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Polycrystalline W-type hexagonal ferrites with chemical formulae Ba0.5Sr0.5 Co2−x Me x Fe16O27 (x = 0, 0.5, Me = Mn, Mg, Zn, Ni) have been prepared using sol–gel autocombustion. It has been reported in our earlier published work that all the samples exhibit a single-phase W-type hexagonal structure which was confirmed by x-ray diffraction (XRD) analysis. The values of bulk density lie in the range of 4.64–4.78 g/cm3 for all the samples which are quite high as compared to those for other types of hexaferrites. It was also observed that Zn-substituted ferrite reflects the highest (14.7 × 107 Ω-cm) whereas Mn-substituted ferrite has the lowest (11.3 × 107 Ω-cm) values of direct current (DC) electrical resistivity. The observed values of saturation magnetization (M s) are found to be in the range of 62.01–68.7 emu/g depending upon the type of cation substitution into the hexagonal lattice. All the samples exhibit a typical soft magnetic character with low values of coercivity (H c) that are in the range of 26–85 Oe. These ferrites may be promising materials for microwave absorbers due to their higher saturation magnetization and low coercivities. Both the dielectric constant and tangent loss decrease with increasing frequency in the lower frequency region and become constant in the higher frequency region. The much lower dielectric constant obtained in this study makes the investigated ferrites very useful for high-frequency applications, i.e. dielectric resonators and for camouflaging military targets such as ships, tanks and aircrafts, etc.

The effect of different degrees of plastic deformation on the frequency dependence of coercive field and losses in NO Fe–(3 wt%)Si was investigated measuring hysteresis loops on a ring-shaped core over a frequency range of 0.5–1000 Hz. The investigation was performed under sinusoidal and triangular external field $H(t)$ . Although $H(t)$ induce a similar quasi-sinusoidal magnetic induction (similar hysteresis loops), the loop area is smaller for that measured by triangular applied field resulting in smaller magnetic losses. The coercive field and the total losses of all samples increase with increasing frequency; however, the influence of frequency becomes gradually smaller with plastic deformation. The total losses that increase due to plastic deformation at low frequencies become at high frequencies smaller in deformed samples than in an undeformed material. A similar behavior was found in the frequency dependence of the coercivity. Due to a periodic quasi-sinusoidal $B(t)$ , the losses were analyzed using the concept of loss separation. While the quasi-static (hysteresis) loss increases, classical and excess losses decrease with increasing deformation.

The frequency dependence of the hysteresis loop of high-pressure torsion treatment (HPT)-deformed Fe75Al25, industrial steel Fe–3wt%Si, bulk Fe–3wt%Si plates, and nanocrystalline VP800 was measured on ring-shaped samples. Applying a sinusoidal or triangular $H(t)$ delivers a strong frequency dependence of the losses. A sinusoidal or triangular $B(t)$ reduces the total loss [this reduction is larger for triangular $B(t)$ ] and the dependence of loss on frequency becomes smaller. Our results are discussed in terms of Bertotti’s loss separation model. For HPT-deformed Fe75Al25, the total loss $W$ linearly varies with the square root of frequency when a triangular $H(t)$ is applied; however, no significant change in the total loss was found for a triangular induction $B(t)$ up to 500 Hz. The hysteresis, classical, and excess loss values of the Fe–3wt%Si steel plates are different from those of bulk Fe–3wt%Si for all $H(t)$ and $B(t)$ shapes due to different microstructures. In nanocrystalline VP800, the contribution of classical eddy current loss to the total core loss is very small due to high resistivity and small ribbon thickness.

The present work analyzes the influence of electric conductivity on the Magnetic Barkhausen Noise (MBN) signal using a microscopic model which includes the influence of eddy currents. This model is also implemented to explain the dependence of MBN on the frequency of the applied magnetic field. The results presented in this work allow analyzing the influence of eddy currents on MBN signals for different values of the material's electric conductivity and for different frequencies of applied magnetic field. Additionally, the outcomes of this research can be used as a reference to differentiate the influence of eddy currents from that of second phase particles in the MBN signal, which has been reported in previous works.

This work studies the influence of the maximum applied magnetic field on the angular dependence of the energy of the Magnetic Barkhausen Noise signal in three different API5L pipeline steels. The results show that the shape of the angular dependence of the Magnetic Barkhausen Noise energy changes with the increase of the amplitude of the applied magnetic field. This phenomenon is a consequence of the presence of unlike magnetization processes at different magnitudes of the applied magnetic field. The outcomes reveal the importance of controlling the value of the maximum applied field as parameter for the improvement of the MBN angular dependence measurements.

