different technical scheme, we fabricated the MTJs based on an air-stable A-type antiferromagnet, CrSBr, and observed a giant TMR of up to 47000% at 5 K. Meanwhile, because of a relatively high Néel temperature of CrSBr, a sizable TMR of about 50% was observed at 130 K, which makes a big step towards spintronic devices at room temperature. Our results reveal the potential of realizing magnetic information storage in CrSBr-based spin-filter MTJs.
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 DOI:10.1088/0256-307X/40/5/058501 © 2023 Chinese Physics Society Article Text Magnetic tunnel junction (MTJ) is a key element for electrically reading magnetic information, which has been widely used in many spintronic devices,[1] such as hard disk read heads,[2] magnetic sensors,[3,4] magnetic random-access memory,[5] spin-torque oscillator,[6] skyrmion devices,[7] and neuromorphic devices.[8] A classical MTJ usually uses a non-magnetic insulator as the tunneling barrier. High tunneling magnetoresistances (TMRs) have been initially realized in Fe/Al$_{2}$O$_{3}$/Fe[9] and CoFe/Al$_{2}$O$_{3}$/Co[10] MTJs, where the barrier is an amorphous AlO$_{x}$. The highest room-temperature TMR of the AlO$_{x}$ barrier-based MTJ reaches 81% in a Co$_{40}$Fe$_{40}$B$_{20}$/Al–O/Co$_{40}$Fe$_{40}$B$_{20}$ MTJ.[11] According to the ab initio calculations, the Fe/MgO/Fe MTJ is predicted to exhibit a much higher TMR due to the filtering effect of the spin-polarized $\varDelta_{1}$ band of bcc-Fe through the MgO barrier.[12] Experimentally, very high TMRs have been realized in the MTJs with a MgO barrier, such as in CoFeB/MgO/CoFe[13] and Fe/MgO/Fe[14] systems. More than ten years ago, the record-high TMRs of the MgO-based MTJ reached 604% at 300 K and 1144% at 5 K, respectively.[15] Very recently, the epitaxial MTJs, CoFe/MgO/CoFe(001), with better lattice-matching interfaces exhibit an even higher TMR, i.e., 631% at 300 K.[16] Despite successful applications of the classical MTJs, increasing the TMR remains challenging. Two-dimensional (2D) van der Waals (vdW) materials emerge as promising alternatives for realizing high-quality MTJs because of the single crystalline feature as well as atomically sharp and smooth interfaces. With the fast rising of 2D vdW magnets,[17-19] such as Cr$_{2}$Ge$_{2}$Te$_{6}$,[20] CrI$_{3}$,[21] Fe$_{3}$GeTe$_{2}$,[22] and CrTe$_{2}$[23] together with the vdW tunnel barriers, e.g., h-BN, WSe$_{2}$, In$_{2}$Se$_{3}$, it is possible to design and realize full-vdW MTJs. Such vdW MTJs have been reported based on Fe$_{3}$GeTe$_{2}$/InSe/Fe$_{3}$GeTe$_{2}$[24] and Fe$_{3}$GeTe$_{2}$/h-BN/Fe$_{3}$GeTe$_{2}$,[25] which show TMRs of 41% at 10 K and 300% at 4.2 K, respectively. Recently, a room-temperature (RT) vdW ferromagnet Fe$_{3}$GaTe$_{2}$ has been reported,[26] and a full-vdW MTJ based on it exhibits an RT TMR of 85%.[27] However, only relatively low TMRs have been observed in 2D vdW MTJs based on the classical structure of MTJs until now, which still can not compete with the state-of-the-art MgO-based MTJs. Unlike the classical MTJs, a spin-filter MTJ (sf-MTJ)[28,29] based on an A-type vdW antiferromagnetic semiconductor or insulator produces a giant TMR, which is much higher than the classical MTJs at low temperatures.[13-16,30-33] The advantages of using an A-type vdW antiferromagnet lie in its simplicity because the conventional trilayer sandwich MTJ structure is replaced by one medium with multiple spin-filter processes.[28] Two magnetic configurations of the magnetic layers can be easily realized by changing the external magnetic field due to the weak antiferromagnetic coupling of the two adjacent layers. A few research groups have obtained giant values of TMR in sf-MTJs based on graphene/CrI$_{3}$/graphene, such as 19000% at 2 K,[30] 10000% at 10 K,[31] and 10000000% at 1.4 K.