Shape-induced phenomena in finite-size antiferromagnets

It is common knowledge that the direction of the easy axis in a finite-size ferromagnetic sample is controlled by its shape. In the present paper we show that a similar phenomenon should be observed in compensated antiferromagnets with strong magnetoelastic coupling. The destressing energy which originates from the long-range magnetoelastic forces is analogous to the demagnetization energy in ferromagnetic materials and is responsible for the formation of the equilibrium domain structure and the anisotropy of macroscopic magnetic properties. In particular, the crystal shape may be a source of additional uniaxial magnetic anisotropy which removes the degeneracy of the antiferromagnetic vector or the artificial fourth-order anisotropy in the case of a square cross-section sample. In the special case of antiferromagnetic nanopillars, shape-induced anisotropy can be substantially enhanced due to lattice mismatch with the substrate. These effects can be detected by magnetic rotational torque and antiferromagnetic resonance measurements.