Ballistic transport of spin waves incident from cobalt leads across cobalt–gadolinium alloy nanojunctions

• Model is presented for spin wave scattering at CoGd disordered alloy nanojunctions.
• Computations yield the localized and resonant magnon modes on the nanojunctions.
• The spin waves ballistic reflection and transmission spectra are computed.
• Calculated resonance assisted maxima are observed in the transmission spectra.
• These maxima shift to lower frequencies with increasing nanojunction thickness.

Calculations are presented for the scattering and ballistic transport of spin waves (SW) incident from cobalt leads, on ultrathin ferrimagnetic cobalt–gadolinium ‥Co][Co(1−c)Gd(c)]ℓ[Co‥‥Co][Co(1−c)Gd(c)]ℓ[Co‥ nanojunction systems. The nanojunction [Co(1−c)Gd(c)]ℓ[Co(1−c)Gd(c)]ℓ itself is a randomly disordered alloy of thickness ℓ hcp lattice planes between matching hcp planes of the Co leads, at known stable concentrations c≤0.5c≤0.5 for this alloy system. To compute the spin dynamics, and the SW scattering and ballistic transport, this alloy nanojunction is modeled in the virtual crystal approximation (VCA), valid in particular at the length scale of the nanojunction for submicroscopic SW wavelengths. The phase field matching theory (PFMT) is applied to compute the localized and resonant magnons on the nanojunction. These magnons, characteristic of the embedded nanostructure, propagate in its symmetry plane with spin precession amplitudes that decay or match the spin wave states in the semi-infinite leads. The eigenvectors of these magnon modes are calculated for certain cases to illustrate the spin precession configurations on the nanojunction. The VCA-PFMT approach is also used to calculate the reflection and transmission spectra for the spin waves incident from the Co leads on the nanojunction. The results demonstrate resonance assisted maxima for the ballistic SW transmission spectra due to interactions between the incident spin waves and the nanojunction magnon modes. These properties are general for variable nanojunction thicknesses and alloy stable concentrations c≤0.5c≤0.5. In particular, the positions of the resonance assisted maxima of spin wave transmission can be modified with nanojunction thickness and alloy concentration.