[33] However, the Néel temperature ($T_{\rm N}$, $\sim $ 60 K) of CrI$_{3}$ is relatively low, and the chromium halides usually degrade in the atmosphere. For practical applications, one has to find A-type antiferromagnets with higher $T_{\rm N}$ and better stability. CrSBr is a vdW A-type antiferromagnetic semiconductor ($E_{g} \sim 1.5$ eV).[34,35] Unlike most 2D vdW materials, CrSBr is an air-stable material that can be stored under ambient conditions without degradation.[36] More importantly, CrSBr has a relatively high $T_{\rm N}$ ($\sim $ 150 K for the bulk).[36,37] On the other hand, ab initio calculation suggested that CrSBr is very suitable for fabricating sf-MTJs, and a high TMR is predicted.[34] Considering that only the A-type antiferromagnet is necessary for realizing the sf-MTJ, one may even simplify the above full-vdW sf-MTJ stacks by replacing graphene with conventional metallic electrodes. In this Letter, we employ CrSBr as the core structure to make sf-MTJs by fabricating vertical stacks of Pt/Au/CrSBr/Pt. The Pt/Au/CrSBr/Pt sf-MTJs exhibit a giant TMR of up to 47000% at 5 K. Moreover, we find that there is a TMR of about 50% at 130 K, which is very close to $T_{\rm N}$. It is so far the record-high working temperature in the sf-MTJs, and it provides a new platform for spintronic devices based on vdW materials.
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Fig. 1. Spin-filter magnetic tunnel junctions (sf-MTJs). (a) Schematic of the device for the sf-MTJ, with A-type antiferromagnet CrSBr as the spin-filter layer between Pt/Au and Pt electrodes. (b) Schematic of the magnetic states for CrSBr, layered-antiferromagnetic state (left panel, $H < H_{{\rm flip}}$) and fully ferromagnetic state with in-plane magnetization (right panel, $H>H_{{\rm flip}}$). (c) Optical microscopy image of the MTJs (left), including three junctions with different diameters (12, 24, and 18 µm). The cross-sectional STEM images (middle and right) of the measured device with different length scales.
Figure 1(a) shows the schematic structure of the sf-MTJ, which consists of two metal electrodes separated by multilayer CrSBr. The single-crystalline bulk CrSBr was purchased from HQ Graphene Ltd. We chose a route of making the CrSBr devices that is more compatible with conventional spintronic metallic stacks. Firstly, a bilayer of Pt (4.5 nm)/Au (1.5 nm) was deposited onto the Si/SiO$_{2}$ wafers by dc magnetron sputtering. Here, the Au layer was used to obtain large-area and high-quality CrSBr flakes by the Au-assisted mechanical exfoliation method.[38] Next, the thin CrSBr was pre-exfoliated from bulk CrSBr in the glove box (the concentration of O$_{2}$ and H$_{2}$O are below 0.1 ppm) and stuck on the Si/SiO$_{2}$/Pt/Au substrates. After transferring the crystals from the glove box into the sputtering chamber and exfoliating the crystals under a high vacuum, we obtain the relatively thin CrSBr flakes on Pt/Au with clean and fresh surfaces.[39] A Pt ($\sim $ 4 nm) layer was subsequently deposited onto CrSBr with an ultra-low sputtering power (3–5 W) and a relatively high deposition pressure ($\sim $ 1 Pa). Ultraviolet lithography and Ar ion milling were used for fabricating the MTJs. The layout of sf-MTJ devices is shown in the left panel of Fig. 1(c). All the electrical measurements were carried out in a physical property measurement system (PPMS, Quantum Design). Figure 1(b) shows the crystal structure of CrSBr viewed along with the $a$-axis, where the $b$-axis is the in-plane easy axis of CrSBr. The Cr spins in the CrSBr are ferromagnetically coupled in the single layers and antiferromagnetically coupled between neighboring layers, forming the so-called A-type antiferromagnetic structure, as shown in the left panel of Fig. 1(b). Hence, two layers of CrSBr have opposite magnetic moments under the zero or small magnetic field. In this configuration, all the electrons, spin-up or spin-down, must encounter a higher barrier height after getting polarized by passing through the first layer because the next layer has an opposite spin orientation, giving rise to a higher tunneling resistance. When a magnetic field is applied