We present an experimental study of the premartensitic and martensitic phase transitions in a Ni 2 MnGa single crystal by using ultrasonic techniques. The effect of applied magnetic field and uniaxial compressive stress has been investigated. It has been found that they substantially modify the elastic and magnetic behavior of the alloy. These experimental findings are a consequence of magnetoelastic effects. The measured magnetic and vibrational behavior agrees with the predictions of a recently proposed Landau-type model ͓A. Planes et al., Phys. Rev. Lett. 79, 3926 ͑1997͔͒ that incorporates a magnetoelastic coupling as a key ingredient.

Granular films composed of well defined nanometric Co particles embedded in an insulating ZrO 2 matrix were prepared by pulsed laser depositon in a wide range of Co volume concentrations ͑0.15Ͻ x Ͻ 0.43͒. High-resolution transmission electron microscopy ͑TEM͒ showed very sharp interfaces between the crystalline particles and the amorphous matrix. Narrow particle size distributions were determined from TEM and by fitting the low-field magnetic susceptibility and isothermal magnetization in the paramagnetic regime to a distribution of Langevin functions. The magnetic particle size varies little for Co volume concentrations x Ͻ 0.32 and increases as the percolation limit is approached. The tunneling magnetoresistance ͑TMR͒ was successfully reproduced using the Inoue-Maekawa model. The maximum value of TMR was temperatureindependent within 50-300 K, and largely increased at low T, suggesting the occurrence of higher-order tunneling processes. Consequently, the tunneling conductance and TMR in clean granular metals are dominated by the Coulomb gap and the inherent particle size distribution.

The ac electrical response is studied in thin films composed of well-defined nanometric Co particles embedded in an insulating ZrO 2 matrix which tends to coat them, preventing the formation of aggregates. In the dielectric regime, ac transport originates from the competition between interparticle capacitive C p and tunneling R t channels, the latter being thermally assisted. This competition yields an absorption phenomenon at a characteristic frequency 1/(R t C p ), which is observed in the range 10-10 000 Hz. In this way, the effective ac properties mimic the universal response of disordered dielectric materials. Temperature and frequency determine the complexity and nature of the ac electrical paths, which have been successfully modeled by an R t -C p network.

An x-ray photoelectron spectroscopy ͑XPS͒ analysis of Nb/Al wedge bilayers, oxidized by both plasma and natural oxidation, is reported. The main goal is to show that the oxidation state-i.e., O:͑oxidize͒Al ratio-, structure and thickness of the surface oxide layer, as well as the thickness of the metallic Al leftover, as functions of the oxidation procedure, can be quantitatively evaluated from the XPS spectra. This is relevant to the detailed characterization of the insulating barriers in ͑magnetic͒ tunnel junctions.

We have studied the effect of ageing within the miscibility gap on the electric, magnetic, and thermodynamic properties of a nonstoichiometric Heusler Cu-Al-Mn shape-memory alloy, which undergoes a martensitic transition from a bcc-based structure (␤ phase͒ towards a close-packed structure (M phase͒. Negative magnetoresistance that shows an almost linear dependence on the square of magnetization with different slopes in the M and ␤ phases was observed. This magnetoresistive effect has been associated with the existence of Mn-rich clusters with the Cu 2 AlMn structure. The effect of an applied magnetic field on the martensitic transition has also been studied. The entropy change between the ␤ and M phases shows negligible dependence on the magnetic field, but it decreases significantly with annealing time within the miscibility gap. Such a decrease is due to the increasing amount of Cu 2 MnAl-rich domains that do not transform martensitically.