Polarized neutron reflectivity

Abstract We use polarized neutron reflectivity to study the evolution of the... 

Keywords Magnetic multilayers, interlayer exchange coupling, exchange bias, magnetization reversal, X-ray reflectivity, polarized neutron reflectivity, domain walls, magnetic roughness. 

Figure 6. Polarized neutron reflectivities of fN 0 = 5 nm, 20 nm and 60 nm samples during the reversal process. The spin flip (R+, R +) and non spin flip (if4-1", R") reflectivities are simultaneously modeled to obtain the magnetization configuration as shown in the inset. The lines are the computed reflectivities for different scattering cross-sections based on this model.

Results obtained from polarized neutron reflection measurements have been reported on granular samples consisting of Fe particles in an AI2O3 matrix. However, this experiment deals with surface studies and is not well adapted to the analysis of fine particle properties. Nevertheless, some features can be deduced. 

In neutron reflectivity, neutrons strike the surface of a specimen at small angles and the percentage of neutrons reflected at the corresponding angle are measured. The an jular dependence of the reflectivity is related to the variation in concentration of a labeled component as a function of distance from the surface. Typically the component of interest is labeled with deuterium to provide mass contrast against hydrogen. Use of polarized neutrons permits the determination of the variation in the magnetic moment as a function of depth. In all cases the optical transform of the concentration profiles is obtained experimentally. 

As at room temperature Bragg reflections contain both nuclear and magnetic structure factors, the nuclear structure was refined from a combination of polarized and unpolarized neutron data. Contrary to the ideal structure where only three atomic sites are present, it has been shown [11, 12] that some Y atoms were substituted by pairs of cobalt. These pairs, parallel to the c-axis are responsible for a structure deformation which shrinks the cobalt hexagons surrounding the substitutions. The amount of these substituted Y was refined to be 0.046 0.008. Furthermore, the thermal vibration parameter of Coi site appeared to be very anisotropic. The nuclear structure factors Fn were calculated from this refined structure and were introduced in the polarized neutron data to get the magnetic structure factors Fu. 

The advantage of using polarized neutrons to determine weak magnetic reflections is clearly apparent. For example, if T =0.01 then the magnetic contribution to the intensity in an unpolarized beam experiment is 0.01 percent, but R Ri 1.04, i.e., there is a 4 percent effect on changing the incident neutron polarization. It is necessary to know the nuclear scattering amplitude accurately if an accurate magnetic amplitude is to be obtained and extinction corrections in particular must be accurately performed. 

Such an experiment is not easy to perform and is very time consuming because of the very small magnetic structure factors for the forbidden reflections. The investigation of MnF2 was important however, both in demonstrating the application of polarized neutron techniques to antiferromagnetic materials and in revealing directly the covalent spin density transferred to the fluorines [the existence of this spin density was already known of course, from NMR measure-... 

In the case of neutron diffraction there is no polarization effect, but the Lorentz factor applies so that the neutron reflecting power is ... 

Shirai M, Nomura M, Asakura K, Iwasawa Y (1995) Development of a chamber for in situ polarized total-reflection fluorescence X-ray absorption fine structure spectroscopy. Rev Sci Inst 66 5493-5498 Sinha SK (1996) Surface roughness by X-ray and neutron scattering methods. Acta Phys Polonica A 89 219-234... 

Initial wavelength selection is done by a mechanical (rotating) velocity selector, since energy resolution is not an important issue here. Polarized neutrons may be produced by reflection by a magnetic multilayer mirror. Essential parts are the so-called flippers. These devices, consisting of flat coils, can change the direction of the neutron spin (which is extremely sensitive to external magnetic fields). 

For a given model of the structure normal to the interface, no matter how complex, it is possible to calculate the neutron reflectivity exactly using the same formulae, apart from the difference in the refractive index, as for light polarized at rightangles to the plane of reflection. For a multilayer structure the optical matrix method [4] can then be used, in which the interface is divided into as many layers as are required to describe it with adequate resolution. This method lends itself especially well to machine calculations and is therefore the most widely used method of analysing neutron reflectivity. However, it does not reveal the relatively simple relation between reflectivity and interfacial structure, which can be done more clearly using the kinematic approximation. In the kinematic approximation the reflectivity profile is given by [5,6]... 

The aim of this chapter is to compare and contrast adsorption kinetics of model cationic surfactants at air-water and solid-liquid interfaces, so as to draw general conclusions and identify dominant processes. Recently, strides have been made in understanding surfactant adsorption kinetics, and in this area development and application of new surface selective techniques has been key. Methods of relevance in this chapter are neutron reflectivity (NR), ellipsometry, and optical reflec-tometry (OR). These techniques are based on scattering and/or interference of neutron radiation or polarized laser light, and hence the principal advantages are that they directly probe surface layer structures and adsorption densities. In the text the terms surface excess, adsorbed amount, and surface density are used interchangeably to express two-dimensional concentrations, either at air-water or solid-liquid surfaces. The main surfactants considered are the family of n-alkyltrimethylam-monium bromides C ,TAB, of alkyl chain carbon number m. 

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