Large Spin Polarization from symmetry-breaking Antiferromagnets in Antiferromagnetic Tunnel Junctions

Abstract

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.

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).

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 H_IP. A linear background from the diamagnetism of the silicon substrate has been subtracted. c Anomalous Hall resistance (R_AHE) of Mn₃Sn measured at 10 K under out-of-plane magnetic fields H_OOP. d Optical microscopy image of an AFMTJ device. The scale bar corresponds to 10 µm. e Magnetoresistance of an AFMTJ (R_MTJ) measured at 10 K after zero-field cooling. The resistance change corresponds to a 113% TMR ratio.

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.

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 M_Pin. d Schematic of the pinned (M_Pin) and switchable (M_Free) 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.