Papers by Charanjit Bhatia
Journal of Applied Physics, Jan 28, 2015
Pole tip recession (PTR) is one of the major issues faced in magnetic tape storage technology, wh... more Pole tip recession (PTR) is one of the major issues faced in magnetic tape storage technology, which causes an increase in the magnetic spacing and hence signal loss during data readback. Despite efforts to reduce the magnetic spacing, PTR, and surface wear on the heads by using protective overcoats, most of them either employ complex fabrication processes and approaches do not provide adequate protection to the head or are too thick ($10-20 nm), especially for future high density tape storage. In this work, we discuss an approach to reduce the PTR and surface wear at the head by developing an ultrathin $7 nm bilayer overcoat of silicon/silicon nitride (Si/SiN x) and carbon (C), which is totally fabricated by a cost-effective and industrial-friendly magnetron sputtering process. When compared with a monolithic C overcoat of similar thickness, the electrically insulating Si/SiN x /C bilayer overcoat was found to provide better wear protection for commercial tape heads, as demonstrated by Auger electron spectroscopic analyses after wear tests with commercial tape media. Although the microstructures of carbon in the monolithic and bilayer overcoats were similar, the improved wear durability of the bilayer overcoat was attributed to the creation of extensive interfacial bonding of Si and N with the C overcoat and the alumina-titanium carbide composite head substrate, as predicted by time-of-flight secondary ion mass spectrometry and confirmed by in-depth X-ray photoelectron spectroscopy analyses. This study highlights the pivotal role of enhanced interfaces and interfacial bonding in developing ultrathin yet wear-durable overcoats for tape heads. V
ACS Applied Materials & Interfaces, Feb 7, 2012
Slippery and Wear-Resistant Surfaces Enabled by Interface Engineered Graphene
Nano Letters, 2019
Friction and wear remain the primary cause of mechanical energy dissipation and system failure. R... more Friction and wear remain the primary cause of mechanical energy dissipation and system failure. Recent studies reveal graphene as a powerful solid lubricant to combat friction and wear. Most of these studies have focussed on nanoscale tribology and have been limited to a few specific surfaces. Here, we uncover many unknown aspects of graphene's contact-sliding at micro- and macroscopic tribo-scales over a broader range of surfaces. We discover that graphene's performance reduces for surfaces with increasing roughness. To overcome this, we introduce a new type of graphene/silicon nitride (SiNx, 3 nm) bilayer overcoats that exhibits superior performance compared to native graphene sheets ( mono and bi-layer) i.e. display the lowest microscale friction and wear on a range of tribologically poor flat surfaces. More importantly, 2 layer graphene/SiNx lubricant (< 4 nm in total thickness) shows the highest macroscale wear durability on tape-head (topologically variant surface) that exceeds even most of previous thicker (~7-100 nm) overcoats. Detailed nanoscale characterization and atomistic simulations explain the origin of the reduced friction and wear arising from these nanoscale coatings. Overall, this study demonstrates that engineered graphene-based coatings can outperform conventional coatings in a number of technologies.
Journal of Magnetism and Magnetic Materials, Sep 1, 2018
Tailoring writability and obtaining better signal-to-noise ratio performance by tuning the magnet... more Tailoring writability and obtaining better signal-to-noise ratio performance by tuning the magnetic and microstructural properties of FePt media is of great interest in the bid to achieve high areal densities for next generation hard disk drives (HDDs). Conventional ways to tune FePt media are primarily by either the insertion of additional layers, or by inclusion of segregants such as B, C, Ni, SiO2 etc. Here we describe an approach that involves modifying growth kinetics by N inclusion, by plasma treatment in FePt bilayers (consisting of a hard and a soft FePt layer). The soft FePt layer aids in obtaining easy writability, while nitrogen plasma treatment of the interface of the two FePt layers facilitates suitable tailoring of the microstructure. This approach contributed to lowering of grain dimension as well as reduction in domain sizes in the FePt bilayers, but with the unwanted side-effects of reduction in squareness and ordering. However, we further propose a method to alleviate these concerns by the deposition of the soft FePt layer in partial nitrogen atmosphere, that restored the squareness and ordering while retaining the smaller domains obtained by nitrogen plasma treatment. Thus, the proposed approach provides a potential direction towards meeting the mutually conflicting requirements of easy writability and better signal-to-noise ratio performance in FePt media.
