Magnetic phase diagram of ${\mathrm{Eu}}_{1-x}{\mathrm{Ca}}_x{\mathrm{Co}}_2{P}_2$ determined using muon spin rotation and relaxation

The present study investigated the magnetic nature of a solid solution system consisting of ${\mathrm{EuCo}}_2{P}_2$ and ${\mathrm{CaCo}}_2{P}_2$ using a muon spin rotation and relaxation (${\mu }^{+}\mathrm{SR}$) technique, which is sensitive to local magnetic environments. The former compound ${\mathrm{EuCo}}_2{P}_2$ is known to enter an incommensurate helical antiferromagnetic (AF) phase below 66 K with neutrons, which was confirmed by the present ${\mu }^{+}\mathrm{SR}$. The magnitude of the ordered Eu moments proposed with neutrons was found to be consistent with that estimated by ${\mu }^{+}\mathrm{SR}$. Furthermore, the latter lattice-collapsed tetragonal phase compound ${\mathrm{CaCo}}_2{P}_2$ is known to enter an $A$-type AF phase below 90 K, and ${\mu }^{+}\mathrm{SR}$ measurements on single crystals revealed the presence of a spin reorientation transition at around 40 K, below which the $A$-type AF order is likely to be completed. Although all ${\mathrm{Eu}}_{1-x}{\mathrm{Ca}}_x{\mathrm{Co}}_2{P}_2$ compounds were found to enter a magnetic phase at low temperatures regardless of $x$, a static ordered state was formed only at the vicinity of the two end compounds, i.e., $0\le x\le 0.4$ and $0.9\le x\le 1$. Instead, a disordered state, i.e., a random spin-glass state, short-range ordered state, or highly fluctuating state was found in the $x$ range between 0.4 and 0.9, even at the lowest measured temperature (2 K). Together with the magnetization data, our findings clarified the magnetic phase diagram of ${\mathrm{Eu}}_{1-x}{\mathrm{Ca}}_x{\mathrm{Co}}_2{P}_2$, where a ferromagnetic exchange interaction between Co ions through the ${\mathrm{Eu}}^{2+}$ ion competes with a direct AF interaction among the Co ions, particularly in the $x$ range between 0.57 and 0.9. This competition yielded multiple phases in ${\mathrm{Eu}}_{1-x}{\mathrm{Ca}}_x{\mathrm{Co}}_2{P}_2$.