S.E. Urreta

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.

Magnetization mechanisms in ordered arrays of polycrystalline Fe100−xCox nanowires

Magnetization reversal processes and coercivity mechanisms in polycrystalline Fe100−xCox nanowire arrays, resulting from an AC electrodeposition process, are investigated. The array coercivity is described on the basis of polarization reversal mechanisms operating in individual wires, under the effect of inter-wire dipolar interactions described by a mean field approximation. For individual wires, a reversal mechanism involving the nucleation and further expansion of domain-wall like spin configuration is considered. The wires have a mean grain size larger than both the nanowire diameter and the exchange length, so localized and non-cooperative nucleation modes are considered. As the Co content increases, the alloy saturation polarization gradually decreases, but the coercive field and the relative remanence of the arrays increase, indicating that they are not controlled by the shape anisotropy in all the composition range. The coercive field dependence on the angle between the applied field and the wire ...

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.

Hysteresis Properties of Hexagonal Arrays of FePd Nanowires

FePd nanowires (NWs) with different compositions have been grown into anodized aluminum oxide templates (AAO) by AC electrodeposition at room temperature. The effects of nanowire composition and morphology on the hysteresis properties of the ordered array of NWs are investigated. All the NWs are polycrystalline; the Fe-rich wires have a bcc structure while the Pd rich ones are fcc. FePd NWs are ferromagnetic and the spontaneous magnetization is found to be parallel to the nanowire long axis (out of plane easy axis) and both, the saturation polarization and the coercive field increase with the iron content.

Cooperative nucleation modes in polycrystalline CoxPd1−x nanowires

Polycrystalline CoxPd1−x (x = 1, 0.60, 0.45, 0.23, and 0.11) cylindrical nanowires (o = 18–35 nm, about 1 μm length) are produced by AC electrodeposition into hexagonally ordered alumina pores. Single-phase nanowires of an fcc Co-Pd solid solution, with randomly oriented equiaxed grains (7–12 nm) are obtained; in all the cases, the grain size is smaller than the wire diameter. The coercive field and the reduced remanence of Co-rich nanowire arrays are hardly sensitive to temperature within the range varying from 4 K to 300 K. On the other hand, in Pd-rich nanowires both magnitudes are smaller and they largely increase when cooling below 100 K. This behavior also depends on the mean grain size. These facts are systematized considering two main aspects: the non-trivial temperature and composition dependence of the crystalline anisotropy and the saturation magnetostriction in Co-Pd alloys; and a random anisotropy effect, which defines a nucleation localization length that may involve more than a single grain...

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.

Curie Temperature and Hopkinson Effect in Twin Roller Melt Spun

The temperature dependence of the magnetic polarization near the Curie temperature TC in Ni2MnGa stoichiometric alloys, directly processed from the melt in a twin-roller melt-spinning device, is investigated. The effect of the solidification rate on the Hopkinson peak detected is evaluated in samples quenched at three different tangential wheel speeds of 10, 15, and 20 m/s. The resulting microstructures were previously characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and by transmission electron microscopy (TEM). EDS results indicated that all the alloys have the composition Ni2MnGa; at room temperature and above this temperature, a cubic L21 ferromagnetic ordered austenitic phase is observed. The Curie temperatures and the magnitude of the Hopkinson effect are estimated from the magnetic polarization versus temperature curves measured in a Faraday balance, in the range 300 K-400 K. As expected for samples with identical composition, the Curie temperatures remain insensitive to the processing route. At low fields (10 mT), the magnitude of the Hopkinson effect is larger in samples quenched at lower rates and it practically vanishes in all the alloys for applied fields near 100 mT.

{\hbox{Ni}}_{2}{\hbox{MnGa}}

Magnetoresistive Cu90Co10 alloys have been produced by twin roller melt spinning at tangential wheel speeds between 10 m/s and 30 m/s to obtain different solute and Co-rich precipitate distributions. X-ray diffraction indicates that the ribbons are polycrystalline with a fcc Cu(Co) (200) textured matrix; no evidence of a spinodal-like composition modulation could be detected with this technique. Transmission electron microscopy observations performed in samples quenched at 10 m/s indicate the existence of small, coherent, Co-rich precipitates of mean size of about 11 nm and bigger (~40 nm) Co-oxide particles. The small coherent precipitates form colonies inside the grains, mediated by precipitate free zones. Samples quenched at 30 m/s are single magnetic phase (superparamagnetic), but the hysteresis loops of the other as cast ribbons are well fitted by a superparamagnetic contribution and a ferromagnetic one, the latter arising from the Co-rich precipitates. Room temperature coercivities, of about 30-50 mT, are lower than those predicted for a mechanism of coherent rotation in the ensemble of non-interacting, Co-rich precipitates.

Shape Memory Alloys

FeCo nanowires 20 nm diameter are synthesized by AC electrodeposition using AAO membranes as templates. The hysteresis and structural properties of the as deposited nanowire arrays are characterized. The orientation dependence of the coercive field measured in the array is compared with the predictions of a magnetization reversal model involving the nucleation and propagation of a transverse wall. The model is promissory because it predicts the experimental curve shape quite well.