Evidence of giant magnetoresistance effect in heterogeneous nanogranular films produced by ultrashort pulsed laser deposition (original) (raw)
Related papers
Ultrafast pulsed laser deposition as a method for the synthesis of innovative magnetic films
Applied Surface Science, 2009
Exchange-coupled monocomponent magnetic films constituted of disk-shaped Ni and Fe nanoparticles were produced by ultrafast pulsed laser deposition, in vacuum. These films show a peculiar cauliflower-like structure, made of granular agglomerates of nanoparticles sticking to one another with a significant shape and orientation anisotropy. Both as-deposited Ni and Fe films present hysteresis loops with a high in-plane remanence ratio (0.61 and 0.81 at 250 K, respectively), relatively low values of the saturation and coercive fields and a steep slope near coercivity. At temperature of 10 K and 250 K, the magnetization curves confirm the strong influence of the production technique on the topologic structure of these films, and consequently on their magnetic properties. In perspective, the striking and intriguing properties of these nanogranular films appear very promising for potential application as permanent magnets and in data storage technology.
Journal of Alloys and Compounds, 2013
Structural, microstructural, morphology and magnetoresistive properties of Fe x Cu y Ag z granular thin films exhibiting giant magnetoresistance were investigated. Several series of samples with wide concentration range were prepared by DC magnetron sputtering. The binary FeCu and FeAg systems are the end components of the ternary Fe x Cu y Ag z system. The compositions of the samples were determined by energydispersive X-ray analysis. X-ray diffraction patterns showed Ag(1 1 1) and Cu(1 1 1) peaks and the Fe peaks were not observed. The d-spacings calculated from the Ag(1 1 1) and Cu(1 1 1) peaks are smaller and greater than the standard bulk value for Ag and Cu, respectively, indicating a partial intermixing of Fe, Cu and Ag atoms. The electron diffraction patterns obtained on selected samples showed many Cu and Ag reflections and they also did not show any reflection from Fe. The electron micrographs show the granular structure of these films with typical granule size of 4-5 nm. The surface image of the FeAg films observed using a scanning electron microscope showed the presence of droplet-like Ag particles on the film surface. On the other hand the Fe x Cu y and Fe x Cu y Ag z films are smooth without any Ag particles on the film surface. Surface morphology of some samples studied by atomic force microscopy showed that the films are smooth and their typical surface roughness is 0.5-2.0 nm. Magnetoresistance measurements on the samples up to 20 kOe showed typical superparamagnetic behaviour. We have obtained a maximum giant magnetoresistance ratio of 3.8% in these films measured at 300 K for an in-plane magnetic field of 20 kOe indicating an increase in the value of magnetoresistance due to Ag substitution in the FeCu films. Percolation limit for the Fe grains has also been obtained from the magnetoresistance data.
Magnetic properties of Fe∕MgO granular multilayers prepared by pulsed laser deposition
Journal of Applied Physics, 2009
Granular multilayers ͓Fe͑t nm͒ / MgO͑3 nm͔͒ N with 0.4 nmഛ t ഛ 1.5 nm were prepared by sequential pulsed laser deposition. Transmission electron microscopy ͑TEM͒ images show that increasing t causes the growth of the sizes of Fe nanoparticles and broadening of the particle size distribution. For t Ͼ 0.81 nm, continuous Fe layers are formed. The evolution of the shapes and sizes of the particles is reflected in the magnetic properties of the investigated films. A crossover from superparamagnetic to ferromagnetic behavior upon formation of a continuous Fe layer is observed. The fit of zero field cooled and field cooled susceptibility measurements and magnetization curves using Curie-Weiss law and a weighted sum of Langevin functions, respectively, allows the estimation of the average granule size for the films with t Ͻ 0.61 nm. The results of the estimations correlate with the data obtained from TEM images. Reduction of saturation magnetization for Fe nanoparticles and an increase of the coercivity up to 1200 Oe at low temperatures were found. It is attributed to the formation of Fe-core/FeO x-shell structured nanocrystals. The oxide shell gives rise to a strong contribution of surface anisotropy. Isotropic tunneling magnetoresistance up to ϳ3% at room temperature and in magnetic field up to 18 kOe was found for the film with t = 0.61 nm. For higher t, an anisotropic magnetoresistance typical for continuous ferromagnetic films was observed.
