Distribution of dipole fields in disordered crystalline and amorphous ferromagnetic alloys

By means of computer simulation we have calculated the distribution functions of dipole fields in disordered crystalline and amorphous ferromagnetic alloys A_1-xB_x. It is shown that for all cubic lattice sites in simple cubic, body-centred cubic and face-centred cubic materials as well as for amorphous materials the envelopes of the distribution functions may be obtained in a satisfactory approximation by considering only the nearest contributing atoms. Whereas in crystalline materials we have a complicated structure of the distribution functions for arbitrary values ofx, we obtain simple Gaussian distributions for the case of amorphous materials, using Heimendahl's model of the amorphous structure. The influence of isotropic and anisotropic short-range order is discussed in detail.     

Resistivity and magnetoresistivity of amorphous rare-earth alloys

The resistivity and magnetoresistivity of amorphous rare-earth alloys are studied starting from the general approach of Van Peski-Tinbergen and Dekker. The random axial crystal-field and the magnetic correlations between the rare-earth ions are consistently taken into account. The characteristic features of the available experimental data are explained both of the case of random ferromagnetic and antiferromagnetic order.     

Magnetic properties of amorphous rare-earth—Iron alloys

The magnetic properties of various amorphous alloys of the type R_1-xFe_x (R = Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu) have been determined in the concentration range 0.3 ? x ? 0.5. With the exception of the Gd and Lu alloys pronounced thermomagnetic history effects were observed in the temperature dependence of the magnetization. These effects are due to a strong temperature dependence of the coercive force (H_c) which is found to obey an exponential law of the form H_c ∞ exp(-αT). ⁵⁷Fe Mössbauer spectra were obtained on the alloys of a composition close to 40 at% Fe. From the combined results of the Mössbauer spectroscopy and magnetic measurements it is derived that the Fe moment decreases if one passes through the rare-earth series. It is postulated that this decrease is due to small differences in the compositional short-range order in the amorphous alloys caused by the heat of mixing becoming more negative in the same sense.     

Distribution of magnetic dipole fields in amorphous ferromagnetic and speromagnetic alloys and in crystalline and amorphous spin glasses

For binary alloys with only one sort of magnetic atoms we have calculated, by means of computer simulations, the distribution of magnetic dipole fields at regular lattice sites and suitably defined interstitial sites for the magnetic ground state of amorphous ferromagnetic and speromagnetic alloys and crystalline and amorphous spin glasses. Thereby we have considered spatially random arrangements of atoms and have neglected all magnetic correlations in the case of speromagnets and spin glasses. We have derived symmetric and isotropic distribution functions, which are fairly well described by Gaussian distributions for large concentrations of magnetic atoms only. The distribution functions are characterized by their second moments, which are important for the discussion of NMR and μ⁺ SR relaxation rates in these materials.     

Note on the magnetic properties of amorphous rare-earth nickel alloys

The magnetic properties of several amorphous rare-earth nickel alloys close to 30 at% Ni have been studied in the temperature range 4.2–300 K. The Curie temperatures range from 2 K in Pr₆₉Ni₃₁. Alloys in which the rare-earth component has an orbital moment show large intrinsic coercive forces below T_c. These coercive forces vary considerably with temperature, the temperature dependence of the alloys Dy₆₉Ni₃₁ and Tb₆₉Ni₃₁ is satisfactorily described by means of an exponential law of the form H_c(_T) = H_c(0) exp(-αT).     

169Tm Mössbauer effect studies of rare-earth amorphous alloys

Tm-M amorphous alloys with M=Cu, Al, Ag, and Fe have been studied using the Mössbauer effect in¹⁶⁹Tm over a temperature range of 0.05 K to room temperature. The spectra for all alloys are strikingly similar, in contrast to those of the crystalline counterparts. The most notable feature of all spectra taken at temperatures below about 30 K is the existence of two distinct subspectra, one with high magnetic hyperfine field that shows no sign of electronic relaxation effects, and the other with low (or zero) field. The origin of the two component subspectra and their temperature dependences is still an open question.     

