  Magnetic topological semimetals (TSMs) allow for an effective control of the
topological electronic states by tuning the spin configuration, and therefore
are promising materials for next-generation electronic and spintronic
applications. Of magnetic TSMs, Weyl nodal-line (NL) semimetals likely have the
most tunability, and yet they are the least experimentally studied so far due
to the scarcity of material candidates. Here, using a combination of
angle-resolved photoemission spectroscopy and quantum oscillation measurements,
together with density functional theory calculations, we identify the
square-net compound EuGa4 as a new magnetic Weyl nodal ring (NR) semimetal, in
which the line nodes form closed rings in the vicinity of the Fermi level.
Remarkably, the Weyl NR states show distinct Landau quantization with clear
spin splitting upon application of a magnetic field. At 2 K in a field of 14 T,
the transverse magnetoresistance of EuGa4 exceeds 200,000%, which is more than
two orders of magnitude larger than that of other known magnetic TSMs. High
field magnetoresistance measurements indicate no saturation up to 40 T. Our
theoretical model indicates that the nonsaturating MR naturally arises as a
consequence of the Weyl NR state. Our work thus point to the realization of
Weyl NR states in square-net magnetic materials, and opens new avenues for the
design of magnetic TSMs with very large magnetoresistance.
