Debabrata Mishra

CoFe2O4−Fe3O4 Magnetic Nanocomposites as Photocatalyst for the Degradation of Methyl Orange Dye

We report the investigation of temperature-dependent magnetic properties and photocatalytic activity of CoFe2O4−Fe3O4 magnetic nanocomposites (MNCs) synthesized by hydrothermal process. Room-temperature magnetic hysteresis (M-H) loops result enhanced saturation magnetization of 90 emu/g and coercivity (HC) of 530 Oe for CoFe2O4−Fe3O4 MNCs. With decreasing temperature to 20 K, HC increases from 500 Oe to 6800 Oe, and the M-H loops exhibit exchange coupling feature between CoFe2O4 and Fe3O4. Low- and high-temperature-dependent magnetization measurements confirm that the blocking temperature lies above 300 K and the presence of two magnetic phase transitions corresponding to CoFe2O4 and Fe3O4, respectively. The photocatalytic activity of the MNCs has been examined on the reduction of methyl orange (MO), a colored compound used in dyeing and printing textiles. The observed results suggest that the CoFe2O4−Fe3O4 MNCs act as an excellent photocatalyst on the degradation of organic contaminants and degrade 93% of MO in 5 hours of UV irradiation. The photocatalytic activity of MNCs is attributed to remarkably high band gap energy and small particle size. Also, the MNCs with a reproducible photocatalytic activity are well separable from water media by applying external magnetic field and acts as a promising catalyst for the remediation of textile wastewater.

Thickness dependent magnetic properties of amorphous FeTaC films

We report on the study of thickness and temperature dependent magnetic properties of amorphous FeTaC (t = 20–200 nm) thin films prepared on thermally oxidized Si substrate at ambient temperature. Room temperature coercivity remains constant (∼1.5 Oe) for t between 20 and 50 nm, but increases rapidly (>18 Oe) when t > 50 nm. Also, the shape of M-H loop changes from rectangular to flat loop with increasing film thickness; and at larger thicknesses (>50 nm), the central range of constant slope in the flat loop extended largely along with the enlargement of hysteresis around the origin. This is mainly due to the development of perpendicular anisotropy with increasing the film thickness causing a transition from in-plane orientations of the spins to a magnetic stripe domain structure, which degrades the magnetic properties at larger thickness. Low temperature thermomagnetization curves obtained under zero-field-cooled (ZFC) and field-cooled (FC) conditions depicts a bifurcation between ZFC and FC data at large...

Enhanced soft magnetic properties in multilayer structured amorphous Fe-Ta-C films

We report the investigation of enhanced soft magnetic properties in amorphous Fe-Ta-C thin films at larger thickness (∼200 nm), multistep magnetization reversal process and disappearance of magnetic disorder at low temperatures using [Fe-Ta-C(y)/Ta(x)]n=0−4/Fe-Ta-C(y) multilayer structured films prepared on thermally oxidized Si substrates. As-deposited films showed amorphous structure. Room temperature coercivity decreased largely from 24 Oe, for a single layer film, to 0.12 Oe, for multilayer films with n = 4 with the same total Fe-Ta-C thickness, signifying that the intervening Ta layers play a critical role in reducing coercivity. Magnetic hysteresis loops for the multilayer films with n > 2 exhibit pronounced multistep magnetization reversal processes for temperatures below 70 K depending on the number of multilayers and smooth narrow hysteresis curves for temperatures above 70 K. Low temperature thermomagnetization curves obtained under zero-field-cooled (ZFC) and field-cooled (FC) conditions showed...

Enhanced soft magnetic properties in magnetic field annealed amorphous Fe(Co)–Zr–B alloys

We report the effects of longitudinal magnetic field annealing (MFA) on the microstructure, magnetic domain structure, and magnetic softness of Co substituted amorphous Fe89−x−yCoyBxZr11 alloys. A two-phase structure characterized by bcc Fe(Co) nanocrystals (size 100 Oe) due to the formation of Fe(Co)–Zr compounds. Large MS (173 emu/g) and low HC (0.4 Oe) were obtained for an Fe69Co10B10Zr11 alloy annealed at 823 K. Lorentz microscopy results reveal that the average size of the domains decreases with increasing Co content and magnetic ripple structures are observed in s...

Magnetic properties of mechanically alloyed Fe100?xZrx (20 ? x ? 35) powder

Structural, kinetic and magnetic properties of amorphous/non-equilibrium solid solution powders of Fe100?xZrx (20 ? x ? 35) prepared by the mechanical alloying process are reported here. Magnetic properties of the as-milled powders have been investigated as functions of the Zr concentration, applied magnetic field, relaxation time and temperature. As the Zr concentration is increased in Fe100?xZrx, the activation energy of Fe?Zr crystallization increases, whereas the saturation magnetization and effective magnetic anisotropy show oscillatory variation. Temperature dependent field-cooled and zero-field-cooled (ZFC) magnetization measurements show the presence of spin-glass like phase at low temperature even at an applied magnetic field of 1?T. Thermomagnetic behaviour of the samples strongly depends on the strength of the external applied field. Magnetic relaxation measurements carried out at different temperatures under ZFC condition show that the magnetization relaxes slowly with time in a logarithmic manner up to 103?s. This slow magnetic relaxation is attributed to stress induced effective anisotropy that sets the energy of the domain walls and frustration resulting from the co-existence of ferromagnetic and finite antiferromagnetic interactions.

