  Lorentz-violating type-II nodal lines exhibit attracting physical properties
and have been hot discussed currently. However, their investigations have been
mostly limited in nonmagnetic system because of lacking ideal spin-polarized
candidates with clean type-II nodal line states. Here, for the first time, we
report the family of X2YZ4 (X=K, Cs, Rb, Y=Cr, Cu, Z=Cl, F) compounds are such
ideal candidate materials by using the member of K2CuF4 as an example. We show
the material is a ferromagnetic half-metal with weak anisotropy, which host
fully spin-polarized conducting electrons. In the conducting spin channel, the
band crossing form a pair of type-II nodal lines, protected by mirror symmetry.
These type-II nodal lines are different with former proposed examples because
they have a 100% spin polarization. In addition, we also show the material can
realize switchable topological states, which can be easily controlled by
external magnetic field. It is noticed that, the material: i) is stable and can
be synthesized in experiments; ii) has clear magnetic structure; and iii)
manifests clean type-II nodal line state and clear drumhead surface states.
Therefore, the proposed X2YZ4 compounds are expected to be an excellent
platform to investigate the novel physical properties of both type-II nodal
line states with complete
