Magnon auto-oscillation in domain walls

Magnons are the fundamental excitations in magnetic materials, and they can transport angular momentum without actual charge currents. Therefore, they are attractive for applications in energy efficient information technology, offering high operating frequencies up to the THz range. Here we present a novel scheme for magnon generation using spin currents and domain walls.
When a charge current is applied to a bilayer consisting of a heavy metal and a ferromagnetic metal, the spin currents originating from the spin Hall effect in the heavy metal apply a spin transfer torque on the magnetization of the ferromagnetic layer. This allows driving efficiently auto-oscillations of magnetization [1]. We focused on domain walls as local nano magnon channels [2]. Since it is possible to move domain walls by electrical currents [3], domain walls are attractive for nano-sized reprogrammable circuits.
A 370 nm wide boomerang structure was fabricated from a Pt/CoFeB bilayer (Fig.1). The sample was magnetized by applying an external magnetic field H. After the saturation, the external magnetic field was set to 0 Oe, and a dc current was applied to the sample. The magnon intensity at the apex of the boomerang structure was measured by Brillouin light scattering microscopy [4]. Figure 2 shows the dc current dependency of the magnon spectrum on the remanent state. Magnons were detected for currents between 3.6 mA and 4.5 mA, while no magnons were observed for any negative dc currents. A domain wall is generated at the apex for the remanent state because of the shape anisotropy of the boomerang structure. We succeeded to excite magnons under zero magnetic field due to the autooscillation of the magnetization by spin transfer torque.

Reference
[1] A. N. Slavin and V. Tiberkevich, IEEE Trans. Magn. 45, 1875 (2009).
[2] K. Wagner et. al., Nat. Nanotech. 11, 432 (2016).
[3] S. S. P. Parkin et. al., Science 320, 190 (2008).
[4] T. Sebastian et. al., Front. Phys. 3, 35 (2015).