Cobalt ferrite (CoFe2O4) and manganese substituted cobalt ferrite (Co0,25Mn0,75Fe2O4) were synthesized by coprecipitation and sol–gel methods. X-ray diffraction revealed that manganese ions slightly enlarged the crystal lattice with respect to pure cobalt ferrite. Magnetization, linear magnetostriction, and piezomagnetic coefficient values tended to decrease with the ion substitution. Linear magnetostriction dropped from a value of −111.5 ppm for cobalt ferrite to values of −22.6 and −30.6 ppm for the substituted samples obtained from coprecipitation and sol–gel, respectively. The sample prepared by sol–gel technique resulted in a higher magnetization, magnetostriction, and piezomagnetic coefficient than that synthesized by coprecipitation method.

Divalent (Me) ions substituted W-type hexagonal ferrites with chemical formula Ba0.5Sr0.5Co2−xMexFe16O27 (x = 0, 0.5 and Me = Mn, Mg, Zn, and Ni) were prepared by sol–gel autocombustion method at 1300 °C for 5 h. Thermal and IR spectral studies were carried out by means of differential thermal/thermogravimetric analysis and Fourier transform infrared spectroscopy respectively. The microstructure was investigated by using the powder X-ray diffractometer and the field emission gun scanning electron microscope. The results showed that divalent ions substituted W-type hexagonal ferrites were single phase and appeared as hexagonal platelet-like shape which is better shape for microwave absorption. M–H loops of a selected sample were measured up to maximum applied field of 9T in a temperature range of 4.2–400 K. The values of saturation magnetization were calculated by the law of approach to saturation. The room temperature value of coercivity is found to be a few oersteds which is favorable for EM materials. Based on the results of electromagnetic parameters, reflection losses were calculated which are found to depend on the thickness of the absorber.

The effects of Al-substitution on electrical, magnetic and high frequency dielectric parameters of single phase BaCo2AlxFe16−xO27 (x = 0–1) hexaferrites have been discussed in the present work. The XRD patterns reveal the single phase formation of these ferrites. Bulk density, X-ray density and porosity as a function of Al contents were calculated. Incorporation of Al into the W-type hexagonal structure was confirmed by energy-dispersive X-ray spectroscopic (EDXS) technique. The DC resistivity was observed to increase with increasing Al contents due to the unavailability of Fe3+ ions. The M–H loops were measured up to a maximum applied field of 9 T. The saturation magnetization (Ms) values were calculated by the law of approach to saturation. The coercivity (Hc) values for all the samples lie in the range of a few hundred oersteds which is one of the necessary conditions for EM materials, security, switching and sensing applications. The high frequency dielectric constant and dielectric loss in the frequency range of 0.025–1.5 GHz were observed to decrease as a function of Al contents which are favorable for high frequency applications and microwave devices like dielectric resonators.

Magnetic measurements, X-ray diffraction and scanning electron microscopy (SEM) experiments were carried out in the as-cast Gd ( Co 1 − x Cu x ) 5 samples with different Co/Cu content. Already in the as cast state, this system shows high coercive field for x=0.3 and a magnetization driven by nucleation of reversal domain. SEM micrograph and microanalysis show possible spinodal decomposition in the as-cast state, hence regions with different Co/Cu-content are observed, while the Gd-content almost does not change. High resolution X-ray diffraction patterns show a main CaCu5-type structure with traces of a secondary phase and distorted peak profiles as function of the Cu content. The evolution of the microstructure is discussed in relation with the Cu incorporation into the CaCu5-type structure. The Cu addition avoids the formation of the 2:7 phase within the 1:5 matrix, favoring the formation of a more homogeneous Gd(Co,Cu)5 phase. The relation between the observed microstructure and the magnetic behavior is also discussed.

The magnetic and microwave characterization of single phase hexaferrites of entirely new composition Ba1-xSrxCo2AlFe15O27 (x=0.2–1.0) for application in a microwave absorber, have been reported. The samples synthesized by sol–gel method were investigated by differential thermal analyzer, Fourier transform infrared spectroscope, X-ray diffractometer, field emission gun scanning electron microscope, vibrating sample magnetometer and vector network analyzer. Platelet grains exhibit well defined hexagonal shape which is a better shape for microwave absorption. M–H loops for a selected sample were measured for a temperature range of 4.2–400 K. Moreover M–H loops for all Sr-substituted samples were also measured at room temperature up to a maximum applied field of 9 T. Saturation magnetization values were calculated by the law of approach to saturation. The room temperature coercivity for all the samples is found to be a few hundred oersteds which is necessary for electromagnetic materials and makes these ferrites ideal for microwave devices, security, switching and sensing applications. The complex permittivity, permeability and reflection losses of a selected ferrite–epoxy composite were also investigated over a frequency range of 0.5–13 GHz.

