Ratnesh Gupta

Influence of ion implantation on the magnetic properties of thin FeCo films

Modifications of 73-nm-thick polycrystalline FeCo films by magnetic-field-assisted implantation of Ne, Xe, and Au ions have been investigated. For magnetic characterization, the longitudinal magneto-optic Kerr effect and magnetic force microscopy (MFM) in the remanent state have been used, while structural information has been gained from glancing-angle x-ray diffraction and Rutherford backscattering spectroscopy. The irradiated films show a soft-magnetic behavior with large magnetic anisotropy. The Ne ions initially induce an increasing coercivity, possibly due to radiation defects accumulated during the ion implantation, while higher Ne fluences anneal out the defects and reduce the coercivity. For the heavy ions the deposited energy density is high enough to reduce the coercivity at small fluences and then to increase it slightly for increasing fluence. Correlations between the magnetic anisotropy, coercivity and the ion species and fluence have been established. The MFM pictures feature the largest ch...

The effect of swift heavy ion irradiation on perpendicular magnetic anisotropy in Fe-Tb multilayers

The effect of swift heavy ion irradiation of Fe-Tb multilayers has been studied using x-ray reflectivity and conversion electron Mossbauer spectroscopy measurements. The energy deposited in the multilayer in the form of electronic excitations results in significant modification of the interfaces. During 80 MeV Si ion irradiation the interfacial roughness is found to increase linearly with irradiation dose. Mossbauer measurements provide evidence of segregation of Fe and Tb atoms in the intermixed region, which may be the cause of the observed increase in the interfacial roughness. The as-deposited multilayer exhibits a large perpendicular magnetic anisotropy, which shows a monotonic decrease with irradiation dose. The observed decrease in perpendicular magnetic anisotropy is a combined effect of the stress relaxation in the bulk of layers and an increase in the interfacial roughness. 150 MeV Ag ion irradiation causes additional intermixing at the interface, accompanied by a large reduction in perpendicular magnetic anisotropy.

Phase transitions in Co thin film induced by low energy and high energy ion beam irradiation

Ion irradiation plays an important role to tailor the microcrystalline structure and the corresponding magnetic anisotropy by high local energy deposition and defect accumulation. In the present work, we have investigated the effects of nuclear energy loss and electronic energy loss on the phase transformation and magnetic anisotropy of thin polycrystalline Co films. As-deposited film shows polycrystalline behavior with twofold uniaxial magnetic anisotropy, characteristic to hcp-Co film. 320 keV Au ions induce phase transformation from hcp to fcc. Similar phase transformation have been observed in the case of film irradiated by 120 MeV Au ions. Rate of phase transformation is discussed in terms of electronic and nuclear energy losses and the corresponding changes in the magnetic anisotropic behavior are discussed in the light of uniaxial and fourfold magnetic anisotropy.

On the resistivity minimum in metallic glasses

Abstract Effect of plastic deformation on the low temperature resistivity of amorphous Ni 78 Si 8 B 14 has been studied. The system exhibits a resistivity minimum around 20 K which shifts to lower temperatures with plastic deformation. Present results suggest that the electron-electron interaction effects are responsible for the negative temperature coefficient of resistivity at low temperatures.

Study of Exchange Bias in All Ferromagnetic Fe/Co Soft/Hard Bilayer

We report the unusual and exotic phenomenon of positive exchange bias observed in all ferromagnetic layered soft/hard Fe/Co spring magnet systems. Interestingly, the results are well reproducible both at low (5 K) and room temperatures (300 K). The effect of applied magnetic pinning field of 7 and -5 T reveals a noticeable shift of magnetic hysteresis loops along the field direction. We argue that the reason behind observed positive exchange bias is the establishment of antiferromagnetic (AFM) exchange coupling at the interface as a result of intermixing of Fe into the Co layer. The collective coupling of this intermix layers is AFM which facilitates positive shift of magnetic hysteresis loop.

Influence of preparation methods on the optical and electronic properties of Cr-doped TiO2 thin films

We report the effects of preparation methods on electronic and optical properties of the Cr-doped TiO2 films. X-ray reflectivity pattern suggest that the single layer film have been deposited from all the preparation methods. UV-Vis reflectance technique has been used to obtain its optical properties. Using core level and valence band photoelectron spectra, the doping at the substitutional lattice sites and the stoichiometric transfer of TiO2 in comparison to RF sputtering effectively reduces the band gap energy.

Effect of High-Energy Heavy-Ion Irradiation on Fe/Tb Multilayers

The effect of 80 MeV Si ions on Fe/Tb multilayers has been studied using X-ray reflectivity measurements. A dose of 1015 ions/cm2 causes a significant increase in the interfacial roughness. Irradiation does not cause any additional intermixing between Fe and Tb layers.