One of the investigated systems consisted of two canonical ferromagnetic oxides: a manganite, La_0.7Sr_0.3MnO₃, and a ruthenate, SrRuO₃. Owing to their different Curie temperatures (T_C= 360 K for bulk La_0.7Sr_0.3MnO₃ and T_C= 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 La_0.7Sr_0.3MnO₃ / SrRuO₃ bilayers deposited on TiO₂-terminated SrTiO₃(100) crystals, while keeping the La_0.7Sr_0.3MnO₃ layer 4 unit cells thick and varying the thickness of the SrRuO₃ layers from 8 to 20 unit cells [1]. 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 [2]. 

An outstanding finding is that a strong antiferromagnetic (AF) interlayer coupling existed between the La_0.7Sr_0.3MnO₃ 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 [1]. This is further confirmed by our investigations of superlattices combining another manganite with SrRuO₃, namely Pr_0.7Ca_0.3MnO₃, which also exhibited AF interlayer coupling in in-plane magnetic fields of less than 1 T [3].

 
We extended the investigations of the La_0.7Sr_0.3MnO₃/ 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 La_0.7Sr_0.3MnO₃/SrRuO₃ interfaces [2].   


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 La_0.7Sr_0.3MnO₃ and SrRuO₃, with less intermixing than in the case of the La_0.7Sr_0.3MnO₃ / 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 La_0.7Sr_0.3MnO₃ and SrRuO₃ grown on SrTiO₃(100) substrates. The latter show critical behavior when the thickness decreases below about 5 unit cells. La_0.7Sr_0.3MnO₃ 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, La_0.7Sr_0.3MnO₃ / SrRuO₃ superlattices with 2-unit-cell-thick individual layers have high ferromagnetic ordering temperatures of ≈280 K for La_0.7Sr_0.3MnO₃  and ≈100 K for SrRuO₃. The antiferromagnetic interlayer coupling between La_0.7Sr_0.3MnO₃ 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 La_0.7Sr_0.3MnO₃ layers is very intriguing. There have been similar reports for La_0.7Sr_0.3MnO₃ / SrTiO₃ superlattices with 5-unit-cell-thick layers, which had a Curie temperature of 250 K [4] and for 1 to 2-nm-thick epitaxial La_2/3Ca_1/3MnO₃ layers sandwiched between superconductive YBCO layers [5]. Moreover interfacial ferromagnetism was observed for CaMnO₃/CaRuO₃ superlattices [6]. 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.

[1] Ziese, M., I. Vrejoiu, E. Pippel, P. Esquinazi, D. Hesse, C. Etz, J. Henk, A. Ernst, I. V. Maznichenko, W. Hergert  and I. Mertig.
      Physical Review Letters 104, 167203 (2010).
[2] Hillebrand R., E. Pippel, D. Hesse and I. Vrejoiu. Physica Status Solidi a 208, 2144-2149 (2011).
[3] Ziese, M., I. Vrejoiu, E. Pippel, E. Nikulina and D. Hesse. Applied Physics Letters 98, 132504 (2011).
[4] Fitting Kourkoutis, L., J. H. Song, H. Y. Hwang and D. A. Muller. Proceedings of the National Academy of Sciencies
      of the United States of America 107, 11682-11685 (2010).
[5] Soltan S., J. Albrecht, G. Logvenov and H.-U. Habermeier. Annual Report 2010, 45 (2010).
[6] Freeland J. W., J. Chakhalian, A. V. Boris, J.-M. Tonnerre, J. J. Kavich, P. Yordanov, S. Grenier, P. Zschack, E. Karapetrova, P. Popovich,
      H. N. Lee and B. Keimer. Physical Review B 81, 094414 (2010)

I.Vrejoiu, M. Ziese*

* University of Leipzig, Division of Superconductivity and Magnetism, Leipzig, Germany

Contact: Ionla Vrejoiu