L. Savini

Observation of magnetoresistance in core–shell Fe–Fe oxide systems

A negative magnetoresistance was measured between 15 and 300 K under a maximum field H=70 kOe on two granular systems obtained by compacting Fe nanoparticles surrounded by an oxide shell ∼2 nm thick. The effect depended on the Fe core average size D that was of 8 and 18 nm in the two samples, as by x-ray diffraction. The maximum relative resistance change, about 5%, was observed at 50 K in the sample with smaller D. The results have been interpreted considering intraparticle and interparticle magnetic correlations and microscopic mechanisms similar to those responsible for the magnetoresistance in other granular systems.

Automated resonant mechanical analyzer

An automatic vibrating reed apparatus for internal friction and elastic modulus measurements in solids is described. The apparatus is equipped with a resistive heater for measurements up to 1400 K. A magnetic field (up to 250 kAt/m) is also available in the 300–1000 K range. Measurements as a function of temperature and/or magnetic field can be easily carried out. Data acquisition is controlled by a computer and originally written software allows automatic measurements and real time data analysis offering a user-friendly interface. The acquisition rate is nearly two orders of magnitude faster than that achieved manually. The use of the analyzer is straightforward and does not require a prolonged training of particularly skillful personnel.

Small angle polarised neutron scattering investigation of magnetic nanoparticles

Small angle scattering of polarised neutron (SANSPOL) is a powerful technique for the determination of magnetisation, density and compositional profiles of nanostructured particles. We present here some examples of the magnetic profile determination using the SANSPOL technique and we discuss in detail its advantage with respect to the conventional small angle neutron scattering approach.

Structural configuration and magnetic properties of the rapidly solidified CuCo alloy

The internal friction as a function of temperature and the giant magnetoresistance for varying magnetic field have been measured on Cu90Co10 ribbons obtained by planar flow casting with different quenching rate. The results are discussed considering the presence of quenched-in Co precipitates in the Cu matrix of the as-cast ribbons and different levels of dispersion of Co in the Cu matrix in the two cases.

Structural evolution of a Cu1−xCox alloy during thermal heating studied by acoustic spectroscopy

Abstract A granular CuCo alloy prepared by melt-spinning has a giant magnetoresistance (GMR) effect, whose magnitude depends on thermal treatment, Co concentration, and synthesis conditions. In this paper, the microstructure evolution of Cu 1− x Co x alloy during heating has been investigated by acoustic spectroscopy (internal friction) measurements. It is shown that the nucleation of Co particles occurs at the copper grain boundaries as a consequence of the thermal annealing. Moreover, the high temperature internal friction is directly connected with the presence of Co precipitates in the Cu matrix, depending on the quenching rate and the Co concentration. The results clearly indicate that a careful control of the synthesis parameters is necessary to obtain optimum magnetoresistive behavior.

Scale Dependent Anelasticity and Mechanical Behavior: The Case of Nanocrystalline Metals

A lot of experimental evidence exists now, demonstrating that under suitable circumstances the materials size reduction below critical values leads to physical properties which are significantly modified with respect to those commonly exhibited by conventional materials. Most of the dimensionality effects derive from structural constraints to which a specific physical mechanism is subjected. Critical conditions the scale length of a specific property. Typical examples of structural dimensions are offered by the grain size in bulk materials or the thickness in thin films. The mechanical behavior in the whole range encompassing elastic to plastic regime via anelasticity does not constitute exception to the general rule. Anelastic relaxation processes have been recently observed which are properly described phenomenologically taking into account dimensionality. In nanocrystalline materials, the crystallite size the significant volume fraction of interfaces and the modified dislocation dynamics constitute key parameters which must be carefully controlled to tailor the mechanical behavior. Specific items of scale dependent anelasticity are connected with the possibility offered to combine anelastic relaxation processes depending on atomic diffusion with the grain size. Anelasticity in thin metal films offers also the possibility to explore cases where dislocation dynamics is confined and consequently scale dependent. Interfaces mechanical relaxation is also expected to be modified entering the nanoregime. This item is linked to other relevant features of the mechanical behavior including enhanced microplasticity and superplasticity. This review will report and discuss some recent experimental results obtained by mechanical spectroscopy investigations on nanocrystalline metals and alloys prepared by a variety of synthesis procedures. The results are critically compared to those early obtained on similar conventional materials aiming at understanding some specific aspects of confinement effects on mechanical behavior.

Structure and Thermal Stability of Ball-Milled Fe-FeOxide Mixtures

A comparative study of the milling effect on the equimolar mixture of iron and magnetite powders prepared with two different ball milling devices is presented. Sample microstructure and composition were analyzed by X-ray diffraction and differential scanning calorimetry. Using the shaker device grain size reduction and the mechanically induced formation and thermal stability of wüstite was observed. However, using the planetary device only grain size reduction was observed without chemical reaction to form wüstite. The difference in behaviour is attributed to the different energy transfer characteristics of the two devices. Introduction Mechanical attrition technique by ball milling is widely employed in the production of nanocrystalline metals and composite systems at the nanoscale [1]. A common feature of the materials prepared by this technique is the high degree of metastability deriving from the excess of energy stored into the structure. In the production of compounds, chemical reactions are generally induced and are driven by the energy transferred during the milling process [2]. The aim of this research work is to gain an insight into how the different mechanical energy transfer of two different milling devices (a mixer/shaker mill and a planetary ball mill) may affect the synthesis process and the features of the final products. Moreover, information on the relationship between structural characteristics and thermal stability of the milled samples were obtained. Experimental An equimolar mixture of pure (99.99 at%) commercial Fe powder and commercial magnetite (Fe3O4) powder was milled using both a SPEX8000 mixer/mill and a planetary ball mill. In the commercial Fe3O4 powder a hematite (α-Fe2O3) fraction of about 10 mol% was found. For the SPEX milling, the vials were sealed in air with a powder/spheres mass ratio of 1/10. The O2 content inside the vials was estimated to be ~2.7 mol% with respect to Fe and Fe3O4. Four different samples were milled for 2, 4, 8, and 16 hours, labeled respectively as 2hS, 4hS, 8hS, and 16hS. After milling the vials were below atmospheric pressure indicating that no gas exchange with the atmosphere occurred during milling. For the planetary ball milling, the mill was operated in high vacuum with a powder/spheres mass ratio of 1/7. The powders were milled for 50 hours to produce the 50hP sample. The periodic motion of the vials was characterized by frequencies of ~10 Hz for the planetary mill and ~16 Hz for the shaker mill. * On leave from Departamenti i Fizikes, Universiteti “A. Xhuvani” Elbasan, Rruga “Rinia”, Elbasan Albania. Journal of Metastable and Nanocrystalline Materials Online: 2001-06-01 ISSN: 2297-6620, Vol. 9, pp 37-40 doi:10.4028/www.scientific.net/JMNM.9.37

Magnetic and Elastic Effects in Rapidly Solidified Cu100-xCox Alloys

The Co precipitation in the Cu 100-x Co x (x ≤ 12) granular system obtained by rapid solidification has been investigated by acoustical spectroscopy and magnetic measurements. Particular attention was given in the preparation of samples and annealing procedure in order to identify the optimum structural conditions for the observation of high magnetoresistance at room temperature. The magnetisation curves of the superparamagnetic samples are related to the distribution of Co-rich particles and the elastic energy dissipation coefficient vs. temperature is strongly affected by the microstructure resulting from different Co concentration and thermal treatments. So we obtained information about the distribution of Co both in the as-quenched and annealed samples.