  We have investigated the underlying magnetism in newly discovered single
crystal Kondo lattices CeXAl4Si2, where X = Rh, Ir. We show that the compound
undergoes an incommensurate-to-commensurate magnetic transition at Tc = 9.19 K
(10.75 K in Ir). The spin correlation in the incommensurate phase is described
by a spin density wave configuration of Ce-ions, which locks-in to the
long-range antiferromagnetic order at T = Tc. The qualitative analysis of the
experimental data suggests the role of the Fermi surface nesting, instead of
the lattice distortion causing the Umklapp correction or the soliton
propagation, as the primary mechanism behind this phenomenon.
