A. Garcı́a Prieto

Interfacial magnetic coupling between Fe nanoparticles in Fe–Ag granular alloys

The role of the interface in mediating interparticle magnetic interactions has been analysed in Fe50Ag50 and Fe55Ag45 granular thin films deposited by the pulsed laser deposition technique (PLD). These samples are composed of crystalline bcc Fe (2–4 nm) nanoparticles and fcc Ag (10–12 nm) nanoparticles, separated by an amorphous Fe50Ag50 interface, occupying around 20% of the sample volume, as determined by x-ray diffraction (XRD), x-ray absorption spectroscopy (XAS), and high resolution transmission electron microscopy (HRTEM). Interfacial magnetic coupling between Fe nanoparticles is studied by dc magnetization and x-ray magnetic circular dichroism (XMCD) measurements at the Fe K and Ag L2,3 edges. This paper reveals that these thin films present two magnetic transitions, at low and high temperatures, which are strongly related to the magnetic state of the amorphous interface, which acts as a barrier for interparticle magnetic coupling.

Structural evolution of Co clusters in Co15Cu85 granular alloys by EXAFS spectroscopy

Abstract Metastable Co 15 Cu 85 alloys, produced by melt-spinning, have been annealed at increasing temperatures ( T A =400°C, 450°C, 500°C, 550°C, 600°C, 650°C) in order to study the evolution of the Co clusters and its relation with magnetic and transport properties. A maximum value of 10% in the magnetoresistance at room temperature is found after annealing at 500°C, when the samples are nearly superparamagnetic. A transition to a ferromagnetic behavior is detected with increasing annealing temperature. EXAFS measurements have allowed us to follow in great detail the appearance and evolution of the Co clusters inside the Co–Cu matrix as a function of the annealing temperature. This sensitive technique allows to detect that when annealing at 500°C, the Co segregation process is interrupted, presumably because the solubility of Co in Cu becomes appreciable at that temperature.

Correlation among the structural and magnetic properties of CoCu granular alloys

We have analyzed the magnetic and structural properties of CoCu melt-spun granular alloys annealed at increasing temperatures by means of their room-temperature hysteresis loops and high resolution x-ray diffraction (XRD) experiments. The magnetic analysis has been performed taking into account two contributions to the total magnetization: one from superparamagnetic nanometer Co clusters responsible also of the giant magnetoresistance (GMR) and another ferromagnetic; originated by a Co-rich phase observed by XRD. We have also detected a percentage of Co that remains diluted into the Cu matrix at high annealing temperatures (Tann⩾500 °C) due to the mixcibility of Co and Cu at those temperatures, and this is in the origin of the drop of the GMR.

Time-resolved X-ray diffraction experiments during annealing of Co15Cu85 granular alloy

Abstract Granular alloys composed of nanoclusters of a magnetic element (e.g. Co, Fe, Ni) embedded in a non-magnetic metallic matrix (e.g. Cu, Au, Ag) present giant magnetoresistance (GMR). The GMR is caused by spin-scattering phenomena at the interfaces between magnetic granules and the matrix, and thus is closely correlated to microstructure of the samples. In melt-spun Co x Cu 100− x samples the GMR exhibits a maximum after annealing at the temperature of 450°C largely independent of the sample composition. The dynamics of the Co–Cu segregation process has directly been probed observing in situ annealing process of the samples in time resolved X-ray diffraction experiments. The results of accurate Rietveld analysis definitively demonstrated that the activation of Co–Cu remixing, when heating the samples around 500°C, is largely independent of the annealing kinetics.

Observation of the segregation and the dissolution of the Co and the Cu in CoCu metastable alloys.

Metastable Co(x)Cu100-x(x=5, 10, 15, 20) alloys have been annealed at increasing temperatures in order to study the evolution of the Co cluster and its relation with the magnetotransport properties. The structure was investigated by X-ray Absorption Spectroscopy on the Co K-edge as a function of composition and annealing temperature. An anomalous trend in the structural evolution has been evidenced and related to the preculiar features observed in the magnetotransport properties.

In situ observation of the structural changes induced by thermal annealing on melt-spun Co15Cu85 granular alloys

Abstract Time-resolved X-ray diffraction measurements have been performed on melt-spun Co 15 Cu 85 granular alloy in order to probe the structural changes that occur during the annealing process. These in situ results have been complemented with previous ex situ high resolution X-ray diffraction experiments and related to the magnetoresistance behaviour of the sample.

Study of surface effects on CoCu nanogranular alloys by ferromagnetic resonance

Room temperature ferromagnetic resonance measurements have been performed on Co5 Cu95 melt-spun nanogranular alloys. Results on the ratio of the orbital-to-spin magnetic moment have been compared to previous results obtained by x-ray magnetic circular dichroism, explaining the differences in terms of the degree of magnetic saturation of the samples. Finally, the ferromagnetic resonance results have been related to the giant magnetoresistance (GMR) response of the samples at each stage of annealing, which has been interpreted as a sign of the interfacial origin of the GMR phenomenon.

Magnetic properties of colloidal cobalt nanoclusters

Co nanoclusters were synthesized by an inverse-micelle chemical route. The magnetic and microstructural properties of the nanoparticles have been analyzed as a function of the surfactant (AOT and DEHP) and the drying method. Microstructural analysis has been performed by TEM and XANES; magnetic properties have been studied by hysteresis loops and zero-field cooling – field cooling (ZFC-FC) curves. TEM images show 2 to 4 nm sized particles spherical in shape. XANES measurements point out a significant presence of Co3O4with metallic Co and some Co2+ bound to the surfactant. The presence of antiferromagnetic Co3O4 explains the magnetic transition observed at low T in both ZFC-FC measurements and hysteresis loops. Finally, the presence of magnetic interactions explains the bigger effective cluster size obtained from hysteresis loops fits (6-10 nm) compared to the sizes observed by TEM (2-4 nm).

Magnetization evolution during thermal treatments of CoCu metastable alloys

Abstract We have studied the magnetization changes during the thermal treatment of CoxCu100−x (x=10,15) melt-spun alloys in a Faraday magnetometer. The evolution of the magnetization points out that the Co segregation starts at 370°C and finishes at 550°C. Room temperature hysteresis loops and zero field cooling curves performed in a SQUID magnetometer on samples as-quenched and annealed at 450°C and 500°C reveal the presence of Co particles, around 2 nm in radius, and a ferromagnetic phase. For sample annealed at 550°C, when the Co segregation has already finished, a ferromagnetic behaviour is detected.