Science Advances, Jan 4, 2019
Friction and wear cause energy wastage and system failure. Usually, thicker overcoats serve to co... more Friction and wear cause energy wastage and system failure. Usually, thicker overcoats serve to combat such tribological concerns, but in many contact sliding systems, their large thickness hinders active components of the systems, degrades functionality, and constitutes a major barrier for technological developments. While sub-10-nm overcoats are of key interest, traditional overcoats suffer from rapid wear and degradation at this thickness regime. Using an enhanced atomic intermixing approach, we develop a ~7-to 8-nm-thick carbon/silicon nitride (C/SiN x) multilayer overcoat demonstrating extremely high wear resistance and low friction at all tribological length scales, yielding ~2 to 10 times better macroscale wear durability than previously reported thicker (~20 to 100 nm) overcoats on tape drive heads. We report the discovery of many fundamental parameters that govern contact sliding and reveal how tuning atomic intermixing at interfaces and varying carbon and SiN x thicknesses strongly affect friction and wear, which are crucial for advancing numerous technologies.
Synthesis and corrosion performance of ultrathin carbon nitride overcoats
Tribochemical Wear Of Carbon Films
[1993] Digests of International Magnetics Conference, 1993
Utilizing spin-pumping, inverse spin-Hall and spin-Hall phenomena for synchronization of nano magnetic oscillators
The spin-transfer torque (STT) phenomenon has recently been utilized for magnetic nano spin-trans... more The spin-transfer torque (STT) phenomenon has recently been utilized for magnetic nano spin-transfer torque oscillators (STO) in patterned or nano-contact spin valves and magnetic tunnel junctions (MTJ). For applications, synchronization of multiple oscillators to achieve a higher microwave power and lower bandwidth is required. In order to realize such synchronization, a mutual feedback system among the nano STOs is required. Spin wave mediated interaction of two nano-contact STOs was the first demonstration of STOs synchronization. Despite many efforts, maximum four STOs have been experimentally synchronized so far. A new type of oscillator based on the spin-Hall effect (SHE) has been introduced, in which the oscillation mode is a self-localized spin wave bullet, and its injection locking has been demonstrated as well. The spin Hall oscillators (SHO) can be also realized in nano-magnetic elements.We have proposed a novel method for synchronization of multiple STOs on the basis of spin pumping, inverse spin Hall, and spin Hall phenomena . The proposed oscillator system consists of a series of patterned nano-magnets in junction with a stripe of a normal metal with high spin-orbit coupling. We include an accumulative feedback loop for the modulated part of the current. We perform numerical simulations to show the effect of modulated charge currents in the normal metal due to spin pumping from each nano magnet . Figure 1(c) shows the synchronization achieved for ten nano-magnet oscillators . The main important parameter for achieving the synchronization is the delay of the feedback line . We discuss that for the accumulative delay, the line delay should be close to 0, or q/f (q = 1, 2, ...) in order to achieve synchronization here 1/f~160ps . In addition to the ability to synchronize many nano STOs, the distance between nano-magnets is not bounded to the maximum values reported theoretically for spin-wave mediated synchronization, since the mutual feedback in our system is through charge currents.
A Comprehensive Fundamental Understanding of Atomic Layer Deposited Titanium Oxide Films for c-Si Solar Cell Applications
IEEE Journal of Photovoltaics, Mar 1, 2021
Titanium oxide (TiO<italic><sub>x</sub></italic>) film shows excellent su... more Titanium oxide (TiO<italic><sub>x</sub></italic>) film shows excellent surface passivation of crystalline silicon (c-Si) as well as carrier selectivity. The coexistence of both these properties along with extraordinary optical properties of TiO<italic><sub>x</sub></italic> opens new opportunities to design novel high-performance c-Si solar cells. However, many inconsistent findings have also been reported in the literature that limit the application of TiO<italic><sub>x</sub></italic>. A greater fundamental insight into TiO<italic><sub>x</sub></italic> films therefore needs to be developed in order to produce very high-efficiency c-Si solar cells. In this article, a comprehensive study of atomic layer deposited (ALD) TiO<italic><sub>x</sub></italic> films is presented, including the processing as well as material, electrical and optical properties. The effects of deposition temperature from 100 to 400 °C and postdeposition annealing treatment on the properties of TiO<italic><sub>x</sub></italic> films are examined in terms of the ALD growth process and the composition, stoichiometry, crystallographic structure, bonding environment, elemental, chemical, electrical, and optical properties of the films. The screening of the temperature suggests that it is one of the key parameters that can be used to control the TiO<italic><sub>x</sub></italic> film properties for different applications, by giving excellent c-Si surface passivation with a surface recombination velocity as low as 3.7 cm/s. Further analysis of the underlying passivation mechanism suggests that TiO<italic><sub>x</sub></italic> films in amorphous phase combined with an interface oxide (SiO<italic><sub>y</sub></italic>+TiO<italic><sub>x</sub></italic>) and a small amount of Cl are favorable for achieving good c-Si passivation. These findings significantly improve the fundamental understanding of TiO<italic><sub>x</sub></italic> films from their growth to application, and could enable the control of their properties for future device development.
