Bias Field Orientation Research Articles

Bias field orientation driven reconfigurable magnonics and magnon−magnon coupling in triangular shaped Ni80Fe20 nanodot arrays

Reconfigurable magnonics have attracted intense interest due to their myriad advantages including energy efficiency, easy tunability and miniaturization of on-chip data communication and processing devices. Here, we demonstrate efficient reconfigurability of spin-wave (SW) dynamics as well as SW avoided crossing by varying bias magnetic field orientation in triangular shaped Ni80Fe20 nanodot arrays. In particular, for a range of in-plane angles of bias field, we achieve mutual coherence between two lower frequency modes leading to a drastic modification in the ferromagnetic resonance frequency. Significant modification in magnetic stray field distribution is observed at the avoided crossing regime due to anisotropic dipolar interaction between two neighbouring dots. Furthermore, using micromagnetic simulations we demonstrate that the hybrid SW modes propagate longer through an array as opposed to the non-interacting modes present in this system, indicating the possibility of coherent energy transfer of hybrid magnon modes. This result paves the way for the development of integrated on-chip magnonic devices operating in the gigahertz frequency regime.

Observation of angle-dependent mode conversion and mode hopping in 2D annular antidot lattice

We report spin-wave excitations in annular antidot lattice fabricated from 15 nm-thin Ni80Fe20 film. The nanodots of 170 nm diameters are embedded in the 350 nm (diameter) antidot lattice to form the annular antidot lattice, which is arranged in a square lattice with edge-to-edge separation of 120 nm. A strong anisotropy in the spin-wave modes are observed with the change in orientation angle (ϕ) of the in-plane bias magnetic field by using Time-resolved Magneto-optic Kerr microscope. A flattened four-fold rotational symmetry, mode hopping and mode conversion leading to mode quenching for three prominent spin-wave modes are observed in this lattice with the variation of the bias field orientation. Micromagnetic simulations enable us to successfully reproduce the measured evolution of frequencies with the orientation of bias magnetic field, as well as to identify the spatial profiles of the modes. The magnetostatic field analysis, suggest the existence of magnetostatic coupling between the dot and antidot in annular antidot sample. Further local excitations of some selective spin-wave modes using numerical simulations showed the anisotropic spin-wave propagation through the lattice.

Anisotropic spin waves in two-dimensional triangular shaped bi-component magnonic crystal

Abstract Bi-component magnonic crystals with a strong dipole-exchange interaction across the interface of the constituent magnetic elements have shown promising potentials in magnonics and magnon-spintronics. Here, we have reported an all-optical investigation of spin wave dynamics in an array of periodically arranged bi-component magnonic crystal in the form of triangular-shaped Ni 80 Fe 20 nanoelements embedded in Co 50 Fe 50 matrix using time-resolved magneto-optical Kerr effect magnetometry. The spin wave spectra obtained from the sample reveal a broad band of spin wave modes where they possess a strong and systematic bias magnetic field tunability which is crucial for active control over such system in device applications. Further, the spin wave modes show a six-fold and four-fold rotational anisotropy with the bias field orientation due to combined effects of element shape and lattice symmetry. Micromagnetic simulations reproduce the experimental results qualitatively where the simulated mode profiles unravel the spatial distribution of spin wave frequencies inside both constituent elements while the internal magnetic fields play a crucial role for the observed tunability of spin wave dynamics. Development of such magnetically coupled embedded magnetic nanostructures can pave a new pathway in designing the future magnonic devices and faster microwave communication systems.

Controlled evolution of spin waves in unconventional defective honeycomb antidot lattices

Abstract Efficacious control of spin wave dynamics is demonstrated in two-dimensional magnonic crystals in the form of defective honeycomb antidot lattices. The spin wave dynamics of this system is investigated using broadband ferromagnetic resonance spectrometer. A stark modification in magnonic spectra is achieved with the modulation in the lattice spacing and with the bias magnetic field strength and its orientation. A rich band of spin wave modes are obtained for the lattice with smallest periodicity, which finally converges to two modes in case of the largest one. The spin wave modes in all lattices possess a strong anisotropic behaviour with a six- and two-fold rotational symmetries as a function of the bias field orientation originating from the strong modulation of internal field distribution at different regions of such unorthodox system. The experimentally observed spin wave modes have been qualitatively well reproduced by micromagnetic simulations and the simulated mode profiles unravel the presence of diverse genres of extended and localized standing modes in these unusual magnonic crystals. These observations are significant for tunability and anisotropic propagation of spin waves leading towards the development of efficient magnonic devices competent with the future microwave communication systems.

