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Dileep Kumar

Publications and source records attributed to Dileep Kumar.

At least 19 recordsLinked to original sources
arXiv2025

Energy Control Strategy to Enhance AC Fault Ride-Through in Offshore Wind MMC-HVDC Systems

Modular Multilevel Converter-based High Voltage Direct Current (MMC-HVDC) system is a promising technology for integration of offshore wind farms (OWFs). However, onshore AC faults on MMC-HVDC reduce the power transfer capability of onshore converter station, leading to surplus power accumulation in HVDC link. This surplus power causes a rapid rise in DC-link voltage and may hinder safe operation of OWFs. To address such a situation, this paper presents an AC fault ride-through scheme that combines the storage of surplus power in MMC submodule (SM) capacitors and dissipation of residual power in an energy dissipation device (EDD). The proposed energy control facilitates use of half-bridge MMC SMs with low-capacitance, with their storage capacity leveraged to share the surplus power during faults, with a lower-rated EDD. The proposed scheme is tested on a 640kV/420MW MMC-HVDC system. The results show that proposed control scheme effectively maintains DC link voltages, ensuring connection of OWFs.

eess.SY↗
arXiv2025

Tailoring Magnetic Properties of Zigzag Structured Thin Films via Interface Engineering and Columnar Nano-structuring

We report the emergence of a novel interface-induced shape anisotropy component in zigzag-structured thin films fabricated via Sequential Oblique Angle Deposition (S-OAD). In this study, we systematically investigate cobalt (Co) and Co2FeAl (CFA) thin films by varying column length, number of bilayers, and magneto-crystalline anisotropy (MCA) to explore how structural modulation affects magnetic behavior. Using magneto-optical Kerr effect (MOKE) measurements in conjunction with synchrotron-based grazing-incidence small-angle X-ray scattering (GISAXS) and 2D X-ray diffraction (2DXRD), we reveal that the interplay between interface-induced, shape, and crystalline anisotropies allows for a tunable magnetic response, ranging from isotropic to anisotropic behavior. The observed uniaxial magnetic anisotropy (UMA) exceeds that of conventional OAD films, while column merging is effectively suppressed through precise multilayer engineering. Structural analysis confirms that periodic, high-density interfaces at the junctions of oppositely tilted columns are central to this anisotropy control. These findings demonstrate that interface engineering and columnar nanostructuring within zigzag nanostructures offer a powerful route for tailoring magnetic properties in zigzag thin films, enabling their application in next-generation spintronic and magnetic sensor technologies.

cond-mat.mtrl-sci↗
arXiv2025

Investigations of Metal-Organic structures for applications in Organic Spin Valve devices

Organic spintronics has drawn the interest of the science community due to various applications in spin-valve devices. But to date, an efficient room-temperature Organic Spin Valve device has not been experimentally realized due to the complicated spin transport at the metal-organic interfaces. These studies are always challenging due to the complicated spin-polarized charge transfer at the metal-organic interfaces. The present study focuses on a comprehensive understanding of the interfacial properties that are essential for advancing device performance and functionality. The Ferromagnetic metals and half-metallic electrodes such as Co, Co2FeAl, etc., and fullerene (C60) bilayer samples are prepared and studied via different structural and magnetic characterizations. Due to the mechanical softness of C60, deep penetration of ferromagnetic metal atoms is observed inside the C60 film. In-situ MOKE measurements reveal the origin of the 23 {\AA} thick magnetic dead layer at the interface, which is attributed to the diffused ferromagnetic clusters exhibiting superparamagnetic behavior. In contrast to the inorganic substrates, magnetic anisotropy tends to develop at 40 {\AA} thick Co film deposited on C60 which enhances with increasing thickness. The XRD measurements confirm the presence of in-plane compressive strain and texturing along the hcp (002) phase in the Co film. The anomaly observed in the hard axis of magnetization is due to high dispersion in the local magnetic anisotropy. The variation of the magnetic anisotropy axis is observed in the Co2FeAlwedge deposited in the proximity of C60. These findings provide valuable insights into the complex interplay between ferromagnetic materials and organic semiconductors offering potential avenues for tailoring magnetoresistance effects and fundamental understanding of organic spintronic devices.

