arXiv subjects

Rodolfo A. Gallardo

Publications and source records attributed to Rodolfo A. Gallardo.

6 recordsLinked to original sources

arXiv2026

Synthetic Altermagnetism Beyond the Crystal Limit

Altermagnetic magnons in crystalline materials exhibit momentum-dependent splitting whose nodal structure and chiral character are governed by the point-group symmetry of the magnetic sublattice rotation. Here, we demonstrate the first synthetic realization of altermagnetic magnonics in a continuum platform composed of antiferromagnetically coupled ferromagnetic films with alternating in-plane exchange anisotropies, showing that the key signatures of altermagnetic magnonics emerge beyond the crystalline setting. Solving the linearized Landau-Lifshitz equation within a dipole-exchange framework, we show that this architecture reproduces the characteristic momentum-dependent splitting, nodal directions, and anisotropic isofrequency contours of A-type altermagnets. Long-range dipolar interactions qualitatively reconstruct this exchange-driven spectrum by lifting the nominal nodal degeneracy, hybridizing opposite-chirality modes, and producing a finite, thickness-dependent wave-vector splitting along directions that are nodal in the exchange-only limit. Extending the bilayer to finite multilayers reveals that synthetic altermagnetism undergoes a parity-dependent reconstruction that separates surface and bulk altermagnetic excitations. These results establish altermagnetic magnon phenomenology as an engineerable collective response of dipole-exchange multilayers beyond microscopic crystal symmetries.

cond-mat.other↗

arXiv2025

Toroidal Moments in Confined Nanomagnets and their Impact on Magnonics

The nonreciprocity created by dipolar coupling, electric currents, and Dzyaloshinskii-Moriya interactions is discussed in cases where the magnon propagation direction has a component parallel to the toroidal moment. A criterion for calculating the toroidal moments is established, addressing the issue of correct origin selection by considering compensated and uncompensated magnetization distributions. This criterion is then applied to various nonreciprocal magnetic systems, with the calculations consistent with those reported in the literature and predicting the existence of nonreciprocity in a more general manner. These results broaden the physical significance of the toroidal moment and facilitate the identification and estimation of nonreciprocity in magnonic systems. This work also clarifies the interrelations between different definitions of the toroidal moment for confined structures, where a surface term arising from surface-bound currents connects these definitions without the need for time-averaging. Comparing these definitions of the toroidal moment applied to different magnetic textures demonstrates that they are always parallel but may differ in magnitude and sign. The discrepancy in the different definitions is deemed irrelevant since its direction, rather than its magnitude, primarily predicts the existence of magnon nonreciprocity.

cond-mat.mes-hall↗

arXiv2019

Spin waves in thin films and magnonic crystals with Dzyaloshinskii-Moriya interactions

The influence of the Dzyaloshinskii-Moriya interaction in ultra-thin ferromagnetic films and chiral magnonic crystals on the behavior of spin waves is reviewed. During the last decade, it has been shown, both theoretically and experimentally, that this anisotropic exchange interaction produces non-reciprocal features on the spin-wave spectrum of a magnetic system, a phenomenon that occurs both for bulk and interfacial Dzyaloshinskii-Moriya coupling. More recently, the concept of a chiral magnonic crystal has been introduced, where the interfacial Dzyaloshinskii-Moriya interaction is periodic. The effect of this periodicity include additional features such as flat bands, indirect gaps, and an unusual spin-wave evolution.

cond-mat.mes-hall↗

arXiv2018

Spin-Wave Modes in Transition from a Thin Film to a Full Magnonic Crystal

Spin-wave modes are studied under the gradual transition from a flat thin film to a 'full' (one-dimensional) magnonic crystal. For this purpose, the surface of a pre-patterned 36.8 nm thin permalloy film was sequentially ion milled resulting in magnonic hybrid structures, referred to as surface-modulated magnonic crystals, with increasing modulation depth. After each etching step, ferromagnetic resonance measurements were performed yielding the spin-wave resonance modes in backward-volume and Damon-Eshbach geometry. The spin-wave spectra of these hybrid systems reveal an even larger variety of spin-wave states compared to the 'full' magnonic crystal. The measurements are corroborated by quasi-analytical theory and micromagnetic simulations in order to study the changing spin-wave mode character employing spin-wave mode profiles. In backward-volume geometry, a gradual transition from the uniform mode in the film limit to a fundamental mode in the thin part of the magnonic crystal was observed. Equivalently, the first and the second film modes are transform into a center and an edge mode of the thick part of the magnonic crystal. Simple transition rules from the $n^{\mathrm{th}}$ film mode to the $m^{\mathrm{th}}$ mode in the 'full' magnonic crystal are formulated unraveling the complex mode structure particularly in the backward-volume geometry. An analogous analysis was performed in the Damon-Eshbach geometry.

cond-mat.mes-hall↗

arXiv2018

Symmetry and localization properties of defect modes in magnonic superlattices

Symmetry and localization properties of defect modes of a one-dimensional bi-component magnonic superlattice are theoretically studied. The magnonic superlattice can be seen as a periodic array of nanostripes, where stripes with different width, termed as defect stripes, are periodically introduced. By controlling the geometry of the defect stripes, a transition from dispersive to practically flat spin-wave defect modes can be observed inside the magnonic band gaps. It is shown that the spin-wave profile of the defect modes can be either symmetric or antisymmetric, depending on the geometry of the defect. Due to the localized character of the defect modes, a particular magnonic superlattice is proposed, wherein the excitation of either symmetric or antisymmetric flat magnonic modes is enabled at the same time. Also, it is demonstrated that the relative frequency position of the antisymmetric mode inside the band gap does not significantly change with the application of an external field, while the symmetric modes move to the edges of the frequency band gaps. The results are complemented by numerical simulations, where an excellent agreement is observed between both methods. The proposed theory allows exploring different ways to control the dynamic properties of the defect modes in metamaterial magnonic superlattices, which can be useful for applications on multifunctional microwave devices operating over a broad frequency range.

cond-mat.mes-hall↗

arXiv2016

The role of the internal demagnetizing field in a surface-modulated magnonic crystal

Magnonic crystals with locally alternating properties and specific periodicities exhibit interesting effects, such as a multitude of different spin-wave states and large band gaps. This work aims for demonstrating and understanding the key role of local demagnetizing fields in such systems. To achieve this, hybrid structures are investigated consisting of a continuous thin film with a stripe modulation on top favorable due to the adjustability of the magnonic effects with the modulation size. For a direct access to the spin dynamics, a magnonic crystal was reconstructed from `bottom-up', i.e., the structural shape as well as the internal field landscape of the structure were experimentally obtained on the nanoscale using electron holography. Subsequently, both properties were utilized to perform dynamic response calculations. The simulations yield the frequency-field dependence as well as the angular dependence of spin waves in a magnonic crystal and reveal the governing role of the internal field landscape around the backward-volume geometry. The complex angle-dependent spin-wave behavior is described for a 360 degree in-plane rotation of an external field by connecting the internal field landscape with the individual spin-wave localization.

cond-mat.mtrl-sci↗