Evolution of anisotropic magnetic properties through helix-to-fan transition in helical antiferromagnetic EuCo₂As₂

A helimagnet comprises a noncollinear spin structure formed by competing exchange interactions. Recent advances in antiferromagnet-based functionalities have broadened the scope of target materials to include noncollinear antiferromagnets. However, a microscopic understanding of the magnetic anisotropy associated with the intricate evolution of noncollinear spin states has not yet been accomplished. Here, we have explored the anisotropic magnetic aspects in a layered helimagnet of EuCo₂As₂ by measuring the magnetic field and angle dependence of the magnetic torque. By adopting an easy-plane anisotropic spin model, we can visualize the detailed spin configurations that evolve in the presence of rotating magnetic fields. This is directly related to the two distinctive magnetic phases characterized by the reversal of the magnetic torque variation across the helix-to-fan transition. Our advanced approach provides an in-depth understanding of the anisotropic properties of noncollinear-type antiferromagnets and a useful guidance for potential applications in spin-processing functionalities.