Research Profile Anjan Barman Professor [email protected] • Two-Dimensional Magnonic Crystals • Development of Magnonic Waveguide • Spin Hall Effect • Magnetic vortex based transistor • Spin waves in Exchange Spring Bilayers We have been working in the fields of nanomagnetism, a. We demonstrated tunable spin wave spectrum in ultrafast magnetization dynamics, magnonics and spin Hall two-dimensional Ni80Fe20 nanodot arrays by varying effect. These are important and rapidly emerging research dot shape. A single collective mode in elliptical dot fields due to their potential applications in extremely fast array transforms into three distinct modes for the data read-write in magnetic recording as well as in spin- half-elliptical, rectangular and diamond dot lattices, transfer torque switching, all-magnetic logic devices and albeit with different peak frequencies and intensities. magnonic crystals for on-chip GHz to sub-THz frequency A drastic change is observed for the triangular dots, communication devices. In addition to big industrial where eight modes covering a broad band are observed. interest, which is craving for new technologies, there is a Using micromagnetic simulations, we characterized the huge fundamental interest in understanding and control modes as different localized, extended and quantized of the microscopic spin configurations and fundamental modes, whose frequencies and spatial profiles are spin excitations in magnetic thin films, multilayers, single determined by a combination of internal field profiles nanostructures and their ordered arrays. There have been a within the nanodots and the stray magnetic field within number of important and outstanding issues in these fields. the lattice. We have addressed some of the above issues in the year 2013-14. b. We further showed that the optically induced spin wave spectra of nanoscale Ni80Fe20 antidot lattices can Two-Dimensional Magnonic Crystals: We developed a be tuned by changing the antidot shape. The spin wave range of two-dimensional magnonic crystals based on spectra also showed an anisotropy with the variation patterned ferromagnetic nanodots and nano antidots of the in-plane bias field orientation. Analyses showed arranged in ordered lattices. These structures were this is due to various quantized and extended modes, nanofabricated by high vacuum thin film deposition and whose nature changes with the antidot shape and focused ion beam milling or electron beam lithography bias field orientation as a result of the variation of the in clean room. Primarily three types of structures were internal magnetic field profile. The observed variation studied this year, nanodots with different non-ellipsoidal and anisotropy in the spin waves with the internal shapes, nano antidots with different shapes - both arranged and external parameters are important for their in square lattices, and elliptical dots arranged in different applications in magnonic devices. lattices. 34 Department of Condensed Matter Physics and Material Sciences c. Tunable two-fold magnetic anisotropy in two- opening resulting from Bragg scattering or anti-crossing dimensional arrays of Ni80Fe20 elliptical elements of modes. We have shown, that the dipolar interactions arranged along their long (LA) or short axis (SA) start to assert their role in the spin wave spectrum are demonstrated from the measurement of time- when the waveguide is scaled up, but even for a period resolved magnetization dynamics. The anisotropy of few hundreds of nanometers the magnonic band field is maximum (minimum) when the elements structure preserves qualitatively the properties found are closely packed along their LA (SA) and attains an in the exchange dominating regime. intermediate value when they are well separated. Micromagnetic simulations revealed that the centre c. We showed that structural changes breaking the mirror mode of the ellipse shows the two-fold anisotropy and symmetry of the waveguide can close the magnonic that the variation in the anisotropy field stems from bandgap. But, the effect of these intrinsic symmetry the strong competition between the shape anisotropy breaking factors can be compensated by a properly of the constituent elements and the inter-element adjusted asymmetric external bias magnetic field, i.e., magnetostatic interaction fields within the arrays. by an extrinsic factor. This allows for the recovery of the magnonic bandgaps occurring in the ideal symmetric Development of Magnonic Waveguide: We have structure. The described methods can be used for numerically and experimentally studied magnonic developing parallel models for recovering bandgaps waveguides based on a thin Ni80Fe20 strip on which one- closed due to an intrinsic defect, e.g. a fabrication dimensional arrays