A. Perumal

Effect of Co and Cu substitution on the magnetic entropy change in Ni46Mn43Sn11 alloy

We report the observation of magnetic entropy change due to martensitic phase transformations of bulk off-stoichiometric Heusler alloys with compositions Ni46Mn43Sn11, Ni44Mn43Co2Sn11, and Ni44Mn43Cu2Sn11, prepared by arc melting method. The martensitic transition of the parent ternary alloy (Ni46Mn43Sn11) shifts to lower temperatures upon Co and Cu substitution. Inverse magnetocaloric effect was observed in these alloys near the martensitic transformation temperature when subjected to an applied magnetic field. Ni46Mn43Sn11, Ni44Mn43Co2Sn11, and Ni44Mn43 Cu2Sn11 alloys exhibited maximum positive magnetic entropy change of 7.9, 11.3, and 18.8 J kg−1 K−1, respectively, under an applied magnetic field of 1.8 T.

Finite Size Effects in Magnetic and Optical Properties of Antiferromagnetic NiO Nanoparticles

We report systematic investigations on structural, magnetic and optical properties of NiO nanoparticles prepared by mechanical alloying. As-milled powders exhibit face centred cubic structure, but average particle size decreases and effective strain increases for the initial periods of milling. Lattice volume increases monotonically with a reduction in particle size. Antiferromagnetic NiO particles exhibit significant room temperature (RT) ferromagnetism with modest moment and coercivity. A maximum moment of 0.0147 μB/f.u at 12 kOe applied field and a coercivity of 160 Oe were obtained for 30 h milled NiO powder. Exchange bias decreases linearly with a decrease in NiO particle size. Thermo-magnetization data reveal the presence of mixed magnetic phases in milled powders and shifts magnetic phase transition towards high temperature with increasing milling. Annealing of milled NiO powder and photoluminescence studies show a large reduction in RT magnetic moment and blue-shifting of band edge emission peak. The observed properties are discussed on the basis of finite size effect, defect density, oxidation/reduction of Ni, increase in number of sublattices, uncompensated spins from surface to particle core, and interaction between uncompensated surfaces and particle core with lattice expansion.

Enhanced room temperature ferromagnetism in antiferromagnetic NiO nanoparticles

We report systematic investigations of structural, vibrational, resonance and magnetic properties of nanoscale NiO powders prepared by ball milling process under different milling speeds for 30 hours of milling. Structural properties revealed that both pure NiO and as-milled NiO powders exhibit face centered cubic structure, but average crystallite size decreases to around 11 nm along with significant increase in strain with increasing milling speed. Vibrational properties show the enhancement in the intensity of one-phonon longitudinal optical (LO) band and disappearance of two-magnon band due to size reduction. In addition, two-phonon LO band exhibits red shift due to size-induced phonon confinement effect and surface relaxation. Pure NiO powder exhibit antiferromagnetic nature, which transforms into induced ferromagnetic after size reduction. The average magnetization at room temperature increases with decreasing the crystallite size and a maximum moment of 0.016 μB/f.u. at 12 kOe applied field and coe...

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...

Spin dynamics and frequency dependence of magnetic damping study in soft ferromagnetic FeTaC film with a stripe domain structure

Perpendicular magnetic anisotropy (PMA) and low magnetic damping are the key factors for the free layer magnetization switching by spin transfer torque technique in magnetic tunnel junction devices. The magnetization precessional dynamics in soft ferromagnetic FeTaC thin film with a stripe domain structure was explored in broad band frequency range by employing micro-strip ferromagnetic resonance technique. The polar angle variation of resonance field and linewidth at different frequencies have been analyzed numerically using Landau-Lifshitz-Gilbert equation by taking into account the total free energy density of the film. The numerically estimated parameters Lande g-factor, PMA constant, and effective magnetization are found to be 2.1, 2 × 105 erg/cm3 and 7145 Oe, respectively. The frequency dependence of Gilbert damping parameter (α) is evaluated by considering both intrinsic and extrinsic effects into the total linewidth analysis. The value of α is found to be 0.006 at 10 GHz and it increases monotonic...

Evolution of atomic order and soft magnetism in mechanically alloyed nanocrystalline Fe–Si powders subjected to heat treatment

We report on the effects of annealing on the structure and magnetic properties of mechanically alloyed Fe75Si25 powders. Mechanical alloying of elemental powders of Fe and Si with atomic composition Fe75Si25 produced disordered α-Fe(Si) with nanocrystalline structure containing large amounts of dislocation defects. With increasing annealing temperatures, a gradual transition of the disordered α-Fe(Si) phase to the ordered DO3 phase accompanied by the growth of crystallites and reduction in dislocation defects were observed. A significant decrease in coercivity from 120 to 50 Oe has been observed up to 773 K. On the other hand, at annealing temperatures in the range 773–1073 K, the coercivity varied as the sixth power of crystallite size and, at higher temperatures, as the reciprocal of crystallite size in the strain relaxed powders. The high coercivity found in the as-milled powder has been attributed to the residual strains present in the powders.

Evaluation of Ni–Mn–In–Si Alloys for Magnetic Refrigerant Application

The influence of excess Ni on the structural, martensitic transformation and magnetic refrigerant properties of Ni_50+xMn_35-xIn₁₄Si₁ (<i>x</i> = 0, 1, and 1.5) alloys was studied by means of X-ray diffraction and magnetization measurements. The martensitic transition temperature increased when Ni was substituted for Mn in Ni_50+xMn_35-xIn₁₄Si₁ and the inverse magnetocaloric effect was observed near the martensitic transformation temperature when the samples were subjected to an applied magnetic field. The maximum magnetic entropy change obtained for the unit magnetic field was 17.2, 19.0, and 2.5 J·kg^-1·K^-1·T^-1 for the alloys with <i>x</i> = 0, 1, and 1.5, respectively. The highest magnetic entropy change obtained was 31.3 J·kg^-1·K^-1 for the alloy with <i>x</i> = 1 at 279 K for a magnetic field change of 0-1.8 T.