Enhancement of Thermally Injected Spin Current through an Antiferromagnetic Insulator
This paper investigates the enhancement of thermally injected spin currents in heterostructures comprising a Normal Metal (NM)/Antiferromagnet (AF)/Yttrium Iron Garnet (YIG) configuration. With the introduction of a thin antiferromagnetic insulation layer such as NiO or CoO, the researchers document a significant enhancement in spin currents up to a tenfold increase. Specifically, their exploration reveals this enhancement is closely linked with the behavior of the Néel temperature of the AF layer and is contingent on the spin-mixing conductance at the NM/YIG interface.
The impetus behind enhancing spin current transmission in spintronic devices stems from its potential for higher energy efficiency and compact architectures, especially considering the limitations posed by the rapid decay of spin currents in materials. Addressing these limitations, the paper proposes the use of antiferromagnetic materials over traditional ferromagnets. The authors utilized an LSSE-based thermal injection method distinct from the high-frequency spin pumping, thereby extending the temperature range for effective spin current measurement.
Among the key experimental observations, the paper reports that the ISHE voltage, which is crucial for detecting spin currents in NMs, peaks near the AF layer's Néel temperature, especially for thin AF layers around 1 nm thickness. Furthermore, the enhancement effect varies between NiO and CoO layers, with NiO displaying a decay length of λ(NiO) = 2.5 nm in Pt/NiO/YIG, underscoring its superior enhancement performance relative to CoO. The decay kinetics are further contrasted with the negligible enhancement observed with non-magnetic insulating AlOx spacers, elucidating the specific effect attributable to AF insulators.
The authors provide a detailed account of their methodological approach, employing theoretical spin current transmission models—which factor in both coherent and non-coherent magnon populations in AF materials—to successfully predict the observed phenomena. The paper's results align the enhanced spin currents with augmented AF magnon activity or fluctuations at the NM/AF interface, especially above the Néel temperature, where long-range antiferromagnetic ordering diminishes but short-range correlations remain active.
Another core finding lies in the linear relationship between the spin current enhancement ratio and the spin-mixing conductance, determined from FMR linewidth measurements across various NM substrates, including elemental 3d, 4d, and 5d metals. This correlation underpins the selection criteria for materials in designing spintronic devices utilizing AF enhancement.
This research paves the way for improved thermal spin injection techniques and a broad understanding of the roles played by spin fluctuations in AF insulators. Future directions could explore a wider array of AF materials and integrate these findings in practical device applications, potentially revolutionizing the development of more efficient spintronic devices.