P Chowdhury

Enhanced room temperature ferromagnetism in electrodeposited Co-doped ZnO nanostructured thin films by controlling the oxygen vacancy defects

We report the growth of un-doped and cobalt doped ZnO nanostructures fabricated on FTO coated glass substrates using electrodeposition method. A detailed study on the effects of dopant concentration on morphology, structural, optical, and magnetic properties of the ZnO nanostructures has been carried out systematically by varying the Co concentration (c.Co) from 0.01 to 1 mM. For c.Co ≤ 0.2 mM, h-wurtzite phase with no secondary phases of Co were present in the ZnO nanostructures. For c.Co ≤ 0.2 mM, the photoluminescence spectra exhibited a decrease in the intensity of ultraviolet emission as well as band-gap narrowing with an increase in dopant concentration. All the doped samples displayed a broad emission in the visible range and its intensity increased with an increase in Co concentration. It was found that the defect centers such as oxygen vacancies and zinc interstitials were the source of the visible emission. The X-ray photoelectron spectroscopy studies revealed, Co was primarily in the divalent s...

Effect of coherent to incoherent structural transition on magnetic anisotropy in Co/Pt multilayers

The crystallographic structure and magnetic anisotropy have been investigated in Co/Pt multilayers deposited by direct current magnetron sputtering on two different buffer layers, Ta and Pt. Detail theoretical and experimental investigations reveal the presence of three effects: magneto-elastic, interface and shape anisotropies, and their competition results in three distinct regions based on the Co layer thickness (tCo). In the region I, with tCo< 6 A, the coherent lattice strain modifies the volume anisotropy through magneto-elastic contribution and it leads to overall positive volume anisotropy energy. With further increase in tCo, lattice mismatch initiates and this results in a decrease in magneto-elastic effect and it enhances the interface anisotropy. The presence of both diminished magneto-elastic effect and enhanced interface anisotropy results in an intermediate region (region II) with negative volume anisotropy energy which is relatively wider (6 A

Anomalous enhancement in interfacial perpendicular magnetic anisotropy through uphill diffusion

We observed interfacial chemical sharpening due to uphill diffusion in post annealed ultrathin multilayer stack of Co and Pt, which leads to enhanced interfacial perpendicular magnetic anisotropy (PMA). This is surprising as these elements are considered as perfectly miscible. This chemical sharpening was confirmed through quantitative energy dispersive x-ray (EDX) spectroscopy and intensity distribution of images taken on high angle annular dark field (HAADF) detector in Scanning Transmission Electron Microscopic (STEM) mode. This observation demonstrates an evidence of miscibility gap in ultrathin coherent Co/Pt multilayer stacks.

Magnetic properties of ordered bi-layer nanowire arrays with different Co crystallographic structures

Cobalt nanowire arrays with varying length were grown by pulse electrochemical route into the pores of alumina template. The structural investigation reveals that at the bottom of the nanowire, it grows with hcp (002) structure up to few hundred nanometer in length and then changes its crystal orientation to hcp (100) phase as it further grows along the wire axis. The variation of measured coercivity and the remanent ratio as a function of nanowire length indicate that each nanowire in array grows with three different segments. The micromagnetic simulations carried out on these structures are then correlated with experimental observations.

Anisotropic magnetic properties of bi-layered structure of ordered Co nanowire array: Micromagnetic simulations and experiments

The angular dependence of magnetic properties were investigated in Co nanowire (NW) arrays with varying length grown by pulse electrochemical route into pore of alumina template. In these investigations, three different NW lengths, 200, 400 and 1000 nm were considered. The structural investigations reveals that shorter NW of length 200 nm have hcp (002) structure, however the existence of both hcp (002) and hcp (100) was observed in longer wire length of 1000 nm, though these phases were well separated in lower and upper segments along the NW axis, respectively. Micromagnetic simulation has been carried out considering the different crystallographic structures as a function of NW length and with the variation of angle between the applied field and the NW axis. Both using experimentally measured magnetization data and micromagnetic simulations, we have inferred that in the nanowire with only hcp (002) structure, the magnetization reversal occurs by coherent rotation, whereas in the case of hcp (100) structure, the reversal phenomena occurs via vortex state and in bilayer structure, the interaction among them leads to a gradual crossover from a coherenet state to vortex along the nanowire axis. Irrespective of the NW length, the angular dependence of H

