Sarjoosing Goolaup
Email Address
elesg@nus.edu.sg
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Publication Partial frequency band gap in one-dimensional magnonic crystals(2008) Kostylev, M.; Schrader, P.; Stamps, R.L.; Gubbiotti, G.; Carlotti, G.; Adeyeye, A.O.; Goolaup, S.; Singh, N.; ELECTRICAL & COMPUTER ENGINEERINGCollective spin wave modes propagating in an array of magnetic stripes coupled by dynamic dipole interaction are investigated by Brillouin light scattering. It is demonstrated that this structure supports propagation of discrete spin waves at any angle with respect to the stripes length. The data are interpreted using a theoretical model based on the Bloch wave approach. It is shown that, due to the one-dimensional artificial periodicity of the medium, the gaps in the spin wave spectrum are partial: the frequency passbands for propagation along the direction of periodicity overlap with the stop bands for propagation along the stripes. © 2008 American Institute of Physics.Publication Magnetostatic coupling in arrays of elongated Ni80Fe 20 rings(2007-11-07) Adeyeye, A.O.; Goolaup, S.; Singh, N.; Wang, C.C.; Gao, X.S.; Ross, C.A.; Jung, W.; Castão, F.J.; ELECTRICAL & COMPUTER ENGINEERINGWe describe the effects of magnetostatic interactions and film thickness on the switching properties and reversal mechanisms of periodic arrays of elongated Ni80Fe20 thin film rings. The magnetization reversal process, the switching field distributions and the transition fields between different equilibrium states are strongly affected by the inter-ring spacing. For closely packed ring arrays, sharp transitions were observed from the onion-to-vortex state due to collective magnetization reversal of the rings. In agreement with micromagnetic simulations, the range of stability of the vortex state is smaller for closely packed ring arrays compared with isolated rings of similar lateral dimensions. © 2007 IOP Publishing Ltd.Publication FePt patterned media fabricated by deep UV lithography followed by sputtering or PLD(2007-06) Qiu, L.J.; Ding, J.; Adeyeye, A.O.; Yin, J.H.; Chen, J.S.; Goolaup, S.; Singh, N.; ELECTRICAL & COMPUTER ENGINEERING; PHYSICS; MATERIALS SCIENCE AND ENGINEERINGContinuous and patterned FePt films (40 nm) were fabricated on silicon (100) substrates using deep ultraviolet lithography with the wavelength of 248 nm followed by sputter deposition or pulsed laser deposition at room temperature, liftoff, and postannealing in vacuum. The structures and magnetic properties of the films were characterized by X-ray diffractometry, X-ray photoelectron spectroscopy, scanning electron microscopy, and alternating gradient force magnetometry. A buffer layer of Ag or MgO was inserted between the Si substrate and FePt to prevent a chemical reaction between Si and FePt. The phase transformation from face-centered cubic to face-centered tetragonal started after annealing at 400°C for continuous films, while for the patterned FePt films, phase formation was retarded. High coercivities of 10-15 kOe have shown potential in hard magnetic applications. The effects of Ag and MgO top layers on the coercivities were also investigated. © 2007 IEEE.Publication Flux closure configuration in ferromagnetic diamond-shaped nanomagnets(2005) Goolaup, S.; Singh, N.; Adeyeye, A.O.; ELECTRICAL & COMPUTER ENGINEERINGPublication Micromagnetics of derivative ring-shaped nanomagnets(2007-03-12) Singh, N.; Goolaup, S.; Tan, W.; Adeyeye, A.O.; Balasubramaniam, N.; ELECTRICAL & COMPUTER ENGINEERINGWe present a systematic investigation of the magnetization reversal process in derivative ring-shaped nanomagnets. As the transition from onion to vortex state in magnetic rings is through the displacement of the domain wall along the circumference of the ring, we have investigated the effect of removing different segments of the ring structure on the magnetization reversal processes. We observed that the magnetic spin configuration is strongly influenced by the axis that is removed, as confirmed by the magnetic force microscopy studies. Our understanding of the reversal process was aided using micromagnetic simulations, which are in very good agreement with experimental results. © 2007 The American Physical Society.Publication