cond-mat.mtrl-sci↗
arXiv2025

Investigation of Fe-Ag and Ag-Fe Interfaces in Ag-57Fe-Ag trilayer Using Nuclear Resonance Scattering under X-ray Standing Wave Conditions

Understanding the interfaces of layered nanostructures is key to optimizing their structural and magnetic properties for the desired functionality. In the present work, the two interfaces of a few nm thick Fe layer in Ag-57Fe-Ag trilayers are studied with a depth resolution of a fraction of a nanometer using x-ray standing waves (XSWs) generated by an underlying [W-Si]x10 multilayer (MLT) at an x-ray incident angle around the Bragg peak of the MLT. Interface selectivity in Ag-57Fe-Ag trilayers was achieved by moving XSW antinodes across the interfaces by optimizing suitable incident angles and performing depth-resolved nuclear resonance scattering (NRS) and X-ray fluorescence (XRF) measurements for magnetic and structural properties. The combined analysis revealed that the rms roughness of 57Fe-on-Ag and Ag-on-57Fe interfaces are not equal. The roughness of the 57Fe-on-Ag interface is 10 Angstrom, while that of the Ag-on-57Fe interface is 6 Angstrom. 57Fe isotope sensitive NRS revealed that hyperfine field (HFF) at both interfaces of 57Fe-on-Ag and Ag-on-57Fe interfaces are distinct, which is consistent with the difference in interface roughnesses measured as root mean square (RMS) roughness. Thermal annealing induces 57Fe diffusion into the Ag layer, and annealing at 325 C transforms the sample into a paramagnetic state. This behavior is attributed to forming 57Fe nanoparticles within the Ag matrix, exhibiting a paramagnetic nature. These findings provide deep insights into interface properties crucial for developing advanced nanostructures and spintronic devices.

cond-mat.mtrl-sci↗
arXiv2024

Depth-resolved Nuclear Resonance Scattering under X-ray standing wave -an approach to study interface magnetism

The isotope selective grazing-incidence nuclear resonance scattering (GI-NRS) technique is demonstrated to be depth-resolved under x-ray standing wave (XSW) conditions to probe the magnetism of the two interfaces of the Fe layers (Fe-on-Tb and Tb-on-Fe interface) independently in Tb/Fe/Tb trilayer structures. Depth resolution was achieved by placing an ultra-thin layer of 57Fe at both interfaces (Tb/57Fe/Fe/57FeTb). Intentionally, both 57Fe layers were assumed to have different hyperfine fields and orientations. Based on theoretical simulations, it is demonstrated that the antinode regions of XSWs generated through the W/Si multilayer structure allow one to independently measure the Fe-on-Tb and Tb-on-Fe interface at different incident angles. These theoretical simulations of NRS patterns at different incident angles correspond to 57Fe layers independently in the Tb/Fe/Tb trilayer. The present work shows the capability of combining XSW and GI-NRS to study buried magnetic interfaces in thin film structures.

cond-mat.mtrl-sci↗
arXiv2024

Buried Interfaces and Spin Orientation in [Co/Pt]10 /Fe multilayer with Orthogonal Magnetic Anisotropy: Effect of Fe Thickness

