Spin-flip Cross Section Research Articles

Effect of electron spin polarization in laser-assisted electron–proton scattering

This work aims to study theoretically the electron–proton scattering for initially spin-polarized electrons in the presence of a circularly polarized electromagnetic field. Using the first Born approximation and the Dirac–Volkov states for dressed electrons, we derive an analytic expression for the differential cross-section with the help of the spin-helicity formalism. Meanwhile, a well-known concept of spin-flip and spin non-flip differential cross sections is applied. The influence of the electromagnetic field on the degree of polarization of the scattered electron is investigated, and the effect of electron spin polarization is examined. We found that the initial spin of the electron can be affected depending on its kinetic energy during the scattering process. This work is a continuation of a recent paper in which we studied the unpolarized electron–proton scattering in the presence of a laser field (Dahiri, et al., 2021).

Dynamical spin susceptibility in La2CuO4 studied by resonant inelastic x-ray scattering

Resonant inelastic x-ray scattering (RIXS) is a powerful probe of elementary excitations in solids. It is now widely applied to study magnetic excitations. However, its complex cross section means that RIXS has been more difficult to interpret than inelastic neutron scattering (INS). Here we report $\ensuremath{\sim}37$ meV resolution RIXS measurements of the magnetic excitations in ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$, the antiferromagnetic parent of one system of high-temperature superconductors. At high energies ($\ensuremath{\sim}2$ eV), the RIXS spectra show angular-dependent $dd$ orbital excitations in agreement with previous RIXS studies but show new structure. They are interpreted with single-site multiplet calculations. At low energies ($\ensuremath{\lesssim}0.3$ eV), we model the wave-vector-dependent single magnon RIXS intensity as the product of the calculated single-ion spin-flip RIXS cross section and the dynamical structure factor $S(\mathbf{Q},\ensuremath{\omega})$ of the spin-wave excitations. When $S(\mathbf{Q},\ensuremath{\omega})$ is extracted from our data, the wave-vector-dependence of the single-magnon pole intensity shows a similar variation to that observed by INS. Our results confirm that suitably corrected RIXS data can yield the genuine wave-vector and energy dependence of $S(\mathbf{Q},\ensuremath{\omega})$ for a cuprate antiferromagnet. In addition to spin waves, our data show structured multimagnon excitations with dispersing peaks in the intensity at energies higher than the single-magnon excitations.

Evidence for the formation of nanoprecipitates with magnetically disordered regions in bulk Ni50Mn45In5 Heusler alloys

Shell ferromagnetism is a new functional property of certain Heusler alloys which has been recently observed in $\mathrm{Ni}_{50}\mathrm{Mn}_{45}\mathrm{In}_{5}$. We report the results of a comparative study of the magnetic microstructure of bulk $\mathrm{Ni}_{50}\mathrm{Mn}_{45}\mathrm{In}_{5}$ Heusler alloys using magnetometry, synchrotron x-ray diffraction, and magnetic small-angle neutron scattering (SANS). By combining unpolarized and spin-polarized SANS (POLARIS) we demonstrate that a number of important conclusions regarding the mesoscopic spin structure can be made. In particular, the analysis of the magnetic neutron data suggests that nanoprecipitates with an effective ferromagnetic component form in an antiferromagnetic matrix on field annealing at $700 \, \mathrm{K}$. These particles represent sources of perturbation, which seem to give rise to magnetically disordered regions in the vicinity of the particle-matrix interface. Analysis of the spin-flip SANS cross section via the computation of the correlation function yields a value of $\sim 55 \, \mathrm{nm}$ for the particle size and $\sim 20 \, \mathrm{nm}$ for the size of the spin-canted region.

Dipolar-coupled moment correlations in clusters of magnetic nanoparticles

Here, we investigate the nature of the moment coupling between 10-nm DMSA-coated magnetic nanoparticles, in both colloidal dispersion and in powder form. The individual iron oxide cores were composed of > 95% maghemite and agglomerated to clusters. At room temperature the ensemble behaved as a superparamagnet according to M\"ossbauer and magnetization measurements, however, with clear signs of dipolar interactions at low temperatures. Analysis of temperature-dependent AC susceptibility data in the superparamagnetic regime indicates a tendency for dipolar coupled anticorrelations of the core moments within the clusters. To resolve the directional correlations between the particle moments we performed polarized small-angle neutron scattering and determined the magnetic spin-flip cross-section of the powder in low magnetic field at 300 K. We extract the underlying pair distance distribution function of the magnetization vector field by an indirect Fourier transform of the cross-section, and which suggests positive as well as negative correlations between nearest neighbor moments, with anticorrelations clearly dominating for next-nearest moments. These tendencies are confirmed by Monte Carlo simulations of such core-clusters.

