FERMI | MagneDyn Elettra | CiPo Magnetizing Silicon: doping dependent spin polarization at Ni/Si interfaces Metal semiconductor interfaces are a fundamental element to the operation of virtually all semiconductor devices. Because of the interplay of different degrees of freedom at the interface of the two different materials, their properties can be modified to a great extent, rendering them the ideal platform for the implementation of novel functionalities. In this context, the integration of magnetic properties into silicon-based materials represents one of the most compelling frontiers in modern electronics. This pursuit, driven by the field of spintronics, aims to leverage the electron’s spin in addition to its charge, paving the way for more energy-efficient and faster devices. However, silicon’s intrinsically non-magnetic nature has long posed a fundamental challenge. In this study, researchers from Elettra and collaborators explored the possibility of inducing a magnetic character into silicon via the proximity of a magnetic thin film grown on it. Furthermore, the study revealed a nontrivial effect of the semiconductor doping to the final magnetic state of the interface. The experiment considered two nearly identical heterostructures based on a ferromagnetic thin film of nickel deposited on a passivated silicon substrate, differing only in the doping concentration of the silicon substrate - a fairly lower doped and a higher one. The researchers employed elementspecific X-Ray Magnetic Circular Dichroism (XMCD) measurements at the CiPo beamline, alongside Magneto-Optical Kerr Effect (MOKE) measurements at the MagneDyn endstation of FERMI, in both the cases the signal was collected via the reflection from the sample. These techniques allow to detect the spin unbalance associated with a specific element by turning the x-ray energy to its characteristic absorption edge; in the case of XMCD the magnetic signal is inferred by comparing the difference in absorption between the excitation from the spinmajority population with respect to the one associated with the spin-minority population. For this experiment, the nickel M2,3 and the silicon L2,3 edges were investigated to observe both the ferromagnetic source and the induced magnetic behaviour of the heterostructure. The data collected at the nickel edge revealed a similar magnetic state for the metallic film, as shown in Fig. 1 (c). Strikingly, what is then the key finding of the study was the presence of an equilibrium magnetic signal at the silicon L2,3 edge, an unexpected result from an element which is nominally nonmagnetic. Moreover, the magnetization signal reversed depending on the doping of the system, signaling a different magnetic alignment. In the low-doped system the spin polarization aligned parallel to the magnetization of the adjacent nickel film, meaning that the coupling of the two materials was ferromagnetic, whereas the high-doped system behaved antiferromagnetically (Fig. 1 d, e, respectively). The striking reversal of the magnetization coupling dopings suggests a competition between different mechanisms at the interface at the origin of the spin imbalance in the proximal layer. One of the possible involved mechanisms is the presence of a magnetic coupling at the interface between the ferromagnetic layer and the substrate via proximity effect, where the ferromagnetic layer MNi modifies the electronic structure of silicon’s accumulation layer, inducing a spin polarization MSi via an exchange splitting µBBeff, Beff being the effective perceived field in silicon, as outlined in Fig. 1 (b). A second possible mechanism is the tunneling of spin-polarized electrons jT from nickel into silicon, creating a net spin asymmetry in the semiconductor (Fig. 1 a). These two effects may be present at the same time and contribute differently to the final magnetic state. To isolate the source of the signal, the researchers ensured that the silicon nitride passivation layer at the interface effectively prevented metallic silicide formation during the sample preparation, ruling out the possibility that the observed magnetism originated from unwanted metallic phases. Through careful analysis of the XMCD spectral features at the Si L2,3 edge, the team was also able to quantitatively estimate the spin magnetic moment induced in silicon. Although small in absolute value -of the order of 0.02 µB per atom - these moments are comparable to those observed in engineered magnetic silicon alloys, underlining the sensitivity and robustness of the measurement approach. The manifestation that the sign and magnitude of induced magnetization in silicon can be tuned by doping adds a new layer of control in designing siliconbased spintronic architectures. The existence of a direct magnetic coupling between the ferromagnetic layer and the underlying semiconductor not only expands fundamental understanding of magnetic proximity effects but also lays important groundwork for future devices that combine established silicon technology with emerging spin-based functionalities. Nonetheless, to fully elucidate the complex interplay and the role of the magnetic proximity effect that gives rise to the magnetic behaviour at the interface, further theoretical investigations are necessary. Elettra Highlights 2024–2025 34
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