Magnons scattering in a ferromagnetic multichannel system: Propagation from one slab to three others

We present a theoretical study of coherent magnon scattering through a four-terminal asymmetric nanojunction connecting ferromagnetic slabs composed of 4, 3 and 2 atomic layers, forming a cross geometry (4/323 multichannel system). The system is modeled using a nearest-neighbor Heisenberg Hamiltonian within the framework of the linear approximation of spin waves, and the phase field matching method (PFMM) transposed into a Landauer-type scattering matrix. The transmission and reflection spectra are calculated for both normal and oblique spinwave incidence, for three possibilities of magnetic exchange in the junction region: Softened, homogeneous and hardened coupling. The results reveal geometry-induced port-selective mode filtering: rather than reflecting the higher-order modes, the channel-number mismatch routes them by transverse order between the output ports. The straight two-layer lead transmits essentially only the fundamental mode, while the higher-order modes are funneled into the two vertical three-layer slabs, and no chiral interaction or external field is required. Mode-specific transmission gaps appear due to the absence of corresponding output channels at low frequencies, and pronounced asymmetric Fano features emerge where localized states in the junction couple to the propagating continuum. Under softened exchange, the localized spin states are truly bound; under homogeneous and hardened exchange, they become resonant quasi-bound states embedded in the continuum, directly controlling Fano resonance positions and amplitudes. The resonances shift toward the higher frequencies, and the oblique incidence component ([Formula: see text] provides an additional geometrical lever that reduces the total magnonic conductance by about 17% by uniformly increasing all propagation thresholds. These results establish quantitative design principles for magnonic mode-selective filters and demultiplexers based on purely structural channel asymmetry.