Devajyoti Mukherjee

Magnetic anisotropy and field switching in cobalt ferrite thin films deposited by pulsed laser ablation

We report the observation of contrasting magnetic behavior in cobalt ferrite (CFO) thin films deposited on single crystalline magnesium oxide (MgO) and strontium titanate (STO). Epitaxial films on MgO (100) with a lattice mismatch of 0.35% showed out-of-plane anisotropy whereas the films on STO (100) with a lattice mismatch of 7.4% displayed in-plane anisotropy. Stress anisotropy calculated from angle-dependent x-ray diffraction analysis confirmed that the change in anisotropy originates from the lattice mismatch. An additional low-field switching characteristic is observed in the M-H loops of the CFO films, which became prominent with lowering temperature as also evidenced from the rf transverse susceptibility measurements. The obtained results revealed that the low field switching is associated with the film-substrate interface.

Nanocolumnar interfaces and enhanced magnetic coercivity in preferentially oriented cobalt ferrite thin films grown using oblique-angle pulsed laser deposition.

Highly textured cobalt ferrite (CFO) thin films were grown on Si (100) substrates using oblique-angle pulsed laser deposition (α-PLD). X-ray diffraction and in-depth strain analysis showed that the obliquely deposited CFO films had both enhanced orientation in the (111) crystal direction as well as tunable compressive strains as a function of the film thicknesses, in contrast to the almost strain-free polycrystalline CFO films grown using normal-incidence PLD under the same conditions. Using in situ optical plume diagnostics the growth parameters in the α-PLD process were optimized to achieve smoother film surfaces with roughness values as low as 1-2 nm as compared to the typical values of 10-12 nm in the normal-incidence PLD grown films. Cross-sectional high resolution transmission electron microscope images revealed nanocolumnar growth of single-crystals of CFO along the (111) crystallographic plane at the film-substrate interface. Magnetic measurements showed larger coercive fields (∼10 times) with similar saturation magnetization in the α-PLD-grown CFO thin films as compared to those deposited using normal-incidence PLD. Such significantly enhanced magnetic coercivity observed in CFO thin films make them ideally suited for magnetic data storage applications. A growth mechanism based on the atomic shadowing effect and strain compression-relaxation mechanism was proposed for the obliquely grown CFO thin films.

Role of epitaxy in controlling the magnetic and magnetostrictive properties of cobalt ferrite-PZT bilayers

Epitaxial thin films of cobalt ferrite (CFO) single layer and CFO–lead zirconium titanate (PZT) bilayers were deposited on single crystal MgO (1 0 0) and SrTiO3 (STO) (1 0 0) substrates by pulsed laser deposition. The structural properties were characterized using x-ray diffraction and atomic force microscopy. The magnetic properties of the as-grown thin films were measured at 10 and 300 K in both parallel and perpendicular magnetic fields. The CFO–PZT bilayer films showed enhanced or reduced values of magnetization as compared with those of the CFO single layer films depending on the substrate of deposition. A strain compression–relaxation mechanism was proposed in order to explain the structure–property relationships in the CFO–PZT bilayer thin films.

Enhanced giant magneto-impedance effect in soft ferromagnetic amorphous ribbons with pulsed laser deposition of cobalt ferrite

We have grown a 50 nm thick magnetic layer of cobalt ferrite (CFO) on two Co-based amorphous ribbons of compositions Co65Fe4Ni2Si15B14 (type I) and Co69Fe4Ni1Mo2B12Si12 (type II) by a pulsed laser deposition technique. A comparative study of the influence of this CFO layer on the giant magneto-impedance (GMI) effect and field sensitivity (η) of the ribbons is presented. Our results reveal that the presence of the CFO layer enhanced both the GMI ratio and the field sensitivity of the ribbons. Relative to the plain ribbons, the GMI ratio and field sensitivity of the CFO-coated ribbons increased by 97% and 42% for type I and by 34% and 50% for type II, respectively. The enhanced effects are attributed to the modifications on the ribbon surface and closure of magnetic flux paths due to the CFO layer.

Controlled Ti seed layer assisted growth and field emission properties of Pb(Zr0.52Ti0.48)O3 nanowire arrays.

We report on the directed upright growth of ferroelectric (FE) Pb(Zr0.52Ti0.48)O3 (PZT) nanowire (NW) arrays with large aspect ratios of >60 using a Ti seed layer assisted hydrothermal process over large surface areas on ITO/glass substrates. In a two-step growth process, Ti seed layer of low surface roughness with a thickness of ~500 nm and grain size of ~100 nm was first deposited by radio frequency (RF) sputtering which was subsequently used as substrates for the growth of highly dense, single crystalline PZT NWs by controlled nucleation. The electron emission properties of the PZT NWs were investigated using the as-grown NWs as FE cathodes. A low turn-on field of ~3.4 V/μm was obtained from the NW arrays, which is impressively lower than that from other reported values. The results reported in this work give direction to the development of a facile growth technique for PZT NWs over large surfaces and also are of interest to the generation of high current electron beam from FE NW based cathodes for field emitter applications.

