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Saha S.,Thematic Unit of Excellence on Nanodevice Technology | Barman S.,Thematic Unit of Excellence on Nanodevice Technology | Ding J.,National University of Singapore | Adeyeye A.O.,National University of Singapore | Barman A.,Thematic Unit of Excellence on Nanodevice Technology
Applied Physics Letters | Year: 2013

Spin-waves in single Ni80Fe20 (Py) and Co bi-component units embedded in two-dimensional arrays thereof are investigated by all-optical time-resolved magneto-optical Kerr effect microscope. Two bands of modes observed for the bias field applied along the long-axis of the bi-component unit convert into four distinct bands, and the mode frequencies change significantly as the field is rotated to the short-axis. Micromagnetic simulations reproduce the experimental results, and comparison with single Py and Co elements and single bi-component unit reveals significant variations of mode frequencies and mode profiles, indicating opportunities for magnonic band engineering in this type of bi-component arrays. © 2013 AIP Publishing LLC.


Mandal R.,Thematic Unit of Excellence on Nanodevice Technology | Laha P.,Thematic Unit of Excellence on Nanodevice Technology | Das K.,Thematic Unit of Excellence on Nanodevice Technology | Saha S.,Thematic Unit of Excellence on Nanodevice Technology | And 3 more authors.
Applied Physics Letters | Year: 2013

We show that the optically induced spin wave spectra of nanoscale Ni 80Fe20 (permalloy) antidot lattices can be tuned by changing the antidot shape. The spin wave spectra also show an anisotropy with the variation of the in-plane bias field orientation. Analyses show this is due to various quantized and extended modes, whose nature changes with the antidot shape and bias field orientation as a result of the variation of the internal magnetic field profile. The observed variation and anisotropy in the spin waves with the internal and external parameters are important for their applications in magnonic devices. © 2013 AIP Publishing LLC.


Klos J.W.,Adam Mickiewicz University | Kumar D.,Thematic Unit of Excellence on Nanodevice Technology | Krawczyk M.,Adam Mickiewicz University | Barman A.,Thematic Unit of Excellence on Nanodevice Technology
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

We demonstrate that the magnonic band structure, including the band gap of a ferromagnetic antidot waveguide, can be significantly tuned by a relatively weak modulation of its structural parameters. We study the magnonic band structure in nanoscale spin-wave waveguides with periodically distributed small antidots along their central line by two independent computational methods, namely, a micromagnetic simulation and a plane-wave method. The calculations were performed with consideration of both the exchange and dipolar interactions. For the exchange dominated regime, we discuss, in details, the impact of the changes of the lattice constant, size, and shape of the antidots on the spin-wave spectra. We have shown that a precise choice of these parameters is crucial for achieving desired properties of antidot waveguides, i.e., a large group velocity and filtering properties due to existence of magnonic band gaps. We discuss different mechanisms of magnonic gap opening resulting from Bragg scattering or anticrossing of modes. We have shown that the dipolar interactions start to assert their role in the spin-wave spectrum when the waveguide is scaled up, but even for a period of few hundreds of nanometers, the magnonic band structure preserves qualitatively the properties found in the exchange dominating regime. The obtained results are important for future development of magnonic crystal based devices. © 2014 American Physical Society.


Klos J.W.,Adam Mickiewicz University | Kumar D.,Thematic Unit of Excellence on Nanodevice Technology | Krawczyk M.,Adam Mickiewicz University | Barman A.,Thematic Unit of Excellence on Nanodevice Technology
Scientific Reports | Year: 2013

We theoretically study the spin-wave spectra in magnonic waveguides periodically patterned with nanoscale square antidots. We show that structural changes breaking the mirror symmetry of the waveguide can close the magnonic bandgap. The effect of these intrinsic symmetry breaking can be compensated by adjusted asymmetric external bias magnetic field, i.e., by an extrinsic factor. This allows for the recovery of the magnonic bandgaps. The described methods can be used for developing parallel models for recovering bandgaps closed due to a fabrication defect. The model developed here is particular to magnonics, an emerging field combining spin dynamics and spintronics. However, the underlying principle of this development is squarely based upon the translational and mirror symmetries, thus, we believe that this idea of correcting an intrinsic defect by extrinsic means, should be applicable to spin-waves in both exchange and dipolar interaction regimes, as well as to other waves in general.


Barman A.,Thematic Unit of Excellence on Nanodevice Technology | Haldar A.,Thematic Unit of Excellence on Nanodevice Technology
Solid State Physics - Advances in Research and Applications | Year: 2014

The research in magnetization dynamics has growing interest which is driven by both fundamental quests and technological demands. Ultrafast dynamics of magnetization provide important information about the material or device properties. It also poses challenges to investigate dynamics down to femtosecond time scale with nm spatial resolution. Time- and space-resolved magneto-optical Kerr microscopy is a very powerful technique to probe ultrafast responses in the time domain. A brief theoretical background of magnetization dynamics and the measurement technique have been introduced. Evolutions of this rapidly emerging technique and its applications in thin films, multilayers to magnetic micro- and nanostructures have been discussed. In this respect several pioneering works have been introduced and explained. © 2014 Elsevier Inc.


