Modulated magnetism and anomalous electronic transport inCe3Cu4As4O2

The complex magnetism and transport properties of tetragonal ${\mathrm{Ce}}₃{\mathrm{Cu}}₄{\mathrm{As}}₄{\mathrm{O}}₂$ were examined through neutron scattering and physical property measurements on polycrystalline samples. The lamellar structure consists of alternating layers of ${\mathrm{CeCu}}₄{\mathrm{As}}₄$ with a single square Ce lattice and ${\mathrm{Ce}}₂{\mathrm{O}}₂$ bilayers. Peaks in the specific heat at the N\'eel temperature ${T}_N=24$ K, ${T}₂=16$ K, and ${T}₃=1.9$ K indicate three magnetic phase transitions or distinct crossover phenomena. For $T<{T}_N$ neutron diffraction indicates the development of ferromagnetic $ab$ sheets for both Ce sites, with alternating polarization along $\mathbf{c}$, and a wave vector ${\mathbf{k}}₁={\mathbf{c}}^*$. For $T<{T}₂$, quasi-two-dimensional low-energy spin fluctuations with ${\mathbf{k}}₂=\frac{1}{2}{\mathbf{a}}^*$ and polarized perpendicular to ${\mathbf{k}}₂$ are suppressed. The data are consistent with quasi-two-dimensional antiferromagnetic order in the ${\mathrm{CeCu}}₄{\mathrm{As}}₄$ planes polarized along the ${\mathbf{k}}₂$ wave vector. ${T}₃$ marks a spin-flop transition where the ${\mathbf{k}}₁$ staggered magnetization switches to in-plane polarization. There are significant transport anomalies associated with the transitions, in particular a substantial reduction in resistivity for $T<{T}_N$. At $T=100$ mK the magnetic correlation length exceeds $75\phantom{\rule{0.28em}{0ex}}\AA{}$ and the ${\mathbf{k}}₁$ modulated staggered moment is $0.85\phantom{\rule{4pt}{0ex}}{\ensuremath{\mu}}_B$, which matches the $0.8\phantom{\rule{4pt}{0ex}}{\ensuremath{\mu}}_B$ saturation magnetization achieved for ${\ensuremath{\mu}}₀H=7$ T at $T=2$ K. We trace the unusual sequence of magnetic transitions to competing interactions and anisotropies in the alternating quasi-two-dimensional magnetic layers.