Intermetallic systems based on rare earth, like ReR2Si2 (Re= Rare Earth; R= Transitional metal)systems, have a mixed valence and thanks to this they show fascinating properties such as formation of heavy fermions and unconventional superconductivity. Magnetically, rare earth intermetallics are well described within local magnetism models due to the strong localisation of the 4f shell at the atomic sites. Recently it has been found that on the Silicon terminated surfaces of HoRh2Si2 and GdRh2Si2 there are two types of spin-polarized 2-Dimensional electron states (2-DES). One is a surface Schockley state, and the other a resonant surface state. The latter forms Dirac fermions at the Gamma point, and it has been shown that they can couple to the magnetic degrees of freedom of the 4f electrons of the rare earth. Improving our understanding of the magnetic structure of such systems, as a function of temperature, may allow us to induce non-trivial electronic and magnetic properties into nanostructures deposited on top of these systems, which could be interesting for future technological applications.
In my thesis work I have conducted a series of experiments at ID32 of ESRF on ReR2Si2 systems, with Re=Gd, Eu, Sm, Ho and R=Rh, Ir, using polarized Soft X-Ray radiation. To this end I used X-Ray Resonant Magnetic Scattering, which allowed us to obtain informations on the magnetic structure of the samples.
With results of these experiments I was able to uncover the magnetic structures of the systems in study. In particular I found that, for the compounds made of Gd and Sm after the phase transition from paramagnetic to anti-ferromagnetic, the magnetic structures is the same for the two of them: the magnetic moments arrange in such a way on the a-b plane to form ferromagnetic sheets which arrange anti-ferromagnetically along the c-axis. The Eu sample forms ferromagnetic sheets anti-ferromagnetically arranged along the c-axis too, but in this case the magnetic structure in an incommensurate one, i.e. the periodicity of these ferromagnetic sheets does not match the crystalline planes periodicity, and the ordering wave vector varies with temperature. The Ho sample has a strong magnetic Bragg peak, but doesn’t present some of the features common in the others, in this case the magnetic moments form ferromagnetic sheets on the c-b plane, with direction perfectly aligned to the c-axis direction, confirming what was previously found using neutrons.
These results nicely illustrate how the magnetism of these local moment systems sensitively depends on the indirect exchange between the sites as well as the details of the hybridisation between the local 4f states and itinerant valence states. Application of chemical pressure or electronic doping by chemical substitution could therefore be a promising route to control the bulk magnetism of these systems and thereby also the spin-polarisation of the 2-DES at the surface.