Patterned magnetic nanostructures are potential candidates for future energy efficient, on-chip c... more Patterned magnetic nanostructures are potential candidates for future energy efficient, on-chip communication devices. Here, we have experimentally and numerically studied the role of nanochannels to manipulate spin waves in Ni80Fe20 connected nanodot arrays of varying filling fraction. Rich spin-wave spectra are observed in these samples, where the number of spinwave modes decreases with increasing filling fraction due to the retrenchment of the demagnetizing field. The nanochannels affect the spin-wave modes of the connected dots through dipole-exchange coupling. For all modes the vertical nanochannels couple the nanodots, except for the highest frequency modes where all nanochannels act as coupler. This feature is further explored in the simulation, which reveals that only the highest frequency mode can propagate through all the nanochannels, analogues to an electronic demultiplexer. This study will be useful to understand the role of nanochannels in patterned magnetic nanostructures and their applications in spin-wave based communication devices.
Harnessing high-frequency spin dynamics in threedimensional (3D) nanostructures may lead to parad... more Harnessing high-frequency spin dynamics in threedimensional (3D) nanostructures may lead to paradigm-shifting, next-generation devices including high density spintronics and neuromorphic systems. Despite remarkable progress in fabrication, the measurement and interpretation of spin dynamics in complex 3D structures remain exceptionally challenging. Here, we take a first step and measure coherent spin waves within a 3D artificial spin ice (ASI) structure using Brillouin light scattering. The 3D-ASI was fabricated by using a combination of two-photon lithography and thermal evaporation. Two spin-wave modes were observed in the experiment whose frequencies showed nearly monotonic variation with the applied field strength. Numerical simulations qualitatively reproduced the observed modes. The simulated mode profiles revealed the collective nature of the modes extending throughout the complex network of nanowires while showing spatial quantization with varying mode quantization numbers. The study shows a well-defined means to explore high-frequency spin dynamics in complex 3D spintronic and magnonic structures.
We report microwave-power-driven strong magnon-magnon coupling in Ni 80 Fe 20 nanocross array wit... more We report microwave-power-driven strong magnon-magnon coupling in Ni 80 Fe 20 nanocross array with large anticrossing gaps up to 1.03 GHz. We also observe microwave-driven large nonlinear shift in ferromagnetic resonance (FMR) frequency with its sign dependent on the strength of bias magnetic field. A drastic enhancement of inter-nanocross-dynamic dipolar interaction results in the anticrossing, while variation in internal spin texture leads to the nonlinear FMR shift. The tunable coupling strength and nonlinearity by microwave power ushers externally controlled nonlinear magnonic devices.
We have theoretically studied how resonant spin wave modes in an elliptical nanomagnet are affect... more We have theoretically studied how resonant spin wave modes in an elliptical nanomagnet are affected by fabrication defects, such as small local thickness variations. Our results indicate that defects of this nature, which can easily result from the fabrication process, or are sometimes deliberately introduced during the fabrication process, will significantly alter the frequencies, magnetic field dependence of the frequencies, and the power and phase profiles of the resonant spin wave modes. They can also spawn new resonant modes and quench existing ones. All this has important ramifications for multi-device circuits based on spin waves, such as phase locked oscillators for neuromorphic computing, where the device-to-device variability caused by defects can be inhibitory.
Magneto-elastic (straintronic) switching of bistable magnetostrictive nanomagnets is an extremely... more Magneto-elastic (straintronic) switching of bistable magnetostrictive nanomagnets is an extremely energy-efficient switching methodology for (magnetic) binary switches that has recently attracted widespread attention because of its potential application in ultra-low-power digital computing hardware. Unfortunately, this modality of switching is also very error prone at room temperature. Theoretical studies of switching error probability of magneto-elastic switches have predicted probabilities ranging from 10-8-10-3 at room temperature for ideal, defect-free nanomagnets, but experiments with real nanomagnets show a much higher probability that exceeds 0.1 in some cases. The obvious spoilers that can cause this large difference are defects and non-idealities. Here, we have theoretically studied the effect of common defects (that occur during fabrication) on magneto-elastic switching probability in the presence of roomtemperature thermal noise. Surprisingly, we found that even small defects increase the switching error probabilities by orders of magnitude. There is usually a critical stress that leads to the lowest error probability and its value increases enormously in the presence of defects. All this could limit or preclude the application of magneto-elastic (straintronic) binary switches in either Boolean logic or memory, despite their excellent energy-efficiency, and restrict them to non-Boolean (e.g. neuromorphic, stochastic) computing applications. We also studied the difference between magneto-elastic switching with a stress pulse of constant amplitude and sinusoidal time-varying amplitude (e.g. due to a surface acoustic wave) and found that the latter method is more reliable and generates lower switching error probabilities in most cases, provided the time variation is reasonably slow.
Ferromagnetic antidot arrays have emerged as a system of tremendous interest due to their interes... more Ferromagnetic antidot arrays have emerged as a system of tremendous interest due to their interesting spin configuration and dynamics as well as their potential applications in magnetic storage, memory, logic, communications and sensing devices. Here, we report experimental and numerical investigation of ultrafast magnetization dynamics in a new type of antidot lattice in the form of triangular-shaped Ni80Fe20 antidots arranged in a hexagonal array. Time-resolved magneto-optical Kerr effect and micromagnetic simulations have been exploited to study the magnetization precession and spin-wave modes of the antidot lattice with varying lattice constant and in-plane orientation of the bias-magnetic field. A remarkable variation in the spin-wave modes with the orientation of in-plane bias magnetic field is found to be associated with the conversion of extended spin-wave modes to quantized ones and vice versa. The lattice constant also influences this variation in spin-wave spectra and spi...