In this paper we present the results of nano-high indentation hardness measurements, nano-wear me... more In this paper we present the results of nano-high indentation hardness measurements, nano-wear measurements be as well as tribochemical wear tests on a series of hydrogenated us and nitrogenated carbon films. It is shown that the mechanical ~~~i~~~. generates force and displacemen properties, such as hardness and abrasive wear resistance, while important, are not the primary determining factors in the wear durability of the head disk interface in magnetic hard disk drives.
Individual magnetization reversal of a square dot matrix by common current excitation
Journal of Physics D, Jun 30, 2015
We have proposed a method for magnetization reversal of individual sites of a 2 by 2 matrix of pe... more We have proposed a method for magnetization reversal of individual sites of a 2 by 2 matrix of perpendicularly magnetized dots by common current excitation. The spin-polarized current signal consists of a dc-biased ac part followed by a pure dc one. The amplitude of the dc and ac parts of the current, as well as the phase and duration of the ac current, determine the reversal sites through the magnetostatic interaction among the dots. We show that the individual selectivity in magnetization reversal occurs through two consecutive steps, dephasing of the matrix dyadic pairs dynamics followed by nonlinear dephasing of the individual elements. This method can be utilized to increase the storage density of magnetic random access memory by enabling common access for four or more bits.
Application and modeling of single contact electron beam induced current technique on multicrystalline silicon solar cells
Solar Energy Materials and Solar Cells, Feb 1, 2015
The first demonstration of single contact electron beam induced current (SCEBIC) technique on mul... more The first demonstration of single contact electron beam induced current (SCEBIC) technique on multicrystalline silicon (mc-Si) solar cells is reported. A lumped single-diode analytical model is also proposed to theoretically explain the SCEBIC phenomenon within solar cells as well as the current transient characteristics of the major model parameters, such as shunt resistance Rsh, junction capacitance Cj and parasitic capacitance Cs. The accuracy of the analytical model is then verified using PSPICE simulations, which show a close match with the experimental results. It is found that a large value of parasitic capacitance Cs is necessary to achieve good SCEBIC signal strength with a relatively low signal-to-noise ratio (SNR), and this is realized experimentally by adopting a metal enclosure in the measurement setup. In addition, an advantage of SCEBIC over the conventional double-contact method is also demonstrated by characterizing partially processed solar cells, which clearly illustrates the high degree of flexibility of SCEBIC in solar cell characterization.
Heat-assisted magnetic recording (HAMR) demonstration using C-shaped nano-apertures
Proceedings of SPIE, Mar 4, 2015
ABSTRACT
IFAC Proceedings Volumes, 2011
In this paper, the effect of nonlinear distortion on system identification is studied. In particu... more In this paper, the effect of nonlinear distortion on system identification is studied. In particular, the effect of nonlinear friction on frequency response measurement is examined. Design of multifrequency excitation signal that minimizes nonlinear effect on frequency response measurement has been explored and the best linear approximation model has been defined. An identification method using differential No Interharmonic Distortion (NID) excitation with optimized crest factor is proposed. The method is used to identify the best linear model of a hard disk drive actuator.
