Laura B. Steren

From the superparamagnetic to the magnetically ordered state in systems of transition metal clusters embedded in matrices

Abstract One of the interests in systems of ultrafine particles embedded in matrices is to adjust the size of the nanoparticles and the mean distance between them separately to study cluster interactions in a controlled environment. The synthesis of iron and cobalt clusters embedded in an insulating or metallic matrix has been realized by the co-deposition of both beams arriving at the same time on a 45°-tilted substrate. The nanoparticles were produced from an intense cluster beam of selected size (centered around 300 atoms per cluster) produced from a laser vaporization source. We used a Knudsen cell to evaporate the matrix. From the ratio of the deposition rates of both beams, we were able to continuously adjust the atomic concentrations of the clusters in the matrix from 3 to 70%. In situ electrical conductivity measurements confirmed a 3D percolation threshold of around 25%. The typical size distribution of embedded clusters determined from HRTEM observations revealed nanocrystallized grains with a very narrow dispersion in size. Two local environments emerge from EXAFS simulations: core atoms in the cluster with the density of bulk and surface atoms with a dilated parameter. In agreement with structural considerations, we clearly observed by magnetoresistance and magnetization measurements versus temperature and concentration a magnetic percolation threshold of around 25% for Co clusters in a Ag matrix (maximum of 12% GMR) corresponding to the transition from the superparamagnetic to the magnetically ordered state.

Giant magnetoresistance in magnetic nanostructures. Recent developments

Abstract Giant magnetoresis (GMR) effects have now been observed in several types of magnetic nanostructures in which the relative orientation of the magnetic entities can be changed by an applied field. However, previous investigations on multilayers with the current perpendicular to the layer planes (CPP) and on granular systems have raised fundamental problems. Here we describe the results of recent studies on low field GMR in structures including layers and clusters and CPP-GMR.

Giant magnetoresistance in hybrid magnetic nanostructures

Abstract We report on magnetization and magnetoresistance measurements in hybrid structures, composed of soft magnetic layers of permalloy and hard magnetic clustered-layers of cobalt separated by silver. We also present a study of the dependence of the giant magnetoresistance effect with the angle between the magnetizations of successive magnetic layers.

Giant magnetoresistance in magnetic nanostructures

Abstract Since its discovery in magnetic multilayers, the giant magnetoresistance has become one major research topic in both applied and pure magnetism. At first, new theories had to be devised to explain the oscillating indirect exchange coupling and to understand how spin-dependent transport properties give rise to the giant magnetoresistance. Also, because the magnetic multilayer where, at the time, a new class of materials, a lot of attention has been devoted to the study of their structural properties. In this paper, we review some experimental results and theoretical models on GMR in magnetic multilayers. Then, we present an overview of the giant magnetoresistance in new nanostructures - granular alloys and hybrid systems - with magnetic entities in the form of clusters instead of layers.

Giant magnetoresistance in hybrid nanostructures

Abstract We describe the Giant Magnetoresistance (GMR) properties of multilayered structures including continuous layers of permalloy (Ni80Fe20) and discontinuous layers of cobalt separated by layers of copper or silver. These hybrid structures exhibit large GMR ratios with high slopes at low field (as high as 7%/Oe). They are also very convenient for an accurate determination of the angular dependence of the GMR and interesting to discuss the origin of the GMR. We finally analyse GMR measurements performed with the current perpendicular to the layers and separate the bulk and interface contributions.

Magnetoresistance effect in (La, Sr)MnO3 bicrystalline films

The angular dependence of the magnetoresistance effect has been measured on bicrystalline La(0.75)Sr(0.25)MnO(3) films. The measurements have been performed on an electronically lithographed Wheatstone bridge. The study of the angular dependence of both the magnetoresistance and the resistance of single-crystalline and grain-boundary regions of the samples allowed us to isolate two contributions of low-field magnetoresistance in manganites. One of them is associated with the spin-orbit effect, i.e. the anisotropic magnetoresistance of ferromagnetic compounds, and the other one is related to spin-disorder regions at the grain boundary. Complementary x-ray diffraction, ferromagnetic resonance and low temperature magnetization experiments contribute to the characterization of the magnetic anisotropy of the samples and the general comprehension of the problem.

Structural and electrical characterization of ultra-thin SrTiO3 tunnel barriers grown over YBa2Cu3O7 electrodes for the development of high Tc Josephson junctions

The transport properties of ultra-thin SrTiO(3) (STO) layers grown over YBa(2)Cu(3)O(7) electrodes were studied by conductive atomic force microscopy at the nano-scale. A very good control of the barrier thickness was achieved during the deposition process. A phenomenological approach was used to obtain critical parameters regarding the structural and electrical properties of the system. The STO layers present an energy barrier of 0.9 eV and an attenuation length of 0.23 nm, indicating very good insulating properties for the development of high-quality Josephson junctions.

Magnetic reorientation and thermal stability in MnAs/GaAs (100) micro patterns driven by size effects

Size effects and their consequences in the thermal stability of the magnetization of the micro-sized MnAs/GaAs(100) ribbons were studied by magnetic force microscopy. We found out that the orientation in which size is reduced plays a key role in the magnetic configuration and thermal stability of the micro-sized patterns. On the one hand, when reducing the size in the [0001] α-MnAs direction, the system shows an improvement in the thermal stability of the remanent magnetization. On the other hand, when the size is reduced in the [11-20] α-MnAs direction, the system goes through a magnetic reconfiguration from in-plane magnetized to out-of-plane magnetized, which also improves the thermal stability.

Combined effects of vertical and lateral confinement on the magnetic properties of MnAs micro and nano-ribbons

The micromagnetic domain structure of MnAs films gave place to an intense research activity in the last few years due to its potential application in magneto-electronic devices such as domain-wall track memories and logic circuits. These applications require a full knowledge of miniaturization effects on the magnetic properties of the material. In this work, X-ray photoemission electron microscopy has been used for imaging magnetic domains in lithographically fabricated MnAs ribbons, addressing the dependence of the domain configuration on film thickness and ribbon width. Our experiments show a transition from head-on to regular stripe domains below a critical width/thickness ratio wc ≈ 6. Micromagnetic simulations suggest that this transition is correlated to the magnetic structure of the surface plane. Depending on the ribbon width and thickness, the magnetic configuration is shown to evolve from flux-closure domain structure to a state of almost homogeneous magnetization, observed for narrower ribbons. The evolution of the domain structure, magnetic fraction, and magnetization with temperature has been studied across the ferromagnetic/paramagnetic transition. Our experiments show that the magnetic configuration in ribbons exhibits higher stability to temperature variations than in as-cast films.

Anisotropic magnetic-field-induced phase transition in MnAs nanoribbons

MnAs thin films present a phase coexistence of regularly arranged ferromagnetic (α) and paramagnetic (β) stripes below the Curie temperature when grown onto GaAs(100) substrates. In this letter, we report the observation of a magneto-structural phase transition induced by magnetic field on MnAs nanoribbons below the Curie temperature. A transformation of high-resistance paramagnetic regions into low-resistance ferromagnetic ones is observed above temperature-dependent critical magnetic fields. The phenomenon is hysteretic, highly anisotropic, and size dependent and could be the origin of the high magneto-resistance effect observed at temperatures close to room temperature in these systems.