Tuning of deep level emission in highly oriented electrodeposited ZnO nanorods by post growth annealing treatments

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Fabrication of magnetic nanodot arrays using ultrathin alumina membrane (UTAM)

was explained qualitatively using the micromagnetic simulation.

Effect of Pt layer thickness on perpendicular magnetic anisotropy in ultrathin Co/Pt multilayers

Highly dense and c-axis oriented zinc oxide (ZnO) nanorods with hexagonal wurtzite facets were deposited on fluorine doped tin oxide coated glass substrates by a simple and cost-effective electrodeposition method at low bath temperature (80 °C). The as-grown samples were then annealed at various temperatures (TA = 100–500 °C) in different environments (e.g., zinc, oxygen, air, and vacuum) to understand their photoluminescence (PL) behavior in the ultra-violet (UV) and the visible regions. The PL results revealed that the as-deposited ZnO nanorods consisted of oxygen vacancy (VO), zinc interstitial (Zn i), and oxygen interstitial (Oi) defects and these can be reduced significantly by annealing in different environments at optimal annealing temperatures. However, the intensity of deep level emission increased for TA greater than the optimized values for the respective environments due to the introduction of various defect centers. For example, for TA ≥ 450 °C in the oxygen and air environments, the density of Oi defects increased, whereas, the green emission associated with VO is dominant in the vacuum annealed (TA = 500 °C) ZnO nanorods. The UV peak red shifted after the post-growth annealing treatments in all the environments and the vacuum annealed sample exhibited highest UV peak intensity. The observations from the PL data are supported by the micro-Raman spectroscopy. The present study gives new insight into the origin of different defects that exist in the electrodeposited ZnO nanorods and how these defects can be precisely controlled in order to get the desired emissions for the opto-electronic applications.

Thermally induced perpendicular magnetic anisotropy in CoFeB/MgO/CoFeB based magnetic tunnel junction

Fabrication of ultrathin alumina membranes were carried out using two step anodization from high purity aluminum foil. This membrane provides highly ordered nano pores with diameter of 80 nm and intra-pore distance of the order 110 nm. The membrane with thickness < 100 nm was used as a evaporation mask for the growth of magnetic nanodot arrays of Co, SmCo5 and CoPt on Si substrates. The structural investigations were carried out using Field Emission Scanning Electron Microscopy (FESEM). The elemental composition of nanodot arrays was confirmed using Energy Dispersive X-Ray (EDX) analysis. Magnetic characterizations were carried out to study the in-plane and out of-plane hysteresis behaviors and the coercive fields. The present novel two-step anodization approach is simple, efficient, and provides an inexpensive non-lithographic method to produce high density nano-structured materials for wide range of applications in nanotechnology and spintronics.

The structural and electrical properties of TiO2 thin films prepared by thermal oxidation

Co/Pt multilayer structures with varying Pt layer thicknesses from 1.5 to 12 A were deposited on Ta buffer layer by DC magnetron sputtering technique and both of their magnetic and structural properties were investigated. All the multilayers showed perpendicular magnetic anisotropy (PMA) and the enhancement in PMA is observed with increase in the Pt layer thickness. The structural investigation using XRD reveals the presence of single peak in all these multilayers, which indicate the formation of coherent Co/Pt (111) structure. The origin of PMA in these multilayer structure is due to the strain induced in the Co layer. From the full-width-half-maximum (FWHM) of the XRD patterns it is observed that the grain size is increasing with increasing the Pt layer thickness. The influence of the enhancement of the grain size is then correlated with the measured M-H hysteresis behavior and subsequently the anisotropy energy.