摘要:

Neutron scattering measurements are reported on the mixed valence compounds Ce1−xThxand TmSe. The &khgr;″(Q,&ohgr;) as derived from the inelastic spectra of Ce0.74Th0.26shows a peak in the &ggr; phase near 20.0 meV and shifts abruptly to greater than 70.0 meV at the transition to the &agr; phase. The temperature independence of the susceptibility within the &ggr; phase cannot be simply reconciled with the temperature dependence of the valence within the &ggr; phase. TmSe is shown to order in a type I antiferromagnetic structure belowTN∼3.2 K. The magnetic phase diagram is understood as a successive domain reorientation and a metamagnetic phase transition forT<3 K with increasing field. The mixed valence nature manifests itself in a reduced moment and a markedly altered crystal field. Another sample of TmSe with a lattice parameter implying 100&percent; Tm3+exhibits spin correlations characteristic of a type II structure but never achieves long range order.

ISSN:0021-8979    

DOI:10.1063/1.324753

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

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 4f5and 4f6configurations, then our measurements imply that the induced moment from the 4f5configuration is dominated by the Van Vleck contribution to the susceptibility arising from an admixture of states with differentJvalues or that the 4f5configuration 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.

ISSN:0021-8979    

DOI:10.1063/1.324754

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

In the present experiment polarized neutron scattering techniques have been used to assess the spatial distribution and temperature dependence of the magnetization induced in asinglecrystal of &ggr;‐Ce by a magnetic field of 60 kG. The measured form factor is found to be in agreement with the free ion 4fmagnetic form factor of Ce3+. Comparison of the localized susceptibility, obtained in the present experiment, with bulk susceptibility measurements indicate that there is a rather large positive conduction electron contribution to the static susceptibility of &ggr;‐Ce. The localized moment is found to follow, between 300 and 115 K, a Curie‐Weiss law with a paramagnetic Curie temperature of (−44±2.5) K. The measurements indicate that at approximately 100 K some ions of the &ggr;‐Ce crystal undergo a change in their electronic configuration.

ISSN:0021-8979    

DOI:10.1063/1.324755

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Heat capacity, magnetic susceptibility, and electrical resistivity measurements have been made on a wide ranging set of Ce‐La and Nd‐La alloys. In the case of Ce it is found that less than 5 at. &percent; of La will prevent the &bgr; (dhcp) to &agr; (collapsed fcc) transition atT≲50 K. In the case of Nd‐La only the dhcp allotrope is formed. Two magnetic ordering temperatures have been found for many of the samples. These are believed to be caused by antiferromagnetic ordering on the two different atomic sites in the dhcp structure, cubic and hexagonal. In both sets of alloys the two ordering temperatures coalesce into one for La concentrations ≳30&percent;. Additional magnetic features in Ce‐La alloys are explained by the formation of the fcc phase.

ISSN:0021-8979    

DOI:10.1063/1.324756

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Various physical properties on single crystals of TmSe with varying but defined chemical composition have been measured. It is found that deviations from stoichiometry strongly influence the low temperature properties of these compounds and certain trends in the behaviour are demonstrated.

ISSN:0021-8979    

DOI:10.1063/1.324757

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

27Al NMR spectra have been observed in polycrystalline CeAl2for temperatures between 2 and 295 K. Frequency shifts, quadrupole couplings and linewidths have been obtained in the paramagnetic phase aboveTc=3.8 K. The isotropic shiftK′idue to Ce3+spin polarization indicates a marked decrease of the27Al hyperfine fieldHhf=K′i/&khgr; with increasing temperature. A non‐dipolar contribution to the anisotropic shift scales with the observedK′i. Both these results can be accounted for only by an anisotropic local moment‐conduction electron coupling. BelowTcthe NMR line broadens appreciably, attaining a field‐independent value at very low temperatures. This observation is consistent with the onset of magnetic ordering, although the absence of a large shift and the magnitude of the linewidth are not understood.

ISSN:0021-8979    

DOI:10.1063/1.324758

出版商:AIP    

年代:1978

数据来源: AIP

ISSN:0021-8979    

DOI:10.1063/1.324760

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The paper summarizes the results of neutron scattering from a weak itinerant ferromagnet MnSi (the itinerant electron system) and Invar alloys: Fe65Ni35and Fe3Pt (the quasi‐localized spin system). Well defined spin wave excitations as well as the excitations in the Stoner continuum could be detected at 5 K for MnSi in the induced ferromagnetic state. The spin dynamics in this weak itinerant ferromagnet were found to be compatible with a simple theory of an itinerant ferromagnet within RPA. The Stoner boundary is located at about 2.5 meV and the magnetic properties of MnSi are mainly determined by the Stoner excitations. In the case of Invar alloys, spin wave excitations could be detected up to 80 meV. The itinerant character of the magnetic properties is suggested not to be due directly to the Stoner excitations but to interactions between the spin waves and the Stoner modes in some higher order mechanisms.

ISSN:0021-8979    

DOI:10.1063/1.324700

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The conduction electron susceptibility &khgr; (q) is considered to play an important role in the magnetism of rare‐earth metals and alloys. In order to obtain information about &khgr; (q), studies of the spin waves in the alloy Tb0.76Y0.24have been carried out in a magnetic field. The magnetic structure of this alloy was found to remain spiral down to liquid helium temperature with zero field. The spin‐wave dispersion curve in this structure has already been studied along thec*‐direction, and the results were analyzed successfully in terms of a susceptibility function corresponding to a one‐dimensional system with a slight modification. In order to obtain somewhat independent information about &khgr; (q), the spin‐wave dispersion curve for the ferromagnetic phase has been studied. A field of 14 kG was necessary to transform the structure into a ferromagnet at liquid helium temperature. Spin‐wave energies which are calculated in terms of the susceptibility function determined from the measurements in the spiral structure agree rather well with the observed energies. A large damping and softening of the spin wave has been observed near the wave vectorq=0.16 which characterized the spiral configuration. The origin of the phenomenon may be related to the instability of the ferromagnetic structure.

ISSN:0021-8979    

DOI:10.1063/1.324761

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Spin wave and crystal field excitations in single crystal HoFe2have been studied using inelastic neutron scattering. At 10 K [100] is the easy magnetic direction and at this temperature the acoustic spin wave mode exhibits a 0.60 meV gap due to anisotropy. The acoustic mode is degenerate at the zone boundary with a flat optic mode of energy 8.3 meV. The lack of dispersion in this mode is a consequence of the small (<.01 meV) Ho‐Ho exchange energy. A third mode is a highly dispersive optic mode which represents excitations of the iron sublattice spins and has a stiffness parameter almost identical to iron metal. A nearest neighbor linear spin wave model has been applied which represents the data of the three lowest modes well. This model predicts the remaining three allowed modes to be at energies above 200 meV. There is no measurable anisotropy in the spin wave modes with varying propagation directions. At room temperature, higher states of the Ho crystal field multiplet become populated and weaker dispersionless excitations between these levels are observed. The observed excitations are broadened beyond instrumental resolution because of contributions from several levels close together in energy. Crystal field calculations including exchange have been performed and are compared to the energies of the observed optic mode transitions.

ISSN:0021-8979    

DOI:10.1063/1.324762

出版商:AIP    

年代:1978

数据来源: AIP