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Motivated by the discovery of helical magnetic structure in RbEuFe₄As₄ [1], we study interlayer ordering of magnetic moments in a material composed of spatially-separated superconducting and ferromagnetic layers [2]. We consider the interplay between the normal and superconducting indirect exchange interaction mediated by tunneling between the conducting layers. The normal interlayer interaction can be related with the 2D density of states of an isolated layer. For shallow bands, such interaction is ferromagnetic and short-range. On the other hand, the superconducting contribution always gives antiferromagnetic interaction and extends over several layers if the interlayer hopping energy exceeds the superconducting gap. The frustration between the normal and superconducting parts may lead to a spiral magnetic configuration. The 90^ο angle between the neighboring-layers moments observed in RbEuFe₄As₄ is caused by four-fold in-plane anisotropy. This model explains a physical origin of the magnetic spiral in RbEuFe₄As₄.
[1] Z. Islam and O. Chmaissem, private communication, K. Iida et al., Phys. Rev. B 100, 014506 (2019).
[2] A. E. Koshelev, arXiv:1910.01609.