MAGNETIC PROPERTIES AND CRYSTALLIZATION OF THE RAPIDLY QUENCHED (Fe1-xNdx) 81.5B18.5 ALLOYS

Magnetic properties and crystallization behaviors of amorphous (Fe_1-xNd_x) _81.5B_18.5 alloys were studied. The crystallization temperature is found to rise at first and then drop monotonically with x, having a maximum value of 976K at x=0.11 (9at% Nd). The (Fe_1-xNd_x) _81.5B_18.5 alloys prepared at a quenching rate of v_s = 6.6m /s are amorphous, and exhibit good glass formability. Both the coercive field H_c and energy product (BH)_max depend strongly on Nd concentration. Amorphous (Fe_1-xNd_x) _81.5B_18.5 alloys with higher Nd concentration have a high coercive field at low temperature, due to the large random uniaxial anisotropy of Nd. The room-temperature H_c and (BH)_max obtained on optimal annealing con-ditions show two maxima as a function of Nd concentration x. The highest room-temperature coercive field H_c =22 kOe within the Nd concentrations around x=0.368 and the maximum energy product(RH)_max= 13.3 MG·Oe at x =0.055 are observed. The hard magnetic properties of these crystallized samples are related to the presence of the bard magnetic Nd₂Fe₁₄B phase.     

EFFECT OF THE SUBSTITUTION OF Ga ON THE STRUCTURE AND MAGNETIC PROPERTIES OF Dy2Fe17 COMPOUND

Dy₂Fe_17-xGa_x (x = 0,1,2,3,4, 5 and 6) compounds were prepared by arc melting. These compounds are of single phase, having a hexagonal Th₂Ni₁₇-type structure fox x=0 and rhombohedral Th₂Zn₁₇-type structure for x≥1. The substitution of Ga for Fe in the Dy₂Fe₁₇ leads to a linear increase of unit-cell volumes. The saturation magnetization M_s at 1.5K is found to decrease linearly with increasing Ga concentration, from 65emu/g for x= 0 to 5emu/g for x = 6; and the Fe magnetic moment μ_Fe is almost independent of Ga concentration. The Curie temperature T_C is found first to increase with increasing Ga content x, and goes through a maximum value of 559 K at about x = 3, then decreases. The sharp increase of T_C at lower Ga content may result from the increase of unit cell volumes. Dy₂Fe_17-xGa_x compounds with x≤5 exhibit easy plane anisotropy at room temperature, and those with x = 6 possess easy-axis.     

Cr掺杂对BaxSr1-xRuO3体系磁性和输运性质的影响

本文采用固相反应法制备了BaxSr1-xRuO3和BaxSr1-xRu0.9Cr0.1O3两组样品,对比研究了它们磁性和电输运性质.当掺杂量少于0.2时,体系处于有少许畸变的钙钛矿的3C结构,Cr的掺入很大幅度提高了体系的铁磁转变温度Tc,电输运性质上也由坏金属行为转变为Mott绝缘体的行为,并且在铁磁相变附近出现电阻拐点.当掺杂量大于等于0.2时,体系发生结构相变,由畸变钙钛矿3C结构变为6 M的单斜结构.这时,Cr的掺入对体系的影响并不明显.这些结果表明引起SrRuO3铁磁转变温度升高的Ru+5-O-Cr+3相互作用对晶格结构有强烈的依赖关系.     

Changes in the hyperfine interactions in the Fe80Nb3Cu1B16 metallic glass under tensile loading

The influence of tensile stress on the changes in hyperfine parameters obtained by Mössbauer spectroscopy of the ferromagnetic amorphous NANOPERM-type Fe₈₀Nb₃Cu₁B₁₆ alloy has been investigated. The bulk changes are obtained by Mössbauer spectroscopy in a transmission geometry using γ-rays; the surface properties are studied in a scattering geometry using conversion electrons. The Mössbauer spectra are analyzed by two distributions of hyperfine inductions, which result in high-field and low-field components corresponding with a model of clusters formed by predominantly iron atoms and intermediate phase containing mainly the Nb, Cu and B atoms. The obtained results are completed by the bulk and surface magnetic measurements, with observations of surface morphology by AFM and microstructure by SEM. The investigations yield approximately linear increase of hyperfine parameters and slight deterioration of the bulk as well as surface magnetic characteristics with tensile loading.     

