Subterahertz spin pumping from an insulating antiferromagnet

Abstract: Spin-transfer torque and spin Hall effects combined with their reciprocal phenomena, spin-pumping and inverse spin Hall (ISHE) effects, enable the reading and control of magnetic moments in spintronics. The direct observation of these effects remains elusive in antiferromagnetic-based devices. We report sub-terahertz spin-pumping at the interface of a uniaxial insulating antiferromagnet MnF2 and platinum. The measured ISHE voltage arising from spin-charge conversion in the platinum layer depends on the chirality of the dynamical modes of the antiferromagnet, which is selectively excited and modulated by the handedness of the circularly polarized sub-THz irradiation. Our results open the door to the controlled generation of coherent pure spin currents at THz frequencies.

• The paper reveals multiple dynamical modes in MnF2, demonstrating controlled spin dynamics via circularly polarized sub-THz irradiation.
• It distinguishes coherent spin pumping from thermal effects by analyzing the ISHE voltage’s chirality dependence.
• It quantifies spin-mixing conductance at the antiferromagnetic/metal interface, indicating efficient spin interconversion comparable to ferromagnets.

Spin-Pumping in Insulating Antiferromagnets: Novel Insights into Sub-Terahertz Dynamics

The paper "Sub-Terahertz Spin-Pumping from an Insulating Antiferromagnet" provides an in-depth analysis of spin dynamics in antiferromagnetic (AF) materials, particularly focusing on the generation and detection of spin currents at sub-terahertz frequencies. Conducted by Vaidya et al., this research advances our understanding of AF materials' potential in spintronics by identifying and characterizing the spin-pumping mechanism from MnF$_2$ into platinum (Pt).

Theoretical Foundation and Experimental Setup

The research investigates antiferromagnets, which, unlike ferromagnets, exhibit negligible net magnetization below their Néel temperature. This property makes AF materials resistant to external magnetic perturbations, allowing for dense packing without crosstalk. The significant exchange interactions in AF materials elevate spin excitations to the THz frequency range, a regime challenging to explore due to technical limitations. The study attempts to leverage AF materials' characteristics to actively generate spin currents through a procedure dubbed spin-pumping, reciprocated by the inverse spin Hall effect (ISHE) in adjoining Pt layers.

Key Experimental Observations and Theoretical Implications

The experiment used an AF MnF$_2$ and Pt bilayer subjected to sub-THz microwave irradiation. The ISHE voltage measured in Pt depends intricately on the chirality of MnF$_2$ dynamical modes, an outcome controlled by the circular polarization of the sub-THz radiation.

1. Spin Dynamics Characterization: The researchers discovered the presence of multiple dynamical modes within MnF$_2$ dictated by the applied magnetic field, namely low- and high-frequency modes (LFM and HFM), the spin-flop (SF) mode, and the quasi-ferromagnetic mode (QFM). These excitations demonstrated disparate chiral properties that could be modulated with high precision by manipulating the microwave irradiation's polarization.
2. Coherent vs. Incoherent Contributions to Spin Currents: By examining the ISHE signal's polarization dependency, the study provided strong evidence that the observed voltage signals result primarily from coherent spin-pumping rather than the incoherent spin Seebeck effect traditionally induced by temperature gradients.
3. Quantification of Spin-Mixing Conductance: From the coherent portion of spin-pumping, the spin-mixing conductance was estimated and found comparable to ferromagnetic interfaces, indicating efficient spin interconversion at the interface.

Implications for Spintronics and Future Prospects

The findings from this study hold substantial implications for spintronic device engineering, specifically in advancing the manipulation of AF order parameters through electrical methods. By mastering the coherent generation and propagation of spin currents at higher frequencies, this research pushes the boundaries toward developing THz spintronic devices that are both energy-efficient and immune to external magnetic influences.

Moreover, the ability to control the polarization of spin currents through AF materials encourages further exploration of spintronics' theoretical underpinnings, particularly regarding AF materials with unique lattice symmetries. Future work may explore integrating such AF-based systems into complex spintronic circuits, paralleling advancements already evident in ferromagnetic configurations.

In conclusion, the research by Vaidya et al. provides a comprehensive framework for further investigating and potentially exploiting the spin dynamics of AF materials. While challenges remain, particularly in understanding the full scope of non-equilibrium dynamics and the effect of material mismatching, the experimental and theoretical advancements presented offer a promising path forward.