Modulated magnetism and anomalous electronic transport in $\rm Ce_3Cu_4As_4O_2$

Abstract: The complex magnetism and transport properties of tetragonal Ce$3$Cu$_4$As$_4$O$_2$ were examined through neutron scattering and physical properties measurements on polycrystalline samples. The lamellar structure consists of alternating layers of $\rm CeCu_4As_4$ with a single square Ce lattice and oxygen-linked Ce bi-layer $\rm Ce_2O_2$. Extending along $\bf c$, a tube-like Fermi surface from DFT calculations points to a quasi-two-dimensional electronic system. Peaks in the specific heat at the Ne\'{e}l temperature $T{N}=24$ $\rm K$, $T_{2}~=~16 $ $\rm K$ and $T_{3}~=~1.9$ $ \rm K$ indicate three magnetic phase transitions or distinct cross-over phenomena. For $T<T_{N}$ neutron diffraction indicates the development of ferromagnetic ab sheets for both Ce sites, with alternating polarization along $\bf{c}$, a wave vector ${\bf k}{1}={\bf c}^*$. For $T<T{2}$, quasi-two-dimensional low-energy spin fluctuations with ${\bf k}{2}=\frac{1}{2}{\bf a}^*$ and polarized perpendicular to ${\bf k}{2}$ are suppressed. The data are consistent with quasi-two-dimensional antiferromagnetic order in the $\rm CeCu_4As_4$ planes polarized along the ${\bf k}{2}$ wave vector. $T{3}$ marks a spin-flop transition where the ${\bf k}{1}$ staggered magnetization switches to in-plane polarization. While the narrow 4f bands lie deep below the Fermi surface, there are significant transport anomalies associated with the transitions; in particular a substantial reduction in resistivity for $T<T{N}$. At $T=100$ $ \rm mK$ the ${\bf k}_1$ modulated staggered moment is $0.85~\mu_B$, which matches the $0.8~\mu_B$ saturation magnetization achieved for H $~=~7$ $ \rm T$ at $T~=~2$ $ \rm K$. From low T Lorentzian fits the correlation length is in excess of 75 \AA. We argue the unusual sequence of magnetic transitions results from competing interactions and anisotropies for the two Ce sites.