Efficient tunability of magnetization dynamics in two-dimensional circular and triangularshaped N... more Efficient tunability of magnetization dynamics in two-dimensional circular and triangularshaped Ni80Fe20 antidots arranged in hexagonal lattice is demonstrated using a combination of all-electrical and all-optical detection techniques. A broad band of modes is observed for both the lattices. A strong variation in the spin-wave spectra is obtained with the strength and orientation of the bias magnetic field. A crossover between two higher frequency branches is observed with the variation of bias magnetic field strength in circular antidot lattice, whereas no such crossover is observed in the triangular antidot lattice. In addition, the spin-wave modes in both lattices show strong six-fold anisotropic behaviour presumably due to the variation of internal field distribution originating from a combination of the lattice arrangement and the shape of the antidots as a function of the bias magnetic field orientation. Micromagnetic simulations qualitatively reproduce the experimentally observed spin-wave modes and the simulated mode profiles reveal the presence of extended and quantized standing spin-wave modes in these lattices. Also, some lower frequency localized edge modes, obtained in the triangular antidot lattice due to the asymmetric demagnetized regions at sharp corners, are not observed in the circular antidot lattice. These observations are significant for large tunability and anisotropic propagation of spin waves in GHz frequency magnetic devices.
Magneto-elastic (or "straintronic") switching has emerged as an extremely energy-efficient mechan... more Magneto-elastic (or "straintronic") switching has emerged as an extremely energy-efficient mechanism for switching the magnetization of magnetostrictive nanomagnets in magnetic memory, logic and non-Boolean circuits. Here, we investigate the ultrafast magneto-dynamics associated with straintronic switching in a single quasi-elliptical magnetostrictive Co nanomagnet deposited on a piezoelectric Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) substrate using time-resolved magneto-optical Kerr effect (TR-MOKE) measurements. The pulsed laser pump beam in the TR-MOKE plays a dual role: it causes precession of the nanomagnet's magnetization about an applied bias magnetic field and it also generates surface acoustic waves (SAWs) in the piezoelectric substrate that produce periodic strains in the magnetostrictive nanomagnet and modulate the precessional dynamics. This modulation gives rise to intriguing hybrid magneto-dynamical modes in the nanomagnet, with rich spin wave texture. The characteristic frequencies of these modes are 5-15 GHz, indicating that strain can affect magnetization in a magnetostrictive nanomagnet in time scales much smaller than 1 ns (~100 ps). This can enable ~10 GHz-range magneto-elastic nanooscillators that are actuated by strain instead of a spin-polarized current, as well as ultrafast magneto-electric generation of spin waves for magnonic logic circuits, holograms, etc.
Three-dimensional magnetic nanostructures are now attracting intense interest due to their potent... more Three-dimensional magnetic nanostructures are now attracting intense interest due to their potential as ultrahigh density future magnetic storage devices. Here, we report on the study of ultrafast magnetization dynamics of a complex three-dimensional magnetic nanostructure. Arrays of magnetic tetrapod structures were fabricated using a combination of two-photon lithography (TPL) and electrodeposition. All-optical time-resolved magneto-optical Kerr microscopy was exploited to probe the spin-wave modes from the junction of a single tetrapod structure. Micromagnetic simulations reveal that the nature of these modes originates from the intricate three-dimensional tetrapod structure. Our findings enhance the basic knowledge about the dynamic control of spin waves in complex three-dimensional magnetic elements which are imperative for the construction of modern spintronic devices.
Interfacial Dzyaloshinskii-Moriya interaction (IDMI) is important for its roles in stabilizing th... more Interfacial Dzyaloshinskii-Moriya interaction (IDMI) is important for its roles in stabilizing the skyrmionic lattice as well as soliton-like domain wall motion leading towards new generation spintronic devices. However, achievement and detection of IDMI is often hindered by various spurious effects. Here, we demonstrate the occurrence of IDMI originating primarily from W/CoFeB interface in technologically important W/CoFeB/SiO2 heterostructures using Brillouin light scattering technique. Due to the presence of IDMI, we observe asymmetry in the peak frequency and linewidth of the spin-wave spectra in the Damon-Eshbach (DE) geometry at finite k wave-vectors. The DMI constant is found to scale as the inverse of CoFeB thickness, over the whole studied thickness range, confirming the presence of IDMI in our system without any extrinsic effects. Importantly, the W/CoFeB interface shows no degradation down to sub-nanometer CoFeB thickness, which would be useful for devices that aim to use...
Nanomagnets form the building blocks for a gamut of miniaturized energy-efficient devices includi... more Nanomagnets form the building blocks for a gamut of miniaturized energy-efficient devices including data storage, memory, wave-based computing, sensors, and biomedical devices. They also offer a span of exotic phenomena and stern challenges. The rapid advancements of nanofabrication, characterization, and numerical simulations during the last two decades have made it possible to explore a plethora of science and technology applications related to nanomagnet dynamics. The progress in the magnetization dynamics of single nanomagnets and one- and two-dimensional arrays of nanostructures in the form of nanowires, nanodots, antidots, nanoparticles, binary and bi-component structures, and patterned multilayers have been presented in detail. Progress in unconventional and new structures like artificial spin ice and three-dimensional nanomagnets and spin textures like domain walls, vortex, and skyrmions has been presented. Furthermore, a huge variety of new topics in the magnetization dynam...
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Papers by Sourav Sahoo