Excitation of the 2S State of Atomic Hydrogen by Electron Impact

The excitation cross sections of the 2S state of atomic hydrogen at 10 low incident electron energies (10.30 and 54.5 eV) have been calculated using the variational polarized method. Nine partial waves are used to get convergence of cross sections in the above energy range. The maximum of the cross section is 0.137 π a 0 2 at 11.14 eV which is close to the experimental result 0.163 ± 0.2 π a 0 2 at 11.6 ± 0.2 eV. The present results are compared with other calculations, many of them are based on the close-coupling approximation, including the R-matrix method. Differential cross sections at 13.6 eV incident energy have also been calculated. Spin-flip cross sections have been calculated and compared with those obtained using the close-coupling approximation.

Effect of Dzyaloshinski-Moriya interaction on elastic small-angle neutron scattering

For magnetic materials containing many lattice imperfections (e.g., nanocrystalline magnets), the relativistic Dzyaloshinski-Moriya (DM) interaction may result in nonuniform spin textures due to the lack of inversion symmetry at interfaces. Within the framework of the continuum theory of micromagnetics, we explore the impact of the DM interaction on the elastic magnetic small-angle neutron scattering (SANS) cross section of bulk ferromagnets. It is shown that the DM interaction gives rise to a polarization-dependent asymmetric term in the spin-flip SANS cross section. Analysis of this feature may provide a means to determine the DM constant.

New experimental upper limit of the electron–proton spin-flip cross-section

In a previous publication, measurements of the depolarization of a stored proton beam by interaction with a co-propagating unpolarized electron beam at low relative energy have been presented and an upper limit of about 3 ×107 b for the electron–proton spin-flip cross-section was determined. A refined analysis presented in this paper reduces the previous upper limit by a factor of three by the introduction of a new procedure that also makes use of non-identified particles.

Speromagnetism at the ferrimagnetic compensation point in an Fe64Er19B17 metallic glass — the head of a dandelion, or the spokes of a wheel?

Magnetization and neutron scattering measurements with polarization analysis were made on an Fe64Er19B17 glass in an applied field of 2 Tesla to investigate its ferrimagnetic compensation at Tcomp = 112±2 K. The measurements prove that the glass has a non-collinear magnetic structure. The two non-spin flip cross-sections interchange between 100 K and 125 K, showing the magnetic structure flips at Tcomp. They are identical at 112 K where the forward limit of the spin flip crosssections, |dσ±∓/dΘ|Q=0, was also greatest. We discuss these observations, and compare competing models for the magnetic structure of the glass at Tcomp.

Analyticity, unitarity and one-loop graviton corrections to Compton scattering

We compute spin-flip cross section for graviton photoproduction on a spin-1/2 target of finite mass. Using this tree-level result, we find one-loop graviton correction to the spin-flip low-energy forward Compton scattering amplitude by using Gerasimov-Drell-Hearn sum rule. We show that this result agrees with the corresponding perturbative computations, implying the validity of the sum rule at one-loop level, contrary to the previous claims. We discuss possible effects from the black hole production and string Regge trajectory exchange at very high energies. These effects seem to soften the UV divergence present at one-loop graviton level. Finally, we discuss the relation of these observations with the models that involve extra dimensions.

Observation of cross-shaped anisotropy in spin-resolved small-angle neutron scattering

We report the results of spin-resolved small-angle neutron scattering (SANS) experiments on the two-phase nanocrystalline alloy NANOPERM. At a saturating applied magnetic field of $1.27\phantom{\rule{0.28em}{0ex}}\mathrm{T}$ we observe a cross-shaped angular anisotropy in the non-spin-flip SANS cross section ${\ensuremath{\Sigma}}^{++}$. This feature---for this class of materials only visible at saturation in ${\ensuremath{\Sigma}}^{++}$---is attributed to the specific ratio of nuclear to magnetic scattering being smaller than unity. Analysis of the non-spin-flip and spin-flip cross sections provides the nuclear and magnetic SANS and allows us to estimate the magnitude of the respective scattering-length density contrast at the interphase between the nanoparticles and the amorphous magnetic matrix.

Sperimagnetism in Fe78Er5B17 and Fe64Er19B17 metallic glasses: I. Moment values and non-collinear components

Magnetization measurements have been made on a Fe64Er19B17 glass, which exhibits ferrimagnetic compensation at Tcomp = 112 K, and polarized beam neutron scattering measurements have been made on Fe78Er5B17 and Fe64Er19B17 glasses to supplement the measurements made earlier on Fe64Er19B17. The magnetization data were analysed with a phenomenological model, to find the magnetic moments and their components needed to interpret the neutron data. Four spin-dependent scattering cross-sections were obtained in absolute units from each neutron experiment, to determine the atomic-scale magnetic structures of the two glasses. The finite spin-flip cross-sections confirmed that these (Fe,Er)83B17 glasses are non-collinear ferrimagnets. The cross-sections were calculated using a model based on random cone arrangements of the magnetic moments. The moment values and the random cone angles were refined in the calculations, which produced good agreement between the calculated curves and the experimental data. The forward limit of the spin-flip cross-sections |∂σ±∓/∂Ω|Q=0 of the Fe64Er19B17 glass which peaked at Tcomp and the temperature variation of the total scattering amplitudes (b∓p∥(Q)) suggested that the random cone angles open fully so that the collinear components p∥(Q) tend to zero at Tcomp. The ferrimagnetic compensation is therefore characterized by an equality of the magnetic sublattices; the reversal of the magnetic structure and a compensated sperimagnetic phase which appears at Tcomp.

Polarizing a stored proton beam by spin flip? – A high statistic reanalysis

Prompted by recent, conflicting calculations, we have carried out a measurement of the spin flip cross section in low-energy electron-proton scattering. The experiment uses the cooling electron beam at COSY as an electron target. A reanalysis of the data leeds to a reduced statistical errors resulting in a factor of 4 reduced upper limit for the spin flip cross section. The measured cross sections are too small for making spin flip a viable tool in polarizing a stored beam.

Neutron spin-flip scattering of nanocrystalline cobalt

We report results of longitudinal (one-dimensional) neutron polarizationanalysis on polycrystalline bulk Co with an average crystallite size ofD = 10 nm. The spin-flip small-angle neutron scattering (SANS) data are analyzed in theapproach-to-saturation regime within the framework of micromagnetic theory. Inparticular, we provide a closed-form expression for the spin-flip SANS cross section . From the data analysis, we find a room-temperature value ofA = (2.6 ± 0.1) × 10 − 11 J m − 1 for the exchange-stiffness constant, which agrees well with earlier data.

Non-collinear ferrimagnetism in aFe64Er19B17 metallic glass

Polarized neutron beam measurements on aFe64Er19B17 metallic glass have shown directly that it has a non-collinear magnetic structure. It can bedescribed using a model in which the magnetic moments on the iron atoms point in arandom cone that is ferrimagnetically coupled to a random cone of erbium moments, in themanner suggested from bulk measurements. The spin-flip cross-sections weresuccessfully calculated using an optimized choice of the values of the magnetic momentsμFe,μEr and the randomcone angles θFe, θEr. The non-spin-flip cross-sections have an unusual variation with the scattering vectorQ, which has not been observed before with transition metal–metalloid glasses. At 1.5and 60 K the cross-section contains a pre-peak at a smaller value ofQ (1.3 Å − 1) than the pre-peaks which have been observed in the structure factors of some transitionmetal glasses. At 180 K the form of these cross-sections remains the same but the twochannels have interchanged, so contains the pre-peak. This interchange shows that a complete inversion of the magneticstructure occurs between 60 and 180 K—presumably at the compensation temperatureTcomp≈120 K. Attempts to simulate these cross-sections using the methods applied to (Fe,Tb)B glasseswere unsuccessful because none of the known partial structure factors contains a pre-peakwhich can imitate the observed one. The possible origins of the pre-peak arediscussed.

Longitudinal polarization analysis in small-angle neutron scattering

Due to recent progress in the development of 3He spin filters, it has only now become possible to perform routinely longitudinal (one-dimensional) neutron-spin analysis (POLARIS) in small-angle neutron scattering (SANS) experiments. It is the purpose of this article to provide a brief introduction into the technique and to discuss first experimental data. In particular, for the most common scattering geometry where the applied magnetic (guide) field is perpendicular to the incident neutron beam, we write down the equations for the non-spin-flip and spin-flip SANS cross sections of a bulk ferromagnet, and we discuss the various angular anisotropies and asymmetries along with some selected experimental results on an FeCr based soft magnetic nanocrystalline alloy. In particular, we show that the analysis of the spin-flip data allows one to obtain the magnitude-squares of the three vector (Fourier) components of the magnetization.

Polarizing a stored proton beam by spin flip?

We discuss polarizing a proton beam in a storage ring, either by selective removal or by spin flip of the stored ions. Prompted by recent, conflicting calculations, we have carried out a measurement of the spin-flip cross section in low-energy electron–proton scattering. The experiment uses the cooling electron beam at COSY as an electron target. The measured cross sections are too small for making spin flip a viable tool in polarizing a stored beam. This invalidates a recent proposal to use co-moving polarized positrons to polarize a stored antiproton beam.

Proton scattering at intermediate energies onNi58: How well is it understood?