A series of Al-substituted Co 2 W hexaferrites BaCo 2 Al x Fe 16 − x O 27 ( x = 0, 0.2, 0.4, 0.6, 0.8, 1) synthesized by sol-gel autocombustion method have been investigated. The samples were characterized by DTA/TGA analysis, FTIR spectroscopy, X-ray Diffractometer and field emission gun scanning electron microscopy (FEGSEM). Magnetic behavior of a selected sample was examined up to a magnetic field of 9T between a temperature range of 4.2 K–400 K. The values of saturation magnetization were calculated by the law of approach to saturation. The coercivity of the investigated sample has a value of about hundred oersteds at room temperature which is favorable for EM materials. The complex permittivity and complex permeability of a composite sample have been measured in the frequency range of 0.5–13 GHz. Based on the theoretical calculation of reflection loss; the microwave absorption properties have been investigated. The results indicate that the attenuation peak frequency of these compounds can be tuned for the desired frequency by varying the thickness of the absorber.

The quasilogarithmic magnetic viscosity of the perpendicularly anisotropic Nd-Fe-B films is determined by measuring the time dependent magnetization in coercive fields at 300 K-4.2 K. The distribution of energy barrier heights in the films is believed to be the origin of this stretched exponential magnetic relaxation. Another feature exhibited in the anisotropic Nd-Fe-B films is the nonmonotonic temperature dependence of the magnetic viscosity coefficients, which are extracted by the decay slopes of the linear quasilogarithmic magnetic aftereffect curves. (C) 2012 Elsevier B.V. All rights reserved.

Microscopic, element selective structural and magnetic properties of cobalt ferrite produced by sol–gel (SG) and by ball milling (BM) methods were studied with X-ray absorption near-edge structure (XANES) spectroscopy and X-ray emission spectroscopy (XES). All investigated samples exhibit only Fe3+ and Co2+ cations. The relative distribution of the Co2+ and Fe3+ cations in the tetrahedral and octahedral sites depends on the production method of the samples. For the samples presenting a similar but small grain size (

In this work, the structural and magnetic properties of Tb3 (Fe28� xCox )V 1.0 (x¼ 0, 3, 6) compounds have been investigated. The structural characterization of compounds by X-ray powder diffraction is an evidence for a monoclinic Nd3(Fe, Ti)29-type structure (A2/m space group). The refined lattice parameters a and b (but not c) and the unit cell volume V, obtained from the XRD data by the Rietveld method, are found to decrease with increasing Co concentration. The unit cell parameters behavior has been attributed to the smaller Co atoms and a preferential substitution of Fe by Co. The anisotropy field (Ha) as well as critical field (Hcr) was measured using the singular point detection (SPD) technique from 5 to 300 K in a pulsed magnetic field of up to 30 T. At T¼ 5 K, a FOMP of type 2 was observed for all samples and persists at all temperatures up to 300 K. For sample x ¼0, Hcr¼ 10.6 and 2.0 T at 5 and 300 K, respectively, is equal to that reported earlier. The occurrence of canting angles between the magnetic sublattices during the magnetization process instead of high-order anisotropy contributions (at room temperature are usually negligible) has been considered to explain the survival of the FOMP at room temperature. The anisotropy and critical fields behave differently for samples with x¼ 0, 3 compared with x¼ 6. The observed behavior has been related to the fact that the Co substitution for Fe takes place with a preferential entrance in the inequivalent crystallographic sites of the 3:29 structure. The contribution of the Tb-sublattice in the Tb3(Fe, V)29 compound with uniaxial anisotropy has been scaled from the anisotropy field measured on a Y3(Fe, V)29 single crystal with easy plane

USANSPOL is a novel ultra-small-angle scattering technique with polarised neutrons for investigation of magnetic materials. It represents a polarised neutron extension to traditional USANS which works with unpolarised neutrons. The high angular resolution of this technique relies on the narrow reflection width of perfect crystal reflections and is employed in a double-crystal diffractometer. Corresponding to the μrad resolution of the set-up, micro-structures of the order of a few tenths of a micrometre up to a few tens of micrometres may be investigated. Neutron polarisation is achieved by insertion of birefringent magnetic prisms between the monochromator crystal and the sample. Rocking the analyser crystal produces a scattering pattern for both neutron spin states in a single measurement but well separated in reciprocal space. By this technique, we have recently studied various amorphous Galfenol soft-magnetic ribbons which were produced by spinning from melt at different manufacturing conditions. USANSPOL allows for a determination of domain sizes of the non-magnetised samples and a study of the growing of magnetically homogeneous regions with increasing externally applied magnetic field. The manufacturing process of the ribbons is reflected in the magnetic micro-structure of the different specimens.

Five sets of soft-magnetic metals, such as pure Fe, pure Ni, Fe-3 wt% Si, Fe-6.5 wt% Si and Fe-17 wt% Co, were subjected to high pressure torsion (HPT) up to strain levels where a saturation of the microstructural refinement is observed. Following HPT at 77, 293 and 723 K, transmission electron microscopy (TEM) was used to study the grain size and grain shape of the severely deformed metals. The coercivity HC was characterized in a magnetic closed system by using ring shaped samples. Magnetic measurements obtained on ring shaped samples give a much higher accuracy for determining the coercivity. Depending on the material the mean microstructural sizes in the steady state vary from 300 nm at 723 K to 30 nm at 77 K, respectively. The coercivity of the deformed materials first increases with decrease in grain size. Once the crystallite size is far below 100 nm the coercivity shows a strong decrease.

