Syed Shahbaz Ali

Nonlocal magnetoresistance due to Lorentz force in linear transport region in bulk silicon

Magnetoresistance (MR) of lightly doped bulk silicon has been studied in linear transport region. Both nonlocal and local MR results from deflection of in-plane transport of carriers by Lorentz force. However, nonlocal MR is nearly one order of magnitude larger than local one. We ascribe the enhanced nonlocal MR to polarity-conserved charges accumulated on boundaries near anode and cathode, which alters potential distribution and meanwhile increases current flowing in nonlocal region. This mechanism of nonlocal MR can be generalized to other materials with high or moderate mobility.

Exchange-biased hybrid ferromagnetic–multiferroic core–shell nanostructures

Artificial exchange-biased two-phase core-shell nanostructures consisting of ferromagnetic (Ni) and multiferroic (BiFeO3) materials were manufactured by a two-step method. An exchange bias effect was observed and studied, which indicates that it is possible to fabricate ferromagnetic-multiferroic nanostructures to utilize the combined ferroelectric and antiferromagnetic functionalities of bismuth ferrite.

Perpendicular magnetic anisotropy in Ta|Co40Fe40B20|MgAl2O4 structures and perpendicular CoFeB|MgAl2O4|CoFeB magnetic tunnel junction

Magnetic properties of Co40Fe40B20 (CoFeB) thin films sandwiched between Ta and MgAl2O4 layers have been systematically studied. For as-grown state, Ta/CoFeB/MgAl2O4 structures exhibit good perpendicular magnetic anisotropy (PMA) with interface anisotropy Ki = 1.22 erg/cm2, which further increases to 1.30 erg/cm2 after annealing, while MgAl2O4/CoFeB/Ta multilayer shows in-plane magnetic anisotropy and must be annealed in order to achieve PMA. For bottom CoFeB layer, the thickness window for PMA is from 0.6 to 1.0 nm, while that for top CoFeB layer is between 0.8 and 1.4 nm. Perpendicular magnetic tunnel junctions (p-MTJs) with a core structure of CoFeB/MgAl2O4/CoFeB have also been fabricated and tunneling magnetoresistance ratio of about 36% at room temperature and 63% at low temperature have been obtained. The intrinsic excitations in the p-MTJs have been identified by inelastic electron-tunneling spectroscopy.

Perpendicular magnetic tunnel junction and its application in magnetic random access memory

Recent progresses in magnetic tunnel junctions with perpendicular magnetic anisotropy (PMA) are reviewed and summarized. At first, the concept and source of perpendicular magnetic anisotropy (PMA) are introduced. Next, a historical overview of PMA materials as magnetic electrodes, such as the RE—TM alloys TbFeCo and GdFeCo, novel tetragonal manganese alloys Mn—Ga, L10-ordered (Co, Fe)/Pt alloy, multilayer film [Co, Fe, CoFe/Pt, Pd, Ni, Au]N, and ultra-thin magnetic metal/oxidized barrier is offered. The other part of the article focuses on the optimization and fabrication of CoFeB/MgO/CoFeB p-MTJs, which is thought to have high potential to meet the main demands for non-volatile magnetic random access memory.

Post magnetic field annealing effect on magnetic and structural properties of Co80Pt20 nanowires and nanotubes fabricated by electrochemical method

Highly ordered Co80Pt20 nanowires (NWs) and nanotubes (NTs) have been synthesized by low cost DC electrochemical deposition method with constant stirring during the fabrication process in anodic aluminum oxide nano-templates with average pore diameter of about 200 nm. The structural and magnetic properties of nanostructures have been investigated before and after simple and magnetic field annealing. Magnetic field of 1 T has been applied during annealing process in the direction perpendicular to NWs and NTs axis. X-Ray Diffraction analysis shows face centered cubic (fcc) as the dominant phase for Co80Pt20 NWs and post annealing led to better crystallinity with retained fcc phase. Furthermore, magnetic properties, including saturation magnetization (Ms), squareness ratio (Mr/Ms), and coercivity (Hc), have been investigated as a function of annealing temperature by Vibrating Sample Magnetometer.

Exchange bias in two-step artificially grown one-dimensional hybrid Co-BiFeO3core-shell nanostructures

One-dimensional core-shell nanostructures consisting of a ferromagnetic cobalt core and a multiferroic BiFeO3 (BFO) shell were fabricated by an artificial two-step methodology. The coupling between the ferromagnetic core and multiferroic shell manifests a significant exchange bias effect which gives a clear demonstration of the anti-ferromagnetic functionality of the BFO shell material. Exchange biases of 30 Oe and 60 Oe are observed at 300 K and at 5 K, respectively. Superparamagnetic contributions at lower temperatures play an important role in contributing to overall magnetic behavior. Dominant shape anisotropy causes parallel alignment of the easy magnetization axis along the axis of core-shell nanowires. A coherent mode of the magnetization reversal mechanism is observed by the angular dependence of coercivity (H c). This versatile two-step methodology can be employed to fabricate and investigate many other hybrid nanostructures leading to a vast scope of investigation for researchers.