In the present work, spin orientation and variation of the strength of coupling in [Co/Pt]ML/Fe multilayer have been investigated as a function of the thickness of the Fe layer. [Co/Pt]ML/Fe multilayer has orthogonal anisotropy with the Fe layer, and [Co/Pt]ML has in-plane magnetic anisotropy and perpendicular anisotropy, respectively. Measurements are performed using in-situ magneto-optical Kerr effect (MOKE) and isotope-sensitive depth-resolved nuclear resonance scattering (NRS) technique. Real-time in-situ MOKE measurement during Fe growth reveals that with an increase in thickness of the Fe layer, moments of Fe layer reorientation from out of the plane to in-plane direction. This is attributed to the decrease in the coupling between the [Co/Pt]ML and Fe layer. For the depth-dependent study, two [Co/Pt]ML/Fe multilayers having the same thickness but different positions of the Fe57 marker layer ([Co/Pt]MLFe/Fe57) and [Co/Pt]MLFe57/Fe ) were studied using the NRS technique. Films with varying external magnetic fields were also studied to investigate coupling strength. Measurements were performed under the x-ray standing wave conditions to enhance resonance yield. It is observed that for the 75 {\AA} Fe in [Co/Pt]ML/Fe multilayer, the coupling varies along the depth of the Fe layer. The coupling is strong at [Co/Pt]ML and Fe interface with spins of the Fe layer aligned in the out-of-plane direction, whereas moments away from the interface are weakly coupled and aligned in-plane along the magnetic easy axis. Due to this gradient in strength of coupling along the depth, a large magnetic field is required to reorient spins at the interface along the magnetic hard axis of the Fe layer; however, spins away from the interface can rotate freely even in the low magnetic field.

cond-mat.mtrl-sci↗
arXiv2024

A Machine Learning Model for Solving Lane-Emden Equation using Legendre Wavelet Neural Network

As we know differential equations are very useful for electrical engineers to solve a variety of problems like: voltage across a capacitor, input versus output voltage, etc. Therefore, the goal of this paper is to find the solutions of non-linear differential equations based on the Lane Emden equation of second order using the Legendre wavelet neural network (LWNN) method. Here all the considered equations are singular initial value problems. To manage the singularity challenge, we have employed an artificial neural network method. This approach utilizes a neural network of a single layer, where the hidden layer is omitted by enlarging the input using Legendre wavelets functions. We have applied a feed-forward neural network method to the proposed problem along with the principle of error backpropagation. The effectiveness of the Legendre wavelet Neural Network method is validated through Lane Emden equations..

math.NA↗
arXiv2024

Growth-Induced Unconventional Magnetic Anisotropy in Co/Fullerene (C60) Bilayer Systems; Insights from a Two-Grain Stoner-Wohlfarth Model

Organic spintronics has drawn the interest of the science community due to various applications in spin-valve devices. However, an efficient room-temperature Organic Spin Valve device has not been experimentally realized due to the complicated spin transport at the metal-organic interfaces. The present study focuses on a comprehensive understanding of the interfacial properties essential for advancing device performance and functionality. The structural and magnetic properties of the ultra-thin Cobalt (Co) films deposited on the fullerene (C60) layer are studied to investigate the origin of magnetic anisotropy in the metal-organic bilayer structures. Due to the mechanical softness of C60, penetration of ferromagnetic Co atoms inside the C60 film is confirmed by the X-ray reflectivity and Secondary Ion Mass Spectroscopy measurements. Grazing incidence small-angle X-ray scattering and atomic force microscopy provided information regarding the structural and morphological properties of the Co/C60 bilayers, angular dependent Magneto-optic Kerr effect measurements with varying Co layer thickness provided information about the growth-induced uniaxial magnetic anisotropy. In contrast to the inorganic silicon substrates, magnetic anisotropy in Co film tends to develop at 25 {\AA} thickness on the C60 layer, which further increases with the thickness of Cobalt. The anomalous behavior in coercivity and remanence variation along the nominal hard axis is explained by a two-grain Stoner-Wohlfarth model with intergranular exchange coupling. It is further confirmed by a non-uniform spatial distribution of magnetic domains investigated through Kerr microscopy. These anomalies could be attributed to the distribution of magneto-crystalline anisotropy and inhomogeneous strain caused by the formation of a diffused layer at the Co/C60 interface.

cond-mat.mtrl-sci↗
arXiv2024

Evolution of Interface Magnetism in Fe/Alq3 Bilayer Structure; Thickness-Dependent Interface Resolved Studies Under X-Ray Standing Wave

