L. Del Bianco

Evidence of spin disorder at the surface–core interface of oxygen passivated Fe nanoparticles

Hysteresis, thermal dependence of magnetization, and coercivity of oxide coated ultrafine Fe particles prepared by inert gas condensation and oxygen passivation have been studied in the 5–300 K range. The results are found to be consistent with a spin-glasslike state of the oxide layer inducing, through exchange interaction with the ferromagnetic core, a shift of the field cooled hysteresis loops at temperatures below the freezing at approximately 50 K.

Synthesis by ball milling and characterization of nanocrystalline Fe3O4 and Fe/Fe3O4 composite system

Nanocrystalline Fe3O4 and a composite system constituted by nanocrystalline Fe and Fe3O4 have been synthesized by ball-milling commercial magnetite and an equimolar mixture of iron and magnetite powders. The physical parameters governing the milling process have been strictly controlled so as to achieve the nanocrystalline state of the precursor material and to avoid chemical reactions. X-ray diffraction and Mossbauer spectroscopy measurements have been carried out both on as-milled powders and on samples previously subjected to annealing treatments in the 100–600 °C temperature range. The results, providing information on the structural and compositional features of the produced samples, are discussed in terms of structural disorder which is healed by subsequent annealing. In the case of the composite system, this analysis indicates that a high mixing degree between the constituent phases has been reached. In particular, the presence of a sextet with anomalous hyperfine parameters in the Mossbauer spectr...

Exchange bias and interface structure in the Ni/NiO nanogranular system

The exchange bias (EB) effect has been studied in Ni/NiO nanogranular samples obtained by annealing in H2, at different temperatures (200 ? Tann ? 300??C), NiO powder previously ball-milled for 20?h. Typically, the samples consist of Ni nanoparticles (mean size of 10?18?nm) embedded in a nanocrystalline NiO matrix. With increasing Tann, the Ni fraction varies from 4% up to 69%. The exchange field depends on the Ni amount, being maximum (~600?Oe), at T = 5?K, in the sample with 15% Ni. In all the samples, the EB effect vanishes at T = 200?K. The structural features of the samples have been investigated by x-ray diffraction, electron microscopy and extended x-ray absorption fine structure and the low-temperature magneto-thermal behaviour has been thoroughly analyzed. The results show the existence of a structurally and magnetically disordered NiO component, which plays the key role in the EB mechanism.

Mechanical behaviour of nanocrystalline iron and nickel in the quasi-static and low frequency anelastic regime

Abstract In this research, we made use of mechanical spectroscopy to study the anelastic behaviour of nanocrystalline Fe and Ni in quasi-static, low-frequency (0.01–10 Hz) regime. The elastic energy dissipation coefficient (Q−1) and the stress relaxation have been measured as a function of frequency and temperature, in a range of temperatures where appreciable grain growth is not expected to occur. The use of such low frequency probes puts into evidence a very strong change in the material response, induced by low temperature annealing (T

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.

Anelastic and structural behavior of ball milled nanostructured iron

Abstract Mechanical spectroscopy and XRD analysis were employed to study the anelastic behaviour and structural evolution of nanostructured iron prepared by mechanical attrition under different conditions. The results show that the thermally induced structural relaxation, without extensive grain growth, leads to strong reductions of the microstrain, dynamic modulus recovery and significant modifications of the anelastic relaxation spectra. These results are compared with those obtained by other structure sensitive techniques. The role of the interfaces and of their structural evolution in determining the observed behaviour of different physical properties is discussed.

Thermal evolution of ball milled nanocrystalline iron

Abstract Mechanical spectroscopy and X-ray diffraction measurements were employed to investigate the thermally induced structural evolution of nanocrystalline iron, prepared by mechanical attrition through different ball milling equipment in different structural states. The evolution of the elastic modulus, grain size and microstrain as a Junction of temperature and after selected isothermal treatments indicates that the rearrangement of structural configuration at the interfaces may stabilise the nanostructure against grain growth.

Electron holography of gas-phase condensed Fe nanoparticles

Electron holography observations were performed on Fe nanoparticles with a mean size of about 50 nm synthesized by gas-phase condensation. Phase maps were obtained which represent the magnetic field both inside and around nanoparticle chains. The results suggest the presence of flux-closure magnetic configurations inside the particles, in agreement with recent micromagnetic calculations.

Anelasticity and structural stability of nanostructured metals and compounds

Abstract Internal friction and dynamic elasticity moduli on thin reeds of nanostructured Al, Fe, FeAl and Fe 3 Al intermetallics prepared by ball milling have been measured. A relaxational damping peak in pure metals at 350–500 K and a modulus increase at 450–500 K without appreciable grain growth has been detected. Moreover, magnetoelastic coupling dependent on grain size has been observed in iron. In the nanophase intermetallic aluminides a strong relaxational damping peak was observed in the 700–800 K range. These results are briefly discussed with reference to the anelastic behaviour of similar coarse grained materials.

Anelastic and magnetoelastic effects and microstructural evolution of the Fe73.5Cu1Nb3Si13.5B9 alloy

Abstract Internal friction, dynamic Youngs modulus and δ E effect have been measured in Fe73.5Cu1Nb3Si13.5B9 ribbons obtained by melt spinning and subsequently submitted to furnace annealing or Joule heating. The measurements have provided informations on the different microstructural states and related magnetic behaviour.