R. M. Moon

Magnetic Structure of Rare‐Earth‐Cobalt (RCo2) Intermetallic Compounds

Saturation magnetization and neutron diffraction measurements have been performed on cubic Laves phase compounds RCo2 in which R is Nd, Tb, Ho, and Er. Neutron powder patterns obtained at room temperature and at 15°K allowed the determination of the magnetic structures. The low‐temperature patterns are of the ferromagnetic or ferrimagnetic type, with large magnetic intensities superimposed on the nuclear peaks. For the compounds of Tb, Ho, and Er, the rare‐earth atoms show nearly the full moment expected for the free tripositive ion and the cobalt moment is about one Bohr magneton. The rare‐earth moments are coupled parallel to each other, but antiparallel to all the cobalt moments. For NdCo2, the observed Nd moment of 2.6±0.2 Bohr magnetons is smaller than the free ion value of 3.27 Bohr magnetons, and it is coupled parallel to the cobalt moment of 0.8±0.2 Bohr magnetons. In the Nd ion, the spin is opposite to the moment (J = L−S), while for the heavier rare earths the spin is parallel to the total momen...

Magnetic Structure of Neodymium

Neutron diffraction measurements have been made on a single crystal of neodymium at temperatures between 1.6° and 20°K. The onset of magnetic ordering occurs at 19°K and the magnetic structure changes at 7.5°K. The crystal structure is hexagonal with a four‐layer stacking sequence of type ABAC. Thus, alternate layers have different nearest‐neighbor environments, either hexagonal or face‐centered cubic. The main features of the diffraction pattern can be interpreted on the basis of an approximate model in which the hexagonal sites order at 19°K with an antiferromagnetic arrangement between alternate hexagonal layers and with a sinusoidal modulation within each layer. The propagation vector for this modulation and the direction of the magnetic moments are both in the basal plane along a b1 reciprocal lattice vector. The magnitude of the propagation vector changes from 0.13 b1 at 18°K to 0.11 b1 at 7.5°K. The cubic sites order at 7.5°K with a similar structure, except that the propagation vector is 0.15 b1 a...

Polarized neutron study of CeSn3 (invited)

Over the past several years there has been considerable interest in materials exhibiting a nonintegral valence. As a result the properties of cerium metal and its compounds have been the subject of many theoretical and experimental investigations. In the present experiment polarized neutron scattering techniques have been used to study the spatial distribution and temperature dependence of the magnetization induced in a single crystal of CeSn3 by a magnetic field of 42.5 kG. In the 40–300 K temperature range the measured magnetic form factor is in good agreement with the 4f magnetic form factor of Ce3+. Below 40 K, on the other hand, the magnetic form factor measurements suggest that the induced magnetization contains a large component of 5d electronic character of eg symmetry, which can account, at least partly, for the increase in the magnetic susceptibility of CeSn3 at low temperatures. The results will be discussed in terms of a model in which the 4f level is positioned slightly above the Fermi energy...

Antiferromagnetism in V2O3

Neutron diffraction measurements on powder and single‐crystal samples of V2O3 have confirmed the existence of long‐range magnetic order in the low temperature (monoclinic) phase. Paoletti and Pickart1 discovered an extra diffraction peak at low temperature which they attributed to nuclear scattering. A polarization analysis experiment on a powder sample showed that this peak was magnetic in origin. In addition, several much weaker magnetic peaks were identified. These peaks index in the monoclinic cell given by McWahn,2 with indices satisfying the relation h+k+l=2n+1. The magnetic peaks show little temperature dependence from 77°K up to the crystallographic transition at 170°K, where they vanish abruptly. On cooling, they suddenly reappear at about 154°K. The data agree with a model in which the V moments are ferromagnetically aligned in (010)m layers (perpendicular to hexagonal a axis) with a reversal between adjacent layers. The ordered moment of 1.2 μB per V atom is oriented perpendicular to the hexago...

