H. R. Child

Some Magnetic Structure Properties of Terbium and of Terbium‐Yttrium Alloys

Neutron diffraction studies have been made on single‐crystal and polycrystalline specimens of terbium. Earlier magnetic and thermal measurements have indicated a transformation to an ordered magnetic state at approximately 230°K, and a subsequent order‐order transformation at approximately 220°K. The neutron measurements show that in the narrow antiferromagnetic region, the magnetic structure of terbium is a helical structure. The interlayer turn angle varies from 20.5° per layer at the Neel point to 18.5° per layer at the lower transition. At this lower temperature the structure transforms, in the absence of any external applied field, to a classical ferromagnetic structure in which the moments are in, or nearly in, the planes perpendicular to the hexagonal axis. At very low temperatures the magnetic moment per atom is very nearly 9.0 Bohr magnetons, the value expected for the ordered tripositive ion. Neutron diffraction measurements have also been made on a series of alloys of yttrium and terbium in ord...

Magnetic Structure versus Electron Number for Some Rare‐Earth Intermetallic Compounds

Neutron diffraction measurements were made on a series of rare‐earth compounds in the Tb(Pd, Ag) and Tb(Ag, In) systems. Most of these compounds exhibit the CsCl type of crystal structure and, for these, magnetic structure determinations were made in order to relate the type of magnetic structure to the number of valence electrons. Two types of antiferromagnetic structures were observed; the (ππ0) type consisting of an antiparallel array of ferromagnetic (110) planes of moments, and the (00π) type with ferromagnetic (001) planes of moments alternating in direction. On the basis of 0, 1, 3, and 3 valence electrons for Pd, Ag, In, and Tb, respectively, the (ππ0) structure is found in the region of 3.5 to 4 valence electrons per unit cell and the (00π) structure in the region of 5 valence electrons per unit cell.

Studies of chemical homogeneity and magnetic domain walls in Fe‐based metallic glasses using small‐angle neutron scattering

Metallic glasses Fe77B23, Fe82B18, and Fe81B13.5Si5.5 have been studied as a function of applied field and annealing condition using small‐angle neutron scattering (SANS). The data were taken at 300 K for momentum transfer q = 4πsinϑ/λ in the range 0.05–0.3 nm−1. In zero field, all the samples produced similar patterns with intense isotropic SANS for q<0.1 nm−1 which could be fitted with Guinier radii RG∼40 nm. This SANS decreased rapidly in intensity with applied field. The change in SANS on annealing correlated directly with changes in bulk coercivity Hc and domain wall density seen in electron microscopy: For stress‐relieved Fe82B18 and Fe81B13.5Si5.5, SANS and Hc decreased slightly, but they increased substantially in Fe77B23 after annealing. This scattering arises primarily from domain walls in accord with dynamical theory. The weak SANS for 0.1<q<0.3 nm−1 suggest that the phase separation and chemical inhomogeneity in these glasses, both as‐cast and annealed, are limited.

Two‐dimensional neutron diffraction of YFe2O4 and CoCr2O4

A new wide‐angle neutron diffraction instrument has been used to study CoCr2O4 and YFe2O4. The instrument is capable of measuring the scattered intensity over a wide region of reciprocal space in a short time, and this feature was used to measure the magnetic diffraction pattern of these materials at temperatures below the magnetic ordering temperature.

Small‐angle neutron scattering from magnetic correlations in Fe0.7Al0.3

Ordered Fe0.7Al0.3 becomes ferromagnetic below 400 K but paramagnetic on cooling below 170 K and then is mictomagnetic below 92 K. A previous neutron study of this material showed magnetic diffuse scattering over this entire temperature range and we present here an extension of this study to very small angles (6×10−3?κ?30×10−3 A−1). We find that the scattering measured at constant K shows a hump at Tc as a function of temperature but also peaks near the inverse Curie temperature and these peaks occur at different temperatures for different κ. This result is reminiscent of spin glass behavior. No significant change occurs in the scattering at the mictomagnetic transition at 92 K. The κ dependence of the scattering can be described by both a Lorentzian (Ornstein‐Zernike type) and by the square of a Lorentzian (Debye type) but a more reasonable fit is found for the latter form. The application of a field of a few kilogauss eliminates the small angle scattering at all temperatures, and for scattering directio...

The Paramagnetic Form Factor of Gadolinium

A sample of Gd2O3 enriched to about 98% in the low absorbing isotope 160Gd was examined by neutron diffraction at temperatures from room temperature to 1.3°K. Measurements of the diffuse background scattering in the high and intermediate temperature patterns have been used to obtain the paramagnetic form factor as a function of sinθ/λ. The observed scattering cross section was converted to f2 values by assuming the full theoretical paramagnetic moment of g[J(J+1)]1/2 for Gd+3 and using this value to correct for the isotropic nonmagnetic part of the scattering. The form factor obtained in this manner is near the calculated Hartree‐Fock free‐atom form factor but appears to be slightly higher over most of the region out to sinθ/λ≅0.3 A−1. Beyond this value, overlapping nuclear reflections in the powder pattern make it impossible to obtain usable values of the background intensity. Magnetic ordering of a complex antiferromagnetic nature appears at about 1.6°K but a large diffuse hump in the pattern remains to...

