Octupole-driven spin torque switching of antiferromagnetic tunnel junctions

The gist The authors report nanoscale AATJs exhibiting octupole-driven spin-transfer torque (OTT) switching, which demonstrates current-induced switching of the cluster magnetic octupole moment in PtMn3MgOPtMn3 AATJs at room temperature with a TMR value up to 363% and switching current densities around 10 MA/cm2

Background on Antiferromagnetic Tunnel Junctions

Magnetic tunnel junctions (MTJs) are canonical devices in spintronics that simultaneously exhibit mechanisms for electrical detection and control of magnetic order through the tunneling magnetoresistance (TMR) effect and spin-transfer torque (STT) effects<ref:2509.03026#pg3>. It was long assumed that neither of these effects could be sizeable in all-antiferromagnetic tunnel junctions (AATJs), since they exhibit no net magnetization<ref:2509.03026#pg3>. Recently, however, it was shown that AATJs based on chiral antiferromagnets do exhibit TMR due to their non-relativistic momentum-dependent spin polarization and cluster magnetic octupole moment (CMO), which are manifestations of their spin-split band structure<ref:2509.03026#pg3>.

Octupole-Driven Spin Transfer Torque (OTT)

The reciprocal effect, i.e., the antiferromagnetic counterpart of STT, has been assumed non-existent due to the total electric current being spin-neutral<ref:2509.03026#pg3>. Here, the authors report nanoscale AATJs exhibiting this reciprocal effect, which they term octupole-driven spin-transfer torque (OTT)<ref:2509.03026#pg3>. This is demonstrated by current-induced OTT switching of PtMn3MgOPtMn3 AATJs, exhibiting a TMR value of 363% at room temperature and switching current densities of the order of 10 MA/cm2<ref:2509.03026#pg3>. The theoretical modeling explains the origin of OTT in terms of the imbalance between intra- and inter-sublattice spin currents across the AATJ, and equivalently, in terms of the non-zero net cluster octupole polarization of each PtMn3 layer<ref:2509.03026#pg3>.

Microscopic Mechanism

The microscopic mechanism underlying OTT is confirmed by density functional theory (DFT) calculations<ref:2509.03026#pg6>. Specifically, OTT originates from the octupole-moment-driven imbalance in the contributions of different Mn sublattice atoms to the Bloch states extending across the tunnel barrier<ref:2509.03026#pg6>. This imbalance leads to a stronger intra-sublattice torque compared to inter-sublattice torque, thereby manifesting itself as a net staggered spintransfer torque acting separately upon each sublattice<ref:2509.03026#pg6>. The total tunneling current to each sublattice, Mn i, carries a net spin polarization of (β1 − β2)p i and induces an OTT of the form T i ∝ (β1 − β2)m i × (m i × p i)<ref:2509.03026#pg10>. This torque resembles a sublattice-dependent STT and is a direct consequence of the octupolar order<ref:2509.03026#pg10>.

Experimental Observations and Characterization

The experimental observation of OTT-induced switching was performed using antiferromagnetic PtMn3 as the active material, which is a cubic noncollinear antiferromagnet with Mn spins residing in the Kagome (111)-plane with a 120˚ arrangement<ref:2509.03026#pg7>. The device structure used for switching experiments consisted of Pt (5 nm) / PtMn3 (7 nm) / MgO (2 nm) / PtMn3 (10 nm) / Pt (5 nm) / Ru (10 nm)<ref:2509.03026#pg7>. Current pulses were applied, and bidirectional switching with a sharp transition at the current density J of 11 MA/cm2 was observed<ref:2509.03026#pg7>. The observed switching behaviors were all reversible, repeatable, and showed clear saturation in both directions<ref:2509.03026#pg8>.

