Siddhartha Mal

Reversible room temperature ferromagnetism in undoped zinc oxide: Correlation between defects and physical properties

We report a systematic study of the structural, chemical, electrical, optical, and magnetic properties of undoped ZnO thin films grown under different conditions as well as the films that were annealed in various environments. Oxygen-annealed films displayed a sequential transition from ferromagnetism to diamagnetism as a function of the annealing temperature. An increase in the green band intensity has been observed in oxygen-annealed ZnO films. Reversible switching of room-temperature ferromagnetism and n-type conductivity have been demonstrated by oxygen and vacuum annealing. Electron paramagnetic resonance data were found to be in agreement with the results of magnetization and conductivity measurements. Possibility of external ferromagnetic impurity as the origin of the unconventional room temperature ferromagnetism in these films has been ruled out by secondary ion mass spectrometer and electron energy loss spectroscopy studies. Correlation between structural, electrical, optical, and magnetic prope...

Controlled p-type to n-type conductivity transformation in NiO thin films by ultraviolet-laser irradiation

We report the systematic changes in structural, electrical, and optical properties of NiO thin films on c-sapphire introduced by nanosecond ultraviolet excimer laser pulses. Epitaxial nature of as deposited NiO was determined by x-ray diffraction phi scans and transmission electron microscopy (TEM) and it was established that NiO film growth takes place with twin domains on sapphire where two types of domains have 60° in-plane rotation with respect to each other about the [111] growth direction. We determined that at pulsed laser energy density of 0.275 J/cm2, NiO films exhibited conversion from p-type semiconducting to n-type conductive behavior with three orders of magnitude decrease in resistivity, while maintaining its cubic crystal structure and good epitaxial relationship. Our TEM and electron-energy-loss spectroscopy studies conclusively ruled out the presence of any Ni clustering or precipitation due to the laser treatment. The laser-induced n-type carrier transport and conductivity enhancement we...

Epitaxial VO2/Cr2O3/sapphire heterostructure for multifunctional applications

In this letter, we report integration of magnetic and ultrafast-transition properties of VO2 films with antiferromagnetic (AFM) Cr2O3 template layer in the epitaxial VO2/Cr2O3/Al2O3 heterostructure The Cr2O3 is an AFM material, which can pin the spin momentum of ferromagnetic VO2, in addition to providing epitaxial template. Thus, the magnetic properties of VO2 films grown with Cr2O3 buffer layer can be improved for multifunctional magnetic tunnel junctions and sensor applications. Electrical resistivity measurements as a function of temperature showed a sharp transition width (1.94 °C), with a small hysteresis width (5.7 °C), and large resistance change (∼3.8×104) across the semiconductor to metal transition (SMT). We discuss the correlations of the magnetic properties and SMT characteristics with epitaxial growth and formation of twins.

Defect mediated reversible ferromagnetism in Co and Mn doped zinc oxide epitaxial films

We have introduced defects in ZnO (undoped and doped with Co and Mn) epitaxial thin films using laser irradiation from nanosecond laser pulses and thermal annealing in oxygen ambient. In contrast to the as grown samples, the laser irradiated films show a significant increase in conductivity, enhancement in UV emission, while maintaining the same wurtzite crystal structure. Room-temperature ferromagnetism (RTFM) is observed in laser-irradiated samples, which increased with the number of laser pulses up to a certain value where magnetic moment saturates. The induced ferromagnetism as well as the enhanced electrical conductivity can be reversed with thermal annealing in oxygen ambient. The magnetization in Co and Mn doped films was found to be strong function of growth conditions and defect concentration. X-ray diffraction and optical absorption experiments suggested a 2+ valance state and tetrahedral coordination for both Co and Mn ions. There is a simultaneous increase in n-type electrical conductivity wit...