Magnetic ordering in Co doped SrNi₂P₂.

SrNi₂P₂ is an intriguing system as it exhibits fully gapped superconductivity and adopts a unique third-collapsed tetragonal (tcT) phase[1,2]. It transitions from uncollapsed tetragonal (ucT) to tcT phase at T_s =< 326 K, where only one-third of P atoms bond across the Sr layer. Substituting Ni with Co completely suppresses T_s, drives sytem to ucT phase, and induces various magnetic ground states. The magnetic phase diagram of Sr(Ni_1-xCo_x)₂P₂ obtained from the magnetization measurements shows that even though both stoichiometric SrNi₂P₂ and SrCo₂P₂ are paramagnetic, Sr(Ni_1-xCo_x)₂P₂ are antiferromangetic for 0.58<≈x<≈0.94 and ferromagnetic for 0.95<≈x<≈0.98. Single-crystal neutron diffraction measurements performed on x = 0.88 sample confirms the long-range antiferromagnetic ordering with helical arrangement of spins along c-direction. Also, feature corresponding to ferromagnetic ordering was observed for x = 0.96. [1] F. Ronning et al PRB 79,134507(2009). [2] S. Xiao et al Nano Lett. 21,7913(2021).