In this paper, we report magnetostriction measurements and an investigation of structural properties on melt-spun ribbons of Fe85.7Al14.3, Fe81.3Al18.7, and Fe77.7Al22.3 ribbons produced at different wheel speeds. The bcc-disordered A2 structure is obtained for 14.3 and 18.7 at %Al whereas the alloy with 22.3 at %Al exhibits the DO3-type structure. With increasing Al concentration, the saturation magnetization decreases, whereas the average lattice constant, longitudinal, transverse, and volume magnetostriction increase. However for each alloy, the crystallite size, lattice constant, and magnetostriction vary randomly with wheel velocity. While the behavior of the longitudinal magnetostriction versus applied field is similar for all samples, the behavior of the transverse magnetostriction is strongly dependent on the wheel speed. These results indicate that the formed structural disorder and the stresses in the ribbons are different with wheel speed.

Longitudinal and transverse magnetostriction measurements were performed on polycrystalline Fe100- xTix alloys, for a large Ti concentration range, from x = 8.8 to 34.7 at.%. Our results showed that substituting Fe by Ti does not significantly enhance the magnetostriction. For all Ti concentrations in the magnetically saturated state, the longitudinal magnetostriction values vary from -2 ×10-6 to 6 × 10-6. On the other hand, Fe100-xTix alloys exhibit a large forced volume magnetostriction, which increases with increasing Ti-concentration. The magnetostriction behavior could be explained considering investigations of the hyteresis loops and the microstructure .

The high field facilities available at the TU Vienna are surveyed. At the TU Vienna exists a quasistatic power installation (10 MW) which allows the generation of a regulated field versus time profile over a pulse time of up to 1 s with a maximum field of 40 T. Additionally, there are four short pulse systems with different condensor batteries (75 kJ, 400 kJ, 440 kJ) which generates pulsed field with pulse durations between 10 and 50 ms and maximum fields up to 60 T. These high field installations are used for magnetization and hysteresis measurements, for magnetoresistance measurements but also for the determination of the anisotropy field or the magnetostriction. Experimental results obtained by different experiments are given.

In order to investigate the effect of the quenching rate on the magnetostriction of Fe82.5Ga17.5 alloy, a set of ribbons was prepared by varying the wheel velocities from 7.5 up to 25 m/s. Additionally Fe85Al15, Fe81Al19 and Fe75Al25 ribbons were prepared under similar conditions. The microstructure shows generally columnar particles and (100) texture perpendicular to the surface of the ribbons. For Fe-Al ribbons, the lattice constant and magnetostriction (which shows volume magnetostriction effect for the higher Al concentrations) increase and saturation magnetization decreases with Al content. However, the average grains size and the lattice constants of Fe82.5Ga17.5 and Fe-Al alloys, show large fluctuations with the wheel velocity. For Fe82.5Ga17.5, ribbons the magnetostriction values and its character with applied field depend strongly on the quenching rate. The dependence of the longitudinal magnetostriction with wheel speed exhibits a maximum of about 116 ppm at 15 m/s, which is related to the higher degree of (100) texture.

The longitudinal and transverse magnetostriction and microstructure of polycrystalline Fe 100− x Ge x ( x = 8, 12, 15, 20) alloys were investigated in order to correlate the magnetostriction with microstructure. In order to obtain different microstructures in the Fe 100− x Ge x alloys, the samples were annealed at 600 °C during 2 h and at 1150 °C for half hour and then quenched in cold water. For Ge concentrations lower than 14 at.%, the longitudinal magnetostriction is positive and increases positively up to 22 ppm at 12 at.% Ge then decreases and vanishes at about 14 at.% Ge. For further Ge concentration increase the longitudinal magnetostriction is negative and reaches −30 ppm for Fe 80 Ge 20 . This behavior, that is very similar to that reported for Fe–Si alloys, is explained by the structural changes caused by different thermal histories of the alloys.

Pipelines exhibit a magnetic anisotropy with the easy axis in the axial direction of the pipe. Three different types of steel API 5L (X52, X56, X60) obtained from out-of-service pipeline sections were investigated with respect to their microstructure, magnetic properties, and magnetostriction. In order to measure the magnetic properties samples were cut parallel as well as perpendicular to the pipe axis. The investigated steels contain 98% iron and 2% of additive elements like Mn, Si, C, and traces of others. The microstructure of steel types X52 exhibited banding along the rolling direction, whereas that of the X56 and X60 steels was negligible. The grain size was between 10 and 25 mum. A similar grain size was observed by raster electron microscopy. The size of the magnetic domains was several micrometers which is distinctly smaller than the observed grain size. The saturation polarization was 2.07 plusmn 0.01 T which is 2% below that of pure Fe. Hysteresis loops were measured in a magnetically closed circuit. Saturation in the transversal direction was significantly more difficult to achieve than in the parallel direction. The loops showed clearly the effect of texture induced anisotropy. The stress sensitivity of the loop is caused by the magnetoelastic energy. The ratio of Jr/Js was below the theoretical value of 0.75 for a cubic isotropic material indicating also the presence of a texture. These results are in agreement with the texture as observed in the microstructure and with the magnetostriction measurements (longitudinal and transversal) which also showed higher values in the transversal direction.