Recoilless fraction, structural, magnetic and thermal properties of Fe73.5Cu1Nb3Si13.5B9 alloy

Differential scanning calorimetry, X-ray diffraction and room temperature Mössbauer spectrum measurements of Fe_73.5Cu₁Nb₃Si_13.5B₉ (Finemet) alloy have been carried out in order to study its structural and magnetic properties as a function of annealing temperature. The DSC profile of as-quenched Finemet showed two exothermic peaks at 530 and 702 °C, corresponding to two crystallization processes. The Finemet alloy remains amorphous at 450 °C with one broad peak in XRD pattern and one broad sextet in Mössbauer spectrum. When the Finemet alloy was annealed at 550 °C, only well indexed body-center-cubic phase was detected. After being annealed at 650 and 750 °C, the XRD patterns showed the coexistence of α-Fe(Si) and Fe-B intermetallic phases with the increase in XRD peak intensities, indicating the growth of crystallites and the decomposition of Fe_73.5Cu₁Nb₃Si_13.5B₉ alloy at elevated temperatures. The Mössbauer spectra of annealed Finemet alloy could be fitted with 4 or 5 sextets and one doublet at higher annealing temperatures, revealing the appearance of different crystalline phases corresponding to the different Fe sites above the crystallization temperature. The appearance of the nanocrystalline phases at different annealing temperatures was further confirmed by the recoilless fraction measurements.     

The electronic structure of Fe1+xNb3-xSe10

Fe_1+xNb_3-xSe₁₀ has a crystal structure consisting of prismatic niobium chains and iron/niobium octahedral chains. A Charge Density Wave (CDW) has been observed at low temperatures, as might be expected from the similarity of the prismatic niobium chains to those present in NbSe₃. Here we report tight binding band structure calculations which indicate that the electrons responsible for conduction and the CDW reside primarily on the prismatic chains. The effects of disorder and stoichiometry in the iron/niobium octahedral chains on the electronic structure and CDW wave vector are discussed.     

Magnetostrictive behaviour of Fe73.5Si13.5B9Nb3−xMoxCu1 alloys

Magnetostriction, dilatation and calorimetric measurements were performed on FINEMET type as-quenched and heat treated ribbons. Nb was gradually replaced by Mo in order to study the influence of the refractory elements exchange on several magnetostriction parameters. The results could be correlated with magnetic and structural transformations throughout thermal treatments and assured that the whole series is suitable for technical applications.     

High field magnetization in Pr(Ni1-xCox)5 single crystals

Magnetization measurements in fields up to 38 T performed at low temperature on single crystals of the hexagonal Pr(Ni_1-xCo_x)₅ compounds for x = 0, 0.05, 0.10 and 0.20 are presented. In PrNi₅ we observe highly original behaviour predicted by the knowledge of the Crystalline Electric Field parameters and arising from the existence of a non-magnetic singlet ground state; namely transitions associated with the field induced “anticrossing” and “crossing” of the two lowest states along the [100] and [120] directions, respectively. The measurements performed on the other compounds have allowed us to study the dependence of this behaviour on Co substitutions.     

EFFECT OF MANGANESE SUBSTITUTION IN R2AlFe16-x Mnx(R=Y,Ho,Nd AND Pr)COMPOUNDS ON STRUCTURAL AND MAGNETIC PROPERTIES

The structural and magnetic properties of R₂AlFe_16-xMn_x(0≤x≤8 for Y,Ho,0≤x≤10 for Pr, 0≤x≤16 for Nd)compounds have been investigated by means of X-ray diffraction and magnetization measurements.The R₂AlFe_16-xMn_x compounds have a hexagonal Th₂Ni₁₇-type structure for R=Y and Ho, and a rhombohedral Th₂Zn₁₇-type structure for Nd and Pr.With increasing x,the unit-cell volumes have a small increase initially,followed by a greater linear increase.The Curie temperature and the saturation magnetizati on of these compounds show marvelous drop with increasing x.Compensation point was observed in the temperature dependence of the magnetization for Ho₂AlFe₁₂Mn₄ compound.     

Permeability-frequency spectra of (Fe1−xcox)73.5Cu1Nb3Si13.5B9 nanocrystalline alloys under different magnetic fields

Under various amplitude of AC magnetic fields domain wall motion is the main mechanism in the magnetization process. This includes domain wall bulging and domain wall displacing. In this paper complex permeability-frequency spectra of (Fe_1−xCo_x)_73.5Cu₁Nb₃Si_13.5B₉ (x=0,0.5$x=0,0.5$) nanocrystalline alloys were measured as a function of the AC magnetic field, ranging from 0.001 to 0.04 Oe. Obvious changes have been found in complex permeability spectra for alloy x=0$x=0$ with the change of the amplitude of AC magnetic field, but variation of AC magnetic field has little effect on complex permeability spectra for alloy x=0.5$x=0.5$. This is attributed to the increased pinning field after substitution of Fe with Co in Fe_73.5Cu₁Nb₃Si_13.5B₉ nanaocrystalline alloy.     

