Electronic nature of the lock-in magnetic transition in CeXAl4Si2

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.