Thermal magnon transport in FM/AFM bilayers

Thermal magnon transport in FM/AFM bilayers investigates how thermal gradients drive spin currents and magnetization accumulation in coupled magnetic heterostructures, revealing chirality-selective magnonic spin transport.

The gist

A key finding is the observation of a thermally triggered spin current in the antiferromagnet, a phenomenon typically absent in bulk antiferromagnets that obey time-reversal symmetry.

Theoretical Framework and Modeling

The study employs a semi-classical approach where the spin operators of a Heisenberg Hamiltonian are replaced by normalized spin vectors, governed by the stochastic Landau-Lifshitz-Gilbert (s-LLG) equation. The effective field, Heff,i, is defined as Heff,i = −∂H/∂Si + ξi. The simulation uses specific parameters: JFM=10 meV (resembling Cr2Te3 and Fe3GeTe2), JAFM = 20 meV (similar to MnTe, Cr2O3 and FeSn), and dz = 0.01 meV for the uniaxial anisotropy. The noise term ξi is a Gaussian white noise fulfilling the fluctuation-dissipation theorem. Magnon dispersion relations are calculated analytically based on the Hamiltonian, yielding dispersions for the FM, fFM = γµs(1 + α2)2dz + 2JFMX/kz(1 − cos(kza)), and for the AFM, fAFM = γµs((2JAFM + 2dz)2 - (1 + α2)2JAFM/Xkzcos(kza)2.

Equilibrium Magnon Properties

In thermal equilibrium, the magnon dispersion relations are calculated. The FM exhibits only one helicity, shown as positive frequencies, whereas the AFM shows both helicities, shown as positive and negative frequencies. The band gap for k → 0 is f0,FM ≈ γµs2dz for the FM and f0,AFM ≈ γµs√8JAFMdz for the AFM (assuming low anisotropy dz ≪ JAFM). The band gap in Fig. 2 is most prominent for the AFM. In equilibrium, all energetically feasible modes are excited equally for symmetric directions with identical amplitudes for ±k-directions leading to no net flow of magnons in real space.

Magnon Injection and Accumulation

To investigate the spin Seebeck effect, a temperature step is applied, exciting the FM only and propagating magnons into the AFM. The magnon accumulation is defined as ∆morder(rz, T) = mz,eq(T) − morder(rz, T). The results show a negative peak in the last heated FM layer corresponding to a maximum reduction of the magnon density, while the interfacial AFM layer shows a positive peak, corresponding to a maximum increase in magnon density compared to the colder bulk region where both, magnon accumulation and magnon density should vanish at 0 K. This asymmetry is attributed to reflections from the interface back into the FM.

Chirality-Selective Transport and Net Magnetization

The study establishes that the FM couples to the AFM via the RH magnon modes only, which are permitted in the AFM but no LH magnons are excited. This imbalance of magnon mode occupations leads to a net magnetization appearing in the AFM. The nonreciprocal transport is characterized by a unidirectional flow of chiral magnons from the FM into the AFM, where predominantly the RH modes of the adjacent FM occur. This behavior makes the bilayer structure behave as a spinwave filter, where only certain modes pass on to the AFM and the restricted modes which are not allowed in the AFM are reflected back inside of the ferromagnet.

Spin Current Sign and Propagation Lengths

The paper investigates two setups: one where FM is thermally excited into AFM, and one vice versa. For magnon injection from the FM to the AFM, a finite magnetization in the AFM is attributed to the mode selectivity which results from the unidirectional flow of RH-magnons. A more realistic linear temperature gradient simulation shows that the sign of this current is preserved across the interface and its amplitude is clearly larger in the FM but is also detectable in the AFM, suggesting that one can induce a finite magnonic spin current into the AFM, without any relativistic effects. The magnon propagation length is visibly larger in the AFM as compared to the FM. The non-equilibrium dispersion relations reveal mode filtering at the interface owing to the directional and chiral preference of the FM on the AFM and vice versa, underpinning a chirality-selective magnon propagation.

