🔬 mesoscale physics

Thermal magnon transport in FM/AFM bilayers

This paper theoretically demonstrates that a spatially varying temperature profile in ferromagnet/antiferromagnet bilayers induces chirality-selective magnon transport and generates a thermally triggered spin current in the antiferromagnet, offering new insights for designing magnonic heterostructures.

Original authors: Moumita Kundu, Ulrich Nowak

Published 2026-10-01

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CC0 1.0

Original authors: Moumita Kundu, Ulrich Nowak

Original paper dedicated to the public domain under CC0 1.0 (http://creativecommons.org/publicdomain/zero/1.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the quest to make faster, cooler, and more efficient computers, scientists are looking beyond the flow of electric charge. For decades, information has been carried by electrons, but this method generates heat and consumes significant power. A promising alternative lies in the magnetic states of materials, where information can be carried by waves of spin rather than moving particles. These waves, known as magnons, can travel through insulating materials that do not conduct electricity, potentially eliminating the energy loss associated with traditional wires. To harness this potential, researchers are investigating how these magnetic waves behave when they move between different types of magnetic materials, specifically at the boundary where a material with aligned magnetic spins meets one with opposing spins. Understanding this interaction is crucial for designing the next generation of spintronic devices, which could store and process data with unprecedented efficiency.

A team of researchers at the University of Konstanz in Germany has taken a deep dive into this problem by simulating a thin layer composed of two distinct materials: a ferromagnet, where all magnetic spins point in the same direction, and an antiferromagnet, where neighboring spins point in opposite directions. Using powerful computer models, they created a virtual environment to observe what happens when heat is applied to one side of this layered structure. In their simulations, they introduced a temperature difference, heating one part of the material while keeping the other cool. This thermal gradient acts like a pump, generating a flow of magnetic waves from the hot region toward the cold region. The researchers were particularly interested in how these waves would behave as they crossed the invisible boundary between the two different magnetic materials.

The simulation revealed a striking asymmetry in how these magnetic waves travel. When the researchers heated the ferromagnetic side, the magnetic waves flowed smoothly into the antiferromagnetic layer. When they heated the antiferromagnetic side, waves also propagated into the ferromagnetic layer, but the behavior differed significantly. The waves traveling from the antiferromagnet into the ferromagnet exhibited a shorter propagation length compared to the reverse direction. This happens because the ferromagnet acts like a filter that only allows waves with a specific "handedness" to pass through. In the world of these magnetic waves, handedness refers to the direction in which the spins rotate as the wave moves. The ferromagnet only supports waves that rotate in one specific direction, while the antiferromagnet can support waves rotating in both directions. When waves travel from the ferromagnet into the antiferromagnet, only the matching waves get through, while the others are blocked or reflected.

This selective filtering has a profound consequence that the researchers observed in their data. In a typical antiferromagnet, the magnetic effects of the two opposing spin directions cancel each other out perfectly, resulting in no net magnetic field. However, in this simulated bilayer, the one-way flow of waves from the ferromagnet created an imbalance. Because only one type of wave entered the antiferromagnet, the cancellation was incomplete. This resulted in the appearance of a small but measurable net magnetic field within the antiferromagnetic layer, a phenomenon that would not occur in a bulk antiferromagnet on its own. The researchers found that this induced magnetism was directly linked to the temperature difference driving the flow of waves.

Furthermore, the team calculated the flow of spin current, which is essentially the transport of magnetic momentum, across the interface. They found that a continuous current of spin flowed from the ferromagnet into the antiferromagnet, driven solely by the temperature gradient. Remarkably, this current persisted even without any external magnetic fields or relativistic effects that are usually required to generate such flows in antiferromagnets. The strength of this current depended on the temperature difference, with larger gradients producing stronger flows. The researchers also noted that the distance these waves could travel into the antiferromagnet was limited, decaying exponentially over a short range of a few tens of atomic layers, though they acknowledged that in real materials with lower internal friction, this distance could be much greater.

The study concludes that the interface between a ferromagnet and an antiferromagnet acts as a sophisticated gatekeeper for magnetic information. By controlling the temperature and the orientation of the magnetic layers, it is possible to direct the flow of spin currents and even induce magnetic states in materials that are normally non-magnetic. These findings, derived entirely from computer simulations, provide a clear theoretical blueprint for how to engineer heterostructures that can selectively transport magnetic information. While the work is currently theoretical, it suggests a pathway toward creating devices where heat can be used to control magnetic signals, offering a new mechanism for data storage and transfer that is both efficient and controllable.

