Low-temperature magnetic ordering in SrEr₂O₄

SrEr$_2$O$_4$ has been characterised by low-temperature powder neutron diffraction, as well as single crystal specific heat and magnetisation measurements. Magnetisation measurements show that the magnetic system is highly anisotropic at temperature above ordering. A magnetic field of 280 kOe applied at $T=1.6$ K does not overcome the anisotropic magnetisation and fails to fully saturate the system. Long range antiferromagnetic ordering develops below $T_N=0.75$ K, identified by magnetic Bragg reflections with propagation vector ${\bf k}=0$ and a lambda anomaly in the specific heat. The magnetic structure consists of ferromagnetic chains running along the \textit{c} axis, two adjacent chains being stacked antiferromagnetically. The moments point along the \textit{c} direction, but only one of the two crystallographically in-equivalent Er sites has a sizeable ordered magnetic moment, 4.5 $\rm \mu_B$ at 0.55 K. The magnetic properties of SrEr$_2$O$_4$ are discussed in terms of the interplay between the low-dimensionality, competing exchange interactions, dipolar interactions and low lying crystal field levels.