of antidots are patterned. We studied defect. The model developed here is particular to the effects of varying different structural parameters and magnonics. However, the method of this development symmetry breaking and external bias field on the magnonic is squarely based upon the translational and mirror band structures including the magnonic band gap in these symmetries associated with a regular crystal structure. waveguides. Thus, we believe that this idea of correcting an intrinsic defect by extrinsic means, should be applicable to a. We explored the possibility of tuning the spin-wave spin-waves in both exchange and dipolar interaction band structure, particularly the bandgaps in a nanoscale regimes, as well as to electron, electromagnetic and magnonic antidot waveguide by varying the shape of acoustic waves in general. the antidots. We interpreted the observed variations by analyzing the equilibrium magnetic configuration Spin Hall Effect: We investigated the thickness dependence and the magnonic power and phase distribution of spin Hall angle (q ) in Co Fe /Pt bilayer using the spin- during spin-wave dynamics. The inhomogeneity in the SH 75 25 torque ferromagnetic resonance (ST-FMR) and modulation exchange fields at the antidot boundaries within the waveguide is found to play a crucial role in controlling of damping (MOD) measurement and compared the results the band structure at the discussed length scales. with those from the Ni80Fe20/Pt bilayers. From the ST-FMR measurements, we found that the intrinsic value of qSH b. We demonstrated that the magnonic band structure for Pt does not depend on the material and thickness of including the band gap of ferromagnetic antidot waveguide can be significantly tuned by a relatively the adjacent ferromagnetic layer. However, due to the large weak modulation of its structural parameters. The influence of some extrinsic effects such as inverse spin Hall investigations were performed with consideration of effect, the measured qSH value strongly depends on the both the exchange and dipolar interactions. For the ferromagnetic material and its layer thickness. The MOD exchange dominated regime we explored, in details, measurement gives higher value of qSH , which is independent the impact of the changes of the lattice constant, size on the ferromagnetic layer thickness but depends on the and shape of the antidots on the spin wave spectra. We have shown that precise choice of these parameters ferromagnetic material and the reason of this inconsistency is is crucial for achieving desired properties of antidot not very clear. We also found that the effective value of Gilbert waveguides, i.e., a large group velocity and filtering damping sharply decreases with the CoFe thickness probably properties due to existence of magnonic band gaps. due to a combination of the Co d3 -Pt 5d hybridization at the We discussed different mechanisms of magnonic gap interface and the spin pumping effect. 35 Research Profile Magnetic vortex based transistor: Transistors constitute perpendicular magnetic anisotropy. We will use magnetic the backbone of modern day electronics. Since their multilayers such as Co/Pd and Co/Pt, which shows a large advent, researchers have been seeking ways to make perpendicular magnetic anisotropy and hence large spin smaller and more efficient transistors. We demonstrated a wave frequencies as the base material for fabrication of sustained amplification of magnetic vortex core gyration in magnonic crystals based on nanoscale antidot lattices. coupled two and three vortices by controlling their relative We will study the effects of i) lattice constants, ii) antidot core polarities. This amplification is mediated by a cascade shape and iii) lattice symmetry on the magnonic spectra. of antivortex solitons travelling through the dynamic stray We will study how magnonic bands can be tuned by varying field. We further demonstrated that the amplification can internal fields resulting from the competition between the be controlled by switching the polarity of the middle vortex demagnetizing and anisotropy fields by varying the above in a three vortex sequence and the gain can be controlled parameters by TR-MOKE microscope and BLS spectroscopy by the input signal amplitude. An attempt to show fan– and interpret the results by plane-wave method and out operation yielded gain for one of the symmetrically micromagnetic simulations. placed branches which can be reversed by switching the core polarity of all vortices in the network. The above We will investigate the results of breaking of translational observations promote the magnetic vortices as suitable symmetry on the magnonic band structures in both in-plane candidates to work as stable bipolar junction transistors and