Effects of Co substitution on the structural and magnetic properties of Sr(Ni1−xCox)2P2

Although ${\mathrm{SrNi}}₂{\mathrm{P}}₂$ adopts the common ${\mathrm{ThCr}}₂{\mathrm{Si}}₂$ structure for $T\ensuremath{\ge}325$ K, being in an uncollapsed tetragonal state rather than a collapsed tetragonal version, it is a special case for the ${\mathrm{ThCr}}₂{\mathrm{Si}}₂$ class: on cooling below 325 K it adopts a one-third collapsed orthorhombic phase where one out of every three P rows bonds across the Sr layers. On the other hand, ${\mathrm{SrCo}}₂{\mathrm{P}}₂$ only exhibits the uncollapsed ${\mathrm{ThCr}}₂{\mathrm{Si}}₂$ structure from room temperature down to 1.8 K. Regardless of their low-temperature structures, neither ${\mathrm{SrNi}}₂{\mathrm{P}}₂$ nor ${\mathrm{SrCo}}₂{\mathrm{P}}₂$ manifests magnetic transitions down to 50 mK and 2 K, respectively. In this work we report the effects of Co substitution in $\mathrm{Sr}{({\mathrm{Ni}}_{1\ensuremath{-}x}{\mathrm{Co}}_x)}₂{\mathrm{P}}₂$, which allows for tuning the transition between the one-third collapsed and the uncollapsed structure. We find a rapid decrease of the one-third collapsed structural transition temperature with increasing Co fraction, until reaching full suppression for $x\ensuremath{\ge}0.1$. Substitution levels in the range $0.11\ensuremath{\le}x\ensuremath{\le}0.58$ show no signs of any transition down to 1.8 K in the magnetization or resistance measurements in the range $1.8\ensuremath{\le}T\ensuremath{\le}300\phantom{\rule{4pt}{0ex}}\text{K}$. However, different magnetically ordered states emerge for $x\ensuremath{\ge}0.65$, and disappear for $x\ensuremath{\ge}0.99$, recovering the known paramagnetic properties of the parent compound ${\mathrm{SrCo}}₂{\mathrm{P}}₂$. These results are summarized in a phase diagram, built upon the characterization by energy dispersive x-ray spectroscopy, x-ray diffraction, temperature-dependent resistance, and field- and temperature-dependent magnetization measurements done on single crystals with different Co fraction. Both the magnetic and structural properties are compared to other systems with ${\mathrm{ThCr}}₂{\mathrm{Si}}₂$ structure that exhibit magnetic ordering and collapsed tetragonal transitions. The magnetic ordering and moment formation are well described by Takahashi's spin fluctuation theory of itinerant electron magnetism [Y. Takahashi, J. Phys. Soc. Jpn. 55, 3553 (1986)].