For decades, ferromagnetic materials have dominated the field of information technology. Recently, however, there has been a boost of new research in antiferromagnetic (AFM) materials. While these materials to-date have primarily been used to manipulate the magnetization reversal in adjacent ferromagnets through exchange bias, researchers today strive to harness the potential applications of antiferromagnetic materials for low power spintronics devices. In particular, new studies have proven AFM insulators to be excellent conductors of DC spin currents with intriguing temperature dependent transmission properties [1-4], however it is unclear to what extent coherent AC spin currents can be transmitted through an AFM material.
In order to understand these mechanisms, we use X-ray detected ferromagnetic resonance (XFMR) [5-8] to study spin dynamics in Ni80Fe20/Ag/CoO/Ag/Co75Fe25 multilayers . XFMR is a powerful tool in the investigation of spin current effects in complex heterostructures, as it enables the observation of magnetization and spin dynamics within each layer by combining ferromagnetic resonance (FMR) with the element-, site-, and valence state-specificity of X-ray magnetic circular dichroism (XMCD).
Using XFMR we were able to excite and observe AC spin current propagation from the Ni80Fe20 source to the Co75Fe25 sink layer at different temperatures around the CoO antiferromagnetic transition. By utilizing the phase information from the precession, we can unambiguously distinguish between different contributions to the magnetic excitation and identify the spin current signal. Our findings show that a coherent AC spin current can be transmitted through antiferromagnetic CoO, and that transmission is strongly enhanced around its Neel temperature.
Additionally, we utilized linearly polarized x-rays for XFMR measurements in terms of a dynamic X-ray magnetic linear dichroism (XMLD), enabling sensitivity not only to directional, but also to axial magnetic order . This new capability introduces the possibility to observe spin excitations even in the absence of a net magnetic moment and enables us to identify the origin of the spin propagation within the antiferromagnet CoO layer.
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