Correspondence between magnetoresistance and magnetization in Co/Cu multilayers studied at higher spacer layer thickness

A series of [Cu(t(Cu))/Co(25 angstrom)](25) multilayers at higher spacer layer thickness have been deposited by dc magnetron sputtering. Magnetoresistance (MR) measurements have been carried out at different temperatures. MR curves showed hysteresis by displaying peaks at magnetic field H. The field values exhibiting the maximum resistance in the magnetoresistance curve (H(p)) were greater than the coercivity (H(c)). The correspondence between the shape of the MR curve and that of the magnetization curve has been established and observed peak splitting in MR curves is attributed to a hardening in the magnetization reversal of some magnetic grains

Superparamagnetism and giant magnetoresistance in sputtered FeCuAg granular films

Magnetization and magnetoresistance studies were carried out on the granular Fe(x)Cu(y)Ag(z) films prepared by dc magnetron sputtering. In this series of samples, we have substituted Ag in the parent Fe-Cu system such that the Ag concentration increases from zero while that of the Cu decreases to zero when we go from one sample to another. The blocking temperatures of the films lie in the range 20-50 K. By using a model, the room temperature magnetization data have been analyzed by determining the contributions from the smaller superparamagnetic particles and larger ferromagnetic particles. Size distributions of the superparamagnetic particles are obtained. The magnetoresistance and (M/M(s))(2) data do not agree for most of the samples investigated. This discrepancy is due to the fact that the (M/M(s))(2) behavior is valid for purely superparamagnetic particles of only one size whereas there are both superparamagnetic and ferromagnetic particles in our samples. When we analyzed the magnetoresistance data using a superparamagnetic model, a good agreement between the experimental data and the simulated data were obtained for some samples. Significant discrepancy observed for other samples may be due to the spin dependent scattering of the electrons traveling from superparamagnetic to ferromagnetic particles or vice versa. (C) 200

Nanogranular Fe-Cu-Ag thin films: Structure, microstructure and giant magnetoresistance

FexCuyAgz granular thin films with several compositions were prepared by dc magnetron sputtering. These films consist of small Fe magnetic particles embedded in a nonmagnetic CuAg matrix. Structure, microstructure, morphology and magnetotransport properties were studied. The compositions of these samples were determined by energy-dispersive X-ray analysis. X-ray diffraction results showed strong Ag(111) peaks and broad Cu(111) peaks in all the samples. The variation of the (111) lattice spacings indicates a partial intermixing of Fe, Cu and Ag atoms. Microstructural studies using transmission electron microscopy (TEM) on a selected sample showed only Ag reflections and no reflection from Cu and Fe. Both XRD and TEM studies did not reveal any diffraction peak due to Fe and Cu for this sample. The fitting of the experimental grain size data obtained from TEM micrograph to the lognormal distribution function has allowed an estimation of the average grain diameter of 3.7 nm. The surface image of the Fe22Ag78 film observed using a scanning electron microscope showed the presence of droplet like Ag particles on the film surface. The Cu substitution results in smooth films without any Ag particles on the surface. Surface morphology by atomic force microscopy shows that the Fe39Cu13Ag48 film has a surface roughness of 0.75 rim. Finally, we have obtained a maximum giant magnetoresistance ratio of 3.2% in these films measured at 300 K for an in-plane magnetic field of 20 kOe

Morphology and resistivity of Al thin films grown on Si(111) by molecular beam epitaxy

Thin films of aluminium metal with varying thickness between 10 and 200nm were grown on (I 1 1) Si substrates at 250 degrees C under UHV conditions using molecular beam epitaxy (MBE). Grown thin films were characterized by in situ Xray photoelectron spectroscopy, and ex situ X-ray diffraction, atomic force microscopy and temperature-dependent electrical resisitivity measurements. The results showed that (i) films grow via 3D-island Volmer-Weber growth mechanism, (ii) with increasing film thickness the average grain size increases and the coalescence takes place for thickness > 60 nm, and (iii) independent of the thickness, films grow with (111) orientation. The room-temperature value of resistivity contrary to the predictions of existing theoretical models is found to increase monotonically up to a thickness of 40 nm. This anomalous feature was understood in terms of the film morphology, whereby charge transport takes place via variable range hopping (VRH). For film thickness = 60 mn the resistivity decreased sharply and the M-I transition disappeared. The bulk value of resistivity (2.59 mu Omega cm) was obtained for thickness > 200 urn. (c) 200

High magnetoresistance and low coercivity in electrodeposited Co/Cu granular multilayers

Co/Cu multilayers were electrodeposited from a single solution electrolyte. By reducing the magnetic layer thickness from 1 to 0.2 nm, samples with heterogeneous mixture of ferromagnetic and superparamagnetic clusters have been made. Magnetization and magnetotransport measurements revealed the structured evolution of the multilayer granular samples. With critical thickness of t(Co)similar to 0.5 nm, room temperature magnetoresistance of 7% was obtained at 2 kOe with hysteresis of 70 Oe. The large magnetoresistance obtained in granular multilayers is attributed to the presence of a range of sizes for the Co particles. (c) 200

Bias and temperature dependent charge transport in high mobility cobalt-phthalocyanine thin films

The temperature dependent current-voltage (J-V) characteristics of highly-oriented cobalt phthalocyanine films (rocking-curve width=0.11 degrees) deposited on (001) LaAlO(3) substrates are investigated. In the temperature range 300-100 K, charge transport is governed by bulk-limited processes with a bias dependent crossover from Ohmic (J similar to V) to trap-free space-charge-limited conduction (J similar to V(2)). The mobility (mu) at 300 K has a value of similar to 7 cm(2) V(-1) s(-1) and obeys Arrhenius-type (ln mu similar to 1/T) behavior. However, at temperatures < 100 K, the charge transport is electrode-limited, which undergoes a bias dependent transition from Schottky (ln J similar to V(1/2)) to multistep-tunneling (conductivity varying exponentially on the inverse of the square-root of electric field)

Defect profiling in organic semiconductor multilayers

Defect depth profile study has been carried out in organic semiconductor (OSC) multilayers to characterize the buried interfaces and layers using beam based positron annihilation spectroscopy. The bilayer and trilayer heterostructures (p-n, p-p and n-p-n) comprise of organic-organic and organic-inorganic (substrate) interfaces. Our study reveals the presence of defects at the interfaces whose concentration is seen to vary with the layer thickness. The S-W correlation has been used to examine the effect of organic materials as well as thickness of the layers on the defect microstructure in multilayers. The nature and type of defects in p-p bilayer are seen to be different as compared to p-n and n-p-n multilayers. Positron mobility in OSC layers has been calculated from the fitted diffusion length which is seen to be of the same order as the effective mobility of charge carrier obtained from the measured current density-voltage (J-V) characteristics. The role of structural defects and the intrinsic electric field at the interfaces on positron systematics is also examined. Positron diffusion modeling together with experimental data suggests that the defect at the interfaces has a stronger influence on the positron systematics than the intrinsic electric field across organic-organic interfaces. (C) 2012 Elsevier B.V. All rights reserved