along $b$-axis and larger than the flip field ($H_{\rm flip}$), all the magnetic moments are aligned with the magnetic field, as shown in the right panel of Fig. 1(b). In this case, the electrons with spins parallel to the field direction encounter the lower barrier height resulting in lower tunneling resistance. The interlayer antiferromagnetic state turns into the interlayer ferromagnetic state. The above discussion can be extended to more layers, and the thicker the CrSBr layer, the smaller the tunneling current for the antiparallel configuration, and hence the higher the TMR. The middle and right images in Fig. 1(c) show the structure of the measured device by high-angle annular dark-field scanning transmission electron microscopy (STEM), which has a high-quality interface and crystalline structure. To quantitatively express tunneling magnetoresistance of the sf-MTJ, we define spin-filter tunneling magnetoresistance (sf-TMR) as \begin{align} {\rm sf{\textrm -}TMR}=\frac{R_{AP} -R_{P} }{R_{P} }\times 100\%, \tag {1} \end{align} where $R_{P}$ and $R_{AP}$ are the resistances when spin alignments are at $P$ (1 T) and $AP$ (0 T) states. Figure 2(a) shows the tunneling resistance of a typical sf-MTJ as a function of the magnetic field. The thickness of the spin filter CrSBr is about 22 nm, measured by an atomic force microscope. The bias voltage is $-$1.0 V, and the temperature is 10 K. A magnetic field ($H$) of $\mu_{0}H = 1$ T along with the $b$-axis of CrSBr is large enough to saturate the moments of different layers, resulting in a low tunneling resistance. The green (orange) line corresponds to the case of decreasing (increasing) $H$. The inset in Fig. 2(a) illustrates the corresponding magnetic states: the ferromagnetic (antiferromagnetic) state corresponds to low (high) resistance at a high (low) magnetic field, respectively. The sf-TMR reaches $\sim $ 7000% according to Eq. (1). In addition, there are some switching steps around $\pm 0.21$ T in Fig. 2(a), and we will discuss them later. Because of the in-plane anisotropy of CrSBr, it is expected that different switching behaviors should exist in the tunneling magnetoresistance loops for $H$ along different axes, as shown in Fig. 2(b). When the magnetic field is applied along the $b$-axis, the saturation field is the lowest ($\sim $ 0.34 T). When the magnetic field is applied along the $a$-axis, the saturation field is relatively larger ($\sim $ 0.94 T), which corresponds to the $H_{\rm flip}$ of the antiferromagnet.[40] The saturation field for the magnetic field along the $c$-axis ($\sim $ 1.73 T) is obviously larger than that along the $a$-axis. These phenomena can be explained by the magnetic anisotropy of CrSBr. Previous experiments and calculations show that CrSBr has triaxial magnetic anisotropy.[41,42] That is an easy axis along the $b$-axis, an intermediate axis along the $a$-axis, and a hard axis along the $c$-axis. By contrast, CrI$_{3}$ only has uniaxial magnetic anisotropy.[21] As a result, CrSBr may offer more degrees of freedom for designing spintronic devices by utilizing its triaxial magnetic anisotropy.
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Fig. 2. Spin-filter effect in the sf-MTJ based on the A-type antiferromagnet CrSBr. (a) Tunneling resistance as a function of the magnetic field under a bias voltage of $-$1.0 V and at a temperature of 10 K. Inset shows the corresponding magnetic states of CrSBr. (b) Normalized tunneling resistance as a function of the magnetic field at 1 V and 30 K. The magnetic field is along with the $a$-axis (black), $b$-axis (red), and $c$-axis (blue) of CrSBr. An optical image of a typical sf-MTJ device is shown in the inset, and the red dashed circle marks the junction. (c) The sf-TMR as a function of bias voltage extracted from $I$–$V$ data is shown in the inset. (d) $\ln(I/V^2)$ as a function of $1/|V|$. The red line fits for F-N tunneling, and the blue line fits for direct tunneling. The insets show band diagrams of the two tunneling.
