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We report a systematic study on the anisotropy symmetry, magnetic domains and magnetic relaxation behavior in Co thin films deposited on MgO (001) substrate by varying (i) the pre-annealing condition and (ii) the speed of substrate rotation during deposition. Substrate annealing prior to deposition leads to the formation of textured thin films. On contrary Co films prepared without substrate pre-annealing exhibit polycrystalline nature. Surface topography imaged by atomic force microscopy (AFM) depicts a profound effect of growth condition on grain size and its distribution. Magnetic hysteresis measurement along with simultaneous domain imaging has been performed by magneto optic Kerr effect (MOKE) based microscope by varying the angle (ϕ) between the easy axis and the direction of applied magnetic field. We observed the existence of cubic and uniaxial anisotropy due to the presence of substrate annealing and oblique angular deposition, respectively. Along the easy axis, magnetization reversal is governed by 180° domain wall motion via branched domains. However, for easy axis

Materials suitable for magnonic crystals demand low magnetic damping and long spin wave (SW) propagation distance. In this context Co based Heusler compounds are ideal candidates for magnonic based applications. In this work, antidot arrays (with different shapes) of epitaxial $\mathrm{Co}_2\mathrm{Fe}_{0.4}\mathrm{Mn}_{0.6}\mathrm{Si}$ (CFMS) Heusler alloy thin films have been prepared using e-beam lithography and sputtering technique. Magneto-optic Kerr effect and ferromagnetic resonance analysis have confirmed the presence of dominant cubic and moderate uniaxial magnetic anisotropies in the thin films. Domain imaging via x-ray photoemission electron microscopy on the antidot arrays reveals chain like switching or correlated bigger domains for different shape of the antidots. Time-resolved MOKE microscopy has been performed to study the precessional dynamics and magnonic modes of the antidots with different shapes. We show that the optically induced spin-wave spectra in such antidot arrays can be tuned by changing the shape of the holes. The variation in internal field profiles, pinning energy barrier, and anisotropy modifies the spin-wave spectra dramatically within the antidot arrays with different shapes. We further show that by combining the magnetocrystalline anisotropy with the shape anisotropy, an extra degree of freedom can be achieved to control the magnonic modes in such antidot lattices.

Skyrmion racetrack memory has a lot of potential in future non-volatile solid state devices. By application of current in such devices, both spin-orbit torque and spin-transfer torques are proven to be useful to nucleate and propagate skyrmions. However, the current applied during nucleation of successive skyrmions may have unwanted perturbation viz. Joule heating and the skyrmion Hall effect, on the propagation of previously generated skyrmions. Therefore, new methodology is desired to decouple the generation and propagation of skyrmions. Here, we present a novel route via micromagnetic simulations for generation of skyrmions from triangular antidot structure in a ferromagnetic nanotrack using local Oersted field. Antidots are holes in a magnetic nanoelement. Multiple skyrmions can be simultaneously generated by incorporating a greater number of antidots. Controlled skyrmion injection can be achieved by tuning the separation between the antidots that are placed at either end of the nanotrack. Here, we propose a novel design to realise skyrmionic racetrcak memory, where one can individually generate and manipulate the skyrmions within the nanotrack.

Magnetization relaxation and surface morphology have been studied for Co thin films prepared by sputtering with oblique deposition and by varying the substrate rotation (ω) at 0, 10 and 20 rpm. The films prepared with ω = 10 and 20 rpm during deposition exhibit high dispersion of local grain anisotropy. The average grain size for the samples prepared with ω = 10, 20 rpm is smaller than that of the sample prepared with ω = 0 rpm. Uniform distribution of grains is observed for the sample prepared with ω = 10 rpm. However, the sample prepared with ω = 20 rpm revealed the formation of non-uniformly distributed island like structures due to the lack of sufficient time for the adatoms to relax. Magnetization relaxation for all the samples was performed using magneto-optic Kerr effect (MOKE) based microscopy in the longitudinal mode. Sample prepared with ω = 0 rpm exhibits slow relaxation due to nucleation followed by fast domain wall motion. On the contrary, for the sample prepared with ω = 10 rpm, uniform grains and high exchange interaction leads to fast relaxation. However, the relaxation is slow in the sample prepared with ω = 20 rpm due to decrease in exchange energy arising from non-uniformity of grains.

The topic of magnetic antidot lattice (MAL) arrays has drawn attention from both fundamental research as well as from application point of view. MAL arrays are promising candidates for making domain engineering in thin films. For various applications it is necessary to understand the magnetization reversal mechanism as well as the relaxation dynamics. In this context we have studied magnetic antidot lattice (MAL) arrays of Co/Pt with perpendicular anisotropy fabricated by combination of photolithography and sputtering deposition. Kerr microscopy domain imaging for the continuous thin film reveals the formation of typical bubble domains of perpendicular media with high anisotropy. However, presence of periodic holes in the MAL arrays lead to nucleation of localised smaller bubbles. We have performed simulations using object oriented micromagnetic framework (OOMMF) which reproduced the experimental results even considering antidot arrays in nano dimension. In literature it has been reported that in MAL arrays with in-plane anisotropy the domain propagation gets significantly hindered by the presence of the holes. However here we show that in perpendicularly magnetized Co/Pt the propagation of the domain walls is not restricted by the presence of the antidots. Further we have performed magnetic relaxation study and found that the global relaxation time for the MAL arrays of Co/Pt is faster as compared to it’s parent thin film. This behavior is opposite to what has been observed in literature for in-plane magnetized MAL arrays.

We have performed micromagnetic simulations to study the formation of skyrmions in ferromagnetic elements with different shapes having perpendicular anisotropy. The strength of Dzyaloshinskii-Moriya interaction (D) and uniaxial anisotropy (K) are varied to elucidate the regime in which skyrmion formation can take place. It is found that for a certain combination of D and K skyrmion formation does not happen. Further we also observed that for large D and small K values, finite size effect dominates which in turn hinders formation of typical N'eel (spherical) skyrmions. However the resulting magnetic phase is skyrmionic in nature and has different shape. We also have found that the shape of the magnetic nano element has a significant role in determining the final magnetic state in addition to the competing D and K values., 7 pages, 7 figures

In this paper, we study the domain wall dynamics in high magnetic flux density (–1.85 T) of soft magnetic ribbons Fe81.2Co4Si0.5B9.5P4Cu0.8 prepared with various annealing conditions. It is observed that annealing these ribbons beyond a temperature (C) crystalizes the alloy with the appearance of α-Fe(-Co) phase. The post annealing leads to a change from amorphous to nanocrystalline phase accompanied by a reduction in internal stress in the ribbons. Domain imaging has been performed under both dc and ac fields to study the domain wall behavior in these ribbons. A wide variety of domain structures are observed in ribbons prepared with different annealing temperature as a result of the change in the intrinsic anisotropy. Under an ac magnetic field, the 'stress patterns' in the amorphous ribbons are robust. However, ribbons with reduced stress (i.e. nanocrystalline) exhibit domain wall propagation which hints at a significant reduction in magnetocrystalline and magnetoelastic anisotropies due to uniform nanocrystallization. The ribbon annealed at C exhibits static domain patterns resulting from strong pinning centers created by Fe-B precipitates.