R. Lopez Anton

Preparation and characterisation of Cu-Co heterogeneous alloys by potentiostatic electrodeposition

Abstract Electrodeposition in the potentiostatic mode has been used to produce Cu–Co metastable alloys. The influence of different plating parameters, such as bath composition, working electrode voltage (vs. reference) and intensity of agitation, on the composition and properties of the deposit has been investigated. A broad range of compositions (2–26 at.% Co) are obtainable. In particular, the Co content in the samples is determined mainly by the Co concentration in the bath and the electrode voltage applied during the deposition. The lack of agitation produces a deposition with a higher Co content. Experimentally obtained magnetic loops have been fitted successfully taking into account a superparamagnetic cluster size distribution. From these fits we conclude that the as deposited samples consist of superparamagnetic clusters and diluted Co atoms which do not contribute to magnetisation. These results demonstrate that electrodeposition is a promising and inexpensive preparation method for obtaining Cu–Co heterogeneous alloys with controllable and adequate properties for technological applications.

Magnetic domains in Ni–Mn–Ga martensitic thin films

A series of martensitic Ni52Mn24Ga24 thin films deposited on alumina ceramic substrates has been prepared by using RF (radio-frequency) magnetron sputtering. The film thickness, d, varies from 0.1 to 5.0 µm. Magnetic domain patterns have been imaged by the MFM (magnetic force microscopy) technique. A maze domain structure is found for all studied films. MFM shows a large out-of-plane magnetization component and a rather uniform domain width for each film thickness. The domain width, δ, depends on the film thickness as in the whole studied range of film thickness. This dependence is the expected one for magnetic anisotropy and magnetostatic contributions in a perpendicular magnetic domain configuration. The proportionality coefficient is also consistent with the values of saturation magnetization and magnetic anisotropy determined in the samples.

High-vacuum annealing reduction of Co/CoO nanoparticles

Porous films of Co/CoO magnetic nanoparticles have been obtained by inert gas condensation and partially oxidized in situ in the deposition chamber. These nanoparticle films were subjected to thermal treatments in high vacuum and the chemical and structural changes monitored by x-ray diffraction, transmission electron microscopy, transport and magnetic measurements (with a focus on the exchange-bias phenomenon), which evidence that for vacuum annealing temperatures above 360 °C, most of the CoO phase is reduced to metallic Co without requiring the presence of an external reducing agent (e.g., H₂) or a plasma. Additionally, there is a certain degree of particle coalescence resulting in the formation of greater nanoparticles as the annealing temperature increases. This yields a smaller proportion of CoO compared to metallic Co and a reduction of the Co/CoO interface density, pinpointed by a drastic decrease of the exchange-bias field. The crucial roles of the vacuum level and the surface-to-volume ratio are evidenced by magnetic measurements, highlighting the potential of magnetometry as a probe for the reduction/oxidation of composite nanostructures.

Unusual magnetic properties in Pr1−xSrxFe0.8Ni0.2O3−δ (x≤0.3)

Pr1−xSrxFe0.8Ni0.2O3−δ (x=0.1, 0.2, and 0.3) polycrystalline phases were prepared by combustion synthesis. Their magnetic properties and Mossbauer effect have been examined in the range 5–300 K. Mossbauer spectroscopy shows the existence of charge disproportionation in several of the samples at low temperatures. Meanwhile, the samples present multiple features pointing out a frustrated magnetic order, with competing ferro- and antiferromagnetic interactions. Additionally, exchange bias (EB) phenomena have been observed. Hence, when the samples are cooled in a static magnetic field down to temperatures below 50 K, the hysteresis loops shift in the opposite direction to the applied biasing field, with shifts up to 10 kOe and more, and a small vertical shift of the loops also appears. This behavior seems linked to spin disorder and to the appearance of ferromagnetic (FM) ordered regions at low temperatures when cooling with a high applied field. The growth of these FM regions is likely due to the aligning ro...

Influence of the preparation method on the properties of Cu-Co heterogeneous alloys

Abstract Cu88Co12 samples have been obtained by electrodeposition and laser ablation and have been annealed at increasing temperatures (450°C, 500°C and 550°C) to compare the influence of the fabrication method on the magnetotransport properties of the samples. Magnetoresistance (MR) and X-ray diffraction (XRD) measurements show different properties between the samples prepared by the two techniques. It was found that the electrodeposited (ED) samples had a larger MR. The properties of the laser-ablated samples depend on the deposition process, showing an anisotropy when prepared with a smaller target rotation speed.

The effect of the deposition parameters on the magnetic and magnetotransport properties of laser ablated Cu–Co thin films

Cu90Co10 samples have been prepared by laser ablation under different deposition parameters and have been subjected to thermal treatment. The microstructure of the samples has been studied through the fitting of the hysteresis loops and magnetoresistance (MR) curves. The as-deposited samples consist of superparamagnetic (SPM) clusters and diluted Co atoms. Upon annealing, greater SPM clusters and a ferromagnetic phase appear. The presence of the ferromagnetic phase is also confirmed by X-ray diffraction (XRD). We have found that the deposition parameters greatly influence the microstructure, affecting the shape of the Co nanoclusters and the percentage of Co atoms contributing to the SPM phase, therefore affecting the magnetotransport properties.

Synthesis and characterisation of electrodeposited Cu90Co10 thin film

Abstract Cu90Co10 samples have been prepared by electrodeposition and have been subjected to thermal treatment. The microstructure of the samples have been studied through the fitting of the hysteresis loops and magnetoresistance curves, considering the possibility of ferromagnetic and superparamagnetic contributions. The as-deposited sample consists of superparamagnetic clusters and diluted Co atoms. Upon annealing, greater superparamagnetic and ferromagnetic clusters appear.

The properties of Co–Cu melt-spun ribbons and thin films: similarity and difference

Abstract Magnetic properties, magnetoresistance and microstructure of the rapidly quenched Co20Cu80 ribbons and Co17Cu83 thin films are comparatively analysed. It is found that the magnetic properties and giant magnetoresistance show qualitatively similar behaviour as a function of annealing temperature, but their microstructures and phase compositions determined by transmission electron microscope are quite different.

Study of GdCo/Si/Co/Si Multilayers by Polarized Neutron Reflectivity

In order to better understand the interesting magnetic behaviour found in the Rare Earth-transition metal/(spacer)/transition metal multilayers, a multilayer of Co/[Si/Gd0.36Co0.64/Si/Co]4/glass was studied by SQUID magnetic measurements and polarized neutron reflectivity. A model for the magnetic profile of the sample was proposed, which fitted well the polarized neutron reflectivity measurements at different magnetic states: ferrimagnetic/antiparallel state, under an applied field of 55 Oe, canted states (315 and 675 Oe) and saturation state (1700 Oe). It was found that even at the expected collinear states (ferrimagnet and saturation states), the magnetization was not totally collinear and there was a small component of the magnetization perpendicular to the applied field

Interlayer Coupling in Co/Ti/(Gd–Co)/Ti Artificial Layered Ferrimagnet

The magnetic properties and the structure of [Co/Ti/Gd0.36Co0.64/Ti]4/Co multilayers are investigated by means of torque magnetometer, vibrating sample magnetometer and transverse magneto-optic Kerr effect (TMOKE) measurements and the atomic force microscopy. Due to interlayer exchange interaction, Co and Gd–Co layers form a macroscopic ferrimagnetic system. The change in the sign of the TMOKE hysteresis loops near the compensation temperature and field induced magnetic phase transitions are found. The latter can be characterized by a critical field which shows a linear variation with the temperature. The magnetic properties of these multilayers from many points of view are similar to those of bulk ferrimagnets.