Neutron Diffraction Study of VF2

A neutron diffraction study of VF2 has been carried out at temperatures between 1.3° and 300°K with powdered and single‐crystal samples. Below its transition temperature, 7°K, associated with the development of the long‐range magnetic order, this rutile structured compound has a spiral spin structure in which the spins lie perpendicular to the tetragonal c axis and spiral along the c axis with a turn angle of 96.0°±0.5°. There is no detectable change in the magnitude of the turn angle between 4.2° and 1.3°K. Based on the magnetic susceptibility and magnetic anisotropy data of VF21 below 4.2°K and Yoshimoris model,2 we have calculated a value for the turn angle of VF2 98°, in good agreement with the neutron scattering result. The neutron diffraction data between 20° and 77°K confirm that there is a large spin correlation between the near neighbors. The parameter, u, designating the fluorine positions in the unit cell, ±(u, u, 0) and ±(½+u, ½−u, ½), was determined to be 0.308±0.001. Comparison of the scatt...

Magnetic Structure Properties of Tb‐Sc Alloys

The results of a neutron‐diffraction study of the magnetic structure properties of the Tb‐Sc alloy system are reported. Five binary alloys with Tb content from 90 at. % to 25 at. % have been examined by powder techniques from 300°K down to liquid‐helium temperatures to determine the magnetic ordering temperatures and the magnetic structures. The dilution by Sc causes the spiral magnetic phase of Tb to remain stable over a temperature interval larger than in the pure metal. This effect is the same general behavior as that found in the Tb‐Y system previously reported. However, the Neel points of the Tb‐Sc alloys fall well below those found for Tb‐Y and the initial turn angles are smaller in this system. TN falls more rapidly for Tb‐Sc alloys than for Tb‐Y alloys as a function of the effective spin variable x=c(g−1)2J(J+1), where c is the atomic percent of Tb, and reaches 0°K at x about 2.5. The 25% Tb alloy shows no visible magnetic order down to 1.3°K in contrast to the Tb‐Y system where magnetic order was observed with as little as 5 at. % Tb. The universal curve of TN vs x previously found for rare‐earth alloys is not obeyed in the Tb‐Sc system.The results of a neutron‐diffraction study of the magnetic structure properties of the Tb‐Sc alloy system are reported. Five binary alloys with Tb content from 90 at. % to 25 at. % have been examined by powder techniques from 300°K down to liquid‐helium temperatures to determine the magnetic ordering temperatures and the magnetic structures. The dilution by Sc causes the spiral magnetic phase of Tb to remain stable over a temperature interval larger than in the pure metal. This effect is the same general behavior as that found in the Tb‐Y system previously reported. However, the Neel points of the Tb‐Sc alloys fall well below those found for Tb‐Y and the initial turn angles are smaller in this system. TN falls more rapidly for Tb‐Sc alloys than for Tb‐Y alloys as a function of the effective spin variable x=c(g−1)2J(J+1), where c is the atomic percent of Tb, and reaches 0°K at x about 2.5. The 25% Tb alloy shows no visible magnetic order down to 1.3°K in contrast to the Tb‐Y system where magnetic order was...

Neutron Diffraction Investigations of the Magnetic Ordering in Rare Earth Nitrides

Neutron diffraction investigations on HoN and TbN at low temperatures show that both compounds become ferromagnetic with Curie temperatures of about 18°K and 43°K, respectively. Although the paramagnetic scattering is consistent with moment values calculated for the free trivalent rare earth ions, the observed ferromagnetic moments are lower than the calculated values and indicate the effect of crystalline field interactions. Diffraction patterns from both compounds at 1.3°K show considerable ferromagnetic short-range-order scattering with characteristics which are different from those associated with critical magnetic scattering.Neutron diffraction investigations on HoN and TbN at low temperatures show that both compounds become ferromagnetic with Curie temperatures of about 18°K and 43°K, respectively. Although the paramagnetic scattering is consistent with moment values calculated for the free trivalent rare earth ions, the observed ferromagnetic moments are lower than the calculated values and indicate the effect of crystalline field interactions. Diffraction patterns from both compounds at 1.3°K show considerable ferromagnetic short-range-order scattering with characteristics which are different from those associated with critical magnetic scattering.

Magnetic Structures of Samarium

Neutron diffraction measurements on single crystals of 154Sm have been performed to determine the magnetic structure and magnetic form factor. The Sm crystal structure consists of a nine layer stacking sequence along the c‐axis with 2/3 of the sites having local hexagonal symmetry and 1/3 having local cubic symmetry. The hexagonal sites order at 106°K in an antiferromagnetic layer‐type structure with a stacking sequence (++0−−0…) along the c‐axis. The zeroes correspond to layers of cubic symmetry and the hexagonal moments are directed along the c‐axis. The magnetic cell is twice the chemical cell in the c‐direction. At 14°K, the cubic sites order in ferromagnetic layers parallel to (101) planes. Considering only the cubic sites, there is a (++−−) sequence along a direction perpendicular to these planes with moments parallel to the c‐axis. The magnetic cell for the cubic‐site structure is four times the chemical cell along both a‐ and c‐directions. No modification of the hexagonal‐site structure is observe...