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Nat Commun
Nat Commun
Nature Communications
2041-1723
Nature Publishing Group UK London

52208
10.1038/s41467-024-52208-6
Article
Large Spin Polarization from symmetry-breaking Antiferromagnets in Antiferromagnetic Tunnel Junctions
http://orcid.org/0000-0002-1739-3694
Chou Chung-Tao 12
http://orcid.org/0000-0001-8827-7570
Ghosh Supriya 3
McGoldrick Brooke C. 2
Nguyen Thanh 4
Gurung Gautam 5
http://orcid.org/0000-0002-6728-5480
Tsymbal Evgeny Y. 6
http://orcid.org/0000-0002-7055-6368
Li Mingda 4
http://orcid.org/0000-0003-3568-5452
Mkhoyan K. Andre 3
http://orcid.org/0000-0001-6892-8102
Liu Luqiao luqiao@mit.edu

2
1 https://ror.org/042nb2s44 grid.116068.8 0000 0001 2341 2786 Department of Physics, Massachusetts Institute of Technology, Cambridge, MA USA
2 https://ror.org/042nb2s44 grid.116068.8 0000 0001 2341 2786 Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA USA
3 https://ror.org/017zqws13 grid.17635.36 0000 0004 1936 8657 Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN USA
4 https://ror.org/042nb2s44 grid.116068.8 0000 0001 2341 2786 Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA USA
5 https://ror.org/052gg0110 grid.4991.5 0000 0004 1936 8948 Trinity College, University of Oxford, Oxford, UK
6 https://ror.org/043mer456 grid.24434.35 0000 0004 1937 0060 Department of Physics and Astronomy & Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, NE USA
7 9 2024
7 9 2024
2024
15 78408 4 2024
29 8 2024
© The Author(s) 2024
2024
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Efficient detection of the magnetic state is a critical step towards useful antiferromagnet-based spintronic devices. Recently, finite tunneling magnetoresistance (TMR) has been demonstrated in tunnel junctions with antiferromagnetic electrodes, however, these studies have been mostly limited to junctions with two identical antiferromagnet (AFM) electrodes, where the matching of the spin-split Fermi surfaces played critical role. It remains unclear if AFMs can provide a finite net spin polarization, and hence be used as a spin polarizer or detector. In this work, we experimentally fabricate single-sided antiferromagnetic tunnel junctions consisting of one AFM electrode (Mn3Sn) and one ferromagnet (FM) electrode (CoFeB), where the spin polarized tunneling transport from AFM is detected by the FM layer. We observe a high TMR at cryogenic temperature (>100% at 10 K) in these asymmetric AFM tunnel junctions, suggesting a large effective spin polarization from Mn3Sn despite its nearly vanishing magnetization. The large TMR is consistent with recent theoretical studies where the broken symmetry in non-collinear AFMs is predicted to lift the spin degeneracy in the band structure. Our work provides strong evidence that spin polarized electrical transport can be achieved from AFMs.

Antiferromagnets have both negligible stray fields, and fast dynamics, making them ideal for fast and densely packed spintronic devices. However, readout of the antiferromagnet state is challenging. Here, Chou et al detect the spin-polarized current emanating from a noncollinear AFM, Mn3Sn.

Subject terms

Spintronics
Magnetic properties and materials
https://doi.org/10.13039/100000028 Semiconductor Research Corporation (SRC) https://doi.org/10.13039/100000185 United States Department of Defense | Defense Advanced Research Projects Agency (DARPA) https://doi.org/10.13039/100000001 National Science Foundation (NSF) DMR-2104912 DMR-2104912 ECCS-2025124 DMR-2309431 DMR-2104912 DMR-2316665 DMR-2011401 ECCS-2025124 DMR-2309431 Chou Chung-Tao Ghosh Supriya McGoldrick Brooke C. Tsymbal Evgeny Y. Mkhoyan K. Andre Liu Luqiao https://doi.org/10.13039/100000015 U.S. Department of Energy (DOE) DE-SC0020148 DE-SC0020148 Nguyen Thanh Li Mingda https://doi.org/10.13039/501100000728 Oxford University | Trinity College, University of Oxford issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