Magnetic Field Annealing Effects on Magnetic Properties of Electrodeposited Co/Cu Multilayered Nanowires

Co/Cu nanowires were fabricated by electrodeposition using anodic alumina templates, with 100 nm diameter nanopores. The thickness of Co and Cu layer was about 45 and 5 nm, respectively. The magnetic properties of multilayered nanowires have been studied before and after simple and magnetic field (MF) annealing. For simple annealing, an increase in coercivity and squareness (SQ) along easy axis of magnetization has been observed. Magnetic field annealing has been done under MF of \(\boldsymbol 1\) T. For comparison, the MF annealing effect on magnetic properties has been studied using fast and slow temperature variations. The corresponding change in saturation magnetization, coercivity, remanent SQ, and the shape of hysteresis loops has been investigated. The coercivity Hc decreases in case of MF annealing with fast temperature variation, whereas it increases in case of MF annealing with slow temperature variation, as compared with as deposited. The enhanced magnetic anisotropy has been observed by slow MF annealing (SMFA), whereas magnetic anisotropy attenuated a little by fast MF annealing. Annealing without MF has also shown the enhanced magnetic anisotropy. SMFA and annealing without MF is likely to improve the interface of Co/Cu.

Spin Hall magnetoresistance in CoFe2O4/Pt films

Pulse laser deposition and magnetron sputtering techniques have been employed to prepare MgO(001)//CoFe2O4/Pt samples. Cross section transmission electron microscope results prove that the CoFe2O4 film epitaxially grew along (001) direction. X-ray magnetic circular dichroism results show that magnetic proximity effect in this sample is negligible. Magnetoresistance (MR) properties confirm that spin Hall MR (SMR) dominates in this system. Spin Hall effect-induced anomalous Hall voltage was also observed in this sample. These results not only demonstrate the universality of SMR effect but also demonstrate the utility in spintronics of CoFe2O4 as a new type of magnetic insulator.

Fabrication, structural and magnetic properties of electrodeposited Fe 80 Pt 20 nanowires and nanotubes

Summary form only given. Magnetic nanostructures have attracted a great deal of attention during the last decade because of their prospective applications not only in microwave absorption, high density recording media, and biosensor applications but also in functional micro and nanodevices. Recently, significant investigations have been motivated to get excess of 1Tbit/in2 in magnetic storage devices but at smaller bit size superparamagnetic limit arises. The process of DC electrodeposition has been employed to synthesize highly ordered Fe80Pt20 nanowires (NWs) and nanotubes (NTs) in anodic aluminum oxide (AAO) templates with average pore diameter of about 200 nm. The structural and magnetic properties of nanostructures has been investigated before and after simple and magnetic field annealing. A significant influence of magnetic field annealing with field strength of 1 T has been observed in the direction parallel and perpendicular to NWs and NTs axis respectively. X-Ray Diffraction analysis showed face centered cubic (fcc) as the dominant phase for Fe80Pt20 NWs and post annealing led to better crystallinity with retained fcc phase. On the other hand, Fe80Pt20 NTs shows amorphous behavior before and after simple and magnetic field annealing. Furthermore, magnetic properties, including saturation magnetization (Ms), squareness (Mr/Ms), and coercivity (Hc), have been investigated as a function of annealing temperature by Vibrating Sample Magnetometer. In conclusion, behavior of coercivity (Hc) was based on prolate ellipsoid chain model. Microstructural and magnetic properties strongly correlate with each other.

Temperature dependence of low frequency noise in magnetic tunneling junctions with Co 40 Fe 40 B 20 /Co 70.5 Fe 4.5 Si 15 B 10 composed free layer

Magnetic tunneling junctions (MTJ) have been widely used in magnetic random access memory (MRAM) and magnetic sensors for the large magnetoresistance (TMR). [1, 2] One of the major issues for high sensitivity sensors is to obtain a low switching field (Hsw). Recently various ferromagnetic metals with low Hsw, such as NiFe [3, 4] and amorphous CoFeSiB [5], have been selected as the sensing layer in MTJ sensors. Moreover, CoFeSiB has a lower saturation magnetization, which is also beneficial for reducing Hsw. [6] With CoFeSiB as the sensing layer, the sensitivity up to 40%/Oe has been achieved in MTJ sensors. [5] Besides the high sensitivity, the noise level of the sensor devices in the low frequency regime is an important parameter, and nanotesla-scaled detectivity has been achieved in MTJ sensors. In this work, the low frequency noise in MTJs with CoFeB/CoFeSiB composed free layer has been studied.