Rawat, Rajdeep Singh

The NX2 device, a low energy plasma focus, at the Nanyang Technological University in Singapore, was used as a soft X-ray (SXR) source for micromachining. The gas used was neon which produced SXRs in a narrow spectral range of 0.9 - 1.6 keV. The SXR yield from repetitive operation of the NX2 device was monitored and measured using a cost effective multi-channel SXR spectrometric system. The system consists of filtered BPX65 PIN diodes, with the associated electronics --- an integrator, sample and peak holder, analogue switch, an A/D converter and a microcontroller. The system enables easy shot-to-shot statistical analysis under repetitive operation at adjustable preset trigger frequencies. A total of 4000 shots were fired at 0.5 Hz, using the same gas filling. The SXR production was at an average yield of 60 J/shot and a maximum single-shot yield of more than 100 J. The SXRs emitted by the NX2 device was used for contact micromachining, producing structures with an excellent aspect ratio of up to 20:1 on 25 μm SU-8 resist.

A 3 kJ plasma focus was operated with a 3He-D2 gas mixture, with partial pressures in the ratio of 2:1, corresponding to an atomic number ratio of 1:1 for 3He and D atoms. The fusion reactions D(3He,p)4He and D(d,p)3H were measured simultaneously using CR-39 polymer nuclear track detectors placed inside a pinhole camera positioned on the forward plasma focus axis. A sandwich arrangement of two 1000 μm thick CR-39 detectors enabled the simultaneous registration of two groups of protons with approximate energies of 16 MeV and 3 MeV arising from the D(3He,p)4He and D(d,p)3H reactions, respectively. Radial track density distributions were obtained from each CR-39 detector and per-shot average distributions were calculated for the two groups of protons. It is found that the D(3He,p)4He and D(d,p)3H proton yields are of similar magnitude. Comparing the experimental distributions with results from a Monte Carlo simulation, it was deduced that the D(3He,p)4He fusion is concentrated close to the plasma focus pinch column, while the D(d,p)3H fusion occurs relatively far from the pinch. The relative absence of D(d,p)3H fusion in the pinch is one significant reason for concluding that the D(3He,p)4He fusion occurring in the plasma focus pinch is not thermonuclear in origin. It is argued that the bulk of the D(3He,p)4He fusion is due to energetic 3He2+ ions incident on a deuterium target. Possible explanations for differing spatial distributions of D(3He,p)4He and D(d,p)3H fusion in the plasma focus are discussed.

Dense plasma focus (DPF) device is conventionally operated in focus mode, to achieve pinch plasma with highest possible temperature and density to maximize the soft and hard x-rays and charged particles. In this paper, we report the first ever application of non-focus mode of DPF device, which is free of magneto-hydrodynamics (MHD) instabilities, for the deposition of zinc oxide thin films (ZnO TFs) on silicon substrates for various number (5, 10, 15 and 20) of non-focused deposition shots (NFDS). The X-ray diffraction (XRD) patterns of as-deposited ZnO TFs confirms the growth along (0 0 2) orientation only. The ZnO TFs are then annealed at 600 °C temperature for 2 h. The XRD patterns of annealed ZnO-TFs confirm the wurtzite phase of ZnO with (1 0 0), (0 0 2) and (1 0 0) planes with improved crystallinity. The up and down shifting of ZnO (0 0 2) diffraction plane indicates the presence of residual stresses which are reduced in annealed ZnO TF. The surface morphology, like shape, size and the distribution of rounded nano-particles, is strongly associated with increasing number of NFDS. Raman analysis shows the development of downshifted E2 (high) and upshifted A1 longitudinal optical (LO) modes centered at 430 cm−1 and 580 cm−1 compared to bulk ZnO (430 and 575 cm−1) indicating the presence of tensile residual stress due to mismatch of thermal expansion coefficient of ZnO TF and Si substrate and due to the presence of oxygen vacancies and Zn interstitials, respectively. The XPS analysis confirms the presence of Zn, Zn–O, C–O, and Zn–OH bonds. The energy band gap and refractive index of annealed ZnO-TF are found to be 3.30 eV and 1.88, respectively. A new method of high quality ZnO TFs synthesis using MHD instability free non-focusing mode of DPF device will open a new alternative synthesis technique.