Ganguly A.,Thematic Unit of Excellence on Nanodevice Technology | Kondou K.,RIKEN | Sukegawa H.,Japan National Institute of Materials Science | Mitani S.,Japan National Institute of Materials Science | And 6 more authors.
Applied Physics Letters | Year: 2014

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. © 2014 AIP Publishing LLC.


Pal S.,Thematic Unit of Excellence on Nanodevice Technology | Klos J.W.,ckiewicz University In Poznan | Das K.,Thematic Unit of Excellence on Nanodevice Technology | Hellwig O.,A Western Digital Company | And 3 more authors.
Applied Physics Letters | Year: 2014

We present an all-optical time-resolved measurement of spin wave (SW) dynamics in a series of antidot lattices based on [Co(0.75nm)/Pd(0.9nm)]8 multilayer (ML) systems with perpendicular magnetic anisotropy. The spectra depend significantly on the areal density of the antidots. The observed SW modes are qualitatively reproduced by the plane wave method. The interesting results found in our measurements and calculations at small lattice constants can be attributed to the increase of areal density of the shells with modified magnetic properties probably due to distortion of the regular ML structure by the Ga ion bombardment and to increased coupling between localized modes. We propose and discuss the possible mechanisms for this coupling including exchange interaction, tunnelling, and dipolar interactions. © 2014 AIP Publishing LLC.


Kumar D.,Thematic Unit of Excellence on Nanodevice Technology | Klos J.W.,Adam Mickiewicz University | Krawczyk M.,Adam Mickiewicz University | Barman A.,Thematic Unit of Excellence on Nanodevice Technology
Journal of Applied Physics | Year: 2014

We present the observation of a complete bandgap and collective spin wave excitation in two-dimensional magnonic crystals comprised of arrays of nanoscale antidots and nanodots, respectively. Considering that the frequencies dealt with here fall in the microwave band, these findings can be used for the development of suitable magnonic metamaterials and spin wave based signal processing. We also present the application of a numerical procedure, to compute the dispersion relations of spin waves for any high symmetry direction in the first Brillouin zone. The results obtained from this procedure have been reproduced and verified by the well established plane wave method for an antidot lattice, when magnetization dynamics at antidot boundaries are pinned. The micromagnetic simulation based method can also be used to obtain iso-frequency contours of spin waves. Iso-frequency contours are analogous of the Fermi surfaces and hence, they have the potential to radicalize our understanding of spin wave dynamics. The physical origin of bands, partial and full magnonic bandgaps have been explained by plotting the spatial distribution of spin wave energy spectral density. Although, unfettered by rigid assumptions and approximations, which afflict most analytical methods used in the study of spin wave dynamics, micromagnetic simulations tend to be computationally demanding. Thus, the observation of collective spin wave excitation in the case of nanodot arrays, which can obviate the need to perform simulations, may also prove to be valuable. © 2014 AIP Publishing LLC.


Kumar D.,Thematic Unit of Excellence on Nanodevice Technology | Sabareesan P.,Thematic Unit of Excellence on Nanodevice Technology | Wang W.,University of Southampton | Fangohr H.,University of Southampton | Barman A.,Thematic Unit of Excellence on Nanodevice Technology
Journal of Applied Physics | Year: 2013

We present the possibility of tuning the spin-wave band structure, particularly the bandgaps in a nanoscale magnonic antidot waveguide by varying the shape of the antidots. The effects of changing the shape of the antidots on the spin-wave dispersion relation in a waveguide have been carefully monitored. We interpret the observed variations by analysing the equilibrium magnetic configuration and the magnonic power and phase distribution profiles during spin-wave dynamics. The inhomogeneity in the exchange fields at the antidot boundaries within the waveguide is found to play a crucial role in controlling the band structure at the discussed length scales. The observations recorded here will be important for future developments of magnetic antidot based magnonic crystals and waveguides. © 2013 AIP Publishing LLC.


Haldar A.,Thematic Unit of Excellence on Nanodevice Technology | Banerjee C.,Thematic Unit of Excellence on Nanodevice Technology | Laha P.,Thematic Unit of Excellence on Nanodevice Technology | Barman A.,Thematic Unit of Excellence on Nanodevice Technology
Journal of Applied Physics | Year: 2014

Spin waves are investigated in Permalloy(Ni80Fe 20)/Cobalt(Co) exchange spring bilayer thin films using Brillouin light scattering (BLS) experiment. The magnetic hysteresis loops measured by magneto-optical Kerr effect show a monotonic decrease in coercivity of the bilayer films with increasing Py thickness. BLS study shows two distinct modes, which are modelled as Damon-Eshbach and perpendicular standing wave modes. Linewidths of the frequency peaks are found to increase significantly with decreasing Py layer thickness. Interfacial roughness causes to fluctuate exchange coupling at the nanoscale regimes and the effect is stronger for thinner Py films. A quantitative analysis of the magnon linewidths shows the presence of strong local exchange coupling field which is much larger compared to macroscopic exchange field. © 2014 AIP Publishing LLC.

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