Zorica Konstantinovic

Competing misfit relaxation mechanisms in epitaxial correlated oxides

Strain engineering of functional properties in epitaxial thin films of strongly correlated oxides exhibiting octahedral-framework structures is hindered by the lack of adequate misfit relaxation models. Here we present unreported experimental evidence of a four-stage hierarchical development of octahedral-framework perturbations resulting from a progressive imbalance between electronic, elastic, and octahedral tilting energies in La(0.7)Sr(0.3)MnO(3) epitaxial thin films grown on SrTiO(3) substrates. Electronic softening of the Mn-O bonds near the substrate leads to the formation of an interfacial layer clamped to the substrate with strongly degraded magnetotransport properties, i.e., the so-called dead layer, while rigid octahedral tilts become relevant at advanced growth stages without significant effects on charge transport and magnetic ordering.

Strain‐Driven Self‐Assembled Network of Antidots in Complex Oxide Thin Films

Structural strain due to lattice mismatch is used to promote the formation of a self-assembled network of antidots in highly epitaxial La(2/3)Sr(1/3)MnO(3) thin films grown on (001) oriented SrTiO(3) substrates by radiofrequency magnetron sputtering. Size, depth, and separation between antidots can be controlled by changing deposition parameters and the miscut angle of the substrate. This morphology exhibits a remarkable magnetic anisotropy and offers unique opportunities for versatile nanostencils for the preparation of nano-object networks that can be of major relevance for the fabrication of oxide-based magnetic and magnetoelectronic devices.

Intrinsic antiferromagnetic/insulating phase at manganite surfaces and interfaces.

In this work we investigate interfacial effects in bilayer systems integrated by La(2/3)Sr(1/3)MnO(3) (LSMO) thin films and different capping layers by means of surface-sensitive synchrotron radiation techniques and transport measurements. Our data reveal a complex scenario with a capping-dependent variation of the Mn oxidation state by the interface. However, irrespective of the capping material, an antiferromagnetic/insulating phase is also detected at the interface, which is likely to originate from a preferential occupancy of Mn 3d 3z(2)-r(2) e(g) orbitals. This phase, which extends approximately to two unit cells, is also observed in uncapped LSMO reference samples, thus pointing to an intrinsic interfacial phase separation phenomenon, probably promoted by the structural disruption and inversion symmetry breaking at the LSMO free surface/interface. These experimental observations strongly suggest that the structural disruption, with its intrinsic inversion symmetry breaking at the LSMO interfaces, plays a major role in the observed depressed magnetotransport properties in manganite-based magnetic tunneling junctions and explains the origin of the so-called dead layer.

Self-Arranged Misfit Dislocation Network Formation upon Strain Release in La0.7Sr0.3MnO3/LaAlO3(100) Epitaxial Films under Compressive Strain.

Lattice-mismatched epitaxial films of La0.7Sr0.3MnO3 (LSMO) on LaAlO3 (001) substrates develop a crossed pattern of misfit dislocations above a critical thickness of 2.5 nm. Upon film thickness increases, the dislocation density progressively increases, and the dislocation spacing distribution becomes narrower. At a film thickness of 7.0 nm, the misfit dislocation density is close to the saturation for full relaxation. The misfit dislocation arrangement produces a 2D lateral periodic structure modulation (Λ ≈ 16 nm) alternating two differentiated phases: one phase fully coherent with the substrate and a fully relaxed phase. This modulation is confined to the interface region between film and substrate. This phase separation is clearly identified by X-ray diffraction and further proven in the macroscopic resistivity measurements as a combination of two transition temperatures (with low and high Tc). Films thicker than 7.0 nm show progressive relaxation, and their macroscopic resistivity becomes similar than that of the bulk material. Therefore, this study identifies the growth conditions and thickness ranges that facilitate the formation of laterally modulated nanocomposites with functional properties notably different from those of fully coherent or fully relaxed material.

Growth kinetics engineered magnetoresistance response in La2/3Sr1/3MnO3 thin films

A route to engineer the intrinsic colossal magnetoresistance (CMR) response in manganite thin films through an accurate control of the growth kinetics is presented. It is shