Neutron Magnetic Form Factor Research Articles

Spin-fluctuation resonance in mixed-valence materials.

We analyze the neutron magnetic form factor of a lattice of fluctuating spins when the narrow band of resonance levels is partially occupied. The form factor is predominantly that of the conduction electrons at the Fermi level. This result shows that spin fluctuation is not the sole source of heavy electrons in a number of mixed-valence and heavy-electron systems, which have been shown experimentally to have f-electron form factors in the Fermi-liquid phase. A resolution of this dilemma is proposed and the proper place where the spin-fluctuation theory applies is identified.

Spin-density asymmetry in NiV alloy at various temperatures

Polarised neutron diffraction has been used to determine the neutron magnetic form factors of a Ni0.959V0.041 random alloy at 4.2, 290 and 378 K. From the analysis of the form factors it is concluded that within experimental accuracy the spatial distribution of the spin density remains unaltered up to 0.9 Tc for this alloy.

Studies of the magnetization densities in Pu compounds; determination of the degree of localization

The electronic configuration of trivalent Pu is nominally a 5f5 state in which there is a strong cancellation of the L (orbital) and S (spin) components in the J=5/2 ground state. This leads to small g factors and magnetic moments, and to unusual neutron magnetic form factors f(Q) with maxima at Q values greater than zero. We present here the results of form-factor measurements (taken on the polarized-neutron diffractometer at the ILL) on single crystals of PuSb, PuTe, and PuFe2. These results show that the form factor, which is a measure of the magnetization density of the 5f electrons around the Pu nucleus, is sensitive to the degree of localization of the 5f electrons. For example, in PuSb the 5f electrons are well localized and the form factor is in good agreement with the predictions of intermediate coupling and crystal-field theory. In PuTe f(Q) falls monotonically from a maximum at Q=0, indicating an almost spin-only system. This is produced by wide 5f bands, which give rise to the observed T-independent magnetic susceptibility. In PuFe2, however, a large maximum occurs in f(Q). We believe this arises from strong hybridization of the 5f and 3d Fe electrons, which leads to a reduction in the Pu orbital component.

A fast and accurate method for the calculation of orbital susceptibility and form factor of paramagnetic transition metals

We describe a new method for calculating the orbital contribution to the susceptibility and neutron magnetic form factor of paramagnetic transition metals. The method has been developed by us over the past four years, and the computational details and results will be published elsewhere. In this paper we discuss the theoretical issues, which include a justification of the basic approximation and the estimate of errors. It is shown that it gives reliable results for the size of the susceptibility and the overall shape of the form factor, but is probably not sufficiently accurate to account for the anisotropy in the orbital moment distribution.

Isoscalar axial-vector form factor at high Q2.

We calculate, using perturbative QCD, the isoscalar axial-vector nucleon form factor at high-momentum transfers. We find that it is 1 to 3 times bigger than the isovector axial-vector form factor, depending on which distribution amplitude is used, and that its size is correlated to the neutron magnetic form factor.

Testing nucleon distribution amplitudes: Relations between neutron and N- Delta form factors.

The proton and neutron magnetic form factors, the axial-vector form factor, and the leading N-..delta.. transition form factor are connected by the nucleon distribution amplitude. If the distribution-amplitude moments obey the QCD sum-rule results, a correlation exists between the neutron magnetic form factor G/sub M//sub n/ and the leading N-..delta.. transition form factor. For existing proposed distribution amplitudes, one or the other of them will be small and the other not. We comment on what available data say.

Neutron magnetic form factors of ternary Ni-Fe-V alloys

The magnetic form factors of (Ni0.6Fe0.4)1-xVx x=0.095 and 0.159, have been measured at room temperature. The experimental data are compared with a form factor for x=0 obtained by interpolation from literature data. It is concluded that, though the constituents seem to retain their individual properties in binary Ni-Fe alloys, the Ni-Fe matrix is affected as a whole by addition of the V impurities. Therefore in this respect the compounds investigated can be considered as pseudobinary alloys. The addition of V causes an overall decrease of the average host moment but little change in its spatial distribution.

Quasi-elastic scattering of polarized electrons on polarizedHe3

Cross sections and asymmetries for quasi-elastic scattering of longitudinally polarized electrons on polarized $^{3}\mathrm{He}$ are calculated. The model used consists of impulse approximation plus closure approximation to sum over final states. At the quasi-elastic peak the asymmetry is found to be dominated by scattering from the neutron, and judicious choice of target polarization allows sensitivity to either the neutron's electric or magnetic form factor to be maximized. Away from the quasielastic peak, the protons contribute to the asymmetry. The proton's contribution is mainly owing to two partial wave channels, one mixed symmetry $S$ state and one $D$ state which have small total probability in the $^{3}\mathrm{He}$ wave function.

Nucleon wave function and nucleon form factors in QCD

Abstract The values of the few first moments of the nucleon wave function are found by using the method of QCD sum rules. A model for the nucleon wave function is proposed based on a knowledge of these moments. It is shown that with the help of this wave function one obtains the signs and the values of the proton and the neutron magnetic form factors in agreement with experiment. The charmonium decay widths ψ(3100)→ p p , n n , x 2 (3555)→ p p are also calculated. Some predictions for inclusive nucleon production cross sections are presented.

Induced neutron magnetic form factor of LaSn 3

Polarized neutron scattering techniques have been used to study the spatial distribution of the magnetization induced in a single crystal of LaSn 3 by a magnetic field of 42.5 kG at 100 K. We find that the magnetic form factor decreases very rapidly with increasing scattering angle, and bears no resemblance to the spin or orbital free-atom magnetic form factor. The experimental results are in reasonable agreement with band theoretical calculations of the spin magnetic form factor of LaSn 3. We conclude that (a) the spin part is the dominant contribution to the bulk susceptibility of LaSn 3 and (b) there is a substantial amount of Sn-5p electronic character in the wavefunctions near the Fermi level.

