L.M. Fabietti

Mössbauer identification of μ-type metastable phase as the main magnetic component in Nd60Fe30Al10 melt spun alloys

Abstract The local atomic arrangements and the nanoscopically dispersed phases in melt spun Nd 60 Fe 30 Al 10 alloys are investigated by Mōssbauer spectroscopy. It is found that the hard magnetic properties of these alloys are likely to be related to the presence of clusters of a metastable crystalline μ-type phase.

Magnetic viscosity in a nanocrystalline two phase composite with enhanced remanence

Abstract The intrinsic values of the magnetic viscosity and the fluctuation field in a two phase composite Nd 2 Fe 14 B+0.18 vol. αFe were investigated. The composite studied, with equiaxed and isotropically oriented grains had mean grain sizes favoring a strong exchange coupling between the phases. The relations between the intrinsic and the experimentally measured magnitudes are reviewed and the approximations discussed. For applied fields close to the coercivity, the time dependence of the magnetization is not well described by a simple logarithmic law. The intrinsic fluctuation field is found to increase when the reverse internal field increases in all the ranges investigated and two different regimes are identified. These different regimes are discussed considering the distributions of critical fields for nucleation of inverse domains in the hard phase, at hard–soft and hard–hard grain boundaries, respectively.

The influence of microstructure on the martensitic transformation in Cu-Zn-Al melt-spun ribbons

The martensitic transformation was investigated in a set of twin roller melt-spun Cu–Zn–Al shape memory alloys, solidified at tangential wheel speeds between 20 and 40 m/s. The resulting microstructures were analyzed using X-ray diffraction, optical and transmission electron microscopy techniques. The characteristic martensitic transformation temperature, M S, was determined for each condition by conventional resistometric methods. The ribbons are homogeneous in shape and for each quenching rate they exhibit a quite uniform M S temperature. By proper thermal treatments, the different factors affecting M S could be separately examined and from temperature measurements, the contribution of L21 antiphase boundaries evaluated. A calculation of this contribution using pair interchange energies is in good agreement with the experimental results.

Coercive properties of Nd2Fe14B + αFe nanocrystalline composites exhibiting wide distributions of switching fields in the frame of the global nucleation model

Abstract The demagnetization processes in Nd 2 F 14 B+αFe nanocrystalline composites exhibiting square and two-step hysteresis loops are investigated. In all these microstructures, an expanding nucleus model (Global Model) reasonably describes the experimental results in the entire range of the demagnetization loop, provided different sites acting as inverse nucleus are considered. Nucleation is found to take place in both phases and under different coupling conditions exhibiting, in each case, a different reversion regime. Each regime has associated definite values of specific nucleus surface energy and effective local demagnetizing factor, which are similar in all the specimens, in spite of their different microstructures and compositions. These specific surface energy values are close to those expected for domain wall-like configurations resulting from competing interactions in the particular nucleation site, not always related to the intrinsic properties of the phase itself but largely determined by inter-grain interactions.

Nd60Fe30Al10 Glass Forming Magnetic Alloys: A Mechanical Spectroscopy Study at the 300-560 K Temperature Range

The ferromagnetic amorphous phase in rapidly solidified Nd60Fe30Al10 glass forming alloys is investigated in melt spun ribbons (100 µm thick) and in chill cast cylinders (2 mm diameter). The amorphous resulting for these two different quenching rates were characterized by their room temperature hysteresis loops, magnetization and differential calorimetric measurements in the temperature range 300K-900K. The mechanical damping was explored in the 300-560 K temperature range, by measuring the internal friction and the shear modulus in a forced inverted pendulum operating in the frequency range 0.1-10Hz. Simultaneously, the electrical resistance of the samples was measured. The internal friction spectra of both, ribbons and a cylinder, exhibit a local maximum at about 500K, arising in a relaxation mechanism. After some thermal cycles the peak parameters stabilize reaching an apparent activation enthalpy of 1.5 eV and a limit relaxation time τ0 ≈ 0.4-2.5 . 10-17s. In both samples, the electrical resistance largely decreases during the first heating run to 560K and remains unchanged during subsequent thermal cycles. No changes in the elastic modulus or in the damping properties are detected at the Curie temperature of the alloys.

Mössbauer Study of Nd-Fe-Al Nanocomposites

The effects of the cooling rate on the microstructure and magnetic properties of the melt-spun Nd60Fe30Al10 alloys are investigated by Mossbauer Effect (ME) spectroscopy and magnetic hysteresis loops measurements at different temperatures. It is found that the hard magnetic properties of these alloys are likely to be related to the presence of clusters of a metastable crystalline μ-type phase. It is observed that as the cooling rate increases, the amount of this magnetic μ-like phase component decreases in the benefit of an amorphous Nd-rich phase. This phase has an iron concentration, which increases with increasing the wheel speed, is paramagnetic at room temperature and becomes superparamagnetic below approximately 50 K and 100 K for slow and high cooling rates, respectively.