K.G. Efthimiadis

Magnetic properties of FeMnAl alloys

Abstract The magnetical properties of Fe89−xMn11Alx alloys have been studied by Mossbauer spectroscopy and magnetization measurements. The presented examples show both ferromagnetic (x = 14) and re-entrant ferromagnetic (x = 40) behaviour.

Study of the crystallization of amorphous Fe78Si9B13 and Fe76Si8B16 by means of magnetic measurements

Abstract Measurements of the saturation magnetization of amorphous Fe 78 Si 9 B 13 and Fe 76 Si 8 B 16 as a function of temperature and also as a function of the duration of heating the materials at selected temperatures were made. The results suggest that the crystallization of Fe 78 Si 9 B 13 takes place in two stages, whereas the one of Fe 76 Si 8 B 16 in three. Since, during the initial stage of crystallization, regions of b.c.c. Fe(Si) are formed, measurements of the saturation magnetization of the Fe a Si 1− a ( a = 1,0.9 and 0.75) were also carried out, in order to render possible the calculation of the activation energy of the initial crystallization.

On the influence of Al on the magnetic ground state of 3d ferromagnetic alloys

Abstract This paper describes variations in the mean atomic magnetic moment with Al admixtures in transition element alloys, as functions of the number of 3d + 4s electrons and of the atomic concentration of Al. From the experimental results presented, it seems that the presence of the admixtures results in the completion of the 3d zone of the transition elements. The population increase of the 3d band is interpreted as a consequence of its narrowing and its displacement in the energy spectrum.

High coercivity Gd-substituted Ba hexaferrites, prepared by chemical coprecipitation

A series of Gd-substituted Ba hexaferrites with nominal formula (Ba1−xGdx)O⋅5.25 Fe2O3 (x=0–0.30) were prepared by the chemical coprecipitation method from nitrate precursors and heating at T=800–1200°C for 2h. The samples have been examined by x-ray diffraction, vibrating-sample magnetometer, and scanning electron microscopy methods. Gd substituted samples form single phase materials with the M-type hexaferrite structure at all heating temperatures, in the range of x⩽0.10–0.20. The saturation magnetization (at 1.8T) varies slightly with x in most cases and, for x=0.05–0.10, it increases up to 66.7Am2∕kg, exceeding the value of the unsubstituted hexaferrite. A strong enhancement of the coercivity is observed for all substituted samples, with maximum values Hc=457kA∕m for the single-phase x=0.10 sample annealed at 1000°C and Hc=477kA∕m for the x=0.25 sample annealed at 1100°C which contains Fe2O3 and GdFeO3 impurities. As the variation of coercivity with either substitution rate (x) or annealing temperatur...

Crystallization of amorphous Fe75 − xCuxSi9B16

Abstract In this work the influence of Cu admixtures on the crystallization process of amorphous FeSiB alloys is studied, based on measurements of the saturation magnetization of the series Fe75 − xCuxSi9B16 (x = 0, 1, 2, 2.8 and 3.5) during their heating in selected temperatures. The main conclusion is that the presence of the Cu admixtures accelerates the crystallization, due to the microsegregation that it induces in the alloys.

On the anomalous electrical resistivity in some Ni3[FecAl1-c] alloys

Abstract The electrical resistivity temperature dependence of several Ni3[FecAl1-c] is examined. The negative resistivity coefficient appearing at elevated temperatures is discussed by means of the experimental findings of L12 superstructure formation and the qualitative assumption that the phenomenon is attributed to the order-disorder transformation.

Magnetic stimulation of the spine: the role of tissues and their modelling

Numerical modelling of magnetic stimulation in the spine is a scarce subject in the literature, although it has been gaining clinical acceptance. In the present work we present the results from a simplified computational model of the spine. The results indicate that it is necessary to use a numerical technique for solving the problem, which takes into account tissue dispersion and both dielectric properties (conductivity and permittivity), since a difference of 14% in the induced electric fields was found when displacement currents were included. With respect to the role of tissues in stimulation efficiency, it was confirmed that water-rich tissues lead to a shielding effect of the spinal cord. However, this effect becomes smaller at the height of the intervertebral discs, resulting in an increase of the field inside the spine.

On Magnetic Properties and Thermal Stability of Fe(M)‐Si‐B (M = V, Ni, Cu, Nb, Mo, Pd, Ag) Amorphous Alloys

Experimental results are presented regarding the variation of the atomic magnetic moment, the Curie temperature and the crystallization temperature of Fe 78-c M c Si 9 B 13 (M= V, Nb, Mo) and Fe 75-c M c Si 9 B 16 (M= Ni, Cu, Pd, Ag) amorphous alloys. Efforts were made to explain qualitatively the phenomena, based on the behavior of the amorphous alloys when various admixtures substitute Fe atoms.

Influence of Cu admixtures on the crystallization of amorphous Fe75Si9B16

Abstract The crystallization of the amorphous alloys Fe 75−x Cu x Si 9 B 16 (x ≤ 4) is completed in three stages: (a) dendritic growth of bcc Fe[Si,Cu], (b) eutectic crystallization of bcc Fe[Si,Cu] + bct Fe 3 B and, finally, (c) formation of bct Fe 3 B. The metastable compound Fe 3 B formed during the second and third stages is eventually dissolved into bcc Fe and bct Fe 2 B. As Cu atoms replace Fe atoms, the kinetics of the whole process is modified because of microsegregation: the first stage is decelerated, the second is accelerated, and the third is accelerated up to 2 at% Cu, but is decelerated at higher concentrations.

The Effect of Coil Modeling on the Predicted Induced Electric Field Distribution During TMS

We investigated the effect on the predicted induced field inside the head tissues of the numerical model adopted for simulating the excitation coil during transcranial magnetic stimulation (TMS). A commercially available eight-shaped coil was modeled in four different ways. The electric field inside a detailed (2 mm resolution) and anatomically realistic numerical phantom of the human head was calculated with the impedance method. The results revealed differences of more than 170% in the maximum local value of electric field, depending on the coil model. It was shown that the predicted amount of involved tissue in the brain varied as well, pointing to the importance of correct source modeling before individualized patient treatment planning can be introduced. The calculations showed clearly that an eight-shaped TMS coil should not be modeled as two single thin-wire loops, since this approach may yield inaccurate induced field distributions.