The local structure in melt-spun Fe 85 Ga 15 ribbons with a width ∼3 mm and thickness ∼60 μm produced in argon atmosphere was studied by analyzing EXAFS and XANES data. The following results were obtained: Ga–Ga bonds were not detected excluding the tendency to form clusters of Ga atoms; Ga substitutes Fe creating a local strain of about +1% on the first shell Fe–Ga bond, whereas on the second Fe–Ga shell strain quickly relaxes down to +0.3%; XANES spectra are compatible with a random substitution of Fe atoms by Ga atoms in the A2 structure. From the AFM investigation, we observed that at the surface (free side) of the ribbon the particles are elongated along the ribbon (∼2 μm×∼5 μm) and each particle is formed by small grains of average size of 200 nm.

Several sets of steel samples, like cold-rolled low carbon steel with different rolling reductions and thermal aged Fe–1 wt% Cu alloys, are investigated with respect to magnetic parameters such as coercivity, remanence and shape of the hysteresis loop. Measurements are performed in a magnetic open system, which allows magnetic saturation of the material and in a magnetic closed system using ring or picture frame shaped samples. The results obtained for ring or picture frame shaped samples give a much better accuracy for determining the coercivity. The coercivity (remanence) of the cold-rolled carbon steel samples increases (decreases) systematically with the rolling reduction. The duration of the thermally aging procedure performed at 450 °C has nearly no influence on the coercivity value. However, if thermally aging is applied on a cold-rolled sample, the coercitivity decreases with increasing aging time.

This work presents new eddy current measurements in pulsed fields. A commercial point pick-up coil is used to detect the induction signal along the radius of Cu and Al samples with cylindrical shape and diameters between 5 and 35 mm. Local eddy current measurements were performed on the surface of conducting materials due to the small dimensions of the coil. A simple electrical circuit, used as a model, is proposed to describe the local eddy current effect in pulsed fields. The proposed model allows to calculate the phase shift angle between the signal proportional to eddy currents and the applied external field in a pulsed field magnetometer.

Investigations of the magnetic properties of NbFe 2 were performed by measuring the temperature dependence of hysteresis loops, initial AC-susceptibility and zero field cooled (ZFC)–FC magnetizations in a wide field range of 50 Oe⩽ μ 0 H ⩽8000 Oe. Two well-defined peaks, at 4.8 and 21.4 K, were found in the AC-susceptibility curve, and only the first peak is sensible to a frequency change of the applied field. The peak changes to high temperature and its intensity decreases with increasing frequency, thus showing a relaxation phenomenon. In ZFC magnetization at 50 Oe, the peaks occur at 3.4 and 21.4 K. With increasing DC field, both peaks are shifted to low temperatures; however, the first peak disappears at fields higher than 100 Oe and a field higher than 1250 Oe suppresses the second peak. A third maximum merges from the magnetization curve at around 7 K in a field of 500 Oe, in which case, the maximum is suppressed by an external field higher than 4000 Oe. One can suggest that the ordered state of the stoichiometric NbFe 2 is a spin density wave (SDW) and both ferro- and antiferromagnetic spin fluctuations coexist.

This paper describes a new measuring system to perform local area magnetization measurements on the surface of a magnetic sample in a pulsed field magnetometer at room temperature. A commercial point pick-up coil is used to detect the induction signal along the radius of samples with cylindrical or disc shapes. The relationship between the induced voltage and the sample geometry is treated and a calibration procedure is proposed to obtain the magnetization values in physical units for the case when the point pick-up coil is at the centre of the sample. The local demagnetizing factor is obtained using the measurement of the local anisotropy field applying the single-point detection (SPD) technique. The measurement of this SPD singularity detected in the transverse reversible susceptibility, using a sample with cubic geometry, is demonstrated for the first time in this type of magnetometer. The application of the system for the investigation of commercial permanent magnets is illustrated, and the advantages and disadvantages are discussed.

X-ray diffraction and transmission electron microscopy (TEM) experiments were performed in heat-treated Sm(Co,Cu) 5 samples with different Co/Cu content. The major phase observed in the diffraction patterns exhibits a hexagonal CaCu 5 type structure in all the studied compositions. The behavior of the diffraction profiles, as function of the Cu content, is believed to be due to an inhomogeneous distribution of Co and Cu through out the samples. Heavily planar faulted regions have been observed by means of TEM and are also associated with Sm and Co rich phases within the matrix. A CaCu 5 type structure with local Sm rich regions seems to be responsible for the observed magnetic behavior.