The excitation-energy region up to 23 MeV in $^{58}\mathrm{Ni}$ has been investigated with the ($\stackrel{\ensuremath{\rightarrow}}{p},{\stackrel{\ensuremath{\rightarrow}}{p}}^{'})$ reaction at an incident energy of 172 MeV and forward angles favoring the excitation of the spin-dipole mode in the continuum. The measured cross sections, spin-flip cross sections, and spin-flip probabilities are compared to microscopic reaction-model calculations using various effective projectile-target interactions. The nuclear structure input, which takes into account one-particle one-hole (1p1h) as well as two-particle-two hole (2p2h) excitations, allows for a reasonable description of the continuum region well above the particle-decay thresholds. However, considerable problems remain in the description of spin-flip resonances at lower excitation energies, which cannot be explained by a quenching of the spin-isospin interaction.

Evidence of collinear ferrimagnetism in (Fe, Tb)B metallic glasses from polarized beamneutron scattering

The atomic-scale magnetic structures of two terbium-substituted(Fe0.83−xTbx)B0.17 metallic glasses have been determined by polarized beam neutron scattering measurementsusing the IN20 spectrometer at the Institut Laue-Langevin, Grenoble. The fourspin-dependent, neutron scattering cross-sections were measured in absolute units for thetwo glasses. The spin-flip cross-sections and were found to be small; they were independent of the scattering vectorQ, independent of the temperature and were also of the same magnitude as the nuclearincoherent cross-section, within experimental errors. These observations indicate that themagnetic structure in these glasses must be collinear. The non-spin-flip cross-sections werefound to have a hitherto unobserved shift of the first peak between the and channels. A (collinear) ferrimagnetic state which is consistent with the spin-flip cross-sections istherefore proposed for these glasses. The magnetic moments on the terbium atoms are alignedantiparallel to those on the iron atoms, and the values of both moments reduce to zero byx = 0.50 terbium, in agreement with magnetization data. A calculation of the non-spin-flipcross-sections based on this model correctly predicts the shift of the first peaks. Thisbehaviour arises because of the ferrimagnetic correlations between the magnetic momentsand the very strong magnetic scattering from the terbium atoms.

Nonuniform and sequential magnetization reversal via domain structure formation for multilayered system with grain size induced enhanced exchange bias

We report on the magnetization reversal in series of exchange-biased multilayers NiFe(10.0 nm)/[ Ir20Mn80(6.0 nm)/Co80Fe20(3.0 nm)] N studied by specular reflection and off-specular scattering of polarized neutrons. All specimens are sputtered and post-annealed at 530 K (i.e. above the IrMn Neel temperature of 520 K) in Ar atmosphere before cooling to room temperature in the presence of a field of 130 Oe which induces the unidirectional anisotropy. We find HEB is dependent upon the number of bilayers N as it gradually increases from 0.33 kOe for N=1 to a considerably higher value of upto ≈0.9 kOe for N=10. X-ray specular and diffuse scattering data reveal no significant variation of the lateral correlation length and only a weak dependence of the vertical rms interface roughness on N. Atomic and magnetic force microscopy, however, show a strong reduction of the grain size accompanied by distinct changes of the ferromagnetic domain structure. The enhancement of the exchange bias effect is presumably related to the shrinking of the related domain size in the antiferromagnet due to the structural evolution in the multilayers. Polarized neutron reflectometry (PNR) measurements are done at different applied fields sweeping both branches of the hysteresis loop. The spin-flip (SF) cross section of both the N=10 and 3 samples show diffusely scattered intensity appears gradually as the field approaches HEB and is most intense where the net magnetization vanishes. The disappearance of diffuse scattering in saturation indicates that the off-specular intensity is related to the reversal process. The reversal proceeds sequentially starting with the bottom (top) CoFe layer for decreasing (increasing) field and is related to the evolution of the grain size along the stack. The reversal of each CoFe layer is for both field branches due to domain wall motion. Thus as a main result, we observe a sequential and symmetric magnetization reversal in exchange-biased multilayers. The concomitant in-plane magnetization fluctuations revealed by off-specular spin-flip scattering indicate a more complex reversal mechanism than hitherto considered. Moreover, although the grain size decreases from N=3 to 10 by a factor of about four the reversal mechanism remains similar.

Symmetric elastic and spin-flip low-energy collisions of the hydrogen-isotope mesic atoms in the adiabatic hyperspherical approach

The reduced adiabatic hyperspherical (RAHS) basis suggested previously is used to calculate elastic and spin-flip cross sections in the processes (aμ)F+a → (aμ)F′+a, a=(p, d, t), for collision energies 10−3≤ɛ≤102 eV. The rapid convergence of the method is demonstrated: to achieve an accuracy of ⋍1% in the calculated cross sections, it is sufficient to use N≤10 of the basis RAHS functions. The comparison of the obtained results with the previous ones is presented.