Spinwave transport through a four-terminal cross-junction: the role of next-nearest neighbor exchange

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Abstract
Generally, models of spin-wave transport through magnetic junctions only consider the exchange between nearest neighbors. However, real thin films also exhibit coupling between second-order neighbors ( NNN ), ferromagnetic (FM) in ordinary films and antiferromagnetic (AFM) in frustrated ones, the influence of which at the device level has been observed. In this work, we examine how the NNN -contribution reshapes the spin wave response of a four-terminal asymmetric cross junction, with slabs of four, three, three and two atomic layers wide (the 4–3–3–2 system). The device is treated within the Heisenberg ferromagnet and solved by two methods: the analytic phase field matching theory (PFMT) and the tight-binding scattering solver Kwant. At a realistic strength, diagonal bonds leave the transport band gap-free and strictly reciprocal, while renormalizing it in a sign-dependent manner. FM NNNs raise the top of the conduction band, increasing the cutoff frequency of the wider slab from 7.41 to 7.57 J , and slow down the antisymmetric channel of the narrow slab by destructive interference of its two longitudinal hopping paths; AFM NNNs lower the band top and accelerate this channel. The frequency at which each slab closes, and thus ceases to route a signal, shifts accordingly. Whereas, if the coupling is stronger, the same diagonal links open a width-selective stopping band that transforms the cross into a frequency-selective reflector, a qualitatively new transport function. The sign of the second neighbor exchange thus shapes the spectral landscape of the junction, quantitatively at the realistic strength and qualitatively when driven harder, and is not a negligible correction.

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