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Source: Forschungszentrum JulichJülich researchers discover new formula for changing the electronic and magnetic properties of oxide interfaces (advanced materials)

Jülich, December 15, 2020 – Most materials are either magnetic or they are not. Scientists at Forschungszentrum Jülich have now deciphered a new mechanism that enables the electronic and magnetic properties of a material to be changed in a targeted and reversible manner. The effect is based on the transfer of ions at the interface between two oxides – the researchers were able to demonstrate the existence of this process experimentally for the first time. Both oxides alone typically show neither magnetism nor significant electrical conductivity. Only in combination do both properties appear at their interface. The exact causes of their strength are still unclear. However, the researchers succeeded in changing the magnetic order at the interface by shifting ions. This makes the material more magnetic. They could also control the electrical properties via the electron distribution.

Control of electronic properties through electron and ion transferCopyright: Rose et al., Advanced Materials (2020), DOI: 10.1002 / adma.202004132 (CC BY-NC-ND 4.0) Such flexible material systems are for various new IT concepts such as neuromorphic computing or spintronic approaches are relevant. One possible application would be a “multifunctional” transistor that can control not only electrical current but also, possibly, “spin currents” (spintronics). Such a component could then be controlled by applying a voltage as well as a magnetic field. The investigated “electronic” interface system consisting of the oxides LaAlO3 and SrTiO3 was discovered back in 2004 and has generated worldwide interest. Both materials exchange both electrons and atomic components in the form of charged ions at the contact surface, as the Jülich researchers have now been able to show for the first time. As a result, new electronic properties develop at the interface. A similar phenomenon is classically known from semiconductors. However, the purely electronic effect there is exclusively limited to the exchange of electrons. The researchers at the Jülich Peter Grünberg Institute (PGI-7 / PGI-6) were able to demonstrate experimentally for the first time that, in addition to the exchange of electrons, the “ionic” charge transfer for the Change in the electronic properties of the LaAlO3 / SrTiO3 system is responsible. The concept makes it possible to adjust the conductivity of the interface and at the same time to generate magnetic properties. Using various “mixtures” of electronic and ionic charge transfer, they were able to create different interfaces that differ in terms of their electronic and atomic structure. The researchers were able to create interfaces with high conductivity and weak magnetism, for example, or lower conductivity and stronger magnetism. Experimental evidence was obtained using X-ray photoelectron spectroscopy under atmospheric conditions (near-ambient pressure X-ray photoelectron spectroscopy, NAP-XPS ). The method is still quite new and allows – as has now been demonstrated – direct access to the ionic processes at atomically defined interfaces. In the experiment, the movement of cations across the interface can be investigated and controlled dynamically via temperature and oxygen atmosphere. From this data, specific conclusions can be drawn about the relationship between the ionic structure and the resulting electrical and magnetic properties.Process control through oxygen contactThe ion transfer is controlled via contact with oxygen. This causes strontium ions (Sr) to move out of the interface. Every missing Sr ion binds two electrons, which can then no longer contribute to the electrical conductivity, so that the electrical conductivity drops. At the same time, this process creates crystal defects that influence the magnetic order of the other electrons. Thus the system becomes more magnetic as it loses conductivity. Researchers had previously postulated that these cations can move freely in this way, but the majority considered it to be practically impossible. The experimental proof of this process in the present study therefore sets a milestone in the understanding of ionic processes at oxide interfaces. The research project was led by the PGI-7 and PGI-6 with the participation of RWTH Aachen University, the PGI-1 and international collaborations with researchers Charles University in Prague, Advanced Light Source (ALS) in Berkeley and the Pacific Northwest National Laboratory (PNNL) in Richland. Original publication: M. Rose, B. Šmíd, M. Vorokhta, I. Slipukhina, M. Andrä, H. Bluhm, T. Duchoň, M. Ležaić, SA Chambers, R. Dittmann, DN Mueller *, F. Gunkel * Identifying Ionic and Electronic Charge Transfer at Oxide HeterointerfacesAdvanced Materials 2004132 (published online 2 December 2020), DOI: 10.1002 / adma.202004132

Further information: Peter Grünberg Institute, Electronic Materials (PGI-7) Contact: Dr. Felix GunkelPeter Grünberg Institute, Electronic Materials (PGI-7) Tel .: +49 2461 61-5339 E-Mail: f.gunkel@fz-juelich.de

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