Fabrics with special wettability have drawn growing attention in recent years in the area of oil–water separation due to their low cost, good flexibility, and ease of handling. However, an efficient and fast method to enable the required wetting state on fabrics still remains a challenge. In this work, a one‐step, rapid, and chemical‐free hydrogen plasma treatment is reported to prepare a superhydrophobic and oleophilic polyester fabric. The as‐prepared fabrics display a static water contact angle of 153.2° with excellent oil–water separation capability. The mechanism of surface transformation is discussed through chemical analyses, which indicate a significant removal of carboxyl group from the pristine hydrophilic surface. This developed method is envisaged to be used for on‐demand large‐scale production of materials for emergency oil cleanup through either separation or selective adsorption.

Mutually coupled spin-Hall nano-oscillators (SHNO) can exhibit a binarized phase state, offering pathways to realize Ising machines and efficient neuromorphic hardware. Conventionally, phase binarization is achieved in coupled identical SHNOs via injecting an external microwave at twice the oscillator frequency in the presence of a strong biasing magnetic field. However, this technology poses potential challenges of higher energy consumption and complex circuit design. Moreover, differences in the individual characteristic frequencies of SHNOs resulting from fabrication-induced mismatch in SHNO dimensions may hinder their mutual synchronization. Addressing these challenges, we demonstrate purely dc current-driven mutual synchronization and phase binarization of two nonidentical nanoconstriction SHNOs without biasing magnetic field and microwave injection. We thoroughly investigate these phenomena and underlying mechanisms using micromagnetic simulation. We show how the localized fundamental mode of the spin wave emerging from the magnetization auto-oscillation reinforces the mutual synchronization, while the second-harmonic spin wave induces the phase binarization in the coupled SHNO pair. We further demonstrate the bias field free synchronized SHNO pair efficiently performing a reservoir computing benchmark learning task: sin- and square-wave classification, with 100% accuracy, utilizing the current-tunable phase binarization phenomenon. Our results showcase promising magnetization dynamics of coupled bias field free SHNOs for future computing applications.

The prospect of electrically controlled writing of ferromagnetic bits is highly desirable for developing scalable and energy-efficient spintronics devices. In this direction, various efforts have been made to achieve electrically controlled magnetization switching utilizing an artificial multiferroic system. To date, the magnetization switching has been realized in a diverse nanopatterned magnetic system. However, the demonstration of electric field-induced strain-controlled magnetization switching in artificial spin ice (ASI) coupled with a piezoelectric material is still unexplored. In the present work, we perform micromagnetic simulations to investigate the electric field-induced strain-mediated magnetization switching in an ASI based multiferroic system. Here, the piezoelectric strain-controlled magnetization switching has been studied by applying the electric-field pulse at different angles with respect to the axes of the system. Remarkably, magnetization switches by 180∘ only if the external electric-field pulse is applied at some specific angles, close to the anisotropy axis of the system (≈30∘–60∘). Our detailed analysis of the demagnetization energy variation reveals that the energy barrier becomes antisymmetric in such cases, facilitating complete magnetization reversal. Moreover, we have also proposed a possible magnetization reversal mechanism with two sequential electric-field pulses of a relatively smaller magnitude. We believe that the present work could pave the way for a future ASI-based multiferroic system for scalable magnetic field-free low power spintronics devices.

