Subhadra Gupta
University of Alabama
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Featured researches published by Subhadra Gupta.
IEEE Transactions on Magnetics | 2010
Eugene Chen; D. Apalkov; Z. Diao; A. Driskill-Smith; D. Druist; D. Lottis; V. Nikitin; X. Tang; S. Watts; S. Wang; Stuart A. Wolf; Avik W. Ghosh; Jiwei Lu; S.J. Poon; Mircea R. Stan; W. H. Butler; Subhadra Gupta; Claudia Mewes; Tim Mewes; P.B. Visscher
Spin-transfer torque random access memory (STT-RAM) is a potentially revolutionary universal memory technology that combines the capacity and cost benefits of DRAM, the fast read and write performance of SRAM, the non-volatility of Flash, and essentially unlimited endurance. In order to realize a small cell size, high speed and achieve a fully functional STT-RAM chip, the MgO-barrier magnetic tunnel junctions (MTJ) used as the core storage and readout element must meet a set of performance requirements on switching current density, voltage, magneto-resistance ratio (MR), resistance-area product (RA), thermal stability factor (¿) , switching current distribution, read resistance distribution and reliability. In this paper, we report the progress of our work on device design, material improvement, wafer processing, integration with CMOS, and testing for a demonstration STT-RAM test chip, and projections based on modeling of the future characteristics of STT-RAM.
Journal of Applied Physics | 2012
Anusha Natarajarathinam; Zeenath Reddy Tadisina; Tim Mewes; S. Watts; Eugene Chen; Subhadra Gupta
Magnetic behavior of CoFeB at various thicknesses ranging from 2 nm to 8 nm capped with different materials, such as MgO, Ta, Ru, and V have been studied. The films were sputter-deposited and subsequently characterized by magnetometry and broadband ferromagnetic resonance (FMR). There are magnetically dead layers at the interface observed with Ru and Ta capping layers, while MgO and V have almost no effect on the magnetization of the CoFeB. As the ferromagnetic layer is made thinner, the effective magnetization decreases, indicating an interfacial perpendicular anisotropy. Particularly in the case of MgO, V/Ru, and V/Ta capping layers, interfacial perpendicular anisotropy is induced in CoFeB, and the Gilbert damping parameter is also reduced. The origin of this perpendicular magnetic anisotropy (PMA) is understood to be caused by the interface anisotropy between the free layer and the capping layer. The effect of post-deposition annealing and CoFeB thickness on the anisotropy and damping of V/Ta capped sa...
Journal of Applied Physics | 2010
Zeenath Reddy Tadisina; Anusha Natarajarathinam; Billy D. Clark; Alton L. Highsmith; Tim Mewes; Subhadra Gupta; Eugene Chen; S. Wang
CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy (PMA) free and reference layers composed of Co/M (where M=Pd or Ni) multilayers have been optimized for high PMA and high tunneling magnetoresistance (TMR). The effects of Co thickness, Pd thickness, and the number of Co/Pd bilayers on the anisotropy and coercivity of the [Co/Pd]n multilayer films have been studied for both free and reference layers. The damping parameter α of CoFeB capped multilayers was determined using broadband ferromagnetic resonance. The transport properties of the patterned MTJ stacks were measured from 10 to 400 K. A maximum TMR of 10% at 10 K (5%–10% at 300 K) was obtained for these perpendicular MTJs, regardless of whether or not they were magnetically annealed for MgO–CoFeB crystallization. This indicates that the fcc-bcc-fcc transitions from the fcc multilayers to the bcc CoFeB/MgO/CoFeB do not promote the “giant MgO TMR effect” caused by symmetry filtering.
Journal of Applied Physics | 2012
A. Natarajarathinam; R. Zhu; P. B. Visscher; Subhadra Gupta
We have previously reported on fully perpendicular Co/Pd multilayer (ML)-based CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs). However, Co/Pd ML-based MTJs have rarely exhibited tunneling magnetoresistance (TMR) ratios greater than 10%. This has been attributed to the inability to pull a sufficiently thick CoFeB layer perpendicular on top of MgO, as well as the incomplete bcc templating of CoFeB from MgO owing to the adjacent fcc Co/Pd MLs. Other researchers have used amorphous thin Ta bridge layers to transition between the bcc and fcc layers. Thin CoFeB with various seed or capping layers have also shown fully perpendicular anisotropy, and thus have gained interest in use as a free layer. Here we report on fully perpendicular MTJs of this type, with a thin CoFeB free layer and a Co/Pd ML-based SAF pinned layer with a thin amorphous Ta bridge layer to transition from bcc CoFeB to the fcc multilayers. The experimentally measured M-H loops show excellent agreement with micromagnetic simulations. Current-...
