= Easy-plane anisotropy
{wiki}
<Easy-plane anisotropy> makes spin orientations within one plane energetically preferable to orientations normal to it. For example, a classical single-site term $D(S^z)^2$ with $D>0$ penalizes out-of-plane components and leaves the $xy$ plane degenerate. At sufficiently low <energies>, an in-plane angle can then be the main orientational <degree of freedom>.
The resulting approximate angular description connects a magnet to the <XY model>, but the microscopic form matters. For a quantum <spin> one-half, $(S^z)^2$ is proportional to the identity, so that single-site term alone does not generate orientational anisotropy. Anisotropic exchange interactions provide another route. Stating the spin size and <Quantum Hamiltonian> prevents transferring a classical mechanism to a quantum model where it is ineffective. Residual in-plane <anisotropy> can further select a finite set of directions rather than a continuous circle.
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