Magnetic Form Factor of Chromium

Neutron‐diffraction measurements of the magnetic Bragg peaks from single crystals of chromium with small additions of manganese have been used to determine the magnetic form factor of chromium. The addition of manganese simplifies the experimental problem by stabilizing the simple antiferromagnetic structure. A single magnetic peak is observed at reciprocal lattice positions, rather than a group of six satellite peaks as in pure chromium. Also, manganese increases the Neel point so that the measurements were performed at room temperature. The results are in good agreement with free‐atom Hartree‐Fock calculations for the chromium 3d electrons. A definite indication of nonspherical symmetry was obtained by comparing the (221) and (300) reflections. These reflections come at the same scattering angle, yet the (221) intensity is larger by about a factor of six. This indicates a t2g population of 79%, compared to 60% for a spherical spin distribution. The results are independent of manganese concentration over...

Induced magnetic form factor of Sm in mixed‐valence compounds

The induced moment form factor of Sm has been measured in a number of compounds using the polarized neutron technique. For integral‐valence compounds, experimental and theoretical evidence is presented showing that Sm2+ and Sm3+ usually have large differences in their form factors. For the mixed‐valence systems, SmS at high pressure and Sm0.76Y0.24S, the observed form factor is nearly identical to that of the 2+ ion. If we adopt the viewpoint that the Sm ions are fluctuating between 4f5 and 4f6 configurations, then our measurements imply that the induced moment from the 4f5 configuration is dominated by the Van Vleck contribution to the susceptibility arising from an admixture of states with different J values or that the 4f5 configuration makes a negligible contribution to the form factor. The suppression of the Curie (1/T) terms is consistent with low‐temperature susceptibility measurements on these systems. The implications of these observations are examined within the framework of a simple valence‐fluctuation model.The induced moment form factor of Sm has been measured in a number of compounds using the polarized neutron technique. For integral‐valence compounds, experimental and theoretical evidence is presented showing that Sm2+ and Sm3+ usually have large differences in their form factors. For the mixed‐valence systems, SmS at high pressure and Sm0.76Y0.24S, the observed form factor is nearly identical to that of the 2+ ion. If we adopt the viewpoint that the Sm ions are fluctuating between 4f5 and 4f6 configurations, then our measurements imply that the induced moment from the 4f5 configuration is dominated by the Van Vleck contribution to the susceptibility arising from an admixture of states with different J values or that the 4f5 configuration makes a negligible contribution to the form factor. The suppression of the Curie (1/T) terms is consistent with low‐temperature susceptibility measurements on these systems. The implications of these observations are examined within the framework of a simple valence‐flu...

Absence of Antiferromagnetism in Ti2O3

The possible existence of antiferromagnetism in Ti2O3 has important theoretical implications associated with the explanation of the metal‐to‐semiconductor transition in this material. A previous neutron‐diffraction study, using unpolarized neutrons, indicated that Ti2O3 was antiferromagnetic with the α‐Fe2O3 structure. More recently, contradictory findings were reported which eliminated the α‐Fe2O3 structure but could not rule out the type of ordering found in Cr2O3. We have reexamined this substance, using the new technique of neutron‐polarization analysis. With this technique, the coherent scattering can be separated into nuclear and magnetic parts at a fixed temperature. It is possible to detect small magnetic contributions, even when the magnetic and nuclear peaks have the same scattering vector. At room temperature, we find no evidence for either the α‐Fe2O3 or the Cr2O3 magnetic structure.

Temperature dependence of the field induced magnetic form factor of CeSn3

Polarized neutron scattering techniques have been used to study the spatial distribution and temperature dependence of the magnetization induced in a single crystal of CeSn3 by a magnetic field of 42.5 kG. We find that in the 300–40 K temperature range the measured form factor is in good agreement with the free ion 4f magnetic form factor of Ce3+. Below 40 K large deviations from a 4f magnetic form factor were observed. A satisfactory fit to the data below 40 K can be obtained by assuming that the induced magnetization contains a large component of 5d electronic character of eg symmetry.

Temperature dependence of the field induced magnetic form factor of the intermediate valence compound CePd3

Polarized neutron scattering techniques have been used to obtain information regarding the ground state electronic properties of the intermediate valence compound CePd3. The spatial distribution of the magnetization induced by a magnetic field of 42.5 kG in a single crystal of CePd3 has been studied at 100 and 4.2 K. We find that at 100 K the measured form factor is in good agreement with the free‐ion 4f magnetic form factor of Ce3+. At 4.2 K on the other hand, deviations from a 4f magnetic factor were observed. An adequate fit to the experimental data taken at 4.2 K can be obtained by assuming that the induced magnetization contains a significant component (∼17% of the total) of Ce–5d electronic character.

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...