各向异性场对非晶态合金应力阻抗效应的影响

在非晶态合金材料中引入应力感生各向异性场及磁各向异性场,根据Maxwell方程组及带Gilbert项的Landau-Lifshitz方程,得到了能揭示非晶态合金材料应力阻抗效应的理论,理论结果可定性地解释实验结果. 对不同各向异性场参数下的应力阻抗效应进行了大量计算,获得了一些对理论和应用具有一定参考价值的结论. 关键词： 应力阻抗效应 应力感生各向异性 横向有效磁导率 非晶态合金     

Distribution of electric and magnetic hyperfine fields in Fe-rich gallo-germanates

The crystal field disorder in some tetragonal and trigonal Ca-gallo-germanates, i.e., A₂Fe₂ZO₇ and A₂LnFe₃Ge₃O₁₄, with A?=?Ba or Sr, Ln?=?La or Nd and Z?=?Ge or Si, has been studied by ⁵⁷Fe Mössbauer spectroscopy. The observed distribution of the hyperfine fields is analysed from a theoretical model based on the additive perturbation approximation of crystal fields, a rigid lattice of ionic point charges and the random substitution of Fe³⁺/Z⁴⁺ and A²⁺/Ln³⁺.     

Magnetic hysteresis in rapidly quenched rare-earth alloys

The magnetic and structural properties of rapidly quenched (Pr₈₀Ga₂₀)_100-xFe_x, RTbFe and RTbFe(Co)M alloys are examined over a wide range of chemical compositions, where R ≡ Pr, Sm, MM and M ≡ B and Si. The Ga-containing samples show relatively high coercive fields (up to 3 kOe) in the amorphous state which subsequently disappear after crystallization. On the other hand, the high coercive fields (≈5 kOe) of melt-spun RTbFe samples decrease slightly after crystallization but their magnetic moment increases substantially. Melt-spun RTbFe(Co)M samples are generally magnetically soft in the as-quenched state. Magnetic hardening is produced by annealing the samples around 750°C leading to coercive fields which could not be measured with an ordinary electromagnet (H_c #62; 23 kOe). The best properties have been obtained on a Pr₁₄Fe₇₁B₁₅ sample with a coercive field of 8 kOe and an energy product of 8.5 MGOe. Thermomagnetic data show that a structural transformation takes place upon heating the samples to 750°C. The Curie temperature of the precipitate phase is around 320°C while that of the as-quenched phase is around 160°C. Transmission electron microscope studies show a very fine precipitate structure with a precipitate size below 100 Å. The precipitate phase is believed to be highly anisotropic leading to the observed hard magnetic properties.     

Stability of amorphous alloys

The thermal stability of a number of Ti-base and Zr-base amorphous alloys was studied by means of differential scanning calorimetry. The crystallization temperatures of these amorphous alloys and those reported in the literature for a variety of other amorphous alloys (actinide-base alloys an refractory metal-base alloys) were analysed in terms of the corresponding formation enthalpies. It is shown that there is no correlation between the crystallization temperatures and the formation enthalpies. On the other hand the crystallization temperatures were found to scale more or less linearly to the corresponding formation enthalpies of a hole the size of the smaller type of atom in the binary amorphous alloys.     

Effect of electric current on the magnetic and magnetoelastic properties of amorphous ferromagnetic alloys

The magnetic and magnetoelastic parameters of Fe₆₄Co₂₁B₁₅ and Fe_81.5B_13.5Si₃C₂ amorphous ferromagnetic alloys treated by direct electric current in air are studied as functions of the applied magnetic field and current density. The samples of the alloy have the form of narrow strips with different lengths. It is found that the magnetoelastic parameters of the dc-treated alloys depend on the magnetic field in a qualitatively different way. From the behavior of the magnetic and magnetoelastic parameters of the alloys in the magnetic field a model of magnetization nonuniform distribution in amorphous ferromagnetic alloys subjected to dc treatment is proposed.     