Antiferromagnets (AFMs) have been pursued as potential building blocks for spintronic devices for their fast switching dynamics and absence of long-range dipolar interactions. However, AFMs have not been widely employed in practical magnetic memory or spin logic devices due to the lack of efficient reading and writing mechanisms. It has been recently demonstrated that the ordering parameter of collinear or non-collinear AFMs can be controlled electrically via spin-orbit torque with an energy efficiency similar to ferromagnets (FMs)1–9, providing a promising means to write information into AFMs. On the other hand, reading magnetic states from AFMs remains an obstacle due to the PT symmetry commonly possessed by AFMs10,11 (Fig. 1a), where P and T represent spatial inversion and time reversal operation, respectively. PT symmetry implies that the switching of each individual spin by 180° in a regular collinear AFM is equivalent to an inversion of the crystal structure, or a translation between neighboring spin sublattices, which does not result in any macroscopically observable change. Moreover, Kramers’ theorem dictates that the band structure of these PT-symmetric AFMs be spin degenerate E↑k=E↓k, therefore they will exhibit no spin-ordering-dependent electrical signals10.Fig. 1 Spin-polarized tunneling from non-collinear antiferromagnets.

a Schematic of a typical PT-symmetric antiferromagnet (AFM) and its band structures. The symmetry ensures all bands of a two-fold spin degenerate. b Schematic of the spin texture in one Kagome plane of Mn3Sn crystal and a locally spin-polarized band structure due to the broken PT-symmetry. Purple circles and arrows represent Mn atoms and their moments, orange arrows represent the octupole moment of the non-collinear spin texture formed by the Mn atoms. c Schematic illustration of the spin-polarized Fermi surface in AFM and ferromagnet (FM) in an asymmetric tunnel junction, where the colored arrows represent spin polarization. For simplicity, only the majority band is shown for the FM electrode. The brighter region in the insulator (I) represents regions with high tunneling probability η(k). d Schematics of the device structure of the Mn3Sn/MgO/CoFeB asymmetric tunnel junction (nominal thickness, unit in nm).

To overcome this issue, AFMs with broken PT-symmetry, including altermagnets with collinear spin ordering such as RuO212–14 and non-collinear antiferromagnets such as Mn3X (X = Pt, Ir, Sn, Ge, etc.) have been explored15–17. In Mn3X, because of the frustrated antiferromagnetic exchange of the Kagome lattice, the moment of each Mn atom is canted 120° from its nearest neighbors, resulting in almost zero net magnetic moment. By breaking the PT-symmetry with its crystalline and spin ordering (Fig. 1b), the spin degeneracy of the Mn3X band is lifted in the momentum space (Supplementary section 1), and it exhibits unusual magneto-transport phenomena such as giant anomalous Hall effect (AHE) and anomalous Nernst effect that are comparable in magnitude to FMs18–21. More recently, the tunneling magnetoresistance (TMR) from tunnel junctions made from two symmetric AFM electrodes has been proposed22,23 and experimentally verified24,25, providing a promising way to read AFM magnetic states in nanoscale devices.

