Creation and Modification of Perpendicular Magnetic Anisotropy in Thin Films and Multilayer for Possible Applications

Talapatra, A and Mohanty, Jyoti Ranjan (2018) Creation and Modification of Perpendicular Magnetic Anisotropy in Thin Films and Multilayer for Possible Applications. PhD thesis, Indian institute of technology Hyderabad.

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The thesis deals with the creation and control of perpendicular magnetic anisotropy (PMA) in technologically important material systems e.g. Co/Pt, Co/Pd multilayer (ML), Rare-earth (RE) transition-metal (TM) alloy films, heterostructure and magnetic bit patterned media (BPM) for high density (1T b/inch2 ) data storage application. Co/Pt ML (Si(substrate)/P t(1.3)/[Co(1.2)/P t(0.7)]×50/T a(2)nm) has been experienced where the PMA originates from the strong hybridization between Co (3d element) and Pt (5d elements) at the interface which results in strong spin-orbit coupling. The ML system shows labyrinth-like domain pattern of dimension 110(±2) nm. Local modification of the magnetic domains has been observed employing ultrashort laser pulse, where the tuning of magnetism is limited within the length scale of few µm. The area where the laser is focused shows the formation of disordered sub-domains that minimizes the dipolar energy. The demagnetized random domains appear at the centre of the laser spot and a rim is observed at the edge of the demagnetized area which signifies a deterministic switching. Rotation of domains at the central area with the application of small transverse field (100 mT) proves the region to be magnetically weaker. Micromagnetic simulation replicates the phenomenon by considering a spatial variation of magnetic anisotropy. The optimization of PMA has been studied in Gd19F e81 thin film, a member of RE-TM alloy family. The development of perpendicular anisotropy was observed in terms of stronger contrast of magnetic domains having a characteristic size of 138(±3) nm at the thickness of 100 nm. Global modification of magnetic domains and magnetic properties in amorphous Gd-Fe films has been studied with systematic rapid thermal processing (RTP) at various temperatures (250 oC to 550 oC), and with different time intervals viz., 2, 5, 10 and 20 minutes. The films processed at 250 oC for various time intervals show a successive reduction in magnetic phase contrast and domain size. The domain pattern completely disappeared, and topography dominated mixed magnetic phase has been obtained for the films processed at 450 oC for time intervals greater than 2 minutes. Finally, no stripe pattern was observed when the RTP temperature reached 550 oC and particulate growth is observed over the film surface. The magnetization measurements clearly indicate a reorientation of the magnetization direction from perpendicular to the plane of the surface. Depthresolved structural studies revealed the crystallization of Fe, predominantly nucleating at the film surface and the film-substrate reaction was evidenced at the interface at higher temperature due to the diffusion of Si into the film. As the magnetic moment of free Fe is more than that of Gd-Fe, thus saturation magnetization increases after RTP and hence PMA decreases. The experimental results were complemented by extensive micromagnetic simulations. After the spin re-orientation studies in continuous thin films, the experimental intricacies of patterning nanostructures are highlighted. The effect of experiment induced challenges have been modeled to study their impact on the performance of magnetic BPM. Micromagnetic study of variation of switching field distribution (SFD) in a high-density patterned media is presented as a function of magnetic anisotropy of the system. The study includes manifold effect of magnetic anisotropy in terms of its magnitude, tilt in anisotropy axis and random arrangements of magnetic islands with random anisotropy values. Calculation shows that reduction in anisotropy causes a linear decrease in coercivity because the anisotropy energy tries to align the spins along a preferred crystallographic direction. Tilt in anisotropy axis results in a decrease in squareness of the hysteresis loop and hence facilitates switching. Finally, the experimental challenges like lithographic distribution of magnetic viii islands, their orientation, the creation of defects, etc. demanded the distribution of anisotropy to be random along with random repetitions. It is explained that the range of anisotropy values and the number of bits with different anisotropy play the key role for determining the SFD, whereas the position of the bits and their repetitions do not show a considerable contribution. The final part of the thesis deals with magnetic skyrmion, a promising candidate for future generation ultra-high density data storage with an energy efficient data transfer. We have discussed the possible ways of scaling of skyrmion diameter inside magnetic nanostructure, possessing high PMA. A detailed study has been shown on the dependence of geometry and magnetic properties of the nanostructure over the phase transition from a single domain magnetic state to a multi-domain state via skyrmion configuration. Special emphasis has been given towards the understanding of the role of the defect, its threshold dimension and magnetic anisotropy to tune the skyrmion size. The transformation to an isolated skyrmion and multiple skyrmion lattice from a multi-domain state has been shown as a function of externally applied out-of-plane magnetic field and diameter of the nanostructure. Hence, the thesis provides a broader aspect towards the understanding of creation and control of PMA for the applications in high-density magnetic data storage.

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IITH Creators:
IITH CreatorsORCiD
Mohanty, Jyoti RanjanUNSPECIFIED
Item Type: Thesis (PhD)
Subjects: Physics
Divisions: Department of Physics
Depositing User: Team Library
Date Deposited: 01 Aug 2018 06:52
Last Modified: 21 Sep 2019 05:57
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