Figure 2(c) shows the sf-TMR as a function of bias voltage at 10 K, which is derived from the $I$–$V$ curves shown in the inset. Because the MTJ electrodes on the top and at the bottom are different metals, their differences in work functions and contact barriers may be responsible for asymmetry in bias polarity-dependent resistance and TMR. We also note that the qualities of the interfaces of CrSBr with two electrodes are different. The sputtering of Pt onto CrSBr unavoidably causes damage to top interfaces, which is shown in Fig. 1(c). Since CrSBr is a magnetic semiconductor at low temperatures (blow $T_{\rm N}$), the resistance of Pt/Au/CrSBr/Pt sf-MTJ is mainly determined by the electronic transport behavior through the CrSBr layer. Using the $I$–$V$ data recorded at 10 K in Fig. 2(c) as an example, we plot $\ln (I/V^{2})$ as a function of $1/|V|$ as shown in Fig. 2(d). The electrical tunneling has two types of mechanisms. One is Fowler–Nordhein (F-N) tunneling at high bias voltage, and the other is direct tunneling at low bias voltage. According to the theories of the two tunneling behaviors,[43,44] we can obtain \begin{align} \ln \Big(\frac{I}{V^{2}}\Big)\propto \begin{cases} -\frac{4d\sqrt {2m_{\rm e}^{\ast}\varphi^{3}}}{3\hslash q} \cdot \big(\frac{1}{V}\big),& {\rm F{\textrm -}N~ tunneling}, \\ \ln \big(\frac{1}{V}\big)-\frac{2d\sqrt {2m_{\rm e}^{\ast } \varphi } }{\hslash },& {\rm direct~ tunneling}, \end{cases} \tag {2} \end{align} where $d$ is the barrier thickness, $\varphi$ is the barrier height, $m_{\rm e}^{\ast}$ is electron effective mass, $q$ is the electronic charge, and $\hslash$ is the reduced Planck constant. We take the data at the positive bias voltage with different magnetic fields (0 T and 1 T) as an example. The results can be fitted well by using Eq. (2), which indicates that the two types of tunneling behaviors occur in Pt/Au/CrSBr/Pt sf-MTJ with the change of bias voltage.[31,45] The average barrier height of the antiferromagnetic state ($\mu_{0}H = 0$ T, $\varphi = 0.64$ eV, corresponding to the high resistance state) is larger than the ferromagnetic state ($\mu_{0}H = 1$ T, $\varphi = 0.58$ eV, corresponding to the low resistance state). The differences in these tunneling behaviors are displayed in the inset in Fig. 2(d). With the increasing bias voltage, the induced electric field strongly tilts the energy band of the semiconductor, and the electrons eventually tunnel into its conduction band. That is the occurrence of F-N tunneling, which remarkably reduces the resistance. Figure 3(a) shows the resistance as a function of measurement temperature for different magnetic fields along the $b$-axis of CrSBr. A magnetic field of 1 T is large enough to saturate the magnetic moments of different layers. As can be seen from Fig. 3(a), the turning point denotes the transition between antiferromagnetic and ferromagnetic states. Generally speaking, the competition between the Zeeman energy of each ferromagnetic CrSBr layer under an external magnetic field and the antiferromagnetic exchange coupling energy of these neighboring layers determines the magnetic equilibrium status of the CrSBr multilayers and the tunneling resistance. The magnetic moments of CrSBr can be aligned along the field direction, which is shown as the ferromagnetic state in Fig. 3 if the magnetic field is large enough or antiferromagnetic coupling is negligible at a higher temperature. As temperature decreases, the interlayer antiferromagnetic exchange energy in CrSBr increases more quickly than the magnetization (i.e., Zeeman energy).[36,37] The turning point in the $R$–$T$ plot occurs at the temperature where antiferromagnetic exchange energy overcomes the Zeeman energy, and it signifies the transition from a ferromagnetic state to an antiferromagnetic state. The transition temperature is lower under a larger magnetic field, as shown in Fig. 3(a). Adding up the transition points in Fig. 3(a), we can obtain a field-temperature phase diagram, as shown in Fig. 3(b). The CrSBr is at a paramagnetic state when the temperature is over $T_{\rm N}$ ($\sim $ 150 K).[36,37] Compared with the field-temperature phase diagrams of CrI$_{3}$, CrCl$_{3}$, and CrBr$_{3}$ in the previous work,[32] one can obviously find that CrSBr, as the spin filter in sf-MTJs, has a larger working region.
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Fig. 3. The interlayer magnetic states for the A-type antiferromagnet CrSBr probed by the TMR of the device. (a) Tunneling resistance (with offsets) as a function of measurement temperatures. The magnetic fields are along with the $b$-axis between 0 T and 0.25 T. (b) Field (parallel to $b$-axis) versus temperature phase diagram extracted from results in Fig. 3(a).