Despite the initial demonstration, sizable TMR ratios have so far only been reported in symmetric antiferromagnetic tunnel junctions (AFMTJs) with two identical electrodes24,25, with the difference in tunneling resistance originated from the matched or mismatched spin-split Fermi surface when the magnetic state in one of the electrodes is changed23. Within this framework, the electrical current across the tunneling barrier possesses zero net spin polarization between the two electrodes. It is unclear if a current with net spin polarization can be achieved from AFM materials. In one previous experimental attempt, a negligible TMR (~ 0.2%) was reported in AFMTJs consisting of one AFM and one FM electrode, pointing to the vanishing net spin polarization. The small TMR could be mixed with the contribution from a residual magnetic moment of the AFM. On the other hand, recent theoretical studies predict that an FM-like spin transport with net spin polarization can be achieved using PT-symmetry-breaking AFMs, hence yielding a high TMR in asymmetric AFMTJs26,27. As shown in Fig. 1b, the constraint on the spin degeneracy in the momentum space is lifted in AFMs with broken PT-symmetry. Therefore, the band structure can exhibit local spin polarization pk for different wave vectors k23, while the integration of p(k) over the whole Fermi surface reduces to zero due to the compensated spins in real space. In a magnetic tunnel junction (MTJ), the tunneling rate η(k) for electrons with different k remains non-uniform (Fig. 1c). Generally speaking, electrons located in the center of the projected two-dimensional Brillouin zone make larger contributions to the tunneling current due to the shorter effective tunneling length28,29. Therefore, an FM-like transport with finite spin polarization can be achieved with an AFM electrode, and the corresponding AFMTJ would exhibit TMR as the spin ordering inside the AFM or FM electrode switches under an applied field, as illustrated in Fig. 1c. Since the spin transport from such AFM electrode has a net spin polarization, the TMR is expected to be significant even with amorphous FM counter-electrodes, which is different from previous symmetric AFMTJs which require crystallized and matched electrodes on both sides23.

In this work, to experimentally verify the predicted spin-polarized tunneling from an AFM electrode, we fabricate and measure asymmetric AFMTJs of Mn3Sn/MgO/CoFeB (Fig. 1d). We find that, in contrast to the previous experimental observation, the TMR in AFM/insulator/FM tunnel junctions as large as conventional ferromagnetic MTJs is observed at cryogenic temperatures (> 100% at 10 K and vanishes at 200 K and above). The high TMR ratio obtained in our experiment is consistent with the symmetry-breaking induced spin-split band structure and lends direct evidence to the large net spin polarization from zero-moment magnetic materials. Besides revealing the existence of high effective spin polarization between AFM and FM electrodes, we also find that the TMR in such AFMTJs is not directly correlated with the Hall vector of the bulk of Mn3Sn. By comparing TMR measurement with corresponding magnetization and anomalous Hall effect (AHE) loops, we identify the relation between the magnetoresistance and pinned interfacial layers of Mn3Sn, providing further insights for controlling TMR in such tunnel junctions.

Results and discussion

We develop AFM/insulator/FM MTJs as shown schematically in Fig. 1d, with the non-collinear antiferromagnetic Mn3Sn and ferromagnetic alloy CoFeB employed as two electrodes. The MTJ stack of Si/SiOx//Ru(2)/Mn3Sn(50)/MgO(4)/CoFeB(10)/Ta(5)/Ru(5) (bottom to top, nominal thickness, units in nanometers) is deposited on the oxidized silicon substrate with magnetron sputtering. To minimize Mn diffusion in the tunneling barrier, which is known to be one of the major causes of TMR degradation30,31, we develop the full stack using a three-step recipe of deposition – etching – deposition. Firstly, a bottom layer of Ru(2)/Mn3Sn(50)/MgO(4.5) is deposited and then annealed in situ at 350 °C, to ensure proper crystallization of the Mn3Sn layer. Secondly, after cooling the film to room temperature (RT) we remove the annealed MgO capping layer with in situ Ar plasma, followed by the third step of deposition of MgO(4)/CoFeB(10)/Ta(5)/Ru(5) top electrode (see Experimental Section for growth details). In this whole process, we first cap the Mn3Sn with MgO prior to annealing, then remove the annealed MgO and replace it with a fresh MgO layer so that the tunneling barrier does not experience high-temperature processing, which could lead to significant Mn diffusion. CoFeB, the widely used ferromagnetic material in regular MTJs, is chosen here because it tends to form amorphous structures when sputtered without post-deposition annealing32, which results in a smooth interface with MgO. It also yields a simpler physical picture for characterizing the net spin polarization from the Mn3Sn electrode without the need to consider the exact electronic structure of the FM. We examine the cross-section of the full stack with a scanning transmission electron microscope (STEM) shown in Fig. 2a, where polycrystalline textures are observed in Mn3Sn. We also perform energy dispersive X-ray spectroscopy to map the elemental distribution near the junction, and no significant Mn or Sn diffusion is observed (Supplementary Information Section 2).Fig. 2 Characterizations of Mn3Sn and antiferromagnetic tunnel junction Mn3Sn/MgO/CoFeB.