Exchange bias effect in CoFe2O4-Cr2O3 nanocomposite embedded in SiO2 matrix

A nanogranular system composed of CoFe2O4-Cr2O3 core-shell nanoparticles (embedded in SiO2 matrix) having average particle size of 35 ± 5 nm was prepared by chemical synthesis. Structural characterizations by x-ray diffraction, Transmission Electron Microscope (TEM) show highly crystalline phase of these nanocomposites. High resolution TEM images show distorted and modified grain boundary region. CoFe2O4-Cr2O3 nanocomposites show excellent positive exchange bias effect after field cooling at 100 Oe of applied field. Both the exchange bias field and vertical shift in magnetization loop has been observed in this system below 300 K after field cooling. There is a positive loop shift of about 5 kOe along the field axis at 20 K in this system while the vertical shift is nearly 100% owing to the strong exchange bias effect. The exchange bias effect has been explained in terms of the different magnetic exchange interactions and modified grain boundary effect of CoFe2O4-Cr2O3 nanocomposites. The role of spin glas...

MICROSTRUCTURAL CHANGES UPON ANNEALING AND IT'S EFFECT ON MAGNETIC AND MECHANICAL PROPERTIES OF NANOSIZED COBALT–FERRITE

Nanocrystalline cobalt–ferrite particles of size 20–30 nm have been prepared by a reverse coprecipitation technique under the assistance of ultrasonic irradiation and heat-treatment at different temperatures (from 473 K to 1073 K). Both X-ray diffraction and transmission electron microscope analysis confirms the reduction of strain present in the material with annealing temperature. Enhancement of coercivity and magnetization value has been observed without increase in the particle size for whole range of annealing temperature. Temperature dependent magnetization loop shows considerable magnetic hardening at low temperature. The observed enhancement of the coercivity value has been attributed to the increase in magneto-crystalline anisotropy, surface effects and exchange anisotropy. The mechanical properties of the pure cobalt–ferrite samples and cobalt–ferrite reinforced alumina samples were also examined. The Vickers microhardness and the compressive properties obtained from the stress–strain relation showed higher value with annealing temperatures and higher nanoparticle content.

TEMPERATURE DEPENDENT COERCIVITY AND RELAXATION PHENOMENA IN AMORPHOUS Fe–(Mn)–Zr–B NANOPARTICLES

We report the temperature dependent coercivity behavior and the improvement of room temperature soft magnetic properties in interacting amorphous Fe–Mn–Zr–B powder nanoparticles prepared by mechanical alloying of elemental powders. With increasing Mn concentration from 5 to 20 at.% in Fe–Zr–B alloys, the coercivity (HC) reduces drastically from 170 to 25 Oe. Temperature dependent HC behavior shows exponential variation of HC with temperature. Interparticle interactions dominate the low temperature behavior and surface effect on the nanoparticles is more pronounced at low temperatures. Magnetic relaxation behavior shows slow relaxation in the interacting amorphous nanoparticles and the relaxation process is observed to be thermally activated in the present studies.

High temperature coercive field behavior of Fe–Zr powder

We report the investigation of high temperature coercive field behavior of Fe80Zr20 nanocrystalline alloy powder having two-phase microstructure prepared by mechanical alloying process. Thermomagnetization measurement shows the presence of two different magnetic phase transitions corresponding to the amorphous matrix and nonequilibrium Fe(Zr) solid solution. Temperature dependent coercivity exhibits a sharp increase in its value close to the Curie temperature of the amorphous matrix. This feature is attributed to the loss of intergranular ferromagnetic exchange coupling between the nanocrystallites due to the paramagnetic nature of the amorphous matrix. The temperature dependent coercive field behavior is ascribed to the variations in both the effective anisotropy and the exchange stiffness constant with temperature.

ROLE OF MICROSTRUCTURE AND DOMAIN STRUCTURE ON THE SOFT MAGNETIC PROPERTIES OF MAGNETIC FIELD ANNEALED Fe89-xZr11Bx ALLOYS

We report the evolution of microstructure, domain structure, and soft magnetic properties of amorphous and nanocrystalline Fe89-xZr11Bx(x = 0 - 10) alloys. High-resolution electron microscopy observations reveal that the annealed alloys exhibit a two-phase microstructure. Addition of B enhances the ferromagnetic properties of Fe–Zr amorphous phase in the two-phase structured microstructure, resulting in good soft magnetic properties. Large-sized domains with smooth domain walls are observed in the alloys annealed below 873 K, which exhibit excellent magnetic softness. On the other hand, in the alloys annealed above 873 K, small-sized domains with irregular domain walls and domain wall pinning by Fe–Zr compound are seen. The soft magnetic properties in Fe–Zr–B alloys not only depend on mean grain size, but also on the strength of the intergranular magnetic coupling and structural inhomogeneities.