In the present work, interfacial magnetism at metal organic interface is probed using an isotope sensitive interface resolved nuclear resonance scattering technique which is made depth selective under x-rays standing wave conditions. Using GIWAXS and GINRS measurements, this study evidences the presence of symmetry-based PMA which appears at a lower thickness of Fe having distortion in cubic symmetry and disappears at a higher thickness of Fe as its cubic symmetry retains. The non-zero value of quadrupole splitting evidences the strain at the interfacial region which on increasing thickness of Fe relaxes. The diffusion of Fe is traced using XRF and NRR, deep penetration of Fe in Alq3 layer due to soft nature of the organic film is obtained. This thickness-dependent study enables us to understand the magnetic behavior of buried ferromagnetic metal in the vicinity of organic molecules.

cond-mat.mtrl-sci↗
arXiv2024

Role of stress/strain in tailoring the magnetic and transport properties of magnetic thin films and multilayers

Magnetic anisotropy is a fundamental property of magnetic materials that determines the alignment of the spins along the preferential direction, called the easy axis of magnetization. Magnetic polycrystalline thin films offer several advantages over magnetic epitaxial thin films because of fabrication flexibility, higher coercivity and improved magnetic stability, higher magnetoresistance (useful in magneto-resistive devices such as magnetic field sensors and MRAM cells), cost-effectiveness and thermal stability, etc. In the case of polycrystalline thin films or multilayers, Magneto-crystalline anisotropy (MCA) is not expected due to the random orientation of grains. Therefore, understanding the origin of uniaxial magnetic anisotropy (UMA) is generally difficult and can't be understood in terms of crystal orientation. The origin of UMA in polycrystalline films is often related to the preparation conditions and substrate properties. In the present thesis, we have provided direct in-situ real-time evidence of the stress dependence of magnetic anisotropy through the multibeam optical stress sensor (MOSS) technique. Also, we have tuned the magnetic anisotropy in strength and direction using externally applied stress. To further increase the strength of the magnetic anisotropy, we have developed a new technique that creates a multilayer using a single magnetic material through sequential oblique and normal depositions. This oblique angle deposition technique also helps reduce the penetration of the top ferromagnetic layer inside the organic semiconductor layer in organic spin valve structures. We confirm our results through various in-situ (in UHV and HV) and ex-situ temperature-dependent conventional and unconventional structural, morphological, and magnetic measurements (both lab-based and synchrotron-based) that include MOKE, KERR, GIXRD, AFM, RHEED, and GISAXS, etc. measurements.

cond-mat.mtrl-sci↗
arXiv2024

Mesh-free mixed finite element approximation for nonlinear time-fractional biharmonic problem using weighted b-splines

In this article, we propose a fully-discrete scheme for the numerical solution of a nonlinear time-fractional biharmonic problem. This problem is first converted into an equivalent system by introducing a new variable. Then spatial and temporal discretizations are done by the weighted $b$-spline method and $L2$-$1_\sigma$ approximation, respectively. The weighted $b$-spline method uses weighted $b$-splines on a tensor product grid as basis functions for the finite element space and by construction, it is a mesh-free method. This method combines the computational benefits of $b$-splines and standard mesh-based elements. We derive $\alpha$-robust \emph{a priori} bound and convergence estimate in the $L^2(\Omega)$ norm for the proposed scheme. Finally, we carry out few numerical experiments to support our theoretical findings.

math.NA↗
arXiv2023

Asymmetric magnetism at the interfaces of MgO/FeCoB bilayers by exchanging the order of MgO and FeCoB