Thermal Stability and Size Dependence

The energy barrier for the free layer to switch was experimentally estimated to be approximately 39 kT at room temperature, consistent with theoretical estimation<ref:2509.03026#pg9>. The results show that current-induced switching and TMR both vanish at temperatures near 360 K, reappearing upon return to 300 K<ref:2509.03026#pg8>. Micromagnetic simulations indicate that AATJs with larger pillar diameters will be characterized by non-uniform magnetic textures and non-uniform magnetization dynamics, which can reduce the effective TMR compared to AATJs with uniformly magnetized free and fixed layers<ref:2509.03026#pg56>.

Role of Interface Termination

The experimental spread in TMR values is understood given the microstructural characteristics of the stacks, where individual junctions experience variations in local atomic configuration at the crystalline–amorphous interface, including differences in interface termination<ref:2509.03026#pg30>. Theoretical calculations modeling the effect of interface termination show that TMR is primarily determined by the bulk magnetic structure of the electrodes, with magnetic moment matching in parallel alignment determining the sign of the TMR<ref:2509.03026#pg44>.

Comparison with Other Mechanisms

The asymmetry of switching currents for opposite switching directions, which was consistently observed in all measured devices, rules out SOT as the primary switching mechanism<ref:2509.03026#pg9>. Furthermore, simulations showed that when intra- and inter-sublattice torques have equal forms (β1 = β2), there is no net torque on the sublattices<ref:2509.03026#pg32>. The total torque on sublattice 3 is non-vanishing only for the case of β1 ≠ β2, resulting in an effective OTT efficiency of β eff = β1 − β2<ref:2509.03026#pg33>.

Future Directions

The results establish a new materials platform for antiferromagnetic spintronics and provide a pathway towards deeply scaled magnetic memory and room-temperature terahertz technologies<ref:2509.03026#pg3>. Future research should focus on engineering the band structure to maximize intra-sublattice hybridization of the states at the Fermi level, and investigating surface termination effects to improve TMR uniformity<ref:2509.03026#pg12>. The study also suggests that thermally activated reversal via OTT in PtMn3 may be significantly faster than in ferromagnetic MTJs<ref:2509.03026#pg13>.

Data Availability

The data that support the plots in this paper and other findings of this study are available from the corresponding authors upon request<ref:2509.03026#pg37>. The scientific color maps developed by Crameri et al. are used in this study to prevent visual distortion of the data and exclusion of readers with colour vision deficiencies<ref:2509.03026#pg36>.

Acknowledgements

This research was primarily supported as part of the Center for Energy-Efficient Magnonics (CEEMag), an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award number DE-AC02-76SF00515 and by the U.S. National Science Foundation (NSF) under Award numbers CNS-2425538, DMR-2308691, and DMR-2425567<ref:2509.03026#pg35>. The authors acknowledge useful discussions with Dr. Soho Shim on scanning NV microscopy and with Dr. Charudatta Phatak on TEM analysis<ref:2509.03026#pg35>. MH thanks the Swiss National Science Foundation (SNSF) for the financial support via SNSF Postdoc Mobility Grant No. 222234<ref:2509.03026#pg35>. The work made use of the EPIC facility (RRID: SCR 026361) of Northwestern University’s NUANCE Center, which has received support from the SHyNE Resource (NSF ECCS-2025633), the International Institute of Nanotechnology (IIN) and Northwestern's MRSEC program<ref:2509.03026#pg35>.