STRUCTURE AND SUPERCONDUCTIVITY OF Mg1-xLixB2

The effects of the substitution of Li for Mg in Mg_1-xLi_xB₂(x=0,0.1,0.2) on their structure and superconductivity have been investigated. It has been found by X-ray diffraction that the substitution of Li for Mg with x=0.1 and 0.2 does not cause phase transformation in these samples. However,the measurements of temperature-dependent normalized magnetization indicate the loss of superconductivity with the increase of Li content in these samples.     

Field annealing effect on amorphous (FexCo1-x)85Nb15 alloys

The magnetic anisotropy induced by magnetic field annealing was examined for amorphous (Fe_xCo_1-x)₈₅Nb₁₅ alloys produced by a sputtering technique. An appreciably large uniaxial anisotropy was observed even at x = 0. The anisotropy constant (K_u) obtained was 2.3x10³ erg/cm³ at x=0, and it increased with x, showing a maximum of about 5.4x10³ erg/cm³ near x = 0.2. This compositional change in K_u was explained in terms of the directional ordering of magnetic Fe-Co atom pairs under the negative nonmagnetic interaction energy leading to a precipitation.     

EFFECTS OF THE QUALITY OF SUBSTRATE ON THE STRUCTURE AND SUPERCONDUCTING PROPERTIES OF Yba2Cu3O7-x THIN FILM

We have systematically investigated the structures, morphologies, microwave surface resistance, DC performances and the substrate qualities of three YBa₂Cu₃O_7-x(YBCO) thin films 30mm in diameter. The three samples deposited by the same process have been proved of different properties. It has been shown that the structure and superconducting properties of the thin film depend strongly on the quality of substrate.     

Electrical resistivity of the pseudo-binary system Ce(Fe1-xAlx)2

The Laves phase pseudo-binary system Ce(Fe_1-xAl_x)₂ for x≤0.20 has been studied by means of electrical resistivity measurements from 1.5 to 300 K. It is shown that with Al addition, the long range magnetic order of CeFe₂ is destroyed and that a spin glass phase brings in a minimum in the total resistivity with T_min proportional to x. The freezing temperatures T_f are always smaller than T_min and there appears a negative coefficient of the T^3/2 dependence below T_f. The minimum in d?/dT is well correlated with T_f.     

The existence of giant magnetocaloric effect and laminar structure in Fe73.5−xCrxSi13.5B9Nb3Cu1

Amorphous soft magnetic ribbons Fe_73.5−xCr_xSi_13.5B₉Nb₃Cu₁ (x=1–5) have been fabricated by rapid quenching on a single copper wheel. The differential scanning calorimetry (DSC) patterns showed that the crystallization temperature of α-Fe(Si) phase is ranging from 542 to 569 °C, a little higher than that of pure Finemet (x=0). With the same annealing regime, the crystallization volume fraction as well as the particle size of α-Fe(Si) crystallites decreased with increasing Cr amount substituted for Fe in studied samples. Especially, the interesting fact is that the laminar structure of heat-treated ribbons on the surface contacted to copper wheel in the fabricating process has been firstly discovered and explained to be related to the existence of Cr in studied samples. The hysteresis loop measurement indicated that there is the pinning of displacement of domain walls. The giant magnetocaloric effect (GMCE) has been found in amorphous state of the samples. After annealing, the soft magnetic properties of investigated nanocomposite materials are desirably improved.     

Microstructure and magnetic properties of bulk amorphous and nanocrystalline Fe61Co10Zr2.5Hf2.5Nb2W2B20 alloy

The microstructure and magnetic properties, i.e. the initial magnetic susceptibility, its disaccommodation, core losses and approach to ferromagnetic saturation of the bulk amorphous and partially crystallized Fe₆₁Co₁₀Zr_2.5Hf_2.5Nb₂W₂B₂₀ alloy are studied. From X-ray, Mössbauer spectroscopy and electron microscopy studies we have stated that all samples in the as-quenched state are fully amorphous. However, after annealing the samples at 850 K for 30 min the crystalline α-FeCo grains embedded in the amorphous matrix are found. Moreover, from Mössbauer spectra analysis we have stated that the crystalline phase in those samples exhibits the long-range order. The alloy in the as-quenched state shows good thermal stability of the initial magnetic susceptibility. Furthermore, the intensity of the magnetic susceptibility disaccommodation in the rod is lower than in the ribbon. It is due to low quenching rate during the rod preparation which involves the reduction of free volumes. From the analysis of the isochronal disaccommodation curves, assuming the Gaussian distribution of relaxation times, we have found that activation energies of the elementary processes responsible for this phenomenon range from 1.2 to 1.4 eV. After the annealing of the samples the initial susceptibility slightly enhances and disaccommodation drastically decreases. From high-field magnetization studies we have learned that the size of structural defects depends on the quenching rate (the shape of the samples) and changes after annealing.