Influence of microstructure and interfacial strain on the magnetic properties of epitaxial Mn3O4/La0.7Sr0.3MnO3 layered-composite thin films

Epitaxial Mn3O4/La0.7Sr0.3MnO3 (Mn3O4/LSMO) bilayer thin films were grown on lattice-matched single crystal substrates of SrTiO3 (STO) (100) and MgO (100), with Mn3O4 as the top layer, using a pulsed laser deposition technique. X-ray diffraction (XRD) patterns revealed the single crystalline nature and epitaxial relationship between the layers. A detailed analysis of strains using XRD asymmetric/symmetric scans indicated an increasing in-plane compressive strain in the LSMO layer with increasing thicknesses of the Mn3O4 layer, resulting in a tetragonal distortion of the LSMO lattice in the Mn3O4/LSMO films in comparison to the tensile strains in LSMO single-layer films grown on both STO and MgO substrates. Cross-sectional high resolution transmission electron microscope (HRTEM) images showed atomically sharp interfaces in all films. However, as opposed to a flat interface between LSMO and STO, the Mn3O4 and LSMO interface was undulating and irregular in the bilayer films. Magnetic measurements revealed th...

Role of dual-laser ablation in controlling the Pb depletion in epitaxial growth of Pb(Zr0.52Ti0.48)O3 thin films with enhanced surface quality and ferroelectric properties

Pb depletion in Pb(Zr0.52Ti0.48)O3 (PZT) thin films has remained as a major setback in the growth of defect-free PZT thin films by pulsed laser ablation techniques. At low excimer (KrF) laser fluences, the high volatility of Pb in PZT leads to non-congruent target ablation and, consequently, non-stoichiometric films, whereas, at high laser fluences, the inherent ejection of molten droplets from the target leads to particulate laden films, which is undesirable in heterostructure growth. To overcome these issues, a dual-laser ablation (PLDDL) process that combines an excimer (KrF) laser and CO2 laser pulses was used to grow epitaxial PZT films on SrTiO3 (100) and MgO (100) substrates. Intensified-charge-coupled-detector (ICCD) images and optical emission spectroscopy of the laser-ablated plumes in PLDDL revealed a broader angular expansion and enhanced excitation of the ablated species as compared to those for single-laser ablation (PLDSL). This led to the growth of particulate-free PZT films with higher Pb...

Challenges in the stoichiometric growth of polycrystalline and epitaxial PbZr0.52Ti0.48O3/La0.7Sr0.3MnO3 multiferroic heterostructures using pulsed laser deposition

High quality polycrystalline and epitaxial PbZr0.52Ti0.48O3/La0.7Sr0.3MnO3 (PZT/LSMO) multiferroic thin films were deposited on single-crystal Si (100) and SrTiO3 (STO) (100) substrates using pulsed laser deposition (PLD) technique. The deposition conditions were optimized to overcome some of the challenges during the growth of stoichiometric PZT/LSMO thin films (with LSMO as the bottom layers). The major setback of the preferential evaporation of Pb during the ablation of PZT target, which leads to the growth of non-stoichiometric, Pb-deficient PZT thin films with poor ferroelectric properties, was investigated by studying the laser-target interaction sites and intensified charge-coupled detector (ICCD) imaging of the laser-ablated plumes. X-ray studies revealed that the PZT/LSMO heterostructures deposited under the optimum conditions were highly crystalline. Atomic force microscope images showed uniform grain growth with surface roughness values as low as 1.6 nm. In- and out-of-plane magnetization measu...

Ziz-zag interface and strain-influenced ferromagnetism in epitaxial Mn3O4/La0.7Sr0.3MnO3 thin films grown on SrTiO3 (100) substrates

Epitaxial Mn3O4/La0.7Sr0.3MnO3 (Mn3O4/LSMO) bilayer thin films were grown on single crystal SrTiO3 (STO) (100) substrates, with Mn3O4 as the top layer, using pulsed laser deposition technique. X-ray diffraction revealed the single crystalline nature and epitaxial relationship between the layers, as well as increased lattice distortion of LSMO in the bilayer structure relative to a single layer film of LSMO. Cross-sectional high resolution transmission electron microscope images showed atomically sharp interfaces, but in contrast to the almost flat interface between LSMO and STO substrate, the Mn3O4 and LSMO interface showed a zig-zag structure. Magnetic measurements indicated that the high temperature in situ deposition of a Mn3O4 layer on top of the LSMO film significantly reduced the magnetization of the LSMO film, probably due to the formed ziz-zag interface between LSMO and Mn3O4.

Low temperature synthesis, optical and photoconductance properties of nearly monodisperse thin In2S3 nanoplatelets

We report on the synthesis of nearly monodisperse In2S3 nanoplatelets ∼7 nm thick by a low temperature polyol process using 1-thioglycerol as the capping chemical. The synthesized nanoplatelets crystallized in bulk tetragonal structure and show preferential crystallographic growth within a range of diameters from ∼75–85 nm. Quantum confinement was observed by spectroscopic analyses with a determined band gap of 3.4 eV. Strong UV luminescence corroborated valence-conduction band transition in these confined nanoplatelets. Room temperature photoconductance measurements performed on the prototype device fabricated from the nanoplatelets revealed noticeable enhancement of conductivity under illumination as compared to the dark condition.