J.M. Barandiarán

Continuous cooling approximation for the formation of a glass

Abstract Non-isothermal equations describing the liquid-crystal transformation are derived using the isothermal Avrami equations. A theoretical expression for the critical cooling rate for the formation of a glass is found. Calculations based on this expression are in better agreement with experimental values than those derived from TTT (time-temperature-transformation) curves. A study performed on typical glass forming materials enables the glass forming ability (GFA) to be determined by experimentally measuring crystallization temperatures at different cooling rates which are easily accessible with commonly available technology. The behaviour of the rate constant for crystallization is also obtained from the same data in the experimental range considered. In both cases no previous knowledge of the parameters involved is needed. With some assumptions the values of the viscosity in the crystallization temperature range can be estimated. Although the study was performed for an Avrami index of 4 an extension to other values of n is made under some restricted conditions and a more general treatment is outlined.

Distribution of the magnetic anisotropy in amorphous alloys ribbons

The magnetization curve of an amorphous ribbon along its axis has been calculated in the case of a variation in the intensity of the magnetic anisotropy, which is assumed to have an easy axis perpendicular to the ribbon axis. An expression is given for deriving the anisotropy distribution from the magnetization curve. The influence of small departures of the anisotropy axis from the perpendicular direction is studied. Measurements in as-cast ribbons of Co-rich alloys show that the anisotropy distribution is close to a Gaussian centered near the origin, i.e. almost no macroscopic anisotropy is present. These results are in agreement with the hypothesis made in previous studies to explain the evolution of the magnetization curve of some amorphous alloys with applied stress. The effect of preannealing on the anisotropy distribution induced by a transverse field and by stress annealing is shown to be a smoothing of the internal stresses. >

Influence of Fe in giant magnetoresistance ratio and magnetic properties of La0.7Ca0.3Mn1−xFexO3 perovskite type compounds

In this work polycrystalline perovskite with composition La0.7Ca0.3Fe0.05Mn0.95O3 has been produced by standard ceramic method. A considerable change occurs in magnetic, transport and magnetoresistance properties with respect to the classic composition without Fe. The introduction of a different metal, Fe, in Mn-O layer causes a decrease of about 50 K in the value of TC. In the same way, a decrease of about 10–15% in the average magnetic moment measured at 1 T is also observed. On the other hand, the introduction of Fe does not cause any appreciable change in the value of the lattice parameter. This new compound presents 60% of giant magnetoresistance ratio at 200 K. Magnetization vs. temperature measurements in zero field cooling and field cooling show clear differences at applied fields below 80 kA m−1. Such a behavior, also observed in La0.7Ca0.3MnO3 sample, is not recognizable as a simple ferromagnetic one. A qualitative discussion of the effect in the magnetic and transport properties of these compou...

Magnetic properties of RNi2Si2 compounds (R= rre earth)

Abstract We report an extensive study of the magnetic properties of tetragonal RNi 2 Si 2 compounds (R=Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm), through resistivity, neutron diffraction, susceptibility and magnetization experiments. All compounds exhibit complex incommensurate antiferromagnetic structures, while a transition occurs in TbNi 2 Si 2 between a modulated phase and a simple antiferromagnetic structure, stable at low temperature. The magnitude of the bilinear exchange interactions deviates from the Gennes law and the direction of the ordered magnetic moments presents anomalies across the series, including the probable existence of other types of interactions between the rare earth ions.

Magnetite biomineralization in Magnetospirillum gryphiswaldense: time-resolved magnetic and structural studies.

Magnetotactic bacteria biosynthesize magnetite nanoparticles of high structural and chemical purity that allow them to orientate in the geomagnetic field. In this work we have followed the process of biomineralization of these magnetite nanoparticles. We have performed a time-resolved study on magnetotactic bacteria Magnetospirillum gryphiswaldense strain MSR-1. From the combination of magnetic and structural studies by means of Fe K-edge X-ray absorption near edge structure (XANES) and high-resolution transmission electron microscopy we have identified and quantified two phases of Fe (ferrihydrite and magnetite) involved in the biomineralization process, confirming the role of ferrihydrite as the source of Fe ions for magnetite biomineralization in M. gryphiswaldense. We have distinguished two steps in the biomineralization process: the first, in which Fe is accumulated in the form of ferrihydrite, and the second, in which the magnetite is rapidly biomineralized from ferrihydrite. Finally, the XANES analysis suggests that the origin of the ferrihydrite could be at bacterial ferritin cores, characterized by a poorly crystalline structure and high phosphorus content.

