Magnetic reflections

Magnetic Optical Properties Magnetism, Reflectance Ferrites, Absorbance Molecular magnets, LCDs Storage media MRI... 

Fig. 32. The integrated intensity of magnetic reflections of x-ray resonant exchange scattering measured for NdNi2BjC and SmNi2B2C. Dashed line and full line model calculations for a magnetic moment parallel to the tetragonal a-axis and c-axis, respectively (after Detlefs et al. 1997b).

Neutron elastic scattering is one of the most powerful tools in the determination of the magnetic structure of an ordered material. At the onset of magnetic order additional reflections on the nuclear pattern are created, due to interactions between the electrons of the nuclei. The variation of the intensity of these magnetic reflections with temperature yields the ordering temperature as well as the spontaneous magnetization of the sublattices and thus the critical exponent /5. 

The Rietveld refinements of the low temperature data, 250 - 5 K, converged successfully with use of the room-temperature, I2/m structural model. Table 1. The occupations of the oxygen and Cr/Nb cation sites were fixed at the values obtained from the room-temperature refinement. When occupancy refinement was attempted with the neutron data their least squares shifts were hi ly correlated with those of the thermal parameters. The thermal parameters for all the atoms decreased smoothly with decreasing temperature. Magnetic reflections or magnetic intensity added to existing reflections were not observed at any temperature. Unit cell parameters, atomic positions, occupational and thermal parameters, bond distances and bond angles as a function of temperature are listed in Table 1. 

At the N6el temperature (3.26) wiU become zero (for T>Tn, Sj)=0) and paramagnetic scattering (3.23) is observed at higher temperatures ). The magnitude of Brillouin function and the N6el temperature may be determined from the temperature dependence of the intensity of a magnetic reflection. 

Significant problems often remain however, both in measuring very weak magnetic reflections and in separating magnetic from nuclear intensities. More accurate... 

Fig. 9. Neutron diffraction pattern of NiO at 4.2 K. The magnetic reflections (hkl) all odd] are much weaker tlian the nuclear reflections [Ref. (29)]...

For ferromagnets, nuclear and magnetic reflections occur at the same t, and fj may be established by an applied magnetic field. In particular, for if perpendicular to X... 

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. 

For a particular magnetic reflection indexed as in Equation (50) the magnetic structure factor of the unit cell is ... 

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