Bivas Rana

Tunable magnonic frequency and damping in [Co/Pd]8 multilayers with variable Co layer thickness

We report the experimental observation of collective picosecond magnetization dynamics in [Co/Pd]8 multilayers with perpendicular magnetic anisotropy. The precession frequency shows large and systematic variation from about 5 GHz to about 90 GHz with the decrease in the Co layer thickness from 1.0 to 0.22 nm due to the linear increase in the perpendicular magnetic anisotropy. The damping coefficient α is found to be inversely proportional to the Co layer thickness and a linear relation between the perpendicular magnetic anisotropy and α is established. We discuss the possible reasons behind the enhanced damping as the d-d hybridization at the interface and spin pumping. These observations are significant for the applications of these materials in spintronics and magnonic crystals.

Detection of Picosecond Magnetization Dynamics of 50 nm Magnetic Dots down to the Single Dot Regime

We report an all-optical time-domain detection of picosecond magnetization dynamics of arrays of 50 nm Ni(80)Fe(20) (permalloy) dots down to the single nanodot regime. In the single nanodot regime the dynamics reveals one dominant resonant mode corresponding to the edge mode of the 50 nm dot with slightly higher damping than that of the unpatterned thin film. With the increase in areal density of the array both the precession frequency and damping increase significantly due to the increase in magnetostatic interactions between the nanodots, and a mode splitting and sudden jump in apparent damping are observed at an edge-to-edge separation of 50 nm.

Realization of a micrometre-scale spin-wave interferometer

The recent development of spin dynamics opens perspectives for various applications based on spin waves, including logic devices. The first important step in the realization of spin-wave-based logics is the manipulation of spin-wave interference. Here, we present the experimental realization of a micrometre-scale spin-wave interferometer consisting of two parallel spin-wave waveguides. The spin waves propagate through the waveguides and the superposition or interference of the electrical signals corresponding to the spin waves is measured. A direct current flowing through a metal wire underneath one of the spin-wave waveguides affects the propagation properties of the corresponding spin wave. The signal of constructive or destructive interference depends on the magnitude and direction of the applied direct current. Thus, the present work demonstrates a unique manipulation of spin-wave interference.

MAGNETO-OPTICAL MEASUREMENTS OF COLLECTIVE SPIN DYNAMICS OF TWO-DIMENSIONAL ARRAYS OF FERROMAGNETIC NANOELEMENTS

Magnetic nanodot arrays are interesting systems for future applications in nanotechnology including patterned magnetic media, magnonic crystals, magnetic logic, sensors, STNOs and biomedical applications. All applications require the knowledge base of magnetization processes of magnetic nanodot arrays at various time and length scales. Here, we review the present status of experimental studies of picosecond precessional magnetization dynamics in magnetic nanodot arrays. We discuss the fabrication methods of magnetic nanodot arrays and excitation and detection methods of precessional dynamics by optical means. We further discuss the all-optical excitation and detection of precessional dynamics in Ni80Fe20 (permalloy) nanodot arrays with width between 200 nm and 50 nm and with interdot separation between 50 nm and 400 nm. A transition from strongly collective dynamics to completely isolated dynamics through various weakly collective regimes, variation of precession frequency and damping with the interdot separation, effects of dipolar and quadrupolar interdot interaction, effects of the variation of dot size on the dynamics of single elements and arrays, and anisotropy of collective dynamics have been thoroughly studied by experimental and micromagnetic simulation results. Finally, we discuss the future directions in the research on the dynamics of magnetic nanodot arrays.

Configurational anisotropic spin waves in cross-shaped Ni80Fe20 nanoelements

Optically induced spin waves in Ni80Fe20 (permalloy) cross-shaped nanoelements are studied by time-resolved magneto-optical Kerr effect microscope. A strong anisotropy in the spin wave modes are observed with the orientation angle (ϕ) of the in-plane bias magnetic field. As ϕ deviates from 0° a single resonant mode splits into a numbers of modes, while the powers of the higher frequency modes increase as ϕ increases from 0 to 45°. The lowest frequency mode shows a four-fold configurational anisotropy. The mode of a single cross remains unaffected by the magnetostatic interaction of the neighbouring elements for ϕ = 0°, while the effect increases with ϕ and becomes maximum at 45°, making these elements interesting candidates as building blocks for magnonic devices.

