Corresponding author

Ionela Vrejoiu

Max Planck Institute for Solid State Research

References

1.
Vailionis, A.; Boschker, H.; Liao, Z.; Smit, J.R.A; Rijnders, G; Huijben,  M.; Koster, G.
Symmetry and Lattice Mismatch Induced Strain Accommodation Near and Away from Correlated Perovskite Interfaces
2.
Ziese, M.; Bern, F.; Pippel, E.; Hesse, D.; Vrejoiu, I.

Stabilization of ferromagnetic order in La0.7Sr0.3MnO3/SrRuO3 superlattices

3.
Ziese, M.; Bern, F.; Setzer, A.; Pippel, E.; Hesse, D.; Vrejoiu, I.

Existence of a Magnetically Ordered Hole Gas at the La0.7Sr0.3MnO3/SrRuO3 Interface

In collaboration with:

C. Jia, S.-B. Mi (International Center of Dielectric Research, The School of Electronic and Information Engineering, Xi'an Jiaotong University, China & Peter Grünberg Institute and Ernst Ruska Center for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich, Germany)


M. Ziese (University of Leipzig, Faculty of Physics and Geosciences, Division of Superconductivity and Magnetism, Germany)

Minerva Research Group "Nanoscale Functional Heterostructures"

Effects of SrRuO3 on properties of functional oxide heterostructures

Authors

I. Vrejoiu

Departments

SrRuO3 is a ferromagnetic metallic oxide and is often used as an electrode material for epitaxial ferroelectric heterostructures. We fabricated SrRuO3/PbZr0.2Ti0.8O3/SrRuO3 and SrRuO3/La0.7Ba0.3MnO3/SrRuO3 tri-layers on SrTiO3(100) by PLD. Atomic resolution TEM showed that the first 3 unit cells of the ferroelectric PbZr0.2Ti0.8O3 layer have a reduced polarization, indicating that the bottom SrRuO3 layer may not be the best electrode for ferroelectric tunnel junctions. However, ferromagnetic properties of 2 unit cells thick La0.7Ba0.3MnO3 layers interfaced with SrRuO3 layers are much improved.

SrRuO3 as an electrode material in ferroelectric tunnel junctions                

Ferroelectric tunnel junctions have been a very exciting topic of research since the successful fabrication of ultrathin epitaxial ferroelectric films that allow electron tunneling has been reproducibly demonstrated. Ferroelectric epitaxial layers only a few unit cells thick have been deposited on single crystal substrates coated with various metal oxide electrodes, in order to fabricate ferroelectric tunnel junction devices. Most often the bottom electrode consisted of SrRuO3 because of the good structural compatibility and good metallic properties of the ruthenate. The ferroelectric behavior of the few unit cells thick ferroelectric films was investigated by scanning probe techniques, such as piezo-response force microscopy (PFM). PFM does not provide any quantitative information about the magnitude of the switchable ferroelectric polarization. It has thus not been investigated quantitatively how the polarization of the few unit cells thick ferroelectric layers varies across the ferroelectric layer, especially in the interface regions. Recently developed high resolution transmission electron microscopy (HRTEM) experimental techniques have allowed studying ferroelectric polarization and ferroelectric domains by measuring the off-center displacements of atoms inside ferroelectric layers. We investigated epitaxial SrRuO3/PbZr0.2Ti0.8O3/SrRuO3 heterostructures by aberration-corrected HRTEM. Negative CS imaging (NCSI) technique based on aberration-corrected HRTEM, combined with high-angle annular dark field (HAADF) imaging and annular bright-field (ABF) imaging based on scanning transmission electron microscopy (STEM) allow to determine the full atomic arrangement, including chemical elements with a low atomic number such as oxygen. Additionally the structure of 180° domain walls within the ferroelectric Pb(Zr0.2Ti0.8)O3 layers could be studied. A summary of the main findings is presented in Fig. 1.

