Ground states of hcp vector antiferromagnets: Zn1−xMnxSe

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Abstract
Type III ground states of hcp vector antiferromagnets—appropriate to the wurtzite magnetic semiconductors (MS) such as Zn1−xMnxSe —are shown to be classically unstable at long wavelengths. The Hamiltonian includes antiferromagnetic nearest‐neighbor (NN) and next‐nearest‐neighbor (NNN) isotropic exchange (J1,J2), and NN anisotropic Dzyaloshinsky–Moriya exchange (D). The hexagonal symmetry allows Ja1 for NN in planes perpendicular to the c axis to be different from Jc1 for NN between planes. For three‐component spins a five‐dimensional degenerate manifold of type III ground states is found (Ja1=Jc1, D=0). Their instability is investigated through a continuum formulation of the exchange energy, treating D and ΔJ≡(Ja1−Jc1) as small compared with J1. ΔJ is found to induce a twist of a noncollinear ground state, stabilized by D to zeroth order. By fitting the shift δQ of magnetic peaks seen in neutron scattering experiments on Zn0.45Mn0.55Se the required ΔJ/J1 (∼δQ) is found to be 0.06. This value agrees with estimates of higher order superexchange processes for the two types of pairs.