The electronic structure of Lu

The electronic structure of hcp Lu has been calculated using a linearized augmented plane wave (LAPW) method and the Hedin-Lundqvist local density approximation for exchange and correlation. Although complete self-consistency was hindered by the proximity of the 4f levels to the Fermi energy, the valence bands were converged and the calculation yielded a Fermi surface remarkably similar to that calculated by Keeton and Loucks. Comparison is made with recent de Haas-van Alphen and neutron magnetic form factor experiments.

Neutron magnetic form factors of Ni-rich NiV alloys

The magnetic form factors of Ni1-xVx (x=0.014, 0.041 and 0.046) have been measured at room temperature. A new determination of the form factor of pure Ni has also been performed. The Ni1-xVx form factors and the form factors of Ni0.95Cr0.05 and Ni0.98Ti0.02 are compared with the Ni form factor. It is concluded that in these alloys the long-range disturbance of the magnitude of the Ni moments, caused by the presence of the solute atoms, does not change the distribution of the unpaired electrons within the Wigner-Seitz cell.

Neutron diffraction study of magnetic ordering in Tm 0.5Eu 0.5Se

A single crystal of the magnetic semiconductor Tm 0.5Eu 0.5Se was studied by means of neutron diffraction in the temperature range from 1.8 to 293 K. Long-range magnetic order is detected at temperatures below T c = (18.5±1) K. The measured ferromagnetic moment component of (2.12±0.05) μ B per rare-earth ion at saturation in zero external magnetic field indicates approximately antiparallel alignment of Tm moment and Eu spin (mutual angle 134°). The experimentally determined neutron magnetic form factor confirms the divalent state of both Tm and Eu in Tm 0.5Eu 0.5Se.

Self-consistent spin polarized energy band structure and magnetism in ZrZn 2 and TiBe 2

Results of self-consistent spin-polarized semirelativistic linear muffin-tin orbital energy band studies of TiBe 2 and ZrZn 2 are reported. The calculated magnetic moments, spin magnetization densities and neutron magnetic form factors - and their comparison with experiment - are used to discuss itinerant ferromagnetism in ZrZn 2 and TiBe 1.8Cu 0.2.

Weak and electromagnetic form factors of baryons at large momentum transfer

Perturbative quantum-chromodynamic predictions are given for the weak and electromagnetic elastic and transition form factors of baryons at large momentum transfer $Q$. The leading (helicity-conserving) octet and decuplet form factors can all be expressed as linear combinations of the proton and neutron magnetic form factors. The predictions for the spin structure and relative normalization of the baryon form factors reflect the assumed $\mathrm{SU}{(2)}_{L}\ifmmode\times\else\texttimes\fi{}\mathrm{U}(1)$ structure of the electromagnetic and weak currents, quark and gluon hard-scattering dynamics, and the helicity-flavor symmetry of baryon wave functions at short distances. The results hold to all orders in ${\ensuremath{\alpha}}_{s}({Q}^{2})$ and to leading order in $\frac{m}{Q}$. We also discuss the special features of the contribution of the end-point $x\ensuremath{\sim}1$ region to baryon form factors.

Electronic structure and itinerant magnetism in ZrZn2 and TiBe2

The electronic and magnetic structure of the Cl5 compounds ZrZn2 and TiBe2 has been determined by means of self-consistent spin-polarized LMTO band calculations. The spin-densities and neutron magnetic form factors are compared with neutron scattering data for ZrZn1.9 and TiBe1.8Cu0.2. The moments are found to reside almost entirely on the Zr/Ti atoms, and are found to be 0.14 and 0.21 μB per formula unit respectively.

On magnetic properties of UPb 3 — A neutron diffraction study

The temperature dependence of long-range magnetic order has been determined by neutron diffraction, using profile analysis, on a powder sample of UPb 3. Below T N = 30 K the magnetic structure is described by k = (0 0 1 2 ) . Possible magnetic configurations indistinguishable from a powder pattern are discussed, and information on the neutron magnetic form factor is obtained. The ordered magnetic moment ranges from 1.58 to 1.69 μ B, depending on the assumed 5f n configuration. The measured reduced magnetization as a function of temperature agrees reasonably with calculated values based on a crystal-field model with Γ 5 ground state, including exchange in the molecular field approximation.

Electronic structure and magnetic properties of CeSn3

A simple band model is proposed to explain the neutron magnetic form factor and many other properties of the mixed valence compound CeSn3. The model is not supported by band calculation, and this suggests that the one electron approximation is probably not valid for mixed valence systems.

Field induced magnetic form factor of Zr

The neutron magnetic form factor obtained with H perpendicular to the c-axis is in good agreement with calculations assuming a 65% band theoretical spin density and a 35% atomic-like orbital density. Measurements with two different field directions showed no evidence of the large anisotropy observed in bulk susceptibility experiments.

Dirac-Fock studies of some electronic properties of rare-earth ions

This paper reports results of accurate fully relativistic Dirac-Fock studies of some electronic properties of rare-earth ions. We present, in addition to eigenvalue information including spin-orbit splittings (useful for interpreting ESCA experiments), 〈 r n 〉 values (for n = -3, 2, 4, 6) for the 4f electrons and Slater F k and G k integrals (of interest for hyperfine interaction and crystal field descriptions), values of the Mössbauer isomer shift electronic densities and the 〈 j i 〉 integrals useful for interpreting neutron magnetic form factor measurements.