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Understanding how a spin current flows across metal-semiconductor interfaces at pico- and femtosecond timescales has implications for ultrafast spintronics, data processing and storage applications. However, the possibility to directly access the propagation of spin currents on such time scales has been hampered by the simultaneous lack of both ultrafast element specific magnetic sensitive probes and tailored metal-semiconductor interfaces. Here, by means of free electron laser-based element sensitive Kerr spectroscopy, we report direct experimental evidence of spin currents across a Ni/Si interface in the form of different magnetodynamics at the Ni M2,3 and Si L2,3 absorption edges. This further allows us to calculate the propagation velocity of the spin current in silicon, which is on the order of 0.2 nm/fs.

Converse magnetoelectric coupling in artificial multiferroics is generally modelled through three possible mechanisms: charge transfer, strain mediated or ion migration. Here we demonstrate a novel and highly reliable approach, where electrically controlled morphological modifications control the ferromagnetic response of a magnetoelectric heterostructure, specifically FexMn1-x ferromagnetic films on ferroelectric PMN-PT (001) substrates. The ferroelectric PMN-PT substrates present, in correspondence to electrical switching, fully reversible morphological changes at the surface, to which correspond reproducible modifications of the ferromagnetic response of the FexMn1-x films. Topographic analysis by atomic force microscopy shows the formation of surface cracks after application of a positive electric field up to 6 kV/cm, which disappear after application of negative voltage of the same amplitude. In-operando x-ray magnetic circular dichroic spectroscopy at Fe edge in FexMn1-x layers shows local variations of dichroic signal up to a factor 2.5 as a function of the electrically-driven morphological state. These findings highlight the role of morphology and surface topography as a key aspect in magnetoelectric coupling, whose proof of electrically reversible modification of the magnetic response adds a new possibility in the design of multiferroic heterostructures with electrically controlled functionalities.