Interfaces in FeCoB/MgO/FeCoB magnetic tunnel junction play a vital role in controlling their magnetic and transport properties for various applications in spintronics and magnetic recording media. In this work, interface structures of a few nm thick FeCoB layers in FeCoB/MgO and MgO/FeCoB bilayers are comprehensively studied using x-ray standing waves (XSW) generated by depositing bilayers between Pt waveguide structures. High interface selectivity of nuclear resonance scattering (NRS) under the XSW technique allowed measuring structure and magnetism at the two interfaces, namely FeCoB-on-MgO and MgO-on-FeCoB, yielding an interesting result that electron density and hyperfine fields are not symmetric at both interfaces. The formation of a high-density FeCoB layer at the MgO/FeCoB (FeCoB-on-MgO) interface with an increased hyperfine field (~34.65 T) is attributed to the increasing volume of FeCo at the interface due to boron diffusion from 57FeCoB to the MgO layer. Furthermore, it caused unusual angular-dependent magnetic properties in MgO/FeCoB bilayer, whereas FeCoB/MgO is magnetically isotropic. In contrast to the literature, where the unusual angular dependent in FeCoB based system is explained in terms of in-plane magnetic anisotropy, present findings attributed the same to the interlayer exchange coupling between bulk and interface layer within the FeCoB layer.

cond-mat.mtrl-sci↗
arXiv2023

Engineering magnetic anisotropy and the surface of epitaxial Fe films using ion beam erosion; unveiling self-assembly and tunability

The engineering of surface morphology and structure of the thin film is one of the essential technological assets for regulating the physical properties and functionalities of thin film-based devices. This study investigates the evolution of surface structure and magnetic anisotropy in epitaxially grown ultrathin Fe films on MgO (001) substrates subjected to multiple cycles of ion beam erosion (IBE) after growth. Ultrathin Fe film grows in 3D island mode and exhibits intrinsic fourfold magnetic anisotropy. After a few cycles of IBE, the film displays an induced uniaxial magnetic anisotropy that leads to a split in the hysteresis loop. In addition, clear and conclusive evidence of IBE mediated (2x2) reconstruction of the Fe surface has been observed. We also demonstrate that thermal annealing can reversibly tune the induced UMA and surface reconstruction. The feasibility of the IBE technique by properly selecting ion beam parameters for modification of surface structure has been highlighted apart from conventional methods of tailoring the morphology for tuning of UMA. Thus, the present work paves a way to explore the IBE-induced self-assembling phenomena further.

cond-mat.mtrl-sci↗
arXiv2023

In-situ Growth of Ultrathin Magnetic Films and Tuning the Magnetic Properties by Ion-sculpting

Magnetic anisotropy is a key parameter of magnetic materials as it decides the response in the presence of an external magnetic field. The artificial tailoring of magnetic anisotropy by manipulating surface and interface morphology is attracting widespread interest for its application in spintronic and magnetic memory devices. In this perspective, the primary focus of this thesis has been inducing and tailoring magnetic anisotropy in thin film-based systems via the engineering of structure and surface/interfacial morphology. For this purpose, instead of multistep, expensive lithographic techniques, we have utilized low-energy IBE as a handy, cost-effective, and useful tool for producing magnetic nanostructures and tailoring its surface structure, morphology and magnetic anisotropy. Furthermore, we have also proposed various new and unconventional methods (oblique angle deposition on the rippled substrate, sequential deposition-erosion) for enhancement of UMA and proved its effectiveness via experiments. We have performed most of the present thesis work in-situ utilizing a UHV chamber to get genuine characteristics and understand their interdependencies. However, advanced synchrotron-based techniques such as GISAXS and NFS were also utilized to complement the in-situ observations.

cond-mat.mtrl-sci↗
arXiv2023

Significantly increased magnetic anisotropy in Co nano-columnar multilayer structure via a unique sequential oblique-normal deposition approach

