Sinusoidally modulated spin structure

The antiferromagnetic spin structure of YbNiAl was determined by neutron dif action experiments. Below Tn = 2.9K a sinusoidal modulated structure with an amplitude of /4=1.9(2))1b is observed (Ehlers et al. 1997). The magnetic propagation vector is m = [0.779(1)00], and the ytterbium magnetic moments lie perpendicular to the hexagonal c-axis. These investigations are paralleled by a detailed study of Al nuclear spin-lattice relaxation measurements (Fay et al. 1997). 

Fig. 2. Schematic description of the spin structures in the heavy rare earth metals. The initial high-T ordering, in all except the ferromagnet Gd, is to an incommensurate periodic spin order with propagation direction along the c-axis. CAM (c-axis modulation) is a structure in which the c-axis components of the moment are sinusoidally modulated which evolves into the APD (quasi anti-phase domain) in Er in which a gradual squaring-up of the c-axis moment occurs with additional order appearing in the basal plane. The temperatures of the transitions are from magnetization and neutron diffraction data. (After Legvold 1980.)...

Erbium exhibits a complex magnetic behaviour, with at least four obserrable characteristic temperatures below rN = 85K, the moments order in a sinusoidal c-axis modulated (CAM) structure at rH=54K, there appears a component perpendicular to the c-axis resulting in a helicoidal structure this intermediate phase exhibits a sequence of lock-in transitions of commensurate phases (spin-slip structures) with wave vectors fm = f, 4 observed by iX-ray scattering (Gibbs et al. 1986), the... 

The continuous sinusoidal composition change that occurs during spinodal decomposition can be considered to be a modulation of the solid structure. It is now known that many structures employ modulation in response to compositional or crystallographic variations, and in such cases the material flexibly accommodates changes without recourse to defect populations. (Other modulations, in, for example, magnetic moments or electron spins, although important, will not be discussed here.)... 

Some samples exhibit a long-range modulation along the cube edge superimposed on the simple antiferromagnetic structure with a period of approximately 28 unit cells at room temperature. Modulation has been characterized as a sinusoidal variation in amplitude of resultant spin vector (no order perpendicular to this vector). 

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