Magnetotransport behavior of Co nanowires coupled to Ni80Fe 20 films(2010) Goolaup, S.; Adeyeye, A.O.; Singh, N.; ELECTRICAL & COMPUTER ENGINEERINGWe have fabricated and characterized hybrid-trilayer ferromagnetic structures comprising of 60-nm-thick Ni80Fe20 (Py) film grown on top of patterned array of 60-nm-thick Co nanowires (NWs) with Cu spacer layer. The hybrid structures display unique magnetic and transport properties due to the nonuniform magnetization distribution from the Py layer inducing local stray fields, which could be coupled to the underlying Co NWs. The temperature dependence of the magnetoresistance behavior is dominated by the interplay between the anisotropic magnetoresistance and the giant magnetoresistance effects. © 2010 American Institute of Physics.Publication Magnetic properties of diamond shaped Ni80Fe20 nanomagnets(2005-08-21) Goolaup, S.; Adeyeye, A.O.; Singh, N.; ELECTRICAL & COMPUTER ENGINEERINGWe investigate the magnetic properties of Ni80Fe20 diamond-shaped nanomagnets as a function of the film thickness (t) in the range of 10-120 nm. The samples were fabricated using deep ultraviolet lithography at 248 nm exposure wavelength. We observed that for t ≥ 30 nm, the diamond-shaped structure exhibits flux closure configuration at remanence. A systematic thickness-dependent study reveals that the demagnetizing field across the plane of the structure strongly influences the reversal process. Our experimental results were substantiated by micro-magnetic simulations. © 2005 IOP Publishing Ltd.Publication Field dependence of collective spin modes in transversely magnetized stripes with homogeneous and alternating width(2009) Tacchi, S.; Madami, M.; Gubbiotti, G.; Carlotti, G.; Goolaup, S.; Adeyeye, A.O.; Nguyen, H.T.; Cottam, M.G.; ELECTRICAL & COMPUTER ENGINEERINGBrillouin light scattering is used to study the spin-wave properties of arrays of closely spaced magnetic stripes with both homogeneous and alternating width, thickness of 30 nm, and edge-to-edge spacing of 55 nm. Experiments are performed with both the transferred wave vector and the applied magnetic field in the direction transverse to the length of the stripes, i.e., in the hard magnetization direction. When the transverse applied field is decreased from a relatively large value (0.15 T) down to zero, the overall direction of the stripe magnetization gradually switches from the transverse to the preferred longitudinal orientation, as confirmed by the shape of the Kerr hysteresis loops. The measured frequencies were compared to numerical calculations obtained for Permalloy stripe arrays by a microscopic dipole-exchange theory. A good overall agreement is achieved between experiment and theory, thanks to the inclusion of the dynamical interstripe dipolar coupling. © 2009 American Institute of Physics.Publication Inhomogeneities in spin states and magnetization reversal of geometrically identical elongated Co rings(2007) Gao, X.S.; Adeyeye, A.O.; Goolaup, S.; Singh, N.; Jung, W.; Castaño, F.J.; Ross, C.A.; ELECTRICAL & COMPUTER ENGINEERING; SINGAPORE-MIT ALLIANCEThe magnetic configurations and magnetic reversal processes in arrays of geometrically identical rounded rectangular Co rings have been investigated. Magnetic imaging reveals a range of configurations, including diagonal onion, horseshoe onion, and vortex states. Reversal from the onion to the vortex state can occur via different routes involving domain wall motion within the rings, and the mechanism depends on the applied field orientation. © 2007 American Institute of Physics.Publication Magnetic anisotropy in magnetostatically coupled Ni 80Fe 20 nanowires(2004) Goolaup, S.; Singh, N.; Adeyeye, A.O.; Ng, V.; Jalil, M.B.A.; ELECTRICAL & COMPUTER ENGINEERINGWe have investigated the magnetic property of magnetostatically coupled 80nm Ni 80Fe 20 nanowires fabricated using KrF lithography exposure tools, electron beam deposition and lift-off. The variation of coercivity and squareness as a function of angle was investigated. We observed that both the coercivity and the squareness are strongly dependent on the orientation of the magnetic field relative to the wire easy axis. We observed that shape anisotropy dominates the reversal processes. © 2004 IEEE.