M. Melamud

Moment distribution in amorphous magnetic ribbons by Mössbauer measurements

Mossbauer effect measurements were obtained on smooth‐surface Fe78B13Si9 amorphous ribbons annealed in a transverse magnetic field at high temperature. Absorption spectra were obtained with the ribbon plane tilted at several angles with respect to the direction of the γ rays, in an external field ranging from 0 to 10 Oe. Intensity ratios were analyzed to obtain the magnetization as a function of field. A large zero‐field spread was deduced and excess scatter near the anisotropy field was observed.

Where are the iron atoms and iron moments in RFEmAl12−n? A Wigner‐Seitz analysis

The ternary compounds RFenAl12−n (n=4,5,6; ThMn12 structure) form an extensive family of alloys having complicated magnetic structures, with the rare earths (R) and Fe ordering at different temperatures. This magnetic ordering has been inferred from magnetization, Mossbauer and neutron diffraction measurements. X‐ray results are in disagreement with the magnetic measurements as to which sites are occuped by the Fe, and it is not clear at which of the sites iron has the largest magnetic moment. Recently, we have suggested that the occurrence of a substantial magnetic moment at an atomic site in certain magnetic systems can be related to the occurrence of −72° disclination lines connection atoms on this site. These disclinations are bond lines joining two near‐neighbor atoms which have six nearest neighbors and can be recognized by a sixfold face appearing on the Wigner‐Seitz polyhedra of the two atoms. The Wigner‐Seitz construction and its use in the analysis of this and related problems is presented.

Disclinations: The magnetism and the magnetic anisotropies of the rare‐earth–3d transition‐metal hard magnets (invited)

An important class of hard magnets, involving rare earth and 3d transition metals and sometimes metalloids, includes Nd2Fe14B and Nd2Fe17. We have noted a correlation between the local site magnetism in these two compounds and whether those sites lie on nets of so‐called major ligand lines or disclinations, i.e., bond lines shared by six common nearest neighbors. We have proposed that a criterion for choosing candidate alloys with strong 3d moments is the occurrence of such disclination nets, and using this criterion, have listed several structures having 3d sites with this characteristic. We have also rationalized the crystal field anisotropies as relating to the orientation of the major ligand lines.

Disclinations and magnetism in rare-earth-transition-metal hard magnets

An important class of magnets, involving rare‐earth and 3d transition metals and sometimes metalloids, includes Nd2Fe14B and Nd2Fe17. We have noted a correlation between the local site magnetism in these two compounds and whether those sites lie on nets of so‐called major ligand lines or disclinations, i.e., bond lines shared by six common nearest neighbors. We propose that a criterion for choosing candidate alloys with strong 3d moments is the occurrence of such disclination nets. We have developed a computer program for the calculation of these nets, and using this, several structures having 3d sites with this characteristic have been found.

Disclinations: their relation to the anisotropies of rare-earth hard magnets☆

Abstract In this communication we have observed that the major ligand lines, as seen at rare-earth sites, correlate with the experimental rare-earth anisotropies whereas, previously, we observed 1,2 that their presence at 3d sites in the hard magnets correlates with the occurrence of large 3d moments. The former arises from bonding and charge effects while the latter is, presumably, associated with interatomic exchange and the site volumes attending the presence of such ligand lines. These observations have allowed predictions to be made concerning the magnetic anisotropies which should arise in four crystal structures. These structures should provide environments appropriate to good hard magnets providing that they can be made to accomodate the necessary combinations of rare earth and 3d elements. More importantly, we have observed that there are different classes of major ligand lines with those involving only rare-earth ions tending to dominate, thus controlling the rare-earth site crystal field orientations. In those structures not having such lines, the major ligand lines involving mixes of rare earth and 3d atoms win by default and determine the orientations.

Effect of nitrogen on the properties of hard magnets

The enhancement of magnetic properties of RE‐TM hard magnet materials, such as RE2Fe17, upon nitriding is studied using the Wigner–Seitz (Voronoi) construct. In analogy with other RE‐TM nitrides, it is concluded that nitrogen has a strong preference to occupy the octahedral 9e site in the 2:17 compounds, to the exclusion of the other proposed sites. Additional materials are suggested as candidates for nitriding on the basis of the availability of such an octahedral site. The site preference and its effect on the magnetism in RE2Fe17 is discussed in relation to the Wigner–Seitz cell of the atoms. The magnetic moments on the iron are shown to be correlated with the WS cell volumes, and this dependence differs from that associated with αFe.

Static and dynamic bifurcations in magnetoelastic ribbons (abstract)

The ΔE effect in certain field annealed amorphous ribbon is now about 10, i.e., Young’s modulus E, can be reversibly changed by a factor of 9 with the application of a field of less than 1 Oe. We have reported the field‐induced buckling of a vertically oriented ribbon. The ribbon buckles under its own weight due to the reduction of E with field H. Critical buckling values of H were found to be in good agreement with the eigenvalues of the linearized version of the operator describing the process. Here we present: (a) holographic data where the gradient in the fringe spacing obtained from the hologram of the straight and buckled states is a measure of the curvature; and (b) a rigorous mathematical formalism for extracting from experiment the nonlinear constitutive relation between the curvature θ’(s) and the bending couple m(s) where s is the distance along the ribbon. This process must be carried out (with H as a parameter) from H=0, to values of H somewhat above the anisotropy field, to effect a complete...

Local environments of three forms of Ba2YCu3O6+x

Abstract The Wigner–Seitz local atomic cell construction procedure, provides a topological means for comparing series of related structures in terms of the number and type of nearest neighbors. We used this construction to further our understanding of the chemical bonding environment (and hence, the superconducting properties) in three structurally related materials, Ba 2 YCu 3 O 6+ x , ( x =1, 1/2, and 0). These are high- T c superconductors of T c ≈92 K, 56 K, and a nonsuperconductor, respectively. The local environment around Y is found to be similar in all three compounds, and the topology around Ba is generally similar to that in the ideal perovskite. The Cu sites in the Cu–O plane in all three materials show similar anisotropic bonding behavior. Significant results are: (i) that Ba contributes to the bonding with Cu sites, and (ii) the most noticeable difference in the local environment in these compounds is around the Cu sites in the Cu–O “chains”. We propose a modified charge-transfer model, involving the Cu–O “chains”, “planes” and Ba as an electronic unit, to correlate structure with the observed superconducting properties. The bonding behavior of these oxides also appears to be different from the conventional metal alloy superconductors.

Disclinations, crystal fields, and easy axes of magnetization in the hard magnets (abstract)

There is a need for increased insight into the origins of the easy axes of magnetization of the transition metal-rare earth hard magnets. These systems form in Frank Kasper or closely related crystalline structures. David Nelson has pointed out1 that these topologically close packed phases are threaded with disclination lines. The principal chains among these lines, as they pass through rare-earth sites, appear to define the orientations of the crystal fields at these sites and, in turn, the low-temperature easy axes of magnetization. The currently available experimental data are shown to be in almost perfect accord with this definition of easy axes. Using these ideas, predictions are made for systems and structures for which magnetization anisotropy data have yet to be obtained.

Effect of barium cuprate on high temperature superconductors

BaCuO2 has a pronounced effect on the magnetization data of high-Tc superconductors. By subtracting the estimated magnetic contribution of BaCuO2 present in the superconducting samples, the susceptibility is seen to be purely diamagnetic.