The Coded Aperture Imaging (CAI) technique has been applied with CR-39 nuclear track detectors to image alpha particle source spatial distributions. The experimental setup comprised: a 226Ra source of alpha particles, a laser-machined CAI mask, and CR-39 detectors, arranged inside a vacuum enclosure. Three different alpha particle source shapes were synthesized by using a linear translator to move the 226Ra source within the vacuum enclosure. The coded mask pattern used is based on a Singer Cyclic Difference Set, with 400 pixels and 57 open square holes (representing ρ = 1/7 = 14.3% open fraction). After etching of the CR-39 detectors, the area, circularity, mean optical density and positions of all candidate tracks were measured by an automated scanning system. Appropriate criteria were used to select alpha particle tracks, and a decoding algorithm applied to the (x, y) data produced the de-coded image of the source. Signal to Noise Ratio (SNR) values obtained for alpha particle CAI images were found to be substantially better than those for corresponding pinhole images, although the CAI-SNR values were below the predictions of theoretical formulae. Monte Carlo simulations of CAI and pinhole imaging were performed in order to validate the theoretical SNR formulae and also our CAI decoding algorithm. There was found to be good agreement between the theoretical formulae and SNR values obtained from simulations. Possible reasons for the lower SNR obtained for the experimental CAI study are discussed.

We report the investigation of the spin Hall angle (SHA) of Rhodium (Rh) and the effective magnetic damping constant of Ni80Fe20 in Rh (10 nm)/Ni80Fe20 (10 nm) bilayer system using Spin Torque Ferromagnetic Resonance (STFMR) spectroscopy and density functional theory based analysis. The filtered DC output voltages from Rh (10 nm)/Ni80Fe20 (10 nm) bilayer system at different input signal frequencies and powers for a complete 360º angle were measured. The average magnetic damping constant of Ni80Fe20 is found to be 0.017 and the angle-dependent study shows a 2-fold symmetry with the in-plane anisotropy field of 120 Oe. The SHA of Rh is found to be 0.2 %. The results agrees well with the calculations based on density functional theory where we found spin Hall conductivity (𝜎𝑥𝑦𝑠𝑝𝑖𝑛𝑧) at the charge-neutral Fermi level (EF) is positive and it’s magnitude is ~423(ℏ𝑒) (S/cm).

Developing flexible multiferroic composite with magnetoelectric coupling is highly desirable for the wearable electronic devices, magnetic field sensors, actuators, energy harvesters and memory devices. Here, a flexible artificial multiferroic composite was fabricated using ferromagnetic nickel ferrite (NiFe₂O₄) nanoparticles (NPs) as filler in the ferroelectric polyvinylidene fluoride (PVDF) matrix. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) studies revealed the formation of the inverse spinel phase in NiFe₂O₄ NPs. The vibrating sample magnetometer (VSM) and XRD measurements showed an increase in the magnetic moment and the electroactive β phase fraction, respectively, in PVDF/NiFe₂O₄ composite with the increasing loading concentration of NiFe₂O₄ filler NPs. With the increase in NiFe₂O₄ NPs loading concentration to 40 wt % the magnetoelectric coupling between the ferroelectric (PVDF) and ferromagnetic (NiFe₂O₄ NPs) was confirmed using magnetocapacitance measurement. This work successfully demonstrates the potential of artificial multiferroic PVDF/NiFe₂O₄ composite system, with enhanced dielectric property and room temperature magnetoelectric coupling, for future flexible electronic devices.

The breakdown phase of the UNU-ICTP plasma focus (PF) device was successfully simulated using the electromagnetic particle in cell method. A clear uplift of the current sheath (CS) layer was observed near the insulator surface, accompanied with an exponential increase in the plasma density. Both phenomena were found to coincide with the surge in the electric current, which is indicative of voltage breakdown. Simulations performed on the device with different insulator lengths showed an increase in the fast ionization wave velocity with length. The voltage breakdown time was found to scale linearly with the insulator length. Different spatial profiles of the CS electron density, and the associated degree of uniformity, were found to vary with different insulator lengths. The ordering, according to the degree of uniformity, among insulator lengths of 19, 22, and 26 mm agreed with that in terms of soft X-ray radiation yield observed from experiments. This suggests a direct correlation between CS density homogeneity near breakdown and the radiation yield performance. These studies were performed with a linearly increasing voltage time profile as input to the PF device.