Crystal growth and evolution of magnetism in the EuCuP-EuCuAs solid solution

The hexagonal $\mathrm{Eu}MX$ $(M=\mathrm{Cu}, \mathrm{Ag}, \mathrm{Au}; X=\mathrm{P}, \mathrm{As}, \mathrm{Sb}, \mathrm{Bi})$ compounds host interesting electronic and magnetic properties, with seemingly intertwined topology and transport properties. One key feature of such behavior is the nature of the ordered magnetic structure. In EuCuAs, a topological Hall effect is caused by a conical spin structure that emerges when a field is applied within the easy-plane $(H$ $\ensuremath{\perp}$ $c)$ of the helical ground state that exists below the Neel temperature of ${T}_{N}=14\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. On the other hand, EuCuP is an easy-axis ferromagnet with a Curie temperature ${T}_{C}$ near 31 K. Here, we investigate the evolution of the magnetic properties in ${\mathrm{EuCuAs}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}$ single crystals with $0.16 \ensuremath{\le}x\ensuremath{\le} 0.75$. Crystals grown by cooling slowly in a Sn flux possessed macroscale inhomogeneity of As/P, particularly for arsenic-rich crystals. However, growth in a Sn flux via an isothermal dwell at $600{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ produced crystals that were homogeneous within the resolution of the probes utilized to investigate these crystals. The unit cell volumes, Curie-Weiss temperatures, and magnetic transitions trend linearly with composition and the magnetic anisotropy is reduced in the alloys. The magnetization data of crystals with $x=0.16$ and 0.24 indicate an easy-plane antiferromagnetic ground state while behavior similar to ferromagnetism is observed for crystals with $x\phantom{\rule{4pt}{0ex}}\ensuremath{\ge} 0.41$. The temperature-dependent magnetization data possess multiple transitions for compositions near ${\mathrm{EuCuAs}}_{0.75}{\mathrm{P}}_{0.25}$, revealing a competition of ground states in this arsenic-rich region of the phase diagram. Neutron diffraction data for EuCuP are also presented as a follow up to previous results that revealed a two-step transition at ${T}_{C}$; the observed data were consistent with ferromagnetic order at $T=5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$.