K. Javed

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.

Structural and Magnetic Response in Bimetallic Core/Shell Magnetic Nanoparticles

Bimagnetic monodisperse CoFe2O4/Fe3O4 core/shell nanoparticles have been prepared by solution evaporation route. To demonstrate preferential coating of iron oxide onto the surface of ferrite nanoparticles X-ray diffraction (XRD), High resolution transmission electron microscope (HR-TEM) and Raman spectroscopy have been performed. XRD analysis using Rietveld refinement technique confirms single phase nanoparticles with average seed size of about 18 nm and thickness of shell is 3 nm, which corroborates with transmission electron microscopy (TEM) analysis. Low temperature magnetic hysteresis loops showed interesting behavior. We have observed large coercivity 15.8 kOe at T = 5 K, whereas maximum saturation magnetization (125 emu/g) is attained at T = 100 K for CoFe2O4/Fe3O4 core/shell nanoparticles. Saturation magnetization decreases due to structural distortions at the surface of shell below 100 K. Zero field cooled (ZFC) and Field cooled (FC) plots show that synthesized nanoparticles are ferromagnetic till room temperature and it has been noticed that core/shell sample possess high blocking temperature than Cobalt Ferrite. Results indicate that presence of iron oxide shell significantly increases magnetic parameters as compared to the simple cobalt ferrite.

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.

Enhanced exchange bias in IrMn/CoFe deposited on self-organized hexagonally patterned nanodots

Exchange biased nanostructures of IrMn/CoFe were deposited on anodized alumina with hexagonally patterned nanodot surface structures. Nanodots with diameters of 20, 70, and 100 nm were fabricated to investigate the size effect on the magnetic properties. Magnetometry and the first-order reversal curve method revealed significant enhancements of the exchange bias and coercivity in the nanodots compared with flat films. The enhancements can be attributed to the effective reduction of ferromagnet domain sizes and increased random fields due to the nanostructure morphology and domain wall pinning by the boundaries between adjacent nanodots.

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.

Co/Pt Multilayers on Self-Organized Hexagonal Patterned Nanodots

Anodic alumina with surface of hexagonal patterned nanodots was prepared by a two-step anodizing procedure. Scanning electron microscope and atomic force microscopy results clearly showed the formation of self-organized hexagonal patterned nanostructure. Diameters of the nanodots were controlled by choosing different anodization voltage and types of electrolyte acids. Co/Pt multilayers deposited on the nanodots with different diameters of 20, 70, and 100 nm lead to the formation of magnetic nanostructures with perpendicular anisotropy. Magnetometry and the first-order reversal curve method were used to study the magnetic properties of Co/Pt nanostructures. An out-of-plane magnetic easy axis was observed for the continuous films and the nanodots with diameters of 100 and 70 nm. The magnetic multilayers deposited on 20 nm nanodots appeared to have taken on a hard axis type behavior. The curvature of nanodot arrays induces strong modifications on the magnetic properties of the nanostructures.

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.

Perpendicular Exchange Bias of [Pt/Co] 5 /IrMn Multilayers on Self-Organized Hexagonally Patterned Nanodots

[Pt/Co]5/IrMn multilayers were deposited on hexagonally patterned nanodots on the backside of anodized alumina oxide (AAO) templates and thermally oxidized Si substrates. AAO templates with different diameters of about 50, 60, 130 and 400 nm nanodots were used to fabricate the exchange-biased nanostructures and investigate the size effect on the perpendicular exchange bias. With decreasing nanodots diameter, the multilayers exhibited large variations in both exchange bias field HE and coercivity HC. Compared to the flat film, HE of the nanodots samples was significantly enhanced, which can be attributed to the reduction in antiferromagnetic and ferromagnetic domain sizes induced by the curved morphology and domain wall pinning between two adjacent magnetic nanocaps. Unexpectedly, the 50 nm nanodot sample had higher thermal stability than the flat film sample.