Nikhil Kumar C S
@nitc.ac.in
Assistant Professor
National Institute of Technology
RESEARCH, TEACHING, or OTHER INTERESTS
Engineering, Electrical and Electronic Engineering
Scopus Publications
Scopus Publications
Nikhil Kumar, Paweł Gruszecki, Mateusz Gołębiewski, Jarosław W. Kłos, and Maciej Krawczyk
Wiley
AbstractSpin waves (SWs) are promising objects for signal processing and future quantum technologies due to their high microwave frequencies with corresponding nanoscale wavelengths. However, the nano‐wavelength SWs generated so far are limited to low frequencies. In the paper, using micromagnetic simulations, it is shown that a microwave‐pumped SW mode confined to the cavity of a thin film magnonic crystal (MC) can be used to generate waves at tens of GHz and wavelengths well below 50 nm. These multi‐frequency harmonics of the fundamental cavity mode are generated when the amplitude of the pumping microwave field exceeds a threshold, and their intensities then scale linearly with the field intensity. The frequency of the cavity mode is equal to the ferromagnetic resonance frequency of the planar ferromagnetic film, which overlaps with the magnonic bandgap, providing an efficient mechanism for confinement and magnetic field tunability. The effect reaches saturation when the microstrip feed line covers the entire cavity, making the system feasible for realization.
N. Kumar and A. Prabhakar
Elsevier BV
Abstract Spin polarized electric current, injected into permalloy (Py) through a nano contact, exerts a torque on the magnetization. The spin waves (SWs) thus excited propagate radially outward. We propose an antidot magnonic crystal (MC) with a three-hole defect (L3) around the nano contact, designed so that the frequency of the excited SWs, lies in the band gap of the MC. L3 thus acts as a resonant SW cavity. The energy in this magnonic crystal cavity can be tapped by an adjacent MC waveguide (MCW). An analysis of the simulated micromagnetic power spectrum, at the output port of the MCW reveals stable SW oscillations. The quality factor of the device, calculated using the decay method, was estimated as Q > 105 for an injected spin current density of 7 × 10 12 A/m2.
D. Kumar, K. Konishi, N. Kumar, S. Miwa, A. Fukushima, K. Yakushiji, S. Yuasa, H. Kubota, C. A. Tomy, A. Prabhakar,et al.
The transfer of spin angular momentum to a nanomagnet from a spin polarized current provides an efficient means of controlling the magnetization direction in nanomagnets. A unique consequence of this spin torque is that the spontaneous oscillations of the magnetization can be induced by applying a combination of a dc bias current and a magnetic field. Here we experimentally demonstrate a different effect, which can drive a nanomagnet into spontaneous oscillations without any need of spin torque. For the demonstration of this effect, we use a nano-pillar of magnetic tunnel junction (MTJ) powered by a dc current and connected to a coplanar waveguide (CPW) lying above the free layer of the MTJ. Any fluctuation of the free layer magnetization is converted into oscillating voltage via the tunneling magneto-resistance effect and is fed back into the MTJ by the CPW through inductive coupling. As a result of this feedback, the magnetization of the free layer can be driven into a continual precession. The combination of MTJ and CPW behaves similar to a laser system and outputs a stable rf power with quality factor exceeding 10,000.
G. Venkat, N. Kumar and A. Prabhakar
We simulate spin wave (SW) dynamics in a thin film antidot magnonic crystal (MC) with a ring defect. An external magnetic field is applied perpendicular to the film plane in a forward volume configuration. We initially use the plane wave method to obtain the SW band diagram for the antidot MC structure (without the defect), which gives us an idea of modes that are trapped in the crystal. We then use finite element micromagnetic simulations to study the SW propagation in the MC with the ring defect. The power spectral density of the magnetization at the top of the ring shows peaks that fall within the band gap of the MC. The SW energy in other frequency components is dissipated in the MC and do not reach the other end of the ring.
N. Kumar, G. Venkat, and A. Prabhakar
Institute of Electrical and Electronics Engineers (IEEE)
The dispersion relation of forward volume dipolar spin waves in a magnonic curved waveguide is investigated by solving Walker's equation in cylindrical coordinates with appropriate boundary conditions. Dispersion for exchange spin waves is then calculated using an iterative method. We validate our results by comparing the dispersion relation for higher bending radius with that of a straight waveguide. We also observe good agreement with the dispersion plots from micromagnetic simulations. For a ring, we impose periodic boundary conditions along the azimuthal direction to obtain standing wave mode patterns. The frequency-mode number characteristics of standing waves follow the dispersion relation for propagating spin waves in a curved waveguide. The maximum spin wave amplitude is not necessarily at the center of the waveguide. Exchange interactions cause the maximum to occur at a lower radius for higher mode numbers.
N. Kumar and A. Prabhakar
Institute of Electrical and Electronics Engineers (IEEE)
We investigate the spin wave spectra in a magnonic waveguide using the plane wave expansion method. The structure under the investigation has the form of thin strips of permalloy and cobalt, alternately arranged as a planar waveguide. The structure is assumed to be infinite in length and finite in thickness and width. Spin wave propagation is assumed along the length of the stripe, parallel to the external applied field, in a backward volume configuration. We derive both static and dynamic fields in the magnonic waveguide using the plane wave method, after reducing the linearized Landau-Lifshitz equation to an eigenfrequency problem. The eigenfrequencies corresponding to a wave vector are then numerically calculated and plotted, with the eigenmodes yielding the spatial variation in spin wave amplitudes. The demagnetizing fields, along the length and thickness, were derived from the magnetostatic potential and shows both bulk and edge mode characteristics. In a nonuniform demagnetizing field, low frequency spin waves concentrate their amplitude in a region of low internal magnetic field. These appear as standing wave excitations in the permalloy resulting in zero group velocity, or a flat band structure in the ω(k) dispersion diagram. Finally the dependence of the frequency band gap on the angle between spin wave vector and the applied field is also investigated.