a High-angle annular dark-field STEM image of the Mn3Sn/MgO/CoFeB stack for antiferromagnetic tunnel junctions (AFMTJs) (scale bar corresponds to 3 nm). b Magnetization loop of Mn3Sn measured at 10 K with in-plane applied fields HIP. A linear background from the diamagnetism of the silicon substrate has been subtracted. c Anomalous Hall resistance (RAHE) of Mn3Sn measured at 10 K under out-of-plane magnetic fields HOOP. d Optical microscopy image of an AFMTJ device. The scale bar corresponds to 10 µm. e Magnetoresistance of an AFMTJ (RMTJ) measured at 10 K after zero-field cooling. The resistance change corresponds to a 113% TMR ratio.

To characterize the quality of the Mn3Sn film, we use X-ray diffraction on the bottom stack of Ru/Mn3Sn/MgO (Supplementary Information Section 3), where only peaks that correspond to the hexagonal D019 phase of Mn3Sn are observed33,34, suggesting a polycrystalline, single-phase film. The saturation magnetization of the thin film is measured to be 2 mµB per Mn atom at RT (see Supplementary Information Section 4) and 12 mµB per Mn atom at 10 K (Fig. 2b), with µB being the Bohr magneton, in agreement with the antiferromagnetic phase of Mn3Sn in previous reports18,35,36. Because of broken symmetry and topological Weyl semi-metallic state, Mn3Sn exhibits an extraordinarily large AHE that can be switched with an external magnetic field18. Figure 2c illustrates the Hall resistance as a function of the applied out-of-plane magnetic field. Both the sign and magnitude of the anomalous Hall effect are consistent with the previously reported values18,34 after accounting for the shunting from seed and cap layers (see Supplementary Information Section 5). Large remanence and high coercivity are seen in both the in-plane magnetization (M-H) (Fig. 2b) and out-of-plane AHE (Fig. 2c) hysteresis loops, consistent with the polycrystalline nature of the film.

We pattern the full material stack into micrometer-sized tunnel junctions (Fig. 2d) and study the transport properties. The magnetoresistance result from a typical AFMTJ that has been cooled down to 10 K under zero magnetic field is shown in Fig. 2e. The resistance change around zero magnetic field is due to the soft CoFeB free layer switching, and the magnetoresistance curve reflects a typical minor loop with a TMR ratio of 113%. To demonstrate the repeatability of large TMR, we measure multiple devices in addition to the one shown in Fig. 2e and find qualitatively consistent results (see Supplementary Information Section 6). Besides the zero biasing voltage result, we also examine the bias voltage dependence of TMR, where a decreasing trend with the applied voltage is observed (see Supplementary Information Section 7). This is similar to conventional ferromagnetic MTJs and can be attributed to the hot electron-induced magnetic excitation in magnetic electrodes37–39. Earlier it was predicted that as a non-collinear AFM, Mn3Sn can exhibit spin splitting at different regions of its Brillouin zone23. Symmetric tunnel junctions with two identical AFM electrodes, such as Mn3Sn/MgO/Mn3Sn, had been fabricated to exploit this momentum-resolved, local spin polarization on the Fermi surface and obtain a finite TMR24,25. Our experiment suggests that the symmetric electrode arrangement is not necessary to achieve finite magnetoresistance in AFMTJs, and asymmetric junctions with one AFM and one FM electrode can also exhibit high TMR, due to the large, net tunneling spin polarization from Mn3Sn. This is consistent with the recent theoretical prediction on the finite TMR from single-sided AFMTJs, as a result of spin degeneracy lifting in momentum space27. The momentum-dependent spin polarization p(k) in AFM, in combination with the different tunneling probabilities at the center and edge of the projected 2D Brillouin zone, leads to finite TMR (Fig. 1c). The tunneling current, in this case, is spin-polarized, and the AFM behaves similar to an FM in regular MTJs. Since the TMR from the asymmetric AFMTJ is correlated with the net spin polarization in tunneling current, it is expected to be less stringent in the crystallization condition of the FM counter-electrode compared with the corresponding effect in symmetric AFMTJs (see more discussion in the Supplementary Information section 8). The FM-like spin polarization from the AFM can, in principle, be combined with any FM or other symmetry-breaking AFM electrode, and therefore be used as a general-purpose spin polarizer. Lastly, we note that the spin-polarized transport in non-collinear AFMs of Mn3Sn shares similar origins with the recently discovered collinear family of altermagnets14, where both categories illustrate spin splitting in the momentum space due to the combined crystalline and spin orderings and enable finite magneto-transport properties.

