Sougata Mallick

Effect of substrate rotation on domain structure and magnetic relaxation in magnetic antidot lattice arrays

Microdimensional triangular magnetic antidot lattice arrays were prepared by varying the speed of substrate rotation. The pre-deposition patterning has been performed using photolithography technique followed by a post-deposition lift-off. Surface morphology taken by atomic force microscopy depicted that the growth mechanism of the grains changes from chain like formation to island structures due to the substrate rotation. Study of magnetization reversal via magneto optic Kerr effect based microscopy revealed reduction of uniaxial anisotropy and increase in domain size with substrate rotation. The relaxation measured under constant magnetic field becomes faster with rotation of the substrate during deposition. The nature of relaxation for the non-rotating sample can be described by a double exponential decay. However, the relaxation for the sample with substrate rotation is well described either by a double exponential or a Fatuzzo-Labrune like single exponential decay, which increases in applied field.

Magnetic domain imaging in L10 ordered FePt thin films with in-plane uniaxial magnetic anisotropy

Epitaxial FePt thin films with various thicknesses, having in-plane uniaxial magnetic anisotropy, have been prepared at different deposition temperatures using sputtering technique. L10 ordering in the FePt thin film leads to very large uniaxial magnetocrystalline anisotropy. The magnetocrystalline anisotropy energy increases with the increase in film thickness and deposition temperature. Domain imaging performed by Magneto-optic Kerr microscopy with a longitudinal geometry suggests that the domain structure and magnetization reversal process strongly depend on the film thickness and the deposition temperature. For the films deposited at a certain temperature, there exists a critical thickness where the coercive and saturation fields show maxima.

Relaxation dynamics in magnetic antidot lattice arrays of Co/Pt with perpendicular anisotropy

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.

Size and shape of skyrmions for variable Dzyaloshinskii–Moriya interaction and uniaxial anisotropy

We have performed micromagnetic simulations to study the formation of skyrmions from an initial single bubble domain state 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 the formation of typical Neel (spherical) skyrmions. However, the resulting magnetic phase is skyrmionic in nature and has different shapes. 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.