Magnetoelastic sensors based on soft amorphous magnetic alloys

Abstract Magnetoelastic properties of soft amorphous magnetostrictive alloys are used in high efficiency sensors: when negative magnetostriction (Co-rich) samples are used, a stress sensor can be easily constructed by using the continuous and linear increase of the magnetoelastic anisotropy induced by the applied stress. In this work we report on one of these sensors with a figure of merit of 104 and a range of utilization that extends almost to the theoretical elastic limit of the sample. On the other hand, high positive magnetostriction (Fe-rich) metallic glasses are characterized by the frequency of the longitudinal vibrational mode of the samples, ωr being a function of the applied static field H. Studies performed about the performance of ωr as a carrier of information (i.e., object recognizer) are discussed, and a new sample configuration for this purpose is presented.

Torsion dependence of the magnetization process in magnetostrictive amorphous wire

Abstract The torsion dependence of axial and inverse Wiedemann-effect hysteresis loops for a highly magnetostrictive wire has been experimentally determined for the as-quenched state and after thermal treatment. Particular emphasis has been given to the study of the evolution of the remanence and switching field for bistable behavior with applied torsional strain. The switching process for both kinds of loops is discussed. From the analysis of the results the existence of a small component of the magnetization along a helical direction for the as-quenched state may be concluded.

Magnetic moment and chemical order in off-stoichiometric Ni–Mn–Ga ferromagnetic shape memory alloys

Recent studies have shown that the total magnetic moment in off-stoichiometric Ni–Mn–Ga alloys depends not only on electronic concentration but also on the degree of chemical order in the alloy. We have performed neutron diffraction experiments and magnetization measurements for determining the preferential atomic order and saturation moment in off-stoichiometric compounds (44–52 at.% Ni), having excess Mn and deficient in Ga. These alloys include isoelectronic alloys with different magnetic moments and were chosen in an effort to study the impact of chemical order on the magnetic moment distribution. In this work, we present an improved model of magnetic interaction between Mn atoms, which carry most of the localized magnetic moment of the alloys. The Mn atoms at Ga sites, which are nearest neighbors to properly sited Mn, couple antiferromagnetically to the dominant moment. In contrast, Mn atoms at Ga sites, which are nearest neighbors to Mn at Ni sites, couple ferromagnetically. Mn at Ni sites is always antiferromagnetic (AF). The new model is supported by the exchange variation with the Mn–Mn distance and demonstrates excellent agreement between experimental and calculated magnetic moments. The proposed model is shown to better explain the observed experimental results as compared to the rigid band model and previous localized moment models that assumed AF coupling for all off-site Mn atoms.

Transition from quasistatic to ferromagnetic resonance regime in giant magnetoimpedance

Detailed measurements of giant magnetoimpedance (GMI) in an amorphous ribbon and a magnetic/nonmagnetic multilayer are presented for frequencies up to 3GHz. Through this frequency range, the transition from quasistatic to dynamic regime of GMI can be clearly distinguished, due to the appearance of the ferromagnetic resonance (FMR). The unambiguous experimental evidence presented mediates between conflicting interpretations of GMI: the ones that assume the existence of FMR even for low frequencies and the ones that consider that it is irrelevant. The frequency at which the transition takes place is shown to be related with the width of the resonance, which is substantially different for both samples. It is concluded that the large increase of permeability caused by the ferromagnetic resonance can be advantageous for GMI-based devices only for samples with a small, very well-defined perpendicular anisotropy.

Structural and magnetic changes in FeNbCuSiB amorphous alloys during the crystallization process

Calorimetric and magnetic measurements, x-ray powder diffraction and Mossbauer spectroscopy have been used to study the magnetic and structural changes occurring after each of the two steps of crystallization that take place in FeNbCuSiB-type alloys. Two samples with different boron and silicon concentrations, (x=6, 9), have been studied. They give a somewhat different composition of the crystalline phases appearing after crystallization processes. The most noticeable phenomenon is the observed increase of about 50 K in the Curie temperature of the FeSi crystalline phase between the end of the first crystallization process and the end of the second one, although the composition of this phase remains unchanged. This result is discussed in terms of crystal boundary effects. Also, the Curie temperature of the remaining amorphous phase, in the crystallized samples, is greater than the expected one, due to the coupling with magnetic phases with higher Curie points and inhomogeneities in such a phase.