One of the potential benefits of employing spin-polarized AFM in MTJs is their high immunity to external magnetic fields, which means they can act as a stable polarizer layer in magnetic sensors or memory devices. Figure 3 shows a systematic study of the field dependence of TMR carried out on a similar device (device B). As shown in Fig. 3a, the TMR measurement over a large field range indeed illustrates a typical minor loop behavior up to 40 kOe, where only the CoFeB free layer is switched, and the magnetic state of Mn3Sn fixed layer remains unchanged (see Supplementary Information Section 9 as well). Besides the high field stability, we also note that within the same device, the TMR polarity can change between different thermal cycles. In our experiment, we cool down the devices from RT to 10 K under a nominally zero magnetic field, and afterward, the TMR polarity remains fixed among different field scans at this low temperature. However, when the same device is warmed up to RT and cooled down again, the TMR polarity can show uncorrelated signs from previous thermal cycles, as illustrated in Fig. 3b for the TMR measurements from the same device after two thermal cycles. Accompanied by the thermally randomized polarity, we see that the magnitude of TMR also decreases with temperature and vanishes above 200 K (Fig. 3c and Supplementary Information section 10), a temperature under which Mn3Sn is still in the non-collinear antiferromagnetic phase (Néel temperature ~ 420 K). This suggests that the magnetic states of Mn3Sn contributing to TMR are pinned at low temperatures but activated into fluctuated states at RT. The unpinning of Mn3Sn at a temperature significantly lower than the Néel temperature resembles the blocking temperature of exchange bias in AFM/FM structures40, where the AFM domains become super-paramagnetic due to thermal agitation41.Fig. 3 Strong field resilience of AFM electrode and thermal cycle induced TMR polarity switch.

a Magnetoresistance of an AFMTJ (device B) measured at 10 K without a cooling field. The magnetoresistance stays constant under large applied magnetic fields, indicating the strong immunity of the Mn3Sn layer to the external field. b Magnetoresistance of the same AFMTJ after repeated zero field cooling thermal cycles between RT and 10 K, showing opposite polarities. c Magnetoresistance of the same AFMTJ measured at different temperatures. The TMR decreases with temperature and vanishes at 200 K and above. The data are shifted vertically for clarity.

To achieve control over the TMR polarity, we carry out field cooling measurements by cooling down the device from RT at a constant field (± 90 kOe). After reaching 10 K, the ± 90 kOe magnetic field is removed, and the magnetoresistance is measured. We find that under this field cooling scheme, the TMR polarity behaves consistently (Fig. 4a, b), where the junction always shows higher (lower) resistance when the free layer is parallel (antiparallel) to the cooling field direction. Under the assumption that the cooling field controls the ordering parameter of the AFM (see the insets of Fig. 4a, b), the polarity of our measurement indicates a negative TMR, opposite to most FM-based MTJs. We note that while the polarity of the TMR can be controlled by the cooling field, the magnitude can vary between thermal cycles. This suggests that the cooling field does not saturate the domains and only partially controls the orientation of the pinned domains.Fig. 4 Field cooling induced TMR polarity and magnetization offset.

