Spiral spin arrangement

For antiferromagnetic materials there exists some other spin arrangement (q 0). Here q is the wave vector of some spin spiral structure (see fig. 14.2) whose wavelength A is equal to 2ir/q. Conical spin arrangements need not be considered here since they are only stabilized by the presence of anisotropy. 

For a single crystal at 55 K with the c axis parallel to the axis of observation, the spectrum consists of two lines only because all the spin moments lie along this axis and the other three lines for the 0+ -> 2+ transition have zero probability. At 4-2 K the spins form a spiral arrangement leading to a finite probability for all five lines. Both types of ordering are complex, but a statistical calculation of the transition probabilities agreed well with the experimental data. 

The rare earth metals exhibit a variety of ordered states from ferromagnetic to complicated antiferromagnetic structures collinear, spiral, helical, conical and fan structures, which can be altered by temperature, magnetic fields or by the application of pressure (Nikitin et al., 1972). These complicated arrangements of spins result from the balance in energy between magnetocrystalline anisotropy and exchange forces (Elliott, 1965). Much pressure work has been done on pure rare earth metals and alloys in the last decade especially, and measurements... 

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