Nanocrystalline zinc-substituted cobalt ferrite powders, Co 1− x Zn x Fe 2 O 4 ( x =0, 0.2, 0.4), were for the first time prepared by forced hydrolysis method. Magnetic and structural properties in these specimens were investigated. The average crystallite size is about 3.0 nm. When the zinc substitution increases from x =0 to x =0.4, at 4.2 K, the saturation magnetization increases from 72.1 to 99.7 emu/g and the coercive field decreases from 1.22 to 0.71 T. All samples are superparamagnetic at room temperature and ferrimagnetic at temperatures below the blocking temperature. The high value of the saturation magnetization and the very thin thickness of the disorder surface layer of all samples suggests that this forced hydrolysis method is suitable not only for preparing two metal element systems but also for three or more ones.

The EC supported a network (under the Framework 5 ALFA Programme) designated HIFIELD (Project number II0147FI) and entitled: "Measurement methods involving high magnetic fields for advanced and novel materials". As a result, high field facilities were initiated, constructed or extended at the following laboratories in Latin America: University Cordoba (Argentina), CES, Merida (Venezuela), CIMAV, Chihuahua (Mexico), University Federal de Rio de Janeiro (Brazil).

GdCo9−xSi4+x (x = −0.2 to x = 0.2) and TbCo9Si4 exhibit the tetragonal LaFe9Si4 structure (space group I4/mcm). Magnetically these samples are ferrimagnetic with ordering temperatures close to 50K. The ferrimagnetic magnetization is found to decrease with decreasing Si content, thus, revealing an increasing cobalt sublattice magnetisation. The antiparallel coupling of the Gd- and Co-sublattice magnetisations breaks up at lower critical fields Hl well above 20 T. From the lower critical field Hl as well as from the slope ΔM/ΔH for fields above Hlthe inter-sublattice molecular coefficient can be determined which is approximately constant within this solid solution.

Austromag offers high magnetic fields for various applications in solid state physics. The quasistatic high field facility gets the power (10 MWs) from the line of the city, reaching 40T in a long pulse (up to 1s). Additionally exist various short pulse facilities (pulse duration between 10 and 100 ms) which are energized by a condensor battery. New types of high field magnets (foil coil) were developed. Measurements are possible between 4.2 K and 800 K. The main experiments are: magnetization, magnetoresistence, anisotropy and magnetostriction. New measuring techniques were developed such as pressure dependence of anisotropy and magneto-electric coefficient.

The “law of approach to saturation” is a well-known mathematical model for describing the behavior of ferromagnets at high magnetic field strengths which has the additional advantage that it can be linked to anisotropy through one of the terms in the mathematical expression of the law. In this paper, two recent and more comprehensive models of the magnetization process are compared with the law of approach and with each other in terms of their capability to describe the dependence of the magnetization curve in the high field regime. The comparison leads to relations between these two models and the interpretation of certain aspects of the models in terms of anisotropy. It is shown that the effects of anisotropy can be incorporated directly into these models without any additional assumptions.

The phase relations of the GdCo13−x Six system have been studied by means of scanning microscopy, microprobe analysis and x-ray diffraction. Single-phase samples GdCo9±δSi4∓δ (structure type LaFe9Si4 with space group I 4/mcm) ar ef ormed in a narrow composition range |δ| 0.2, where GdCo9Si4 forms as af ully ordered ternary compound. The magnetic properties of GdCo9Si4 have been investigated by ac susceptibility, magnetization, specific heat and resistivity measurements. These experiments reveal ferrimagnetism below about 47 K, which is analysed in terms of a two-sublattice molecular field model with a local moment Gd subsystem coupling antiparallel to the itinerant ferromagnetic Co 3d sublattice. The 3d–3d exchange of the latter is the driving force for the onset of long range magnetic order. The validity of the twosublattice model is demonstrated by high field measurements, showing that the ferrimagnetic coupling starts to break up at a lower critical field µ0 Hl � 27 T. The magnetic ground state of GdCo9Si4 has been analysed on a microscopic basis via LSDA electronic structure calculations. (Some figures in this article are in colour only in the electronic version)

High-resolution X-ray diffraction patterns of Fe 78.5 Si 20 Ta 1.5 and Fe 80− x Si 20 Nb x ( x =0.5, 3.0, 10) show the existence of a metastable hexagonal phase (NbFe 2− x Si x and TaFe 2− x Si x ) together with bcc-Fe 100− y Si y solid solution and an amorphous matrix. The grain size of the bcc-Fe 100− y Si y phase decreases with increasing Nb content in Fe–Si compounds resulting in nanocrystalline materials. On the other side, the micro-strain, the lattice constant, coercivity and the Curie temperature increase with Nb concentration. Enhancements of the local strain and local magnetic anisotropy were also observed by means of NMR measurements. On the contrary, the addition of Cr causes a decrease in the coercivity and Curie temperature.