a, b Magnetoresistance of an AFMTJ (device B, same as that in Fig. 3) measured at 10 K after field cooling from RT at − 90 kOe (− 90kOe FC) and + 90 kOe (90 kOe FC), respectively. Insets: illustration of the relative orientations between FM magnetic moment and AFM octupole moment near the tunneling interface under different fields. c Mn3Sn magnetization loop measured at 10 K after different field cooling conditions. Red, blue, and black data points are measured after field cooling of 70 kOe, − 70 kOe, and 0 Oe (zero field cooling, ZFC), respectively. The cooling field-induced vertical offset suggests a portion of Mn3Sn net moment is pinned at low-temperature MPin. d Schematic of the pinned (MPin) and switchable (MFree) domains of Mn3Sn after field cooling. The blue and red arrows of ± FC indicate the cooling fields, and the black arrows of ± H represent the applied field after field cooling. The domains close to the interface are pinned, as indicated by the TMR results.

The fact that the polarity of TMR in an AFMTJ is controlled by the field cooling process suggests that the AFM ordering is only changeable above the unpinning temperature. This result is in contrast to the M-H and AHE measurements where Mn3Sn switches reversibly at 10 K with coercivities around 5 kOe (Fig. 2b, c). It is known that in Mn3Sn, the magnetization and AHE are correlated with the AFM ordering parameter as described by the octupole moment42 or Hall vector43, whose switching is accompanied by 180° rotation of each sublattice spin. Therefore, the field-induced switching of AFM ordering at low temperature should, in principle, lead to TMR flipping as well23. To understand the discrepancy in measurements, we examine the influence of field cooling on TMR and magnetization. We measure the magnetization loop of the Ru/Mn3Sn/MgO bottom stack after the film is cooled down under different applied fields from RT to 10 K (Fig. 4c). We find that while the shapes of the magnetization loops corresponding to opposite cooling fields look similar, there are clearly offsets among them, indicating some frozen magnetic moment aligned along the cooling field direction. The existence of cooling field-induced magnetization offsets suggests that not all of the regions in Mn3Sn films can be reversibly switched by a large magnetic field at low temperatures and that there are domains that can only be toggled when the temperature is raised higher36. We also measure this magnetization offset under different temperatures (Supplementary Information Section 11) and notice the offset decreases with the increase of working temperature, similar to the decreasing trend in TMR. Both magnetic moment offset and TMR vanish at temperatures above 200 K. Previously, separations between pinned and rotatable domains have also been observed in the anomalous Hall effect measurement of single layer Mn3Sn, which exhibits a similar unpinning temperature below RT36.

As is well known, TMR is only sensitive to spin polarization of the interfacial layers of magnetic electrodes, while the magnetization or AHE measurement reflects the average behavior of the whole magnetic layer. Therefore, our experimental observations point to the picture that the pinned domains observed in the magnetization loop are located closer to the Mn3Sn/MgO interface, which contributes to TMR, and is decoupled from the field-switchable underlayers of Mn3Sn, as schematically illustrated in Fig. 4d. While the microscopic origin of the drastic difference in magnetic behaviors between bulk and interfacial Mn3Sn requires further investigation, there are a few possible causes. First, it has been reported that Mn3Sn can be heavily strained near an Mn3Sn/oxide interface36, which can lead to a drastically different coercive field43,44, hindering field-induced magnetic switching. Besides, Mn3Sn is known to be chemically active and can be oxidized45. Despite the that our growth and annealing are all carried out in situ, the top surface of Mn3Sn may incur a certain degree of oxidation during the MgO deposition46, which alters the interfacial layer properties, including a significant increase of coercivity47. Future experiments with characterization tools such as X-ray photoelectron spectroscopy will help to elucidate the exact state of the interfacial layers. In addition, we note that unlike FMs, where domain wall formation along the film thickness direction is usually energetically unfavorable, AFMs can exhibit domains along their depth much more easily due to the very narrow domain wall width48,49. In addition, it is well known that the Ar plasma etching process (See Experimental Section for detail) could change the properties of the magnetic electrodes50, and its influence on the Mn3Sn interface layer requires further investigation. Finally, despite the difference in the coercive fields of bulk and interfacial Mn3Sn, it is worth noting that the interfacial Mn3Sn is still in the AFM phase as indicated by the large coercive field (> 40 kOe) obtained from TMR loop (Fig. 3a and Supplementary Information Section 9) and the finite exchange bias in a Mn3Sn/CoFeB control sample (Supplementary Information Section 12).