Ferromagnetic resonance in coupled magnetic nanostructured arrays

The modulated field sweep ferromagnetic resonance (FMR) spectroscopy was used to study the magnetization dynamics in large arrays of interacting cubic nanomagnets. A 60nm thick permalloy (Ni80Fe20) thin films were patterned using a lift-off process into several large arrays of 60 x 60 nm2 nanostructures where the spacing between the magnetic nanocubes was varied to control the strength of dipolar coupling. Electron beam lithography was used for device patterning using lift-off. The permalloy films were deposited using magnetron sputtering. DC magnetic properties were evaluated using alternating gradient force magnetometer (AGFM). In the FMR measurements, the orientation of DC bias magnetic field was varied from the in-plane to out-of-plane with respect to the 2D plane of the arrays. The FMR peak splitting and multiple FMR modes were observed in the evolution of the FMR spectra as the function of the bias field orientation, and were strongly influenced by the nanomagnet geometry and the spacing between the nanomagnets. Two resonance modes, shape and lattice, were particularly well pronounced in the observed FMR spectra. These modes are characterized by effective demagnetizing factors representing different symmetries of the system of interacting nanomagnets. Micromagnetic modeling suggest that micromagnetic texture within the nanomagnets modulated by the stray fields from the neighbors correlates with the FMR spectra.

Effects of antidot shape on the spin wave spectra of two-dimensional Ni80Fe20 antidot lattices

We show that the optically induced spin wave spectra of nanoscale Ni80Fe20 (permalloy) antidot lattices can be tuned by changing the antidot shape. The spin wave spectra also show an anisotropy with the variation of the in-plane bias field orientation. Analyses show this is due to various quantized and extended modes, whose nature changes with the antidot shape and bias field orientation as a result of the variation of the internal magnetic field profile. The observed variation and anisotropy in the spin waves with the internal and external parameters are important for their applications in magnonic devices.

Magnetodynamical response of large-area close-packed arrays of circular dots fabricated by nanosphere lithography

We report a combined experimental and theoretical study of the quasistatic hysteresis and dynamic excitations in large-area arrays of NiFe nanodisks forming a hexagonal lattice with the lattice constant of 390 nm. Arrays were fabricated by patterning a 20-nm-thick NiFe film using the etched nanosphere lithography. We have studied a close-packed (edge-to-edge separation between disks dcp = 65 nm) and an ultraclosed packed (ducp = 20 nm) array. Hysteresis loops for both arrays were qualitatively similar and nearly isotropic, i.e., independent on the in-plane external field orientation. The shape of these loops revealed that magnetization reversal is governed by the formation and expulsion of vortices inside the nanodisks. When we assumed that the nanodisks’ magnetization significantly decreases near their edges, micromagnetic simulations with material parameters deter-mined independently from continuous film measu-rements could satisfactorily reproduce the hysteresis. Despite the isotropic hysteresis, significant in-plane anisotropy of the dynamic response of the ultraclose-packed array was found experimentally by the all-electrical spin-wave spectroscopy and Brillouin light scattering. Dynamical simulations could successfully reproduce the difference between excitation spectra for fields directed along the two main symmetry axes of the hexagonal lattice. Simulations revealed that this difference is caused by the magnetodipolar interaction between nanodisks, which leads to a strong variation of the spatial distribution of the oscillation power both for bulk and edge modes as a function of the bias field orientation. Comparison of simulated and measured frequencies enabled the unambiguous identification of experimentally observed modes. Results of this systematic research are relevant both for fundamental studies of spin-wave modes in patterned magnetic structures and for the design of magnonic crystals for potential applications as, e.g., spin-wave guides and filters.

Instabilities of a spin-valve system with perpendicular polarizer and in-plane bias field

Under a perpendicular spin-transfer-torque (STT) polarizer and in-plane (IP) bias magnetic field, three-dimensional instability thresholds of the critical strength of STT and the field are derived from an effective one-dimensional free energy. The modified astroids derived from the STT and IP fields are quite different from the classical Stoner-Wohlfarth astroid. We find that the STT breaks the symmetry of the astroid seriously when the orientation of bias field is along the easy and hard axes. In particular, the modified astroid not only separates the region with two stable states from the region with only one stable state, but also delimits the region with a dynamical stable state (no stable equilibrium state) when the amplitude of the STT is larger than a critical value required to switch the magnetization at zero-bias field. Finally, the nucleation field, coercivity, and precessional critical field for uniaxial anisotropy are rigorously determined including the effect of the STT, and the hysteresis loops for various orientations of a bias field are computed and discussed.

