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A model describing the effect of magnetic dipolar interactions on the susceptibility of magnetic nanopillars is presented. It is an extension of a recently reported model for three-dimensional randomlike dispersions of nearly spherical nanoparticles in equilibrium [Sánchez et al., Phys. Rev. B 95, 134421 (2017)2469-995010.1103/PhysRevB.95.134421], to well-ordered arrays of nanoparticles out of equilibrium. To test it, a high-quality benchmark consisting of a two-dimensional hexagonal arrangement of quasi-identical parallel nickel nanopillars embedded in a porous alumina template was fabricated. This model is based on an effective demagnetizing tensor, which only depends on a few morphological parameters of the sample, as the nearest-neighbor distance between pillars and the volume fraction of pillars in the specimen. It allows us to obtain the nanopillar intrinsic susceptibility tensor from the magnetic response of the nanopillar ensemble. The values of the in-plane and normal-to-plane susceptibility of the sample are successfully predicted by the model. Furthermore, the model reproduces the susceptibility in the applied field direction, measured for different applied field angles. In this way, it provides a simple and accurate treatment to account for the complex magnetic effects produced by dipolar interactions., Facultad de Ciencias Exactas, Instituto de Física La Plata

The intermetallic compound AlFe in its ordered B2 phase has been ground under Ar in a ball-milling device. The system evolution under mechanical work has been examined as a function of time of milling by x-ray diffraction, transmission Mossbauer spectroscopy, and differential scanning calorimetry. Short and long range disorder was observed. The evolution of the long range order parameter with milling time is analyzed up to the completion of the solid state reaction when a steady state value is reached. The short range order evolution is consistent with a nonstatistical order distribution throughout the sample but mainly distributed in two regions with different degrees of order each. Kinetic analysis of the reordering and recrystallization has been per- formed under continuous heating and isothermal calorimetric regimes. Modeling of these processes is pre- sented as well as the estimated values of both the enthalpy and the activation energy of the reordering process. Results are compared with preceding information on related systems., Facultad de Ciencias Exactas

Metastable [gamma]-FeSi[sub 2] fluorite-type was formed as precipitates in Si(100) by room-temperature [sup 57]Fe implantation, followed by ion-beam-induced epitaxial crystallization at 593 K. In spite of the cubic structure of [gamma]-FeSi[sub 2], the Moessbauer spectrum consists of two broad lines, rather than a single line. It is best described by means of a smooth distribution of electric field gradients with nearly the same isomer shift ([delta][sub IS]=0.198 mm/s) for each gradient. After thermal treatments at 773 K the distribution narrows and a finite probability occurrence frequency for zero gradient appears. As the structure of ideal [gamma]-FeSi[sub 2] consists of Si cubes alternately filled with Fe atoms, it is suggested that the distribution arises from misplacement of Fe atoms to empty Si cubes in the lattice. The dependence with thermal treatments and the evolution towards [beta]-FeSi[sub 2] are discussed.

Based on the evolution of the rapidly quenched Fe 1-x B x alloy density, 57 Fe and 11 B hyperfine fields, 11 B NMR linewidth, and magnetic moment per alloy atom, in the composition range 0.01≤x≤0.25, it is concluded that the short-range order around boron atoms in the amorphous alloys evolve from (orthorhombic) o-Fe 3 B-like at x≅0.12 to (tetragonal) t-Fe 3 B-like at x≅0.22-0.25

Mise en evidence de trois regions differentes: une riche en silicium ou les ions maintiennent un fort caractere covalent dans une structure de reseau aleatoire continu; une region intermediaire ou le caractere covalent diminue; et une region riche en fer ou les effets covalents sont absents

We report here the unexpected observation of significant room-temperature ferromagnetism in a semiconductor doped with nonmagnetic impurities, Cu-doped TiO₂ thin films grown by pulsed laser deposition. The magnetic moment, calculated from the magnetization curves, resulted surprisingly large, about 1.5 μB per Cu atom. A large magnetic moment was also obtained from ab initio calculations, but only if an oxygen vacancy in the nearest-neighbor shell of Cu was present. This result suggests that the role of oxygen vacancies is crucial for the appearance of ferromagnetism. The calculations also predict that Cu doping favors the formation of oxygen vacancies., Facultad de Ciencias Exactas, Instituto de Física La Plata

The first quantitative results on internal oxidation of impurities in a noble-metal host, using the time-differential perturbed angular-correlation technique, are reported here. The system studied was indium-silver, with a few ppm of indium. The existence of a well-defined oxidation front is found for this highly dilute alloy. The kinetics of oxidation were determined for oxidating temperatures of 400 and 500\ifmmode^\circ\else\textdegree\fi{}C. Our results are in good agreement with a parabolic dependence of the depth of the internal oxidation front on the elapsed oxidation time. No unique indium-oxygen configurations were found in the range 280-900\ifmmode^\circ\else\textdegree\fi{}C.

The internal oxidation of $\mathrm{Ag}\mathrm{In}$ alloys has been investigated by time-differential perturbed angular correlation technique on $^{111}\mathrm{In}$ probes. We studied the oxidation kinetics of dilute (a few ppm) $\mathrm{Ag}\mathrm{In}$ alloys at 300\ifmmode^\circ\else\textdegree\fi{}C under different annealing conditions and 1 at.% indium alloys at 300\ifmmode^\circ\else\textdegree\fi{}C and 550\ifmmode^\circ\else\textdegree\fi{}C. In some cases, the time dependence of the oxidation-front advance was ruled by a power law with an exponent different from 0.5. We determined the hyperfine parameters of ${\mathrm{In}}_{2}$${\mathrm{O}}_{3}$ and studied the influence of the oxidation temperature on the indium sesquioxide precipitation in the silver matrix. A consistent description of the oxides formed at different temperatures is given.