In conclusion, we experimentally study Mn3Sn/MgO/CoFeB AFMTJs and observe a TMR ratio as large as 100% at 10 K. The high TMR value shows that large effective spin polarizations can be achieved in the asymmetric tunnel junctions made by AFM and FM electrodes. The result of finite net spin polarization from a zero moment AFM is in agreement with theoretical predictions on PT-symmetry-breaking AFMs, where the spin-splitting band structure, in combination with the wave vector-dependent tunneling probability, leads to FM-like spin transport. In addition, we find that the AFM polarizer layer remains stable under a high magnetic field, and the TMR polarity can only be controlled by thermal processes above the unpinning temperature, indicating a pinned Mn3Sn interfacial layer at low temperature.

The discovery of FM-like spin polarization from AFMs underpins the usage of symmetry-breaking AFM as a general-purpose, field-resilient spin polarizer for controlling spin transport or exciting magnetic dynamics. Future studies that continue to increase the TMR ratio and enhance the unpinning temperature of the non-collinear AFM, e.g., using AFM with higher Néel temperature or with stronger pinning at the surface layer, can possibly extend the observed effects to above RT. Mn3Sn-based AFMTJs also serve as a suitable platform to study electron tunneling from topological band structures in Weyl semimetals.

Methods

Material growth

The full AFMTJ stack consists of Si/SiOx//Ru(2)/Mn3Sn(50)/MgO(4)/CoFeB(10)/Ta(5)/Ru(5) (bottom to top, nominal thickness, units in nanometers). To grow the full stack, a bottom layer of Ru(2)/Mn3Sn(50)/MgO(4.5) is first deposited on top of an oxidized silicon substrate at RT via direct current (dc) and radio frequency (rf) magnetron sputtering with an Ar pressure of 2 mTorr. The film is then annealed in situ at 350 °C for 30 min to reach the needed crystallization. After cooling the film to RT, we remove the annealed MgO capping layer with in situ Ar plasma and then deposit a MgO(4)/CoFeB(10)/Ta(5)/Ru(5) top electrode. The etching rate for the Ar etching is determined by secondary ion mass spectrometry. Another stack with no FM electrode layer, namely Si/SiOx//Ru(2)/Mn3Sn(50)/MgO(4)/Ta(5)/Ru(5), is grown with the same process parameters for X-ray diffraction, magnetometry, and anomalous Hall effect measurements of Mn3Sn. In addition, a stack with no MgO tunnel barrier, namely Si/SiOx//Mn3Sn(50)/CoFeB(10)/Ta(5)/Ru(5), is grown with the same parameters for exchange bias study.

X-ray diffraction measurement

Grazing incidence X-ray diffraction is measured in a Rigaku Smartlab multipurpose diffractometer with a Cu Kα source. The angle of incidence is kept at 2° while the 2θ angle and the angle of reflection perform a coupled scan. The azimuthal angle of the incidence and reflection directions is chosen to be ~ 15° from the Si [100] direction to avoid diffraction peaks from the substrate.

Magnetometry measurement

Superconducting quantum interference vibrating sample magnetometry (SQUID-VSM) measurement is carried out using Quantum Design Magnetic Property Measurement System 3 (MPMS3). A linear background is subtracted from the raw data to account for the diamagnetism of the silicon substrate and the sample stage.

Scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy

Cross-section samples for STEM and energy dispersive X-ray spectroscopy (EDX) studies were prepared on an FEI Helios Nanolab G4 dual-beam focused ion beam (FIB) instrument, using a 30 keV Ga ion beam for thinning the cross-sections. STEM measurements were conducted on an aberration-corrected FEI Titan G2 60-300 (S)TEM microscope equipped with a monochromator, CEOS-DCOR probe corrector, and a SuperX EDX detector. The microscope was operated at 200 keV, with a probe convergence angle of 18.2 mrad and high-angle annular dark-field imaging detector collection angles of 55 and 200 mrad, respectively. A probe current of 120 pA was utilized for imaging and EDX maps acquisitions, done on the Bruker Esprit software.

