Magnetic properties of superlattices with ultrathin manganite layers
Epitaxial thin films of collosal magnetoresistive La0.7Sr0.3MnO3 (LSMO) deposited on SrTiO3(100) show dramatically degraded ferromagnetic properties when their thickness is of only few unit cells. Recently it was reported that interface engineered LSMO films preserve better ferromagnetic behavior, the thinnest ferromagnetic films being 3 unit cells thick. Boschker et al. showed that compositional interface engineering of the La0.67Sr03.3MnO3/TiO2-terminated SrTiO3 substrate could prevent the interface reconstruction driven by polar discontinuity. By inserting a La0.67Sr0.33O layer at the interface the ferromagnetic properties of 5 unit cell thick La0.67Sr03.3MnO3 films were greatly improved . Shapoval et al. used a similar approach and obtained 3 unit cells thick LSMO films with a Curie temperature as high as 250K. Atomically resolved electron microscopy and chemical analysis indicated that ultrathin manganite films start growing with an La-O layer on a strongly Mn/Ti-intermixed interface, engineered by an additional deposition of 2 unit cells of Sr-Mn-O. Such interface engineering results in a hole-doped manganite layer and stabilizes ferromagnetism and metallic conductivity down to the thickness of 3 unit cells .
Superlattices of LSMO and insulating SrTiO3, with LSMO layers down to 4 unit cells, were grown by several groups and were found to have also reduced magnetic moments and Curie temperatures for the onset of ferromagnetism. In La0.7Sr0.3MnO3/SrRuO3 superlattices, LSMO layers as thin as 2 unit cells are ferromagnetic and metallic with Curie temperatures TC close to the transition temperature of bulk LSMO, as proved by measurements of anomalous Hall effect and of magnetization versus temperature and magnetic field measurements . For example Fig. 1(a) and Fig. 1(b) show temperature dependence of the superlattice magnetizations for in-plane applied magnetic fields. The two superlattices have 2 unit cells LSMO layers and differ in the thickness of the ruthenate layers, which are 2 unit cells thick (Fig. 1(a)) and 9 unit cells thick (Fig. 1(b)). The superlattice with 9 unit cells thick SrRuO3 layers exhibits a complex behavior due to the strong magneto-crystalline anisotropy of the rather thick ruthenate layers, but the LSMO layers are nonetheless ferromagnetic below 230K. The onset of ferromagnetic order in the 2 unit cells thick LSMO layers in the superlattice with 2 unit cells thick ruthenate layers occurs at an even higher temperature of 280K.
This is an outstanding result: it did not require any special fabrication stratagem during the pulsed-laser deposition (PLD) of the superlattices, in order to stabilize the ferromagnetic order of ultrathin LSMO layers . The thickness of the neighboring SrRuO3 layers (2 unit cells versus 9 unit cells thick) does not affect the magnetic properties of the 2 unit cells thick LSMO. This indicates that RKKY coupling across the conductive SrRuO3 and epitaxial strain or octahedral tilt mismatch are not dominant driving forces for the stabilization of ferromagnetic order in ultrathin LSMO . Stabilization of ferromagnetic behavior of LSMO in these superlattices may have to do with the Mn-O-Ru interfacial coupling. It is unlikely, however, that a TC stabilization in the LSMO layers around 280K is due to the antiferromagnetic nature of the Mn-O-Ru coupling, since the TC of SRO is just about half of the LSMO value. It is, therefore, more likely that a more general charge transfer mechanism between interfacial Ru and Mn ions is responsible for the effect. The metallic behavior of neighboring SrRuO3 layers may help, charge carriers transferred from the ruthenate layers may act as dopants to the LSMO layers and "pull" them back into a ferromagnetic phase.
In order to explore the role of Mn-O-Ru interfacial magnetic coupling, LSMO/CaRuO3 superlattices have been also fabricated by PLD on SrTiO3(100) and on (LaAlO3)0.3(SrAlTaO6)0.7 (100) crystals. Unlike SrRuO3 that is metallic and becomes ferromagnetic below 160K, bulk CaRuO3 is metallic and paramagnetic at all temperatures. First results on LSMO/CaRuO3 superlattices with nominally 2 unit cells thick LSMO layers indicate that ferromagnetism is preserved with a Curie temperature of about 220K (Fig. 1(c)). This may support our supposition that Mn-O-Ru interfacial bonds are relevant for stabilizing ferromagnetic order in 2 unit cells thick LSMO layers.
Superlattices in which 3 unit cells thick LSMO layers were sandwiched between Nb-doped SrTiO3 layers were fabricated. We are investigating whether interfacing ultrathin LSMO layers with semiconductive oxide layers, which may dope charge carriers into the LSMO, would help recovering their ferromagnetic properties. However, the magnetization measurements of such LSMO/Sr(Ti,Nb)O3 superlattices indicate that in this case 3 unit cells thick LSMO layers are not ferromagnetic (Fig. 1(d)). The charge-transfer mechanism active at LSMO/SrRuO3 interfaces appears not to work in the case of LSMO/Sr(Ti,Nb)O3 interfaces.
Structural characterization of manganite-based superlattices
The structural quality of functional oxide superlattices and the sharpness of their interfaces influence dramatically their physical properties. Scanning transmission electron microscopy (STEM) and EDX investigations were performed to confirm the structural quality of all our superlattices, the uniformity of the layers and look for intermixing at the interfaces. Figure 2 shows high-angle annular diffraction (HAADF) STEM images of two superlattices with nominally 2 unit cells thick LSMO layers and 9 unit cells thick (Fig. 2(a)) or 2 unit cells thick (Fig. 2(b)) SrRuO3 layers. The LSMO layers of these two superlattices are ferromagnetic with high Curie temperature (Fig. 1(a)). In Fig. 2(a) a defect of the SrTiO3 substrate is marked (see the yellow circle), around which only the first layers of the superlattice were structurally distorted. The more remote layers seemed to be, however, undisturbed by the substrate defect.
Figure 3 shows HAADF-STEM images of an LSMO/Sr(Ti,Nb)O3 superlattice with 3 unit cells thick LSMO layers and corresponding energy dispersive x-ray spectroscopy (EDX) line scans performed in the same cs-corrected scanning transmission electron microscope (FEI Titan 80-300). The LSMO/Sr(Ti,Nb)O3 interfaces are less sharp than the LSMO/SrRuO3 interfaces. The superlattices were all grown at a substrate temperature of 650°C. The temperature may be slightly too low for achieving stable layer-by-layer growth of Sr(Ti,Nb)O3 and thus explains the rougher interfaces. Nevertheless, EDX measurements proved that niobium was successfully incorporated in the strontium titanate layers, which is essential for robust semiconductive behavior in strontium titanate.