Abstract
Ultraviolet photoemission spectroscopy (UPS) is a powerful tool to study the electronic spin and symmetry features at both surfaces and interfaces to ultrathin top layers. However, the very low mean free path of the photoelectrons usually prevents a direct access to the properties of buried interfaces. The latter are of particular interest since they crucially influence the performance of spintronic devices like magnetic tunnel junctions (MTJs). Here, we introduce spin-resolved extremely low energy photoemission spectroscopy (ELEPS) to provide a powerful way for overcoming this limitation. We apply ELEPS to the interface formed between the half-metallic Heusler compound Co2MnSi and the insulator MgO, prepared as in state-of-the-art Co2MnSi/MgO-based MTJs. The high accordance between the spintronic fingerprint of the free Co2MnSi surface and the Co2MnSi/MgO interface buried below up to 4 nm MgO provides clear evidence for the high interface sensitivity of ELEPS to buried interfaces. Although the absolute values of the interface spin polarization are well below 100%, the now accessible spin- and symmetry-resolved wave functions are in line with the predicted existence of non-collinear spin moments at the Co2MnSi/MgO interface, one of the mechanisms evoked to explain the controversially discussed performance loss of Heusler-based MTJs at room temperature.
Highlights
Ultraviolet photoemission spectroscopy (UPS) is a powerful tool to study the electronic spin and symmetry features at both surfaces and interfaces to ultrathin top layers
In our extremely low energy photoemission spectroscopy (ELEPS) experiments we find that the properties of such Co2MnSi/ MgO interfaces are accessible for MgO overlayers with thickness up to 4 nm (20 ML), making ELEPS a unique method for the nondestructive characterization of buried spintronics interfaces, as found in state-of-the-art Co2MnSi/MgO-based MTJs17
As a basis to understand the results on the Co2MnSi/MgO interface, we start discussing here the results obtained from the bare Co2MnSi(100) surface
Summary
Ultraviolet photoemission spectroscopy (UPS) is a powerful tool to study the electronic spin and symmetry features at both surfaces and interfaces to ultrathin top layers. Sample structures are constrained to FM substrates with ultrathin insulator top layers of 1–2 ML (i.e. less than 0.5 nm)[8], whereas direct access to interfaces buried below thicker insulator layers comparable to www.nature.com/scientificreports actual devices (i.e. 1–2 nm) is experimentally very challenging due to the very low mean free path of electrons in materials[9] This is quite a drawback of UPS, since the interface properties themselves depend on the thickness of the insulating layer[10]. In our ELEPS experiments we find that the properties of such Co2MnSi/ MgO interfaces are accessible for MgO overlayers with thickness up to 4 nm (20 ML), making ELEPS a unique method for the nondestructive characterization of buried spintronics interfaces, as found in state-of-the-art Co2MnSi/MgO-based MTJs17 Taking advantage of such interface sensitivity we report a room-temperature interface spin polarization of
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