Converse magnetoelectric effects in a galfenol and lead zirconate titanate bilayer

Polycrystalline samples of galfenol Fe1−xGax (x=0.17–0.3) have been prepared and used in a bilayer with lead zirconate titanate for studies on the magnetoelectric (ME) effect. The converse ME effect which is the magnetic response of the sample to an applied ac electric field has been investigated. Piezoelectric deformations due the electric field result in an induced magnetization in galfenol that is measured as a voltage U in a pick-up coil wound on the bilayer. A resonance enhancement in the magnitude of U is evident for bending oscillations and longitudinal acoustic oscillations in the bilayer. Data on U as a function of the strength and orientation of bias field H show a maximum in ME coupling strength for in-plane component of H≈250Oe. The data have been analyzed in terms of H dependence of the magnetostriction for galfenol. The converse ME coupling is strong enough for use of the bilayer as a sensor of magnetic and electric fields.

Spin‐wave excitation by spin‐polarized current in magnetic nanostructures

AbstractA current‐driven magnetic point‐contact device can support both a quasi‐linear propagating spin‐wave mode and a nonlinear self‐localized evanescent spin‐wave bullet mode. In this work we show how the bias field orientation determines the nature of the excited modes. By using a micromagnetic framework we can describe the main properties of the observed modes, deriving also a satisfying agreement with analytical theories. It is also pointed out how the nature of these modes differs in: (i) threshold current of excitation, (ii) precession angle, (iii) frequency spectra, (iv) trajectory of the precessional motion, (v) spatial localization of the spin‐wave amplitude. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dispersion control in magnetostatic wave delay lines

A major impediment to the realization of practical magnetostatic wave (MSW) devices is the inherent non-linear delay versus frequency characteristics of MSW delay lines. Most device applications require either nondispersive (delay vs. frequency slope=0) or linearly dispersive (delay vs. frequency slope = constant) characteristics. There are four basic approaches to the elimination and/or linearization of MSW dispersion: (1) modification of the magnetic film properties as a function of film thickness, (2) modification of the ground plane geometry, (3) modification of the bias field uniformity, and (4) perturbation of the propagation path surface with metal electrode reflective arrays. In principle all four approaches can yield either linearly dispersive or nondispersive characteristics with a suitable choice of bias field orientation and critical design parameters. These dispersion control techniques are reviewed, and data from recent experiments demonstrating the strengths and weaknesses of the various approaches are discussed. To date the most extensive experimental work has been done with the multiple layer (magnetic properties varied as a function of film thickness) and ground-plane spacing techniques. At present the ground-plane spacing technique produces results acceptable for some systems applications, while the multiple-layer approach is still under development.

Anisotropic propagation of magnetostatic waves in epitaxial YIG films

A theory for MSW beam steering in anisotropic ferrite film is presented in terms of a generalized dispersion relation in which the magnetic bias field has a general orientation. A computer solution was used to determine steering angle as a function of bias field orientation. Steering angles measured experimentally by an induction probe agree well with theoretical predictions as functions of field misalignment, frequency and propagation direction.

Induction probing of magnetostatic delay line fields

Recently, significant advances have been achieved in the development of tunable microwave (1-20GHz) signal processing elements utilizing magnetostatic waves (MSW) propagating in magnetically biased films of Yttrium Iron Garnet (YIG). The devices use transversal filtering techniques similar to those utilized in surface acoustic waves (SAW) technology. Essential to transversal filtering techniques is a detailed knowledge of the field distribution in the wave propagation media. This work reports initial experimental results of induction probing of the spatial distribution of the MSW field in both magnetostatic surface (MSSW) and forward volumn (MSFVW) wave delay lines. The experimental work of this paper indicates a number of conclusions which are of significance to MSW devices; 1. YIG film anisotropy strongly effects MSW propagation and the relative orientation of the bias field, crystal axes and propagation direction are critical to optimum delay line design. 2. Beam spreading and steering effects are significant and strongly dependent on the relative orientation of bias field, crystal axes, and propagation direction.