Helimagnetism

A very complex phase diagram was predicted in the seventies by Villain for quasi ID helimagnets. Configurations with opposite chirality are degenerate and in the paramagnetic phase the chains are composed by domains with opposite chirality separated by a domain wall (DW). On lowering the temperature a 3D chiral spin liquid phase was predicted, which can be schematized as a 3D ensemble of helices all with the same spin chirality but with random phase between different chains, as shown in Figure 4.6b. 

Canted Antiferromagnetism, Helimagnetism, and Other Noncollinear Orderings... 

In other more complex cases, the spins can take on a configmation that is not well approximated as a perturbed collinear configmation and such is the case for the spiral or helimagnetic ordering depicted in Figme 1. Noncollinear configmations are also observed in the family of so-called geometrically fmstrated materials to be discussed later. 

When, in a helimagnetic structure, a strong uniaxial anisotropy favouring the axis perpendicular to the layers exists, the magnetic moments become... 

Fig. 7.17. Arrangement of magnetic moments in more complex ordered materials (a) a helimagnetic structure (b) a sine wave modulated structure.

Fig. 9. Magnetic phase diagram for 6-axis Dy/Y superlattices and 6-axis films up to 1 pm thick. The lower surface (solid lines) separates the helimagnetic and fan phases, the upper surface (dashed lines) marks the saturation fields. The bulk limits are shown in the foreground.

As a specific example to clarify the effect of reduced film thickness consider the helimagnetic phase of Dy, which has a c-axis wavelength of about 30 A. The structure has a net moment, and is no longer clearly an antiferromagnet, when the film thickness... 

The perturbation of bulk Dy and Er must be regarded as substantial whenever the film thickness falls well below the helimagnetic wavelength of about 30 A. At that point the... 

Fig. 24. Magnetic moment per Dy atom fi>r a Dy/Y superlattice (a) deduced from the helimagnetic peaks (b) shows that a residual, incoherent ferromagnetic intensity is present, accounting for the missing moment.

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