Antiferromagnetic stacking of ferromagnetic layers and doping-controlled phase competition in Ca1−xSrxCo2−yAs2

In search of a quantum phase transition between the two-dimensional ($2\mathrm{D}$) ferromagnetism of ${\mathrm{CaCo}}_{2\ensuremath{-}y}{\mathrm{As}}₂$ and stripe-type antiferromagnetism in ${\mathrm{SrCo}}₂{\mathrm{As}}₂$, we instead find evidence for $1\mathrm{D}$ magnetic frustration between magnetic square Co layers. We present neutron-diffraction data for ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Sr}}_x{\mathrm{Co}}_{2\ensuremath{-}y}{\mathrm{As}}₂$ that reveal a sequence of $x$-dependent magnetic transitions which involve different stacking of $2\mathrm{D}$ ferromagnetically aligned layers with different magnetic anisotropy. We explain the $x$-dependent changes to the magnetic order by utilizing classical analytical calculations of a $1\mathrm{D}$ Heisenberg model where single-ion magnetic anisotropy and frustration of antiferromagnetic nearest- and next-nearest-layer exchange interactions are all composition dependent.