Device fabrication

The micron-sized MTJ device is patterned by photolithography using MLA-150 Maskless Aligner. The full metal stack outside the bottom contact pad region is removed by Ar ion milling. Then MTJ pillar is formed by etching the top layers (MgO/CoFeB/Ta/Ru) elsewhere with an ion miller and a secondary ion mass spectroscopy end-point detector. A SiOx insulation layer with a thickness of 125 nm is deposited using an e-beam evaporator and then lifted off to open a window for the top contact pad to make electrical contact with the MTJ. The top contact pad of Ta(5)/Cu(120)/Pt(5) is deposited by dc sputtering and lifted off. No post-annealing has been used after the full stack deposition. We also fabricate AFMTJs using stacks with post-deposition annealing at 400 °C but observe only very weak TMR (0.5%), likely due to Mn diffusion-induced barrier degradation30.

Electric transport measurement

Tunneling magnetoresistance of MTJs and anomalous Hall effect of Hall bars are measured using a low-frequency lock-in technique in Quantum Design Physical Properties Measurement System (PPMS) with an Electric Transport Option (ETO) module. The AC current amplitude for TMR measurement is fixed at 10 nA. A small time-dependent resistance drift (~ 3% of RMTJ) due to temperature drift and/or contact degradation during measurement is substracted in Fig. 4(b).

Density function theory calculations

Density function theory (DFT) calculations are performed using Quantum ESPRESSO51, taking into account noncollinear magnetism but neglecting spin-orbit coupling (SOC). The plane-wave pseudopotential method with the fully relativistic ultrasoft pseudopotentials52 is employed in the calculations. The exchange and correlation effects are treated within the generalized gradient approximation (GGA)53. The k-point mesh of 12 × 12 × 15 and plane-wave cut-off energy of 52Ry are used for the integration in the irreducible Brillouin zone. The experimental lattice constants of Mn3Sn a = b = 5.665 Å, c = 4.531 Å54, are used in the calculations. The spin expectation value for each Bloch state is calculated as follows: s=ℏ2ψnk,∣,σ,∣,ψnk, where σ is the Pauli matrix and ψnk is the Bloch wave function.

Supplementary information

Supplementary Information

Peer Review File

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-024-52208-6.

Acknowledgements

This work was supported in part by Semiconductor Research Corporation (SRC) DARPA and the National Science Foundation under award DMR-2104912. T.N. and M.L. are partially supported by the U.S. Department of Energy (DOE), Office of Science (SC), Basic Energy Sciences (BES) Award No. DE-SC0020148. Parts of this work were carried out in the Characterization Facility University of Minnesota, which receives partial support from the NSF through the MRSEC (Award Number DMR-2011401) and the NNCI (Award Number ECCS-2025124) programs. S.G. and K.A.M. would like to acknowledge partial support from NSF Grant No. DMR-2309431. G.G. gratefully acknowledges the support of a Junior Research Fellowship from Trinity College, Oxford, U.K. E.Y.T acknowledges support from the National Science Foundation (NSF grant No. DMR-2316665).

Author contributions

C.-T.C. and L.L. planned and designed the experiment. C.-T.C. performed material growth and characterization, device stack growth, and device fabrication. S.G. carried out STEM imaging of the stack with input from K.A.M. C.-T.C., and T.N. set up the measurement system. C.-T.C. carried out the magneto-transport measurements. G.G. and E.T. performed DFT calculations of the Mn3Sn band structure. All coauthors discussed the results. C.-T.C. and L.L. wrote the paper with feedback from all the coauthors.

Peer review

Peer review information

Nature Communications thanks Xianzhe Chen, Guohan Hu, and the other anonymous reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

Data availability

All data generated in this study have been deposited in figshare under the accession code [10.6084/m9.figshare.25562202].

Competing interests

The authors declare no competing interests.

Ethics statement

This work is carried out in accordance with ethical standards such as diversity, equality, and responsible conduct.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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