Doping-induced Ferromagnetic order and its unusual evolution to Helical Antiferromagnetic Order in Sr(Ni$_{1-x}$Co$_x$)$_2$P$_2$
SrNi$_2$P$_2$ represents a unique case of a collapsed structural phase (one-third collapsed, where one out of every three P-P pairs forms a bond) in the A(TM)$_2$X$_2$ family of compounds (A = alkali metal, alkaline-earth metal, or rare earth; TM = transition metal; X = pnictogen). Furthermore, Co doping studies aimed at understanding the interrelationship between this unusual bonding motif and the resulting physical properties produced a magnetically rich phase diagram, specifically on the Co-rich side of the phase diagram. However, important questions remained regarding the detailed nature of the magnetic ground states. To address these issues, we performed single-crystal neutron diffraction measurements on Sr(Ni$_{1-x}$Co$_x$)$_2$P$_2$ with compositions $x = 0.88$, 0.94, and 0.97. For $x = 0.88$ and 0.94, the measurements reveal incommensurate helical magnetic order with a doping-dependent propagation vector $(0,0,\tau)$, similar to that observed in Sr(Ni$_{1-x}$Co$_x$)$_2$As$_2$. In contrast, the $x = 0.97$ composition shows clear signatures of a ferromagnetically ordered ground state, resolving the earlier ambiguity regarding the nature of the low-temperature phase. Furthermore, our results highlight the subtle balance between these competing ground states, whose evolution does not appear to be fully captured by the conventional frameworks of either itinerant or local-moment Heisenberg models typically applied to related 122 systems.