Semanti Pal

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.

Optically induced tunable magnetization dynamics in nanoscale co antidot lattices.

We report the time-domain measurements of optically induced precessional dynamics in a series of Co antidot lattices with fixed antidot diameter of 100 nm and with varying lattice constants (S) between 200 and 500 nm. For the sparsest lattice, we observe two bands of precessional modes with a band gap, which increases substantially with the decrease in S down to 300 nm. At S = 200 nm, four distinct bands with significant band gaps appear. The numerically calculated mode profiles show various localized and extended modes with the propagation direction perpendicular to the bias magnetic field. We numerically demonstrate some composite antidot structures with very rich magnonic spectra spreading between 3 and 27 GHz based upon the above experimental observation.

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.

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...

Effect of the spin-twist structure on the spin-wave dynamics in Fe55Pt45/Ni80Fe20 exchange coupled bi-layers with varying Ni80Fe20 thickness

We have investigated optically induced ultrafast magnetization dynamics of a series of Fe55Pt45/Ni80Fe20 exchange spring bi-layers with varying Ni80Fe20 thickness. Rich spin-wave spectra are observed; whose frequency shows a strong dependence on the Ni80Fe20 layer thickness. Micromagnetic simulations based on a simplified magnetic microstructure were able to reproduce the experimental data qualitatively. The spin twist structure introduced in the Ni80Fe20 layer gives rise to new modes in the composite system as opposed to the bare Ni80Fe20 films.

Time-resolved measurement of spin-wave spectra in CoO capped [Co(t)/Pt(7Å)]n-1 Co(t) multilayer systems

We present an all-optical time-resolved measurement of dipole-exchange spin wave spectra in a series of CoO capped [Co(t)/Pt(7 A)]n-1 Co(t) multilayer systems, where the total Co moment (n × t) is constant. In general, the spectra consist of two intense peaks and additional lower intensity peaks. The observed spin wave modes are modeled by a discrete dipole approximation. The frequency of the spin wave bands depends significantly upon the magnetic anisotropy and the lattice spacing between planes. Both symmetric and anti-symmetric modes are observed from the calculation of the spin-wave profiles across the multilayer in the out-of-plane direction.

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.

All-optical study of tunable ultrafast spin dynamics in [Co/Pd]/NiFe systems: the role of spin-twist structure on Gilbert damping

We investigate optically induced ultrafast magnetization dynamics in [Co(0.5 nm)/Pd(1 nm)](5)/NiFe(t) exchange-spring samples with tilted perpendicular magnetic anisotropy using a time-resolved mag ...

Improvement of chemical ordering and magnetization dynamics of Co–Fe–Al–Si Heusler alloy thin films by changing adjacent layers

We demonstrate an improvement of chemical ordering and magnetic properties of Co2FeAl0.5Si0.5 (CFAS) Heusler alloy thin films, as well as having investigated the correlation between these two, to elucidate the influence of different capping-layers and under-layers. The structural characterization reveals a variation in the surface roughness, grain size as well as in the chemical ordering for different samples. The static magnetometry measurements demonstrate a variation in the magnetic anisotropy behavior. A detailed characterization of the magnetization dynamics was performed using Brillouin Light Scattering (BLS) technique and the magnetic parameters were extracted using analytical modelling of the spin wave spectra. The origin of magnetic anisotropy is found to be correlated with the chemical order and the interfacial properties which can be effectively tailored by changing the adjacent layers. The Gilbert damping constants were extracted from the BLS peaks which also show a broad tunability with the chemical order. A Gilbert damping constant as low as about 0.002 has been found. The observed effects can primarily be attributed to the different melting points of the under-layers and thermal expansion stress between the adjacent layers and CFAS thin films.