<p><strong>Fig. 1: </strong>(a) High resolution HAADF and (b,c)ABF images of PbZr<sub>0.2</sub>Ti<sub>0.8</sub>O<sub>3</sub>/SrRuO<sub>3</sub> interface regions. The polarization vector is upwards as indicated by a vertical arrow in (b). (c) A high-resolution ABF image recorded at the PbZr<sub>0.2</sub>Ti<sub>0.8</sub>O<sub>3</sub>/SrRuO<sub>3</sub> interface showing that the polarization vector of the film is downwards in another domain. (d) Intensity profiles showing the intensity variation along the atomic plane indicated by a vertical red line in (a). Magnified images with superposed structure models of PbZr<sub>0.2</sub>Ti<sub>0.8</sub>O<sub>3</sub> were inserted in (b) and (c). The interfacial Ti/Zr, Ru, and the intermixed (Pb,Sr)O atomic column are indicated by orange, white and blue arrows, respectively.&nbsp; NCSI-HRTEM allowed to obtain: (e) the variation of out-of-plane lattice parameters (<em>c</em>) across the PbZr<sub>0.2</sub>Ti<sub>0.8</sub>O<sub>3</sub>/SrRuO<sub>3</sub> interface; (f) the values (<em>d</em><sub>Ti/Zr&ndash;O</sub>) of the displacement of the O-atom relative to the neighboring Ti/Zr-atom in PbZr<sub>0.2</sub>Ti<sub>0.8</sub>O<sub>3</sub>; (g) the calculated value of<em> P</em><sub>S</sub> for the PbZr<sub>0.2</sub>Ti<sub>0.8</sub>O<sub>3</sub> films. The interface is indicated by a vertical red line. The region in orange shows the deviation of the values of <em>c</em>,<em>d</em><sub>Ti/Zr&ndash;O</sub> and <em>P<sub>s</sub></em> of the first three unit cells from those of the main part of the PbZr<sub>0.2</sub>Ti<sub>0.8</sub>O<sub>3 </sub>layer.</p> Zoom Image

Fig. 1: (a) High resolution HAADF and (b,c)ABF images of PbZr0.2Ti0.8O3/SrRuO3 interface regions. The polarization vector is upwards as indicated by a vertical arrow in (b). (c) A high-resolution ABF image recorded at the PbZr0.2Ti0.8O3/SrRuO3 interface showing that the polarization vector of the film is downwards in another domain. (d) Intensity profiles showing the intensity variation along the atomic plane indicated by a vertical red line in (a). Magnified images with superposed structure models of PbZr0.2Ti0.8O3 were inserted in (b) and (c). The interfacial Ti/Zr, Ru, and the intermixed (Pb,Sr)O atomic column are indicated by orange, white and blue arrows, respectively.  NCSI-HRTEM allowed to obtain: (e) the variation of out-of-plane lattice parameters (c) across the PbZr0.2Ti0.8O3/SrRuO3 interface; (f) the values (dTi/Zr–O) of the displacement of the O-atom relative to the neighboring Ti/Zr-atom in PbZr0.2Ti0.8O3; (g) the calculated value of PS for the PbZr0.2Ti0.8O3 films. The interface is indicated by a vertical red line. The region in orange shows the deviation of the values of c,dTi/Zr–O and Ps of the first three unit cells from those of the main part of the PbZr0.2Ti0.8O3 layer.

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The high-resolution HAADF (Fig. 1(a)) and ABF images (Fig. 1(b)) were acquired simultaneously along the [110] zone axis of PbZr0.2Ti0.8O3 and show a typical interface region of the ferroelectric PbZr0.2Ti0.8O3 on the bottom SrRuO3 electrode layer. The polarization vector is upwards as indicated by a vertical arrow in Fig. 1(b). Figure 1(c) displays a high-resolution ABF image recorded at the PbZr0.2Ti0.8O3/SrRuO3 interface in a domain where the polarization vector of the ferroelectric points downwards. In Fig. 1(d) the intensity profiles are plotted, showing the intensity variation along the atomic plane indicated by a vertical red line in Fig.1(a). Magnified images with superposed structure models of PbZr0.2Ti0.8O3 were inserted in Fig. 1(b) and Fig. 1(c). The interfacial Ti/Zr, Ru, and the intermixed (Pb,Sr)O atomic columns are indicated by orange, white and blue arrows, respectively. The SrRuO3 layer appears to be SrO-terminated and possible intermixing with Pb occurred at the interface. The variation of out-of-plane lattice parameters across the PbZr0.2Ti0.8O3/SrRuO3 interface is plotted in Fig. 1(e) and shows a decreased value for the first 3 unit cells next to the bottom electrode. Accordingly, the values of the displacement of the O-atom relative to the neighboring Ti/Zr-atom (dTi/Zr–O) obtained from NCSI–HRTEM images exhibit a similar behavior for the first three unit cells (Fig. 1(f)). Consequently, the calculated value of the spontaneous polarization PS for the PbZr0.2Ti0.8O3 film, plotted in Fig. 1(g), reaches almost three times lower values in the first unit cells next to the SrRuO3 electrode, with respect to the value away from the interface. This result suggests that the PbZr0.2Ti0.8O3 ferroelectric layer, if thinner than 4 unit cells, has dramatically degraded electrical properties. This may be due to the structural accommodation required to occur in the first unit cells of the tetragonal ferroelectric layer at the interface with the structurally and electrically dissimilar bottom SrRuO3 electrode. This finding is important for designing ferroelectric tunnel junctions where tunneling through the ferroelectric barrier requires the ferroelectric layer be only a few unit cells thick.

