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Novel types of magnetic order generated across epitaxial interfaces of 4d and 5d transition metal perovskite oxides

Subject Area Experimental Condensed Matter Physics
Term from 2017 to 2020
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 335038432
 
Final Report Year 2020

Final Report Abstract

A means to tailor physical properties of matter is to design artificial structures such as epitaxial heterostructures, in which structural accommodations and interfacial coupling occur, which, together with the broken inversion symmetry, strongly influence the physical properties of the heterostructures, often resulting in novel properties. Here we addressed the particular case of epitaxial multilayers built up by interfacing layers of SrRuO3 - a 4d oxide ferromagnet - with layers of the large spin-orbit coupling paramagnetic SrIrO3 and insulating isostructural perovskite oxides such as SrHfO3 and SrZrO3. Multilayers with asymmetric and symmetric interfaces for the ferromagnetic SrRuO3 layers were studied with the expectation that generation of a net interfacial Dzyaloshinskii-Moriya interaction (DMI) is possible in the asymmetric geometry and less favourable in the symmetric geometry. Unlike the metallic multilayers, such as Pt/Co/Ir, where the interfacial DMI is well established and the magnetic interlayer coupling across the non-magnetic spacers can be ferromagnetic, for the perovskite oxide epitaxial multilayers both the interfacial DMI and the interlayer coupling are not yet understood and much less studied. For example, here we demonstrated that ferromagnetic SrRuO3 layers, separated by spacers of SrIrO3/SrZrO3 so thin as 2 monolayers have only a very weak ferromagnetic coupling at all temperatures down to 10 K. The lack of magnetic coupling makes the imaging of the magnetic domains in these multilayers, both by magnetic force microscopy and Lorentz transmission electron microscopy (LTEM), very difficult and thus no information about the chiral behaviour of the domains or possible formation of skyrmions could be inferred. Additionally, LTEM requires special preparation of specimens from the epitaxial multilayers and this turned out to be very challenging for materials sensitive to ion beam irradiation. A technically simple way of probing the formation of skyrmions is to measure the topological Hall resistivity that should occur in the presence of skyrmions as an additional contribution to the ordinary and anomalous Hall effect (AHE). This type of probing relies on the assumption that the topological Hall effect contribution can be extracted unambiguously from the measured total Hall resistivity. Humplike anomalies of the Hall effect resistance loops were often attributed to the formation of skyrmions. Our studies revealed that the detection of skyrmions solely by Hall effect measurements can be misleading in the case of SrRuO3 heterostructures, as the AHE itself has a complex magnetization dependence and is highly sensitive to minute structural and electronic properties variations.

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