The role of defects in the formation of the multidomain state of easy-plane antiferromagnets with magnetoelastic interaction

We analyzed the field dependences of forced magnetostriction in the multidomain state of the easy-plane antiferromagnet CoCl₂ obtained in the following cycles: the introduction-removal of a magnetic field lying in the easy plane, the introduction-removal of a magnetic field lying in the easy plane and directed normally to that introduced earlier, etc. The magnetostriction of the crystal in the multidomain state was shown to contain two components. First, the component reversible in the cycle magnetic field introduction-removal, which makes the major contribution in the crystal under consideration, and, second, a comparatively small irreversible component, that is, the contribution retained after magnetic field removal. In low fields, the reversible magnetostriction component was proportional to the square of the applied magnetic field. Field-induced rearrangement of the multidomain antiferromagnetic state was found to be responsible for singularities of the field dependence of crystal magnetization. In particular, in a near-zero field that lay in the easy plane, the transverse susceptibility decreased twofold compared with its value in fields in which the crystal is already in the monodomain state. At the same time, close to the “monodomainization” field, transverse magnetic susceptibility was maximum. Defects were shown to favor the formation of the reversible multidomain state. Determining factors in this process were elastic and magnetoelastic interactions. The multidomain state of antiferromagnets was described using the domain distribution function over the orientations of domain antiferromagnetic vectors with respect to the magnetic field direction and the magnetic field dependence of this function. The results of our analysis were in close agreement with the experimental data on CoCl₂.