Nanogranular Fe-Cu-Ag Thin Films: Structure, Microstructure and Giant Magnetoresistance
Journal of Nanoscience and Nanotechnology, 2008
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...
Pulsed laser deposition assisted novel synthesis of self-assembled magnetic nanoparticles
Composites Part B-engineering, 2004
We report here a novel thin film processing method based upon pulsed laser deposition to process nanocrystalline materials with accurate size and interface control with improved magnetic properties. Using this method, single-domain nanocrystalline Fe and Ni particles in the 5 -10 nm size range embedded in amorphous alumina as well as in crystalline TiN have been produced. By controlling the size distribution in confined layers, it was possible to tune the magnetic properties from superparamagnetic to ferromagnetic behavior. Magnetic hysteresis characteristics below the blocking temperature are consistent with single-domain behavior. The paper also presents our results from investigations in which scanning transmission electron microscopy with atomic number contrast (STEM-Z) and electron energy loss spectroscopy (EELS) were used to understand the atomic structure of Ni nanoparticles and interface between the nanoparticles and the surrounding matrices. It was interesting to learn from EELS measurements at interfaces of individual grains that Ni in alumina matrix does not form an ionic bond indicating the absence of metal -oxygen bond at the interface. The absence of metal -oxygen bond, in turn, suggests the absence of any dead layer on Ni nanoparticles even in an oxide matrix. q
2009
We present a detailed study on the morphology and magnetic properties of Co nanostructures deposited onto oxidized Si substrates by femtosecond pulsed laser deposition. Generally, Co disks of nanometric dimensions are obtained just above the ablation threshold, with a size distribution characterized by an increasingly larger number of disks as their size diminishes, and with a maximum disk size that depends on the laser power density. In Au/Co/Au structures, in-plane magnetic anisotropy is observed in all cases, with no indication of superparamagnetism regardless of the amount of material or the laser power density. Magnetic force microscopy observations show coexistence of single-domain and vortex states for the magnetic domain structure of the disks. Superconducting quantum interference device magnetometry and x-ray magnetic circular dichroism measurements point to saturation magnetization values lower than the bulk, probably due to partial oxidation of the Co resulting from incomplete coverage by the Au capping layer.
Giant magnetoresistance in nanogranular magnets
EPL (Europhysics Letters), 2008
We study the giant magnetoresistance of nanogranular magnets in the presence of an external magnetic field and finite temperature. We show that the magnetization of arrays of nanogranular magnets has hysteretic behaviour at low temperatures leading to a double peak in the magnetoresistance which coalesces at high temperatures into a single peak. We numerically calculate the magnetization of magnetic domains and the motion of domain walls in this system using a combined mean-field approach and a model for an elastic membrane moving in a random medium, respectively. From the obtained results, we calculate the electric resistivity as a function of magnetic field and temperature. Our findings show excellent agreement with various experimental data.
Nanosecond laser-induced synthesis of nanoparticles with tailorable magneticanisotropy
Journal of Magnetism and Magnetic Materials, 2011
Controlling the magnetic orientation of nanoparticles is important for many applications. Recently, it has been shown that single domain ferromagnetic hemispherical Co nanoparticles prepared by nanosecond laser-induced self-organization, show magnetic orientation that was related to the negative sign of the magnetostrictive coefficient l S [J. Appl. Phys. v103, p073902, 2008]. Here we have extended this work to the Fe 50 Co 50 alloy, which has a positive l S and Ni, which has a negative l S .