Spin orientation of two-dimensional electrons driven by temperature-tunable competition of spin-orbit and exchange-magnetic interactions

Finding ways to create and control the spin-dependent properties of two-dimensional electron states (2DESs) is a major challenge for the elaboration of novel spin-based devices. Spin–orbit and exchange–magnetic interactions (SOI and EMI) are two fundamental mechanisms that enable access to the tunability of spin-dependent properties of carriers. The silicon surface of HoRh2Si2 appears to be a unique model system, where concurrent SOI and EMI can be visualized and controlled by varying the temperature. The beauty and simplicity of this system lie in the 4f moments, which act as a multiple tuning instrument on the 2DESs, as the 4f projections parallel and perpendicular to the surface order at essentially different temperatures. Here we show that the SOI locks the spins of the 2DESs exclusively in the surface plane when the 4f moments are disordered: the Rashba-Bychkov effect. When the temperature is gradually lowered and the system experiences magnetic order, the rising EMI progressively competes with the SOI leading to a fundamental change in the spin-dependent properties of the 2DESs. The spins rotate and reorient toward the out-of-plane Ho 4f moments. Our findings show that the direction of the spins and the spin-splitting of the two-dimensional electrons at the surface can be manipulated in a controlled way by using only one parameter: the temperature.This work was supported by the German Research Foundation (DFG) (Grant Nos. VY64/1-3, GE602/2-1, GRK1621, and SFB1143) as well as by Research Grant No. 15.61.202.2015 of Saint Petersburg State University. We acknowledge Diamond Light Source for beamtime on the beamline I05 under proposal SI11512. The authors acknowledge experimental support from the SIS beamline at Swiss Light Source. The support by the Tomsk State University Academic D.I. Mendeleev Fund Program in 2015 (research grant N 8.1.05.2015), the University of the Basque Country (Grants Nos. GIC07IT36607 and Grant No. IT-756-13), and the Spanish Ministry of Science and Innovation (Grant Nos. FIS2013-48286-C02-02-P and FIS2013-48286-C02-01-P) is also acknowledged.Peer Reviewe