Technical Summary: Thermal Magnon Transport in FM/AFM Bilayers

Problem Statement
The paper addresses the theoretical challenge of understanding thermal magnon transport in heterostructures composed of a ferromagnet (FM) and an antiferromagnet (AFM). While the spin Seebeck effect (SSE) is well-established in bulk materials and magnetic conductors, the behavior of magnons at FM/AFM interfaces remains complex. Specifically, bulk AFMs typically exhibit no net thermal spin transport in the absence of relativistic effects or external magnetic fields because contributions from their two degenerate magnon bands cancel each other out. The study investigates whether the proximity to an FM can break this symmetry, enabling chiral-selective magnon transport and inducing a net spin current in the AFM.

Methodology
The authors employ atomistic spin dynamics simulations based on a semi-classical approach.

Model: A Heisenberg Hamiltonian with nearest-neighbor exchange interactions ( $J$ ) and uniaxial anisotropy ( $d_z$ ) is used. The system consists of an FM layer (modeled after materials like Cr $_2$ Te $_3$ ) and a layered AFM (modeled after MnTe or Cr $_2$ O $_3$ ) with an uncompensated interface and antiferromagnetic coupling.
Dynamics: Spin evolution is governed by the stochastic Landau-Lifshitz-Gilbert (s-LLG) equation, incorporating thermal noise to satisfy the fluctuation-dissipation theorem.
Simulation Setup: The system is simulated with periodic boundary conditions in the $x$ and $y$ directions and open boundaries in the $z$ direction. To prevent magnon reflections at the cold end, absorbing boundary conditions are implemented via an exponentially increasing damping constant.
Thermal Gradients: Two configurations are analyzed: (1) The FM is thermally excited while the AFM is at 0 K, and (2) the AFM is excited while the FM is at 0 K. Both step-function and linear temperature profiles are utilized.
Observables: The study quantifies magnon transport through:
- Magnon accumulation ( $\Delta m_{order}$ ), defined as the deviation from the bulk equilibrium order parameter.
- Net magnetization ( $m_{net}$ ) in the AFM.
- Spin-wave spin current ( $I_s$ ), calculated via equal-time spatial correlations of spin components.
- Non-equilibrium magnon dispersion relations derived from 2D Fourier transforms of spin-wave components.

Key Results

Equilibrium Dispersion: In thermal equilibrium, the FM supports only a single right-handed (RH) magnon mode, whereas the AFM supports two degenerate modes with opposite helicities (RH and left-handed, LH).
FM-to-AFM Injection: When the FM is heated, magnons propagate into the AFM. Due to the interface coupling, only the RH modes of the FM are permitted to enter the AFM. This results in a "chirality-selective" filter where LH modes are suppressed. Consequently, the degeneracy of the AFM modes is lifted, leading to a finite net magnetization and a non-zero spin current in the AFM, despite the absence of relativistic effects or external fields.
AFM-to-FM Injection: When the AFM is heated, both RH and LH modes are excited. However, only the RH modes propagate into the FM, while LH modes are largely reflected or evanescent. This confirms the unidirectional, chirality-dependent nature of the transport.
Spin Current Profiles: Using a linear temperature gradient, the authors demonstrate a continuous spin current flowing from the FM into the AFM. The current sign is preserved across the interface, though its magnitude decays exponentially within the AFM bulk. The asymmetry in magnon accumulation near the interface is attributed to mode filtering and reflection.

Key Contributions

Chirality-Selective Transport: The work identifies that FM/AFM interfaces act as polarization-dependent filters, allowing only specific chiral magnon modes to transmit based on the direction of propagation.
Induced Magnetization in AFM: The study provides a mechanism for generating a net magnetization in a compensated AFM solely through thermal gradients and proximity effects, without requiring spin-orbit coupling or external fields.
Quantification of Spin Current: The paper establishes a method to calculate and detect magnonic spin currents in the AFM layer of a bilayer, linking the current directly to the non-reciprocal magnon dispersion induced by the interface.

Significance and Claims
The authors claim that these findings offer critical insights into the design of heterostructures for magnonic applications. Specifically, the ability to trigger a thermally induced spin current in an AFM via proximity to an FM suggests a pathway for magnonic chirality-selective spin transport. The results indicate that FM/AFM interfaces can function as spin-valve structures or filters where the orientation of the FM controls the polarization and flow of magnons. The authors note that while the simulated propagation lengths are limited by the chosen damping parameters, materials with lower damping would exhibit significantly longer transport distances, making these effects potentially relevant for future spintronic devices. The work underscores the importance of non-reciprocal magnon dispersions in optimizing spin transport in insulating magnetic heterostructures.

Technical Summary: Thermal Magnon Transport in FM/AFM Bilayers

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