Boualem Bourahla

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.

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.

We explore the mechanism of carbothermic reduction in MoO3 powders mechanically activated by carbon black. Given the complexity of this system, which involves a mixture of stable and metastable species and complex in situ processes, we combine theore...

Analytical and numerical formalisms are developed to study the influence of the honeycomb nanocontact configurations on the spin wave scattering and magnon transmission modes in two-dimensional multichannel waveguides. The transmission of magnons propagating through spin interface sites is calculated and analyzed on a ferromagnetic network of spins using the matching technique. In this contribution, two semi-infinite groups of two parallel Heisenberg ferromagnetic chains, joined by honeycomb configuration sites, make the simulated model. The spin reflection and transmission coefficients are derived as elements of a Landauer-type scattering matrix. The results of the numerical simulations demonstrate interferences between incident scattered spin modes and localized spin states in the nanocontact zone, with characteristics of Fano resonances. The simulations are carried out according to the various parameters that characterize the two semi-infinite double chains, which are connected by the honeycomb configurations in the three models of the studied ferromagnetic nanocontact. The obtained results demonstrate the dependence of spin wave conductance on the magnetic exchange, incidence angle and architecture of the spin sites that form the nanocontact.

An analytical and numerical formalism is developed to study the influence of the various positions of substitution atoms B on the scattering and transmission vibration modes in quasi-1D monatomic structures A. The matching technique is employed to calculate the dynamical properties and transmittance spectra of two substituted atomic sites. The theoretical formalism gives a complete description of the lattice dynamics and the vibration-waves propagation in the presence of the impurity sites. Numerical calculations are performed for three different positions of the two substituted atoms B in the low-dimensional structure consisting of double monatomic parallel chains. First, we examine the position where the two B atoms are adjacent in the y-direction, afterwards, we place the two sites side by side in the x-direction. Lastly, the two sites are placed in oblique configuration. The obtained results show that phonons associated to the inhomogeneous structures are strongly dependent on the scattering frequency, elastic force parameters and the position of the atomic substituted sites. In the three considered positions, the presence of the B atoms gives rise to localized vibration effects. The fluctuations observed in the vibration spectra are related to resonances due to the coherent coupling between travelling phonons and the localized vibration modes in the neighborhood of the impurity sites.

A theoretical formalism for calculating the bulk and surface spin modes in Heisenberg semi-infinite lattices is presented on a ferromagnetic cubic network of spins, coupled via nearest and next-nearest neighbors exchange interactions. The magnetic surface can be considered as semi-infinite slabs at the end of the bulk structures. The breakdown of translation symmetry, in the normal direction of the surface, gives rise to localized spin wave modes in its neighborhood. The localized magnon spectrum is derived as elements of a Landauer-type scattering matrix, in the three cubic lattices sc, bcc and fcc. The magnon properties are simulated and determined numerically for each cubic lattice by using the matching technique. The observed fluctuations in the numerical results demonstrate the interference magnon effects between scattered spinwaves and the localized magnon states, generated by the surface region with characteristic Fano resonances. In cubic leads, the localized surface spin states are sensitive to the local magnetic coupling and the incident direction in the surface boundary. In this contribution, the normalized energy of spinwaves arising from the absence of translation symmetry is analyzed for each cubic system as a function of the exchange integral parameters. This addresses the dependence of the surface magnon on the different possibilities of the of the exchange parameters variation from softening to hardening in the neighborhood of the surface region.

In this work, we introduce a computer model and theoretical approach based on the matching technique to investigate the spin precession and the magnetic properties of an ordered magnetic interface joining two ferromagnetic multilayers of type AB, made of 10 spin slabs, obtained by alternative two spin layers A and B. We simulate, particularly, the coherent magnon transmission through spins’ interface, in multilayered thin films, obtained by shearing a part of the film from the other at an angle of 30^∘. The individual and total transmittance of bulk magnons of the thin film, scattering coherently at the shearing interface zone and the localized magnonic spin states, are calculated and analyzed. The transmission and reflection spin modes are derived as elements of a Landauer–Büttiker type scattering matrix. The results highlight the localized spin states on the interface shear domain and their interactions with incident magnons. The evolutions of the magnonic spectra can be presented for arbitrary directions of the incident magnons on the boundary zone, for all accessible frequencies in the propagating bands as well as for the magnetic exchange coupling between each spin A(B) and its adjacent sites and their spin intensity. The results demonstrate the dependence of the magnonic spectra for the perfect multilayered films and at the inhomogeneous domain of the interface shear. The analysis of the spectra illustrates the fluctuations, related to Fano resonances, due to the coupling between travelling magnons and the localized modes in the shear interface domain. The calculated spectra could yield useful information concerning the magnetic parameters of such interface slabs in multilayered films.

In this work, we investigate the scattering phenomena at the inhomogeneous boundary of a magnetic interface in two-dimensional (2D)-triangular ferromagnetic waveguide. We especially examine two ferromagnetic interfaces, named heavy and superheavy solitons in 2D-triangular lattices. The system is supported on a nonmagnetic substrate. Therefore, each spin site in the lattice can be considered free from magnetic interactions with its environment. The spin excitations in the modeled system are studied by the field matching theory (FMT). Their magnons transmission/reflection, localized spin states as well as the local density of spin states (LDOS) are calculated and analyzed. They are derived as elements of a Landauer type scattering matrix. The magnonic spectra are calculated specifically for two distinct interfaces, which join the two semi-infinite perfect 2D-triangular waveguides. The inter-atomic magnetic exchange is varied on the interface domain to investigate the consequences of magnetic softening and hardening for the calculated magnetic properties. The numerical results show the interference effects between the incident magnons and the localized spin states on the interface domain, with characteristic magnetic resonances that vary according to each configuration. The numerical results yield an understanding for the relation between the coherent magnon propagation and the interface configuration in the perfect 2D-triangular system.