Ferromagnetism stabilized in ultrathin La0.7Ba0.3MnO3 epitaxial layers sandwiched between SrRuO3

<strong>Fig. 2: </strong>The ferromagnetic ordering is preserved in La<sub>0.7</sub>Ba<sub>0.3</sub>MnO<sub>3</sub> &nbsp;layers (1 to 5 unit cells thick), sandwiched between SrRuO<sub>3</sub> layers (3&ndash;4) unit cells thick, as proved by SQUID measurements of the magnetization dependence on temperature. <br />(a) La<sub>0.7</sub>Ba<sub>0.3</sub>MnO<sub>3</sub> layers have in-plane easy axis of magnetization, while (b) the SrRuO layers have out-of-plane easy axis. (c) The Curie temperature of La<sub>0.7</sub>Ba<sub>0.3</sub>MnO<sub>3</sub>,<em>T</em><sub>C LBMO</sub>, &nbsp;was &asymp;260 K for 5 unit cells thick and &nbsp;&asymp;160 K for 1 unit cell thick layers. The Curie temperature of SrRuO<sub>3</sub> layers,<em> T</em><sub>C SRO</sub>, varied between 140 K and 80 K. Zoom Image
Fig. 2: The ferromagnetic ordering is preserved in La0.7Ba0.3MnO3  layers (1 to 5 unit cells thick), sandwiched between SrRuO3 layers (3–4) unit cells thick, as proved by SQUID measurements of the magnetization dependence on temperature.
(a) La0.7Ba0.3MnO3 layers have in-plane easy axis of magnetization, while (b) the SrRuO layers have out-of-plane easy axis. (c) The Curie temperature of La0.7Ba0.3MnO3,TC LBMO,  was ≈260 K for 5 unit cells thick and  ≈160 K for 1 unit cell thick layers. The Curie temperature of SrRuO3 layers, TC SRO, varied between 140 K and 80 K. [less]

Epitaxial La0.7Ba0.3MnO3 films typically suffer drastic degradation of the transport properties and ferromagnetic ordering when they are only a few unit cells thick. Symmetry and lattice mismatch-induced strain accommodation occurs at the interface with the dissimilar single crystal substrate [1]. In the past we studied the magnetic and transport properties of La0.7A0.3MnO3/SrRuO3 (A= Sr or Ba) superlattices and we found that ferromagnetic behavior with high Curie temperature could be stabilized in manganite layers as thin as 2 unit cells [2,3]. In order to check if the superlattice configuration plays a crucial role in the peculiar ferromagnetic behavior, we fabricated SrRuO3/La0.7Ba0.3MnO3/SrRuO3 tri-layer heterostructures on SrTiO3(100). The SrRuO3 (SRO) layers were 3-4 unit cells thick and the thickness of the La0.7Ba0.3MnO3 (LBMO) layers was varied between 1 and 5 unit cells. Figure 2 summarizes the results of SQUID measurements of magnetization versus temperature performed on the five samples. The LBMO layers thicker than one unit cell exhibited clear ferromagnetic ordering above 200 K. The one unit cell LBMO layer has a reduced ferromagnetic ordering temperature of about 160 K and much reduced magnetization. Additionally, the antiferromagnetic coupling between the SRO and LBMO layers was evidenced by the behavior of the magnetization curves below the Curie temperature of the SRO layers (i.e., 100-140K). Measurements were performed with the magnetic field applied parallel to the sample surface (Fig. 2(a)) or perpendicular to the sample surface (Fig. 2(b)). The two types of measurements allowed to assign that the LBMO layers have the magnetic easy axis oriented in-plane and the SRO layers have out-of-plane magnetic easy axis. In Fig. 2(c) the Curie temperature for the SRO and LBMO layers of the five samples is plotted. The TC of the SRO layers appears to increase for the samples with thinnest LBMO layers, although all the samples had nominally 3–4 unit cells thick SRO layers. This may be due to ferromagnetic interlayer coupling of the top and bottom SRO layers through the middle LBMO layer, when the LBMO layer is only 1 or 2 unit cells thick.

Our results suggest that magnetic proximity of SRO and LBMO is beneficial for the stabilization of ferromagnetic order in ultrathin manganite layers.

 
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