Mursel Alper

Giant magnetoresistance in electrodeposited superlattices

We have observed ‘‘giant magnetoresistance’’ in short‐period Cu/Co‐Ni‐Cu alloy superlattices electrodeposited from a single electrolyte under potentiostatic control. The superlattices were grown on polycrystalline Cu substrates which were removed before transport measurements were made. Room‐temperature magnetoresistances of over 15% in applied magnetic fields of up to 8 kOe were observed in superlattices having Cu layer thicknesses of less than 10 A.

Growth and characterization of electrodeposited Cu/Cu-Ni-Co alloy superlattices

Abstract We have used a recently developed potentiostatic technique to electrodeposit Cu/Cu-Ni-Co alloy superlattices from a single electrolyte. Our films have been characterized using X-ray diffraction, electron backscatter diffraction patterns and electron probe microanalysis. Good epitaxy and repeat distances as short as 15 A have been achieved.

Magnetoresistance of CoNiCu/Cu Multilayers Electrodeposited from Electrolytes with Different Ni Ion Concentrations

A series of CoNiCu/Cu multilayers was potentiostatically electrodeposited on strong 100 textured Cu substrates from electrolytes with different Ni concentrations. X-ray diffraction patterns showed that all studied samples exhibited a face-centered cubic structure. From scanning electron microscopy images, it was observed that the surface morphology of the films is affected by their Ni content. The compositional analysis by energy-dispersive X-ray spectroscopy demonstrated that as the Ni ion concentration in the electrolyte is increased, the Ni content of the film increases. Magnetoresistance MR measurements were carried out at room temperature in the magnetic fields of 12 kOe. The samples grown from the electrolytes with the Ni concentration of up to 0.3 M exhibited giant magnetoresistance GMR. For the samples grown from 0.3 M Ni, anisotropic magnetoresistance AMR begins to appear as well as GMR. As the Ni concentration in the electrolytes is increased, the AMR effect enhances, whereas the GMR effect weakens. For the 2.0 M Ni concentration, the MR behavior of multilayers converts from GMR to AMR. Furthermore, the magnetization measurements made by a vibrating sample magnetometer showed that the antiferromagnetic coupling between ferromagnetic layers weakens with increasing the Ni concentrations.

Properties of Co–Fe Films: Dependence of Cathode Potentials

Co-Fe films were electrodeposited on Titanium substrates at the cathode potentials changing from - 1.8 to - 2.7 V with respect to saturated calomel electrode (SCE). The structural analysis by X-ray diffraction revealed that all films have a mixed phase of face centered cubic and body centered cubic structure, but the phase ratios change depending on the cathode potentials. The compositional analysis, which was made using an energy dispersive X-ray spectrometry, demonstrated that the Co content of the films slightly increases as the deposition potential increases. The morphological analysis of the films grown at high deposition potential (-2.7 V versus SCE), studied by scanning electron microscopy, indicate that they have larger grains. All Co-Fe films showed anisotropic magnetic resistance up to 4% and its magnitude was affected by the cathode potentials. Magnetic measurements carried out by the vibrating sample magnetometer indicated that the saturation magnetization varied and the coercivity decreased from 46.98 to 31.74 Oe as the cathode potential increased. The easy axis of magnetization was found to be in the film plane for all films. The variation in magnetoresistance and magnetic properties may be related to the structural changes in the films.

Composition Dependence of Structural and Magnetic Properties of Electrodeposited Co-Cu Films

The structural and magnetic properties of Co-Cu films were studied in terms of Co content in the films. The surface morphology of the films showed that the film with the lowest Co content (3 wt. %) had dendritic structures, whereas the surface of the film containing the highest amount of Co (61 wt. %) was more uniform with acicular shapes in some parts. X-ray diffraction patterns of the films showed that their crystal structure is a mixture of dominantly face-centered cubic (fcc) and hexagonal close-packed phases. At the lowest Co content, separate fcc (111) peaks appear, whereas the increase of Co content converts the peaks to be a single broad Co-Cu peak. The vibrating sample magnetometer measurements revealed that the saturation magnetization increases and coercivity decreases due to the decrease of the grain size caused by the increase of the Co content in the film. The structural and magnetic properties of Co-Cu films can be tailored, since these properties are directly related to the ratio of Co to Cu in the film.

Electrodeposition of Multilayered Nanostructures

Electrodeposition is one of the techniques used to produce nanostructured magnetic multilayers as well as single thin films. We have developed an experimental system to produce nanostructure materials such as metal/metal multilayers and multilayer nanowires by electrodeposition. The properties of such structures may depend on many electrochemical parameters such as the electrolyte concentration, the electrolyte pH, the deposition potentials of metals and the current efficiency. The growth mechanisms and the structural and magnetotransport properties of ferromagnetic multilayers grown by electrodeposition from a single electrolyte are studied.

