M. K. Wilkinson

NEUTRON DIFFRACTION INVESTIGATION OF MAGNETIC ORDERING IN DYSPROSIUM

Neutron diffraction measurements on a single crystal of dysprosium show that the magnetic structure in the antiferromagnetic region between 179° and 87°K closely resembles a helical-type arrangement of the atomic moments. In this arrangement the moments within a hexagonal layer are aligned parallel and point in a direction perpendicular to the c axis of the crystal. The moment direction in adjacent layers is rotated by a specific angle which is dependent on the temperature of the sample. A slight modification of this structure exists below about 140°K, and a transition to ferromagnetism occurs at 87°K.

Spin‐Wave Spectrum of Nickel Metal

The spin‐wave spectrum of nickel metal has been measured in the three principal symmetry directions by inelastic neutron scattering using the triple‐axis technique. A large crystal of 60Ni was used to avoid incoherent scattering and reduce contributions from phonon scattering. The spin‐wave dispersion curve rises quadratically in each of the three directions according to the relation E=Dq2 when D is about 430 meV A2. The spin‐wave intensity decreases slowly with increasing energy and then drops suddenly by more than an order of magnitude. This sudden decrease occurs at different energies for the three symmetry directions and in accordance with calculations is interpreted as the intersection of the spin‐wave dispersion curve with a continuum band of excitations. In the [100] direction this intersection corresponds to an energy of about 100 meV and a q of about 0.4 A−1. These results along with band calculations by Hanus indicate a d band splitting of 0.57 eV. The measured spin‐wave spectra are similar to t...

Neutron Diffraction Study of Metallic Erbium

Neutron diffraction measurements were made on erbium single crystals in the temperature range 298°–4.2°K. The material is antiferromagnetic below 80°K and ferromagnetic below 20°K. In the antiferromagnetic region, the magnetic scattering consists of satellite reflections corresponding to a modulation of the magnetic scattering amplitude along the c axis. The spacing and intensity distribution of these satellites show two distinct subregions of antiferromagnetism. In the upper region, between 80° and 52°K, the data suggest a sinusoidal modulation of the magnitude of the c-axis component of magnetic moment with a period of 3.5 c0. Between 52° and 20°K the wavelength of the modulation varies continuously from 3.5° c0 to 4.0 c0. In addition, there is a squaring up of the modulation and a simultaneous ordering of the component of the moment normal to the c axis. Below 20°K the material is basically ferromagnetic with a moment of 7.2 μB directed parallel to the c axis.

Neutron Diffraction Investigations of Ferromagnetic Palladium and Iron Group Alloys

In order to account for the magnetic properties of alloys it becomes important to determine the individual magnetic moments of the constituent atoms. This determination can be accomplished by the combination of neutron diffraction and magnetic induction measurements. Such measurements were made on the following ferromagnetic alloys: Pd3Fe, PdFe, Pd3Co, PdCo, Ni3Co, and NiCo. The average moment values were obtained from magnetic induction measurements while the differences in the atomic moments were determined from either the ferromagnetic diffuse scattering of the disordered alloys or the superlattice reflections of the ordered alloys.

Neutron Diffraction Study of Magnetic Ordering in Thulium

Neutron diffraction measurements have been made on a single-crystal specimen of metallic thulium at temperatures ranging from 293° to 4.2°K. In the temperature range between 38° and 56°K the magnetic structure of thulium is similar to that observed for the high temperature form of erbium; namely, a simple oscillating z-component type antiferromagnetic structure. Below 38°K additional satellite reflections are observed in the diffraction patterns from which it is inferred that the sinusoidal modulation is modified below this temperature. At 4.2°K the normal lattice reflections show an increase in intensity in zero field which is consistent with a mean magnetic moment per thulium atom of approximately one Bohr magneton directed parallel to the c axis. The low temperature magnetic structure which is proposed for thulium is one in which the moments are parallel to the c axis of the crystal but change their orientations according to the sequence 4, 3, 4, 3.... The fundamental period of the modulation remains constant over the entire range of temperature at a value corresponding to 3.5 a 3 periods.

Neutron diffraction investigation of antiferromagnetism in CrCl3

Abstract Neutron diffraction observations have been made on powder and single crystal samples of anhydrous CrCl3 at temperatures from 298 to 4.2°K. This hexagonal layer-type crystal undergoes a transition at 16.8°K to an antiferromagnetic state in which the magnetic moments within each hexagonal layer of metal ions are aligned parallel but adjacent layers of moments are oppositely directed. The axis of spontaneous sublattice magnetization is closely perpendicular to the c-axis. Observations of the (003) antiferromagnetic reflection in an external magnetic field show that the antiferromagnetism can be destroyed with fields of only a few kilo-oersteds, and lead to the conclusion that a net magnetization can be produced with very small magnetic fields.

Antiferromagnetism in Potassium Superoxide KO2

Neutron‐diffraction measurements made on polycrystalline samples of KO2 at several temperatures from 295° to 1.4°K have shown the existence of an antiferromagnetic transition in this compound at 7°K. These measurements have also revealed a crystal structure change slightly above the Neel temperature at about 12°K. Since the packing in the crystal structure involves close contacts between neighboring oxygen molecules, there is probably a correlation between the packing arrangements of these molecules with the onset of magnetic order, and the antiferromagnetic structure could be the result of direct exchange. Only two magnetic reflections definitely can be observed and a unique antiferromagnetic structure cannot be determined. However, the reflections suggest a structure in which there are ferromagnetic sheets of moments parallel to the (00l) planes and the moments on molecules in adjacent planes are antiparallel. The magnetic form factor determined from an analysis of the paramagnetic neutron scattering is...

A Magnetically Pulsed Neutron Beam for Time‐of‐Flight Measurements of Inelastic Scattering

In many types of experiments, time‐of‐flight neutron spectrometers have the advantage of higher counting rates than crystal spectrometers, especially in view of the fact that multiple counters may be used. A method of magnetically pulsing a neutron beam has been developed that avoids the necessity of using high‐speed rotating equipment.The magnetic intensity in a Bragg reflection depends on the orientation of the atomic magnetic moments relative to the scattering vector, and it becomes zero when the moments are directed along the scattering vector. The direction of magnetization in ferrite crystals can be switched rapidly with an external magnetic field so that these crystals can serve both as a neutron monochromator and pulser. Furthermore, the neutron pulse‐width and repetition rate can easily be varied by changing the current pulse that produces the external field.The nuclear contribution for the (111) or (331) reflections can be made very small for 7Li‐Ti ferrite of the correct composition. Large magn...

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.

Spin‐Wave Dispersion Relation for Holmium in the Spiral Magnetic Phase

The spin‐wave dispersion relation for the c direction of holmium metal in its plane‐spiral magnetic phase has been studied by coherent inelastic scattering of thermal neutrons. Constant‐Q measurements of neutron energy loss processes, with the scattered neutron energy held fixed at 8.5 meV, were made at 78° and 48°K. Preliminary least‐squares analyses of the data indicate that the out‐of‐plane anisotropy constant is smaller than that for Dy and Tb, and that five interplanar exchange constants are required in order to fit the data satisfactorily. The resulting exchange constants are larger in magnitude than the anisotropy constant and are both positive and negative in sign as expected for the stability of the spiral structure.