Functional Oxide Superlattices
One of the investigated systems consisted of two canonical ferromagnetic oxides: a manganite, La0.7Sr0.3MnO3, and a ruthenate, SrRuO3. Owing to their different Curie temperatures (TC= 360 K for bulk La0.7Sr0.3MnO3 and TC= 160 K for bulk SrRuO3) and magnetic moments, and to their very dissimilar magnetocrystalline anisotropies, interesting effects had been expected to arise in superlattices with ultrathin layers. Firstly we studied superlattices consisting of fifteen La0.7Sr0.3MnO3 / SrRuO3 bilayers deposited on TiO2-terminated SrTiO3(100) crystals, while keeping the La0.7Sr0.3MnO3 layer 4 unit cells thick and varying the thickness of the SrRuO3 layers from 8 to 20 unit cells . Structural investigations by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), supported by STEM image simulations, proved that the interfaces had at most a 1.5-unit-cell intermixed transition layer, for the superlattices with thicker layers .
An outstanding finding is that a strong antiferromagnetic (AF) interlayer coupling existed between the La0.7Sr0.3MnO3 and SrRuO3 layers at temperatures below 150 K, where both materials are ordered ferromagnetically. The AF coupling is mediated by Mn-O-Ru bonding across the interfaces, as supported by theoretical calculations, which proved that the AF coupling is energetically more favorable . This is further confirmed by our investigations of superlattices combining another manganite with SrRuO3, namely Pr0.7Ca0.3MnO3, which also exhibited AF interlayer coupling in in-plane magnetic fields of less than 1 T .
We extended the investigations of the La0.7Sr0.3MnO3/ SrRuO3 superlattices by reducing the individual layer thickness to less than 3 unit cells. Structural investigations performed by HAADF-STEM in conjunction with electron energy loss spectroscopy (EELS) maps and line scans revealed important details about the sharpness of the La0.7Sr0.3MnO3/SrRuO3 interfaces .
Figure 1 shows HAADF-STEM images and EELS maps and Mn and Ru elemental profiles for a superlattice with nominally 3-unit-cell-thick layers. The investigations proved the structural integrity of the two types of layers and that sharp interfaces formed between La0.7Sr0.3MnO3 and SrRuO3, with less intermixing than in the case of the La0.7Sr0.3MnO3 / SrRuO3 superlattices with thicker layers [1,2].
The motivation for reducing the layer thickness below 4 unit cells was to compare the magnetic and magnetotransport properties of such superlattices with those of bare epitaxial thin films of La0.7Sr0.3MnO3 and SrRuO3 grown on SrTiO3(100) substrates. The latter show critical behavior when the thickness decreases below about 5 unit cells. La0.7Sr0.3MnO3 films thinner than 4 unit cells become antiferromagnetic and insulating and SrRuO3 films thinner than 4 unit cells maintain metallic behavior but exhibit highly reduced ferromagnetic ordering temperature (below 50 K). In contrast to the individual ultrathin films, La0.7Sr0.3MnO3 / SrRuO3 superlattices with 2-unit-cell-thick individual layers have high ferromagnetic ordering temperatures of ≈280 K for La0.7Sr0.3MnO3 and ≈100 K for SrRuO3. The antiferromagnetic interlayer coupling between La0.7Sr0.3MnO3 and SrRuO3 is preserved below 100 K, as proved by the decrease of the overall magnetic moment (Figure 2).
The stabilization of ferromagnetism at temperatures higher than 280 K in the ultrathin La0.7Sr0.3MnO3 layers is very intriguing. There have been similar reports for La0.7Sr0.3MnO3 / SrTiO3 superlattices with 5-unit-cell-thick layers, which had a Curie temperature of 250 K  and for 1 to 2-nm-thick epitaxial La2/3Ca1/3MnO3 layers sandwiched between superconductive YBCO layers . Moreover interfacial ferromagnetism was observed for CaMnO3/CaRuO3 superlattices . This indicates that epitaxial interfacing with a metallic oxide layer and charge-transfer phenomena may facilitate the stabilization of ferromagnetism at temperatures close to the bulk transition temperature.
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I.Vrejoiu, M. Ziese*
* University of Leipzig, Division of Superconductivity and Magnetism, Leipzig, Germany