Differences observed in properties of ternary NiCoFe films electrodeposited at low and high pH

Ternary NiCoFe films were potentiostatically electrodeposited from the electrolytes with low (3.0) and high (3.6) pH levels, and differences in their compositional, structural, magnetic and magnetoresistance properties were studied. The compositional analysis demonstrated that the Ni content in the films decreased, and Co and Fe content increased while electrolyte pH was changed from low to high level. The structural analysis of the films was carried out using the X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The XRD data revealed that the films have a strong (111) texture of the face-centred cubic (fcc) structure at low pH, while for the films at high pH a mixture of dominantly fcc and hexagonal closed packed structure was observed. The SEM studies showed that films grown at low pH level had comparatively larger grains than those at high pH. The magnetic characteristics studied by a vibrating sample magnetometer and magnetotransport properties were seen to be changed by the electrolyte pH. However, all films have in-plane magnetic anisotropy. The differences observed in the magnetic and magnetotransport properties were attributed to the microstructural changes caused by the electrolyte pH.

Study of Electrolyte pH in Production of Cu-Co-Ni Ternary Alloys and Its Effect on Microstructural and Magnetic Properties

Ternary Cu-Co-Ni alloys were electrodeposited in the mediums that have different pH levels to investigate the microstructuremagnetic relationship. The electrolyte pH was considered as high (4.2) and low (2.5) levels. For both levels, the anomalous codeposition occurred in the deposition of the alloys. The face centered cubic crystalline structure was observed in the ternary alloys grown at high and low electrolyte pH levels. In addition, a significant decrease in average grain size of the crystallites, from 60 to 21 nm, occurred when the pH was reduced from 4.2 to 2.5. A dendritic structure was detected in the surface of the alloy deposited from electrolyte with the high pH level, whereas the alloy grown at the low pH has a surface without dendritic clumps. According to magnetic analysis, an increase in the saturation magnetization, caused by the increase of Co content, was detected when the pH was reduced. The coercivity Hc levels were 89 and 80 Oe for the alloys grown at high and low pH, respectively. The change of Hc was attributed to the changes in microstructural properties, i.e., different average grain sizes of the crystallites and surface morphologies, occurred when the electrolyte pH changed. The anisotropic magnetoresistance (AMR) was exhibited by the alloys and the AMR ratio was enhanced from ~3% to ~3.5%-4% when the pH decreased.

Facile electrodeposition CoCu/Cu multilayers: deposition potentials for magnetic layers

The Co(Cu)/Cu magnetic multilayers were produced by electrodeposition technique as a function of the cathode potentials for magnetic layer deposition from a single bath. For proper depositions, cyclic voltammograms were used and the current–time transients were obtained. All potentials were determined with respect to saturated calomel electrode. The Co layers were deposited at cathode potentials of −1.3, −1.5 and −1.7 V, while −0.3 V was used for the Cu layers deposition. All multilayers were polycrystalline in the face-centred-cubic (fcc) structure with both Co and Cu layers adopting the fcc structure. The crystal structure of the multilayers is the same as fcc bulk Cu, but (220) peak splits the two peaks which are Cu(220) and Co(220). Both Co and Cu diffraction lines overlap in the (111) and (200) strong peaks and thus they seem to be a single peak. In the magnetisation measurements, the highest saturation magnetization was found to be 212 kA/m in producing with −1.5 V for Co deposition potential. The coercivities of multilayers are found to be 12.1, 16.9 and 18.3 kA/m for −1.3, −1.5 and −1.7 V cathode potentials, respectively.

Determination of Texture Orientation Related Magnetic Properties of Nickel-Cobalt Films

The determination of texture effects in nickel-cobalt (Ni-Co) films with different thickness, which were obtained by electrodeposition, has been investigated by the measurement of hysteresis loops at different angles. Easy-axis distribution measurements were performed as a function of the squareness Mp(β ) and the correlations were established among the different thicknesses. The composition of Ni- Co films was determined by energy dispersive X-ray spectroscopy. The structural analysis made by X-ray diffraction revealed that all films have a polycrystalline face-centered cubic structure but their texture degrees vary depending on the film thickness. The determination of the easy-axis orientation in 2-D films from the Mp(β ) obtained by the hystersis loops was studied using Fourier series analyses. The coefficient A0 have a value of less then unity while A2 is inversely proportional to the width of the distribution function which may cause the change in the texture preferential orientations. Therefore, the differences observed in the magnetic easy-axis distributions were attributed to the changes in texture orientations caused by the compositional differences at different thicknesses of the polycrystalline films.