Figure 4(a) shows the resistance as a function of the magnetic field along the $b$-axis. The data were measured at different negative bias voltages to obtain the largest values of TMR. The temperature dependence of TMR is shown in the black line of Fig. 4(b). The TMR gradually decreases with increasing temperature. The splitting of the conduction band is caused by the magnetic exchange splitting 2$\varDelta_{\rm ex}$, which is the difference between the up-spin and down-spin energy levels.[29] The electrons of the two different spin orientations experience different barrier heights, which correspond to the different resistances.[29] Meanwhile, $\varDelta_{\rm ex}$ decreases with temperature,[29] i.e., the higher the temperature is, the smaller the $\varDelta_{\rm ex}$ is, and the lower the TMR is. In addition, the spin-flip field also decreases with the elevating temperature, as shown by the red line in Fig. 4(b). This is because the rising temperature reduces the anisotropy energy of CrSBr, while the thermal agitation energy increases.[36,46] The TMR is close to zero with the temperature approaching $T_{\rm N}$. Here, the highest temperature at which we can observe sf-TMR is 130 K. The value is 50%. Furthermore, there are some switching steps in Figs. 2(a) and 4(a), and the possible reasons are: (1) the local inhomogeneity of the CrSBr layer (considering that the diameter of the tunnel junction is about 10 µm), and (2) the layer-dependent spin-flop transitions of the CrSBr layer due to the antiferromagnetic linear-chain model.[31,36] The spin-flop transition can lead to a jump-like reversal between the antiferromagnetic and non-collinear states when the magnetic field increases above the critical value ($H_{\rm flop}$), while the blue line in Fig. 4(b) shows that $H_{\rm flop}$ gradually decreases with the rising of temperature.[31,36] Furthermore, Fig. 4(c) shows the largest value of sf-TMR measured in this work. The value is up to 47000% at 5 K. Note that the developed sf-MTJs in this work significantly extends the working temperature compared to the previous sf-MTJs. Using the material with a higher $T_{\rm N}$ may further extend the working temperature. According to some ab initio calculations,[47] CrTeI is a 2D A-type antiferromagnetic semiconductor at room temperature. Based on our work, one may expect CrTeI enables sf-MTJs to work at higher temperatures. Hence, sf-MTJs based on 2D vdW with higher $T_{\rm N}$ are worth further studying.
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Fig. 4. Temperature dependence of TMR in the CrSBr-based sf-MTJ. (a) Tunneling resistances as a function of the magnetic field at different temperatures. (b) The sf-TMR (black line) as a function of temperature. The spin-flip fields ($H_{\rm flip}$, red line) or the fields of the switching steps ($H_{\rm flop}$, blue line) as a function of different temperatures. All the data are extracted from Fig. 4(a). (c) The largest sf-TMR ($\sim $ 47000%) measured in this work. The bias voltage is $-$0.8 V, and the measurement temperature is 5 K. The green (orange) line corresponds to decreasing (increasing) magnetic field.
In summary, our results demonstrate that the Pt/Au/CrSBr/Pt sf-MTJ fabricated by our new advanced recipe shows excellent TMR properties. We achieve a high TMR of up to 47000% at 5 K in this work. It is not difficult to predict that a higher TMR can be obtained if the measurement temperature is reduced or the thickness of CrSBr is increased. TMR of about 50% is observed at 130 K (near $T_{\rm N}$), which is the highest-record working temperature that shows measurable TMR in sf-MTJs based on 2D vdW materials.[30,31,33,37,45] The CrSBr is an air-stable magnetic semiconductor and has a relatively high $T_{\rm N}$.[36] Its magnetic anisotropy gives rise to different behaviors of resistance with the change of magnetic fields. CrSBr is a semiconductor with a band gap of 1.5 eV,[34,35] which allows many methods to manipulate the magnetism. Moreover, spin-orbit torque driven[48-50] or orbit-transfer torque driven[51] magnetization switching in 2D vdW magnets enables compact and energy-efficiency 2D vdW spintronic devices. These above-mentioned characteristics broaden applications of sf-MTJs and highlight the potentially unrivaled performance of spintronic devices based on 2D vdW materials. Acknowledgments. This work was supported by the National Key Research and Development Program of China (Grant No. 2021YFB3601300), the National Natural Science Foundation of China (Grants Nos. 52161160334, 52271237, 12274437, 12134017, and 12174426), the Science Center of the National Science Foundation of China (Grant No. 52088101), the Beijing Natural Science Foundation (Grant No. Z190009), and the K. C. Wong Education Foundation (Grant No. GJTD-2019-14). 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