IEEE Transactions on Magnetics | 2012
Eugene Chen; D. Apalkov; A. Driskill-Smith; A. Khvalkovskiy; D. Lottis; K. Moon; V. Nikitin; A. Ong; X. Tang; S. Watts; R. Kawakami; M. Krounbi; Stuart A. Wolf; S.J. Poon; Jiwei Lu; Avik W. Ghosh; Mircea R. Stan; W. H. Butler; Tim Mewes; Subhadra Gupta; Claudia Mewes; P.B. Visscher; R. A. Lukaszew
We report our progress on material improvement, device design, wafer processing, integration with CMOS, and testing of STT-RAM memory chips at 54 nm node with cell sizes of 14 and 28 F2 (F=54 nm). A dual tunnel barrier MTJ structure was found to have lower and more symmetric median spin transfer torque writing switching currents, and much tighter parallel to antiparallel switching current distribution. In-plane MTJ devices write endurance data, read and write soft error rates data and simulation fits, and solutions to the long write error rate tail at fast write speeds are discussed.
Journal of Vacuum Science and Technology | 2010
Zeenath Reddy Tadisina; A. Natarajarathinam; Subhadra Gupta
Magnetic CoFeB/MgO/CoFeB-based tunnel junctions with perpendicular magnetic anisotropy Co/M multilayers (M=Ni, Pd, Pt) have been investigated as a function of structural and magnetic properties. Magnetometry, ferromagnetic resonance, x-ray diffraction, stress tests, and local electrode atom probe tomography were carried out primarily on Co/Ni multilayers. A statistical design of experiments was conducted to optimize the perpendicular magnetic anisotropy and damping parameter α of these multilayers. Seed layers, thickness, and thickness ratios are all critical to achieve perpendicular behavior. Perpendicular MgO-based magnetic tunnel junctions with Co/Ni and Co/Pd reference and free layers were fabricated and tested. Sharp MR-H switching characteristics were observed for the Co/Pd multilayers, and a somewhat softer transition was observed for the Co/Ni multilayer with a Cu seed, which did not have as high a perpendicular anisotropy. Tunneling magnetoresistance (TMR) values were limited to about 10%, primar...
Journal of Vacuum Science and Technology | 2009
Xiao Li; Zeenath Reddy Tadisina; Subhadra Gupta; Ganping Ju
Perpendicular anisotropy Co80Pt20 films were patterned into nanopillars of 40–90nm using nanosphere lithography. A monolayer of polystyrene spheres was coated onto the sputtered films and size tailored by reactive ion etching in oxygen. These reduced spheres were used as masks for ion milling of the magnetic films. Increased coercivity and squareness resulted in decreasing nanopillar size. A significant increase in intrinsic coercivity was demonstrated with the decrease in pillar size. The reversal mechanism appears similar to reported results of nucleation of a small reversed volume followed by rapid domain wall motion.
Journal of Applied Physics | 2013
Amritpal Singh; Samuel C. Schwarm; Oleg N. Mryasov; Subhadra Gupta
We have investigated the effect of Ta insertion thickness on perpendicular magnetic anisotropy (PMA) in [Co/Pt multi-layers]/Ta/CoFeB (Hard/Soft) composite free layer. For insertions less than 0.6 nm thick, the ferromagnetic exchange was strong enough to switch the two layers together. The exchange turned from ferromagnetic to anti-ferromagnetic in nature for Ta insertion thickness above 0.7 nm. We observed increase in PMA with increasing Ta insertion thickness. It was noticed that this increase in PMA was caused by the enhancement of CoFeB/MgO interface anisotropy. The interface anisotropy increased by more than 0.9 erg/cm2 as the Ta insertion thickness increased from 0 nm to 0.5 nm. The Hk value for the composite free layer with 0.5 nm thick Ta insertion is around 11 kOe leading to a high thermal stability factor, estimated around 152 for 20 nm nano-dots.
Journal of Applied Physics | 2013
Hao Su; Anusha Natarajarathinam; Subhadra Gupta
Perpendicular anisotropy magnetic nanorods composed of Co/Pd multilayers have been successfully fabricated by glancing angle deposition (GLAD) in a planetary sputtering system. Co and Pd layer thickness, ratio, and bilayer number were optimized for both normal and GLAD depositions. Scanning electron micrographs estimated the nanorods to be about 12 nm in diameter. M-H loops showed that the coercivity for the GLAD nanorods increased from 1.3 kOe for the normally deposited continuous films to 2.9 kOe for the GLAD nanorod array, a 123% increase.
Journal of Vacuum Science & Technology. B. Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena | 2012
Zhenzhong Sun; Dawen Li; Anusha Natarajarathinam; Hao Su; Subhadra Gupta
This study demonstrates a pronounced ion irradiation effect in ion milling of magnetic thin films. In fabrication of bit-patterned media, the ion irradiation could facilitate bit island isolation before complete removal of magnetic materials by ion milling. Combined with block copolymer lithography, sub-20 nm CoPt dots with uniaxial perpendicular anisotropy, resembling Stoner–Wohlfarth-like single domains, were achieved. X-ray diffraction demonstrates that the degradation of the magnetic film by ion irradiation is related to crystal structure damage.