A. Labarta

Magnetic nanoparticles with bulklike properties (invited)

The magnetic behavior of Fe3� xO4 nanoparticles synthesized by either high-temperature decomposition of an organic iron precursor or low-temperature coprecipitation in aqueous conditions is compared. Transmission electron microscopy, x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and magnetization measurements show that nanoparticles synthesized by thermal decomposition display high crystal quality and bulklike magnetic and electronic properties, while nanoparticles synthesized by coprecipitation show much poorer crystallinity and particlelike phenomenology, including reduced magnetization, high closure fields, and shifted hysteresis loops. The key role of the crystal quality is thus suggested, because particlelike behavior for particles larger than about 5 nm is observed only when the particles are structurally defective. These conclusions are supported by Monte Carlo simulations. It is also shown that thermal decomposition is capable of producing nanoparticles that, after further stabilization in physiological conditions, are suitable for biomedical applications such as magnetic resonance imaging or biodistribution studies. V C 2011 American Institute of Physics. [doi:10.1063/1.3559504]

Magnetization and Mössbauer studies of ultrafine Fe-C particles

Abstract The magnetic properties of ultrafine amorphous Fe 1- x C x particles have been studied as a function of temperature and applied field. Magnetization and Mossbauer spectroscopy investigations were combined in order to study superparamagnetic relaxation phenomena. The particles were prepared by thermal decomposition of Fe(CO) 5 . This preparation method is known to produce almost mono-size particles. The mean particle diameter of the Fe-C particles in the present study was about 3.1 nm. The small size resulted in finite size effects, which, e.g. for the temperature dependence of the saturation magnetization gave a significant deviation from the T 3 2 law. The deviation is in accord with recent theoretical calculations of the behaviour of spin waves in ultrafine particles. From the superparamagnetic relaxation studies the minimum relaxation time was estimated to be about 2 × 10 −12 s.

Nanostructural origin of the spin and orbital contribution to the magnetic moment in Fe3−xO4 magnetite nanoparticles

5 nm Fe3−xO4 nanoparticles were synthesized either by high-temperature decomposition in organic phase or in low-temperature aqueous conditions. In the first case oleic acid was covalently bonded to the nanoparticles; in the second case polyvinyl alcohol (PVA) yielded a protective coating without chemical bond. Magnetization measurements and x-ray magnetic circular dichroism showed a saturation magnetization close to bulk magnetite and an orbital moment effectively quenched in covalently bonded nanoparticles. PVA-coated nanoparticles showed a reduced value of the magnetization and ∼3 fold increase in the orbital moment. High resolution electron microscopy suggested that this was related to the nanostructure of the samples.

Magnetic properties of Fe/Cu multilayers

Abstract Fe/Cu multilayers have been prepared under high vacuum conditions. The thickness of the Fe layer is about 5 A while the thickness of the Cu layer is 50 A. A complete study of the magnetic properties have been performed by using a SQUID magnetometer and CEMS. The sample shows a very rich behavior in the low applied field regime. The coexistence of different interactions seems to be present.

Magnetotransport properties of NiFe–Ag granular alloys: Origin of the thermal behavior

The effect of the temperature and magnetic field on the giant magnetoresistivity (GMR) of two FeNi–Ag granular alloys of composition Fe11.4Ni6.4Ag82.2 and Fe7.6Ni16.4Ag76.0 is discussed. Both samples were prepared by rf magnetron sputtering. Parts of them were rapidly annealed at 600, 650, and 750 °C. All samples displayed giant magnetoresistivity which decays from its maximum value with a Tm behavior, with m≈0.8–0.9, suggesting that the decrease in the maximum magnetoresistivity is due to the reduction in the particle magnetization associated with the spin wave excitation, which is a different mechanism to the electron-magnon interaction responsible for the T dependence of GMR in magnetic multilayers. Magnetoresistivity ρM decreases with temperature sharing essentially the same temperature decrease as the square of the macroscopic magnetization M in the whole magnetic field range studied, which is due to the reduction in the particle magnetization and to superparamagnetic effects. The effect of the width...

T ln(tτ0) scaling in small-particle systems: low-temperature behaviour

An alternative method for analyzing the magnetic relaxation data in small-particle systems based on the T ln(tτ0) scaling is presented. Two experimental systems have been studied using this procedure and the resulting master curves have been fitted using different functions for the energy barrier distribution to reproduce the low-temperature relaxation behaviour.

Magnetoelasticity in the Heusler Ni2MnGa alloy

Abstract In this paper we present magnetic and elastic measurements on a NiMnGa single crystal. It is shown that the structural properties of this alloy are strongly influenced by a magnetoelastic interaction.

Magnetic studies of Fe-Y compositionally modulated thin films

Among other interesting problems superlattices or multilayers are very suitable systems to study magnetic properties of ultra thin films, surface effects, periodicity effects, etc. Due to this special properties a wide range of metallic multilayers have been studied with different techniques1,2,3,4,5,6. In our case we are mainly concerned with the iron-silicon system. Early studies on the magnetic properties of amorphous Fe1−xSix thin films were performed by Y. Shimada et al.7 and by D. Bloch et al.8, in this studies the range of composition in which a ferromagnetic ordering exist is available together with some of the structural features, as a function of x, of the FexSi1−x alloys. J.M. Alameda et al.9 have pointed out that the behavior of the saturation magnetization as a function of x between the values 0.66 < x ≤ 0.78 is greatly influenced by the particular conditions of the sample preparation. Clear differences are evident in the magnetic behavior of sputtered samples and evaporated samples pointing out the influence of the short range order effects.

Magnetic history dependence of metastable states in thin films with dipolar interactions

We present the results of a Monte Carlo simulation of the ground state and magnetic relaxation of a model of a thin film consisting of a two-dimensional square lattice of Heisenberg spins with perpendicular anisotropy K, exchange J and long-range dipolar interactions g. We have studied the ground state configurations of this system for a wide range of the interaction parameters J/g, K/g by means of the simulated annealing procedure, showing that the model is able to reproduce the different magnetic configurations found in real samples. We have found the existence of a certain range of K/g, J/g values for which in-plane and out-of-plane configurations are quasi-degenerated in energy. We show that when a system in this region of parameters is perturbed by an external force that is subsequently removed, different kinds of ordering may be induced depending on the followed procedure. In particular, simulations of relaxations from saturation under an AC demagnetizing field or in zero field are in qualitative agreement with recent experiments on epitaxial and granular alloy thin films, which show a wide variety of magnetic patterns depending on their magnetic history.

CoFe-based granular alloys: the role of the metallic matrix

Abstract RF-sputtered CoFe-NM granular alloys (NM=Ag, Cu) with CoFe volume content, x v , ranging from 0.10 to 0.45 have been studied. These two series of samples show similar features depending on the synthesis conditions and post-deposition annealing treatments, revealing the strong dependence of magnetotransport properties on microstructure. Three different regimes have been observed as x v is increased: the classical giant magnetoresistance (GMR) regime at low ferromagnetic contents; at intermediate x v , a domain structure appears, and GMR and anisotropic magnetoresistance (AMR) together with domain wall scattering are observed; and a third regime at x v close but below the volume percolation threshold, where the two latter contributions still coexist, while the GMR contribution has been suppressed by strong magnetic correlations. The role of the metallic matrix is crucial to determine the crossover ferromagnetic contents between these three regimes, which depend on the relative immiscibility of CoFe either in the Ag or Cu matrices and the diffusivity of Ag and Cu. Moreover, the metallic matrix settles the degree of CoFe segregation, sample crystallisation and texture, which are responsible for the magnetotransport properties.