Oblique/normal sequential deposition technique is used to create Co based unique multilayer structure [Co-oblique(4.4nm)/Co-normal (4.2 nm)]x10, where each Co-oblique layer is deposited at an oblique angle of 75deg, to induce large in-plane uniaxial magnetic anisotropy (UMA). Compared to the previous ripple, stress and oblique angle deposition (OAD) related studies on Cobalt in literature, one-order higher UMA with the easy axis of magnetization along the projection of the tilted nano-columns in the multilayer plane is observed. The multilayer retains magnetic anisotropy even after annealing at 450C. The in-plane UMA in this multilayer is found to be the combination of shape, and magneto-crystalline anisotropy (MCA) confirmed by the temperature-dependent grazing incidence small angle X-ray scattering (GISAXS), in situ reflection high energy electron diffraction (RHEED) and grazing incidence X-ray diffraction (GIXRD) measurements. The crystalline texturing of hcp Co in the multilayer minimizes spin-orbit coupling energy along the column direction, which couples with the shape anisotropy energies and results in preferential orientation of the easy magnetic axis along the projection of the columns in the multilayer plane. Reduction in UMA after annealing is attributed to diffusion/merging of columns and annihilating crystallographic texturing. The obtained one-order high UMA demonstrates the potential application of the unique structure engineering technique, which may have far-reaching advantages in magnetic thin films/multilayers and spintronic devices.

cond-mat.mtrl-sci↗
arXiv2023

Evolution of interface magnetism in Fe/Alq3 bilayer

Interface magnetism and topological structure of Fe on organic semiconductor film (Alq3) have been studied and compared with Fe film deposited directly on Si (100) substrate. To get information on the diffused Fe layer at the Fe/Alq3 interface, grazing incident nuclear resonance scattering (GINRS) measurements are made depth selective by introducing a 95% enriched thin 57Fe layer at the Interface and producing x-ray standing wave within the layered structure. Compared with Fe growth on Si substrate, where film exhibits a hyperfine field value of 32 T (Bulk Fe), a thick Fe- Alq3 interface has been found with reduced electron density and hyperfine fields providing evidence of deep penetration of Fe atoms into Alq3 film. Due to the soft nature of Alq3, Fe moments relax in the film plane. At the same time, Fe on Si has a resultant ~43 deg out-of-plane orientation of Fe moments at the Interface due to the stressed and rough Fe layer near Si. The evolution of magnetism at the Fe-Alq3 Interface is monitored using in-situ magneto-optical Kerr effect (MOKE) during the growth of Fe on the Alq3 surface and small-angle x-ray scattering (SAXS) measurements. It is found that the Fe atom tries to organize into clusters to minimize their surface/interface energy. The origin of the 2.4 nm thick magnetic dead layer at the Interface is attributed to the small Fe clusters of paramagnetic or superparamagnetic nature. The present work provides an understanding of interfacial magnetism at metal-organic interfaces and the topological study using the GI-NRS technique, which is made depth selective to probe magnetism of the diffused ferromagnetic layer, which is otherwise difficult for lab-based techniques.

cond-mat.mtrl-sci↗
arXiv2023

Study of magnetism in MgO/FeCoB/MgO trilayers using x-ray standing wave techniques

Interfaces in the MgO-FeCoB-MgO trilayer have been studied with grazing incident nuclear resonance scattering (GINRS) using the x-ray standing waves (XSW) technique. High depth selectivity of the present method allows one to measure magnetism and structure at the two interfaces of FeCoB, namely, FeCoB-on-MgO and MgO-on-FeCoB, independently, yielding an intriguing result that both interfaces are not symmetric. A high-density layer with an increased magnetic hyperfine field at the FeCoB-on-MgO interface suggests different growth mechanisms at the two interfaces. The azimuthal angle-dependent magneto-optic Kerr effect measurements reveal the presence of unusual uniaxial magnetic anisotropy (UMA) in the trilayer. An in-situ temperature-dependent study discovered that this UMA systematically reduces with temperature. The trilayer becomes isotropic at 450C with an order-of-magnitude increase in coercivity. The asymmetry at the interfaces is, in turn, explained by boron diffusion from the FeCoB interface layer into the nearby MgO layer. Stress-induced UMA is observed in the boron-deficient FeCoB layer, superimposed with the bulk FeCoB layer, and found to be responsible for unusual UMA. The temperature-dependent variation in the UMA and coercivity can be understood in terms of variations in the internal stresses and coupling between FeCoB bulk and the interface layer.

cond-mat.mtrl-sci↗