The most important methods for determining the grain size, the grain size distribution and also the actual microstructure of nanocrystalline materials are: X-ray diffraction line profile analysis, transmission electron microscopy, and small-angle neutron scattering. For each of these three methods their specific advantages and disadvantages are discussed and an experimental example is given.

The magnetic properties of amorphous and nanocrystalline hard magnetic materials are summarized. The reduction of the “effective” anisotropy field due to exchange coupling in nanocrystalline materials is demonstrated. This leads experimentally as well as theoretically to a remanence enhancement and to a reduced coercivity. Also the domain structure shows the effect of exchange coupling. Nd–Fe–Al is taken as an example of a new “amorphous” hard magnetic material. For magnetostrictive materials the possibility of reducing the anisotropy in nanocrystalline samples without loosening the high magnetostriction is discussed.

The structural evolution with increasing Nb content in Fe 80− x Si 20 Nb x alloys (0 x ≤ 12) was investigated by means of high-resolution X-ray diffraction. A metastable NbFe 2− x Si x hexagonal phase was found together with cubic bcc Fe 100− y Si y solid solution and the amorphous matrix. The grain size of the bcc Fe 100− y Si y phase and the strain were determined and their dependencies on the Nb content and heat treatment of the samples were investigated. The strain increases and the grain size decreases with an increasing of Nb content reaching a size down to 37 nm. Upon heat treatment of the samples, the hexagonal phase decomposes in two hexagonal phases with similar compositions.

Three different methods (mechanical alloying, coprecipitation and sol–gel) were used for preparing LaFe 12 O 19 hexaferrite and all seem to be suitable for preparation of this phase. A heat treatment in the small temperature range between 1360 and 1390 ∘ C followed by a rapid quenching is necessary for all preparation methods, since without quenching it is not possible to preserve the La-hexaferrite phase. Detailed investigations were performed for the method of coprecipitation varying the La excess, the heat treatment temperature and the quenching method.

The strong influence of Cu substitution on the magnetic properties of SmCo 5 , especially on the coercive field, is well known. Induction-melted samples ( x = 2 , 2.5 , 3 ) show a phase separation within the 1:5 structure. An unusual jump from a low to a high magnetisation state in an increasing external field is observed. Using pulse field, high coercive forces up to about μ 0 J H c ≈30 T and a high critical field μ 0 H U for the unusual jump reaching 45 T at 4.2 K for x = 3 are observed for the first time. H c and H U increase with decreasing temperature. An influence of the field changing rate on the measured H U is found.

The electronic structure of LaxSr1-xFe12O19 (x=0, 0.25, 0.5, 0.75, 1) hexaferrite is calculated using the density functional theory and generalized gradient approximation (GGA). The GGA+U method is used to improve the description of strongly correlated 3d electrons of Fe. The ‘virtual crystal’ approach is employed for the fractional x, its applicability is checked for x=0.5 by comparing it with the supercell method. The electronic charges introduced by the La substitution show no significant preference for any of the iron sublattices. The magnetic moment decreases linearly with the increasing La content in agreement with the experiment.

Results of linear magnetostriction measurements on a Nd80Fe20 mold-cast rod applying a field up to 9 T parallel and perpendicular to the axis of the rod are presented. At 4.5 K, the parallel and perpendicular magnetostrictions at an applied field of μ 0 H = 9 T are −366×10−6 and −180×10−6, respectively, indicating that the pure Nd crystalline phase formed in Nd80Fe20 during the solidification exhibits a crystallographic and a magnetic anisotropy. At 280 K, the Nd80Fe20 rod is magnetically isotropic and exhibits a positive magnetostriction of 36×10−6, at μ 0 H = 9 T . The magnetostrictive properties are discussed based on the temperature dependence of magnetization and thermal expansion in conjunction with the previously reported results about the microstructure of the same specimen.

The effect of Joule heating on the magnetic properties and microstructure was investigated in Fe 73 Al 5 Ga 2 P 11− x C 5 B 4 Si x ( x =1, 3) ribbons with a thickness of about 40 μm. It turns out that, in these alloys, the coercive field, the pinning field and the time dependence of the initial permeability decrease with increasing current density applied during a thermal treatment via Joule heating up to the occurrence of crystallization. At the as-cast state, the sample with x =3 exhibits better soft magnetic properties than those of the sample with x =1; however, after an optimum annealing, both alloys present similar magnetic properties. The temperature dependence of the resistivity shows clear two stages crystallization process for the Fe 73 Al 5 Ga 2 P 8 C 5 B 4 Si 3 alloy. The microstructural and magnetic inhomogeneities of the annealed Fe 73 Al 5 Ga 2 P 8 C 5 B 4 Si 3 alloy with lowest coercivity were investigated by means of a small-angle neutron scattering method. The improvement of the soft magnetic properties caused by Joule heating of materials is due to the internal stress relieve originated during the production of the ribbon and to the enlargement of the domain wall thickness, consequently the reduction of the anisotropy.

