Surface spin waves

In aU scattering techniques, regardless of whether photons, neutrons, or electrons are scattered, the same approach is applied for detecting spin waves. It is based on the measurement of the energy (and the wave vector) loss of the particles upon their scattering from magnetic surfaces. Spin waves in ultrathin films can be studied by... 

Although o)cT is almost invariably too small to yield information about the Fermi surface topology, the modifications to the conduction electrons trajectories produce a modest positive magnetoresistance, which apparently dominates at the lowest temperatures (fig. 6.44). A satisfactory separation of the spin-wave scattering and electron trajectory effects has not yet been achieved. 

The anisotropy will also affect the spin waves (Mills 1991). Measurements of spin wave relaxation along the lines of the work by Vaterlaus, Beutler and Meier (1992) are indicated. There is also a need for mapping the spin wave dispersion, as has been undertaken for Tm (McEwan et al. 1995), but for thin films. We would expect that the lanthanide metals will exhibit spin waves at temperatures above the Curie temperature, much in the same way as has already been observed for Ni (Lynn and Mook 1981, Mook et al. 1973, Mook and Paul 1985, 1988, Mook and Lynn 1986, Steinsvoll et al. 1983, Uemura et al. 1983). Plasmon and other collective electron effects, as well as magnon dispersion, also need to be investigated and such studies are currently only at a preliminary stage for surfaces. 

This picture explains quite well the observed antiferromagnetic fluctuation near Tc [41, 48], the central peak in spin excitation [64], strong spin wave softening and damping [65, 66], relatively low conductivity [39], smeared Fermi edge [67], and small thermopower [61, 62, 68]. In particular the ARPES studies show that the electronic dispersion is well deflned only up to about 1 eV below the Fermi level, and near the Fermi level it becomes totally smeared [67]. This smearing could be evidence of scattering by the split eg levels. Where the JT distortion is locally present, the eg level should be locally split and the potential locally lowered by A/2 = 0.7 eV. This distortion will scatter electrons within A/2 from the Fermi surface, as is observed. 

The thermally excited cone motion, sometimes called the spin mode (this is very similar to the spin wave motion in ferromag-nets), or the Goldstone mode, is characteristic of the nonchiral SmC phase as well as the chiral SmC phase, but is of special interest in the latter because in the chiral case it couples to an external electric field and can therefore be excited in a controlled way. This Goldstone mode is of course the one that is used for the switching mechanism in surface-stabilized ferroelectric liquid crystal devices. The tilt mode, often, especially in the SmA phase, called the soft mode (although hard to excite in comparison with the cone mode, it may soften at a transition), is very different in character, and it is convenient to separate the two motions as essentially independent of each other. Again, this mode is present in the nonchiral SmA phase but cannot be detected there by dielectric methods, because a coupling to an electric field requires the phase to be chiral. In the SmA phase this mode appears as the electroclinic effect. 

Figure 11 (Left) Image of the magnetic excitations in Lao jPbo.jMnOj. (Right) Data plotted in a three-dimensional space defined by two coordinates of momentum in the horizontal plane and the energy vertically. The shaded surface indicates the possible energies and momenta of the scattered neutrons that are accessible in this experiment maximum intensity occurs where this surface intersects the dispersion surface of the spin waves (r.I.u. = reciprocal lattice unit) (data recorded with HET at ISIS).

Brillouin light scattering [73-75]. The electrical field of the photons couples to the spin wave via spin-orbit interachon in this case. The possible wave vector transfer paraUel to the surface is, however, limited by the wave vector of the incident hght, which is of the order of 10 A. Thus, in this method, only modes with a very long wavelength compared to the lattice constant are excited. 

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