Reversed Setup and Spin Current Definition

In the reversed setup, exciting the AFM thermally, magnons propagate from it into the FM. The spin current is defined quantum mechanically as Js = −h¯/Xk s z k vk. For the FM, this thermal gradient creates a spin current which "carries negative angular momenta in the positive z− direction.

As a fastidious and diligent researcher, I have analyzed the provided scientific paper, Thermal magnon transport in FM/AFM bilayers, focusing on its core findings: non-reciprocal magnon transport due to chirality selectivity and its manifestation as a net magnetization in an antiferromagnet (AFM).

Here are the specific improvements that can be made to AI systems based on this research, and what those improved systems could achieve:

)AI System Improvements Based on Magnonic Transport Research

The key insight from this paper is that the interplay between thermal gradients, chirality-dependent magnon modes, and proximity effects in FM/AFM heterostructures leads to a measurable spin current. This suggests a pathway for magnonic spin filters and potentially novel sensing mechanisms.

Here are specific improvements:

1. AI System Improvement: Magnon Transport Simulation Engine (Based on s-LLG & Atomistic Spin Dynamics)

2. AI System Improvement: Chirality-Selective Mode Identification Module

3. AI System Improvement: Non-Equilibrium Spin Current Prediction Model (Spin Seebeck Effect Simulator)

4. Improved AI System 1: Magnon Transport Simulation Engine (Based on s-LLG & Atomistic Spin Dynamics)

5. What it can do:

6. This system would move beyond simple equilibrium calculations by incorporating the full, nonlinear stochastic Landau-Lifshitz-Gilbert (s-LLG) equation, as described in the paper (Eq. 2). It could accurately simulate:

7. Specific Capability: Time evolution of spin dynamics under arbitrary thermal profiles (temperature steps or linear gradients).

8. Specific Capability: Calculation of non-equilibrium magnon accumulation and spin current profiles across interfaces (as shown in Fig. 3 and Fig. 8), including the effects of finite magnon temperature on both FM and AFM sublattices.

9. Improved AI System 2: Chirality-Selective Mode Identification Module

10. What it can do:

11. Specific Capability: Analyze the non-equilibrium magnon dispersion relations (Fig. 5 and Fig. 7) to identify and classify magnonic modes based on polarization (Left-Handed vs. Right-Handed, LH/RH).

12. Specific Capability: Quantify the group velocity asymmetry between chiral modes (e.g., distinguishing the RH mode with positive velocity from the LH mode), which is crucial for understanding non-reciprocal transport.

13. Improved AI System 3: Non-Equilibrium Spin Current Prediction Model (Spin Seebeck Effect Simulator)

14. What it can do:

15. Specific Capability: Predict the existence and magnitude of a finite spin current in an AFM even in the absence of external magnetic fields, based on thermally triggered spin accumulation resulting from FM excitation (the chirality-selective injection mechanism).

16. Specific Capability: Predict how changing the FM polarization (e.g., switching between parallel and antiparallel alignments) would alter the resultant net magnetization observed in the AFM, directly linking structure to magnetic response.

17. Overall Impact & Application of Improved AI Systems:

18. Application: Design of next-generation spintronic heterostructures (spin valves, magnonic filters).

19. Application: Development of highly sensitive, non-volatile thermal sensors capable of detecting subtle spin currents in magnetic materials based on the observed magnon accumulation profiles.

Progress in information processing relies on spintronics, where magnetic states serve as efficient carriers for data storage and transfer. In this work, we theoretically study magnon propagation in a bilayer composed of a ferro-magnet and an antiferromagnet. For this purpose, we probe the spin Seebeck effect by introducing a spatially varying temperature profile. This generates a local magnon excitation and a continuous magnon flux from hot to cold regions which we quantify through the resulting non-equilibrium magnon accumulation. Based on the chirality of these modes, we identify specific constraints for magnon modes traveling either from the ferromagnet into the antiferromagnet or vice versa. A key finding is the observation of a thermally triggered spin current in the antiferromagnet, a phenomenon typically absent in bulk antiferromagnets that obey time-reversal symmetry. These results provide important insights into the design of heterostructures for magnonic chirality-selective spin transport.