The magnetic viscosity coefficient in YxSm1-xCO3Cu2 alloys, with x = 0. 0.25, 0.5, 0.75, and 1, was determined from measurements of the magnetization reversal relaxation close to the coercive field using a vibrating sample magnetometer and from the coercive field relaxation by the sweep-rate method using a pulse-field magnetometer, respectively. The coercive field and the magnetic viscosity coefficient, as determined from both methods, decrease with increasing yttrium concentration. However, the magnetic viscosity coefficient obtained from magnetization relaxation is always smaller (about six times) than that obtained by means of pulsed field measurements. These results are explained on the basis of different time windows of the measurements, proving different flat distributions of energy barriers due to a complex distribution of activation energies. (C) 2004 Elsevier B.V. All rights reserved.

The magnetic domain size of melt-spun Nd 60 Fe 30 Al 10 has been determined by means of small-angle neutron scattering (SANS). An average domain size of 300 nm was found. High resolution electron micrography and magnetic measurements revealed the precipitation of Nd-rich multi-phases embedded in an amorphous matrix. The 57 Fe Mossbauer spectroscopy showing two peaks in the hyperfine field distribution with high and low moment values, respectively.

The magnetic after-effect of initial reluctivity in an amorphous Fe73Al5Ga2P10C5B4Si1 alloy has been investigated by isochronal as well as isothermal measurements. The relaxation intensity is lower than in simpler Fe-based amorphous alloys prepared by rapid quenching methods. This is produced by reduced free volume quenched within the sample during manufacturing and by strong chemical bonds between metalloid atoms and atoms improving the glass-forming ability of the alloy. Changes of pre-exponential factor indicate large changes of amorphous structure after different heat treatments. Different relaxation mechanisms are identified in the samples, reorientation of atom pair axes and short-range metalloid migrations leading to homogenization of the structure and reduction of the free volumes. The most probable activation energy Q* and pre-exponential factor τ0 are lowest for as-prepared samples: Q* = 1.42 eV and τ0 = 3 × 10−16 s. Both these quantities increase after annealing and slow cooling to Q* = 1.79 eV and τ0 = 2 × 10−15 s. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The aim of this work is to describe the further development of the presently existing quasi-static high field facility reaching 40 T in a long pulse (1 s). In order to achieve fields up to 70 T in the future a two coil system will be constructed, where a short pulse (about 5 ms) of 30–40 T will be generated in a small inner coil, which will be additive to a 30 T long pulse. The currently available parts of the existing machine (Austromag) as well as the experience on this area will be used. Beside the increase of the available field the extension of our measurement methods will be described. This covers the development of a pressure cell as well as developments of magnetization measurement systems with improved sensitivity (vibration method, surface coils, cantilever) which are just under construction.

The possibility of replacing Sr by La and Fe by Co and the subsequent improvement of the magnetic properties of the M-type ferrite stimulated our interest in studying other rare-earth ion substitutions. We have found that Sm and Nd substitutions can increase the coercivity of the hydrothermally synthesised Sr hexaferrite powder without causing any significant fall in the saturation magnetisation. In the present work, La-substituted Sr hexaferrite particles were prepared by hydrothermal synthesis and subsequent calcinations. The effects of the initial La/Sr ratio and the calcination temperature on the structure, particle morphology and magnetic properties of La-substituted Sr hexaferrite were investigated by X-ray diffraction, scanning electron microscopy, vibrating sample magnetometry and a pulsed field magnetometer. It was found that, under hydrothermal conditions, the La3+ additives did not substitute exclusively into the SrM structure but this could be achieved to a greater extent by subsequent calcinations at high temperatures. Compared to the effect of Sm and Nd substitutions, La substitution only increases slightly the coercivity for the samples with initial La/Sr ratios up to 1/8, and for the samples with La/Sr ratios such as 1/4 and 1/2, the coercivity is smaller than that of the sample without La. This can be attributed to grain growth during calcination. Most of the La-substituted samples exhibit a similar magnetisation to that of SrFe12O19.

The aim of this work is to give an overview of reproducible pulsed field techniques. Special emphasis is laid on magnetization measurements. A calibration method well suited for pulsed field magnetometers will be described. Pulsed fields can be used for measuring not only the magnetization but also the magnocrystalline anisotropy, and the complete hysteresis loop of permanent magnets can be determined as well.

A pulse field magnetometer with a new flat pick-up coil device is presented for the characterization of magnetic materials in high magnetic fields. The dimensions of this pick-up coil allow the registration of local hysteresis loops. Magnetization measurements and anisotropy field measurements were performed locally on Nd-Fe-B and M-type ferrite permanent magnets at room temperature. Estimation of the local demagnetizing factor by the measurement of H A using the SPD technique is presented. Local hysteresis loops show the eddy current effect in conducting magnetic metallic samples. The field calibration with the surface coil is also discussed.