All-Optical Excitation and Detection of Picosecond Dynamics of Ordered Arrays of Nanomagnets with Varying Areal Density

We have demonstrated optical excitation and detection of collective precessional dynamics in arrays of coupled Ni80Fe20 (permalloy) nanoelements with systematically varying areal density by an all-optical time-resolved Kerr microscope. We have applied this technique to precisely determine three different collective regimes in these arrays. At very high areal density, a single uniform collective mode is observed where the edge modes of the constituent elements are suppressed. At intermediate areal densities, three nonuniform collective modes appear and at very low areal density, we observe noncollective dynamics and only the centre and edge modes of the constituent elements appear.

Tunable spin wave dynamics in two-dimensional Ni80Fe20 nanodot lattices by varying dot shape

We demonstrate tunable spin wave spectrum in two-dimensional Ni80Fe20 nanodot lattices by varying dot shape. A single collective mode in elliptical dot lattices transforms into three distinct modes for the half-elliptical, rectangular, and diamond dot lattices, albeit with different peak frequencies and intensities. A drastic change is observed for the triangular dots, where eight modes covering a broad band are observed. Using micromagnetic simulations, we characterized the modes as different localized, extended, and quantized modes, whose frequencies and spatial profiles are determined by a combination of internal field profiles within the nanodots and the stray magnetic field within the lattice.

Magnetization reversal dynamics in clusters of single domain Ni nanoparticles

We present the magnetization reversal dynamics of clusters of single domain nickel nanoparticles. Experimental results of magnetization reversal of nickel nanoparticles are completely different from that of bulk nickel and single nickel nanoparticles in terms of the overall shape, coercive field, and the saturation magnetization. Simulations show that a cluster consists of minimum 5×5×3 nanoparticles with no physical overlap between the particles reproduces the loop shape and the coercive field but not the saturation magnetization. However, the same cluster with partially overlapped nanoparticles reasonably reproduces all features of the magnetization reversal due to the presence of both magnetostatic and exchange interactions between the particles. Simulated magnetization images show that the reversal of the cluster occurs through the formation of a C-like state, followed by the creation and annihilation of a vortex like structure, an inverse C-state, and finally, a fully saturated reversed magnetic stat...

Anisotropy in collective precessional dynamics in arrays of Ni80Fe20 nanoelements

The anisotropy in the collective precessional dynamics with the variation of azimuthal angle of the bias magnetic field is studied in arrays of permalloy (Ni80Fe20) nanoelements by an all-optical time-resolved Kerr microscope. When the nanoelements are very closely spaced (inter-element separation = 50 nm), a gradual transition from completely uniform collective regime to a completely non-collective regime is observed as the azimuthal angle varies from 0° to 45°. On the other hand, for inter-element separation of 100 nm, a non-uniform collective dynamics is observed at 0° and a non-collective dynamics is observed at 45° but no clear trend in the transition is observed.

Voltage-induced magnetization dynamics in CoFeB/MgO/CoFeB magnetic tunnel junctions

Recent progress in magnetic tunnel junctions (MTJs) with a perpendicular easy axis consisting of CoFeB and MgO stacking structures has shown that magnetization dynamics are induced due to voltage-controlled magnetic anisotropy (VCMA), which will potentially lead to future low-power-consumption information technology. For manipulating magnetizations in MTJs by applying voltage, it is necessary to understand the coupled magnetization motion of two magnetic (recording and reference) layers. In this report, we focus on the magnetization motion of two magnetic layers in MTJs consisting of top layers with an in-plane easy axis and bottom layers with a perpendicular easy axis, both having perpendicular magnetic anisotropy. According to rectified voltage (Vrec) measurements, the amplitude of the magnetization motion depends on the initial angles of the magnetizations with respect to the VCMA direction. Our numerical simulations involving the micromagnetic method based on the Landau-Lifshitz-Gilbert equation of motion indicate that the magnetization motion in both layers is induced by a combination of VCMA and transferred angular momentum, even though the magnetic easy axes of the two layers are different. Our study will lead to the development of voltage-controlled MTJs having perpendicular magnetic anisotropy by controlling the initial angle between magnetizations and VCMA directions.