Magnetism in amorphous Fe-Si alloys

Fe-Si alloys have been obtained by the vapor quenching technique. Resistance measurements, electron microscopy and diffraction results are typical of an amorphous system. The Mössbauer spectra of these amorphous alloys, fitted in terms of a distribution of hyperfine fields, show the existence of a local magnetic order with, however, a proportion of weakly or even no coupled Fe atoms which is temperature and Fe-concentration dependent.     

Amorphous rare-earth alloys: random-anisotropy antiferromagnetism

The model of randomly oriented single-ion uniaxial anisotropy field is extended to the case of antiferromagnetic exchange interaction between rare-earth ions. A self-consistent two-spin cluster approximation is used to calculate the magnetization curves for a model system of spin 1. The results are in good qualitative agreement with experimental data on amorphous RE_xCu_1?x and RE_xAg_1?x alloys (RE = Tb, Dy, Ho). In particular, the observed increase of the “coercive” field with increasing x in RE_xCu_1?x alloys is accounted for.     

Crystal fields of hexameric rare-earth clusters in fluorites

In solid solutions of alkaline-and rare-earth fluorides with a fluorite structure, ions of most elements of the rare-earth (RE) row form hexameric clusters that assimilate the minor component of the solid solutions (fluorine) and build it into the cubic fluorite lattice without changing its shape. An analysis of the EPR spectra of paramagnetic RE ions (Er³⁺, Tm³⁺, Yb³⁺) in clusters of diamagnetic ions (Lu³⁺, Y³⁺) confirms their hexagonal structure, which was established when studying the superstructures of the compounds under study. In such a cluster, a RE ion is in a nearly tetragonal crystal field, with the parameters of this field differing radically from those of single cubic and tetragonal RE centers in crystals with a fluorite structure. In particular, this field causes high (close to limiting) values of the g_∥ factors of the ground states of the paramagnetic RE ions. Computer simulation is used to determine the atomic structure of a hexameric cluster in MF₂ crystals (M = Ca, Sr, Ba). The crystal field and energy spectrum of Er³⁺, Tm³⁺, and Yb³⁺ ions in such clusters are calculated, and the spectroscopic parameters of the ground states of these ions are determined. The calculations confirm the earlier assumption that the unusual EPR spectra of nonstoichiometric fluorite phases are related to RE ions in hexameric clusters.     

Polariton dynamics of a one-dimensional magnetic photonic crystal in crossed magnetic and electric fields

By an example of a semibounded one-dimensional magnetic photonic crystal (MPC) of easy-axis antiferromagnet-nonmagnetic dielectric type placed in mutually orthogonal dc magnetic and electric fields, conditions are found under which the quadratic magneto-optic interaction leads to a number of anomalies in the reflection and propagation conditions for TM or TE waves incident from outside on the surface of the superlattice. It is assumed that the easy magnetic axis is collinear to the electric field.     

Magnetic properties of amorphous alloys

The magnetic properties of existing amorphous iron based materials, i.e., Fe--metalloid, Fe--early transition metals and Fe--rare earth alloys, are briefly discussed for some representative alloys. The spin orientation of amorphous Fe--metalloid alloys has been determined by the angular dependence of hyperfine interactions. It is shown that in iron--early transition metals ferromagnetic order is not long-ranged, but determined by magnetic clusters. The magnetic hyperfine field distributions of Fe-rich iron--early transition metals consist of a high and a low field tail. The magnetic structure has been investigated for two representative Fe--RE (RE = Er, Ce) amorphous alloys. For the first time, the magnetic coupling phenomenon in amorphous/crystalline multilayers has been discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Electrical resistivity of amorphous alloys

Amorphous alloys of Ga, Sn, Pb and Bi with Cu, Ag and Au are produced by evaporation on a cold substrate. The residual resistivity, the temperature dependence of the resistivity, the transition temperature of superconductivity and the temperature of the amorphous-crystalline transformation are measured. We observe e.g. that the residual resistivity increases with the noble metal concentration, and that the temperature coefficient of the resistivity of the Au alloys is always negative. In these two respects amorphous alloys differ in behaviour from the corresponding liquid alloys. These observations can be correlated with the atomic energy levels of the free atoms.