Enhanced Magnon Spin Current Using the Bosonic Klein Paradox

Efficient manipulation of magnons for information processing is a central topic in spintronics and magnonics. An outstanding challenge for long-distance spin transport with minimal dissipation is to overcome the relaxation of magnons and to amplify the spin current they carry. Here, we propose to amplify magnon currents based on the realization of the bosonic Klein paradox in magnetic nanostructures. This paradox involves the antimagnon, carrying opposite spin and energy, the existence of which is usually precluded by ferromagnetic instabilities, as it is an excitation at negative energy. We show that by appropriately tuning the effective dissipation through spin-orbit torques, both magnons and antimagnons are dynamically stabilized. As a result, we find that the reflection coefficient of incident magnons at an interface between two coupled magnets can become larger than one, thereby amplifying the reflected magnon current. Our findings can lead to magnon amplifier devices for spintronic applications. Furthermore, our findings yield a solid-state platform to study the relativistic behavior of bosonic particles, which is an outstanding challenge with fundamental particles.