Author Contributions

PKA, JK and MH conceived the idea<ref:2509.03026#pg37>. PKA designed the AATJ material stack and devices<ref:2509.03026#pg37>. LW, SCM, and JAK fabricated the devices<ref:2509.03026#pg37>. JK, SKC, MSNT and MH carried out electrical measurements with support from JA, SA, MCH, EM, AD, JF, and VKS<ref:2509.03026#pg37>. MH and JK designed the NV magnetometry sample<ref:2509.03026#pg37>. MH and EM grew and characterized the NV magnetometry sample with support from JK<ref:2509.03026#pg37>. AD’A and GDF performed NV center microscopy<ref:2509.03026#pg37>. PGL performed the TEM analysis with support from JK, MH, VPD and MCH<ref:2509.03026#pg37>. MH, JK, and EM performed the XRD and SQUID magnetometry measurements<ref:2509.03026#pg37>. NZ, JACI, JK, and SKC performed the comparison of two-and three-terminal switching geometries<ref:2509.03026#pg37>. MH developed the theoretical description of OTT<ref:2509.03026#pg37>. MH, LDY, ME, WR, EYT, and JMR performed the DFT calculations and theoretical analysis<ref:2509.03026#pg37>. AM, MC and GF developed the micromagnetic code to simulate the three-sublattice model<ref:2509.03026#pg37>. AM, MC, and GF performed the micromagnetic simulations with input from MH and PKA<ref:2509.03026#pg37>. STK and PU performed the theoretical energy barrier calculation<ref:2509.03026#pg37>. JK, MH and PKA wrote the manuscript with contributions from all the other authors<ref:2509.03026#pg37>. All authors discussed the results, contributed to the data analysis, and commented on the manuscript<ref:2509.03026#pg37>. The study was performed under the leadership of PKA<ref:

1. Enhanced material design for antiferromagnetic spintronics: The work establishes a new materials platform for antiferromagnetic spintronics, suggesting that AI can be used to rapidly screen and suggest novel combinations of magnetic materials like PtMn3, IrMn3, and RhMn3 based on desired properties such as high Néel temperatures or specific noncollinear spin structures.

2. Development of octupole-driven spin-transfer torque (OTT) switching algorithms: The theoretical modeling explains OTT in terms of the imbalance between intra- and inter-sublattice spin currents, which AI can use to design predictive models that determine the optimal current density and pulse shape required to switch antiferromagnetic layers.

3. Predictive modeling for magnetic memory scaling: By incorporating micromagnetic simulations into an AI framework, systems can predict how AATJ devices will exhibit non-uniform magnetic textures in larger pillar diameters, allowing designers to anticipate and mitigate the resulting reduction in effective TMR.

4. Optimization of interface engineering for TMR uniformity: AI can analyze theoretical models of interface termination, such as those described in Extended Data Fig. 7, to suggest specific surface treatments or growth conditions that promote higher crystallinity and better control over surface termination, thereby increasing the TMR uniformity across a device array.

5. Real-time characterization of magnetic dynamics: Using the micromagnetic model (Eq. 9) and LLG equations, AI can process experimental time-dependent magnetization data to identify domain wall nucleation and propagation mechanisms in real time, moving beyond simple static measurements.

Magnetic tunnel junctions (MTJs) based on ferromagnets are canonical devices in spintronics, with wide-ranging applications in data storage, computing, and sensing. They simultaneously exhibit mechanisms for electrical detection and control of magnetic order through the tunneling magnetoresistance (TMR) and spin-transfer torque (STT) effects, respectively. It was long assumed that neither of these effects could be sizeable in all-antiferromagnetic tunnel junctions (AATJs), since they exhibit no net magnetization. Recently, however, it was shown that AATJs based on chiral antiferromagnets do exhibit TMR due to their non-relativistic momentum-dependent spin polarization and cluster magnetic octupole moment (CMO), which are manifestations of their spin-split band structure. However, the reciprocal effect, i.e., the antiferromagnetic counterpart of STT, has been assumed non-existent due to the total electric current being spin-neutral. Here, we report nanoscale AATJs exhibiting this reciprocal effect, which we term octupole-driven spin-transfer torque (OTT). We demonstrate current-induced OTT switching of PtMn3MgOPtMn3 AATJs, exhibiting a TMR value of 363% at room temperature and switching current densities of the order of 10 MA/cm2. Our theoretical modeling explains the origin of OTT in terms of the imbalance between intra- and inter-sublattice spin currents across the AATJ, and equivalently, in terms of the non-zero net cluster octupole polarization of each PtMn3 layer. This work establishes a new materials platform for antiferromagnetic spintronics and provides a pathway towards deeply scaled magnetic memory and room-temperature terahertz technologies.