Journal of the American Ceramic Society, Jul 24, 2014
The structural evolution and properties of 0.3Pb(In 1/2 Nb 1/2) O 3-0.38Pb(Mg 1/3 Nb 2/3)O 3-0.32... more The structural evolution and properties of 0.3Pb(In 1/2 Nb 1/2) O 3-0.38Pb(Mg 1/3 Nb 2/3)O 3-0.32PbTiO 3 (0.3PIN-0.38PMN-0.32PT) ferroelectric ceramics with different sintering times have been investigated. The content of the tetragonal phase is increased in samples sintered for more than 6 h, despite that the composition falls in the rhombohedral region of the previously established phase diagram. The results show that the metastable tetragonal phase at room temperature is induced and stabilized by the tensile residual stresses. Excessively long sintering time generally leads to grain coarsening, loss of lead, and deterioration of properties, while the increasing amount of the tetragonal phase, and the large residual tensile stress appear to improve the dielectric and electromechanical properties. This study offers new insights into the sintering of Pb-based ferroelectric ceramics with complex compositions. D. Johnson-contributing editor Manuscript No. 34988.
Thin Solid Films, Dec 1, 2015
Ferroelectric 0.3Pb(In 1/2 Nb 1/2)O 3-0.38Pb(Mg 1/3 Nb 2/3)O 3-0.32PbTiO 3 (PIN-PMN-PT) epitaxial... more Ferroelectric 0.3Pb(In 1/2 Nb 1/2)O 3-0.38Pb(Mg 1/3 Nb 2/3)O 3-0.32PbTiO 3 (PIN-PMN-PT) epitaxial thin films with a perovskite phase were grown on (110)-oriented MgO substrates by sputter deposition method. The structure of the PIN-PMN-PT films was found very sensitive to the growth condition. The single perovskite phase obtained under optimized processing condition was substantially relaxed from lattice-mismatch strain down to at least 11 nm in thickness due to the formation of an ultrathin interfacial layer of twin domains. Compared to other technically important relaxor ferroelectrics such as PMN-PT, the PIN-PMN-PT thin films have a higher Curie temperature and a large coercive field, which are very promising for piezoelectric applications in micro/nanoelectromechanical systems.
Nanomechanical properties of CN/sub x/ overcoats and cathodic arc carbon (CAC) films
IEEE Transactions on Magnetics, Jul 1, 1998
ABSTRACT
Carbon, May 1, 2017
A thickness driven tuning of sp 3 bonding was observed in ultrathin carbon films. The huge change... more A thickness driven tuning of sp 3 bonding was observed in ultrathin carbon films. The huge change in sp 3 growth dynamics for 1-7 nm thick films was noticed which calculated to be significantly larger for filtered cathodic vacuum arc (FCVA)-processed (~ 7% / nm) than sputter-processed (~ 1.1% / nm) carbon films, based on Raman spectroscopy.
Diamond and Related Materials, Apr 1, 2014
The corrosion performance of commercial hard disk media which was subjected to bi-level surface m... more The corrosion performance of commercial hard disk media which was subjected to bi-level surface modification has been reported. The surface treatment was carried out by bombarding the surface of the magnetic media with C + ions at 350 eV followed by 90 eV using filtered cathodic vacuum arc (FCVA). The energy and embedment depth of the impinging C + ions were adjusted by applying an optimized bias to the substrate and simulated by a Stopping and Range of Ions in Matter (SRIM) code which predicted the formation of a graded atomically mixed layer at the carbon-media interface. Cross-section transmission electron microscopy (TEM) revealed the formation of a 1.8 nm dense nano-layered carbon overcoat structure on the surface of the media. Despite añ 33% reduction in the thickness, the bi-level surface modified disk showed corrosion performance similar to that of a commercially manufactured disk with a thicker carbon overcoat of 2.7 nm. This improvement in the corrosion/oxidation resistance per unit thickness can be attributed to the formation of a dense and highly sp 3 bonded carbon layer, as revealed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. This study demonstrates the effectiveness of the bi-level surface modification technique in forming an ultra-thin yet protective overcoat for future hard disks with high areal densities.
arXiv (Cornell University), Jan 22, 2013
A stochastic nonlinear electrical characteristic of graphene is reported. Abrupt current changes ... more A stochastic nonlinear electrical characteristic of graphene is reported. Abrupt current changes are observed from voltage sweeps between the source and drain with an on/off ratio up to 10 3. It is found that graphene channel experience the topological change. Active radicals in an uneven graphene channel cause local changes of electrostatic potential. Simulation results based on the self-trapped electron and hole mechanism account well for the experimental data. Our findings illustrate an important issue of reliable electron transports and help for the understanding of transport properties in graphene devices.
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Papers by Charanjit Bhatia