Anjan Barman

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.

Ultrafast magnetization dynamics in high perpendicular anisotropy †Co/Pt‡ n multilayers

We present time-resolved magneto-optical Kerr effect measurements of picosecond laser induced precessional dynamics in out-of-plane magnetized [Co(4A)∕Pt(8A)]n multilayer films. A fast precession of magnetization (up to 81GHz) emerges 6ps after pumping and decays within a 100ps time scale. The uniform precessional frequency was studied for varying external magnetic bias field and number of Co∕Pt repeats n. The variation of the precession frequency with external bias field is quantitatively understood using the macrospin model of the Landau-Lifshitz-Gilbert equation of motion [Phys. Z. Sowjetunion 8, 153 (1935); Phys. Rev. 100, 1243 (1955)], yielding a large perpendicular anisotropy of up to 1.0×107ergs∕cm3 for our samples. The precession frequency increases sharply with reduced n and almost saturates below n=5, suggesting a commensurate variation of the perpendicular anisotropy. A heavy damping of the precessional motion is observed with increasing n and can be explained by enhanced spin-orbit coupling in...

Dynamics of coupled vortices in a pair of ferromagnetic disks.

We here experimentally demonstrate that gyration modes of coupled vortices can be resonantly excited primarily by the ac current in a pair of ferromagnetic disks with variable separation. The sole gyration mode clearly splits into higher and lower frequency modes via dipolar interaction, where the main mode splitting is due to a chirality sensitive phase difference in gyrations of the coupled vortices, whereas the magnitude of the splitting is determined by their polarity configuration. These experimental results show that the coupled pair of vortices behaves similar to a diatomic molecule with bonding and antibonding states, implying a possibility for designing the magnonic band structure in a chain or an array of magnetic vortex oscillators.

Structural, optical and magnetic properties of sol–gel derived ZnO:Co diluted magnetic semiconductor nanocrystals: an EXAFS study

Structural, local structural, optical and magnetic properties of sol–gel derived Zn1−xCoxO (0 ≤ x ≤ 0.04) nanoparticles have been studied. The crystallite structure, size, and lattice strain have been estimated by X-ray diffraction (XRD) with Rietveld refinement and high-resolution transmission electron microscopy (HRTEM). The small linear increase in lattice parameter ‘a’ and decrease in lattice parameter ‘c’ have been observed which can be attributed to the small distortion of Zn tetrahedron. Extended X-ray Absorption Fine Structure (EXAFS) measurements show that Co-doping creates oxygen vacancies without causing any significant change in the host lattice structure. X-ray Absorption Near Edge Structure (XANES) measurements rule out the presence of metallic Co clusters in the samples. Raman spectroscopy has been employed to study the crystalline quality, structural disorder, and defects in the host lattice. The tetrahedral coordination of the oxygen ions surrounding the zinc ions and wurtzite structure has been studied by FTIR analysis. UV-Vis measurements have been used to study the effect of Co-doping on absorption spectra and hence on the band gap. The band gap initially decreases for low Co-concentration and increases with higher Co-concentration. The PL spectra show six peaks out of which the peak in the ultraviolet (UV) region has been assigned to the near band edge excitonic emission (NBE) and other peaks are related to different defect states. Room temperature ferromagnetism (weak) is observed and magnetization increases with increasing Co-concentration. The grain boundaries, oxygen vacancy and bound magnetic polarons (BMPs) jointly may be responsible for this room temperature ferromagnetism. Variation of resistivity with temperature shows that a thermally activated conduction (Arrhenius) mechanism is valid in the high temperature region whereas Motts variable-range hopping (VRH) mechanism is valid in the low temperature region.

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.

Size dependent damping in picosecond dynamics of single nanomagnets

The authors use time-resolved cavity-enhanced magneto-optical Kerr spectroscopy to study the damping of magnetization precession in individual cylindrical nickel nanomagnets. A wide range of shapes (diameters of 5μm–125nm and aspect ratio: 0.03–1.2) is investigated. They observe a pronounced difference in damping between the micro- and nanomagnets. Microscale magnets show large damping at low bias fields, whereas nanomagnets exhibit bias field-independent damping. This behavior is explained by the interaction of in-plane and out-of-plane precession modes in microscale magnets that results in additional dissipative channels. The small and robust damping values on the nanoscale are promising for implementation of controlled precessional switching schemes in nanomagnetic devices.

Thickness dependence of spin torque ferromagnetic resonance in Co75Fe25/Pt bilayer films

The spin Hall angle of Pt in Co75Fe25/Pt bilayer films was experimentally investigated by means of the spin-torque ferromagnetic resonance and the modulation of damping measurements. By comparing the present results with the Ni80Fe20/Pt system, we found that the ferromagnetic layer underneath the Pt one greatly affects the estimation of the spin Hall angle. We also discuss the spin diffusion length of Pt and the ferromagnetic thickness dependence of the Gilbert damping coefficient.

Microphotonic control of single molecule fluorescence correlation spectroscopy using planar optofluidics.

We demonstrate the implementation of fluorescence correlation spectroscopy (FCS) on a chip. Full planar integration is achieved by lithographic definition of sub-picoliter excitation volumes using intersecting solid and liquid-core optical waveguides. Concentration dependent measurements on dye molecules with single molecule resolution are demonstrated. Theoretical modeling of the FCS autocorrelation function in microstructured geometries shows that the FCS behavior can be controlled over a wide range by tailoring the micro-photonic environment. The ability to perform correlation spectroscopy using silicon photonics without the need for free-space microscopy permits implementation of numerous diagnostic applications on compact planar optofluidic devices.

Proposal for a Standard Micromagnetic Problem: Spin Wave Dispersion in a Magnonic Waveguide

In this paper, we propose a standard micromagnetic problem, of a nanostripe of permalloy. We study the magnetization dynamics and describe methods of extracting features from simulations. Spin wave dispersion curves, relating frequency and wave vector, are obtained for wave propagation in different directions relative to the axis of the waveguide and the external applied field. Simulation results using both finite element (Nmag) and finite difference (OOMMF) methods are compared against analytic results, for different ranges of the wave vector.

Gyration mode splitting in magnetostatically coupled magnetic vortices in an array

We present the experimental observation of gyration mode splitting by the time-resolved magneto-optical Kerr effect in an array consisting of magnetostatically coupled Ni81Fe19 discs of 1 µm diameter, 50nm thickness and inter-disc separations varying between 150 and 270nm. A splitting of the vortex core gyration mode is observed when the inter-disc separation is 200nm or less and the splitting is controllable by a bias magnetic field. The observed mode splitting is interpreted by micromagnetic simulations as the normal modes of the vortex cores analogous to the coupled classical oscillators. The splitting depends upon the strength of the inter-disc magnetostatic coupling mediated by magnetic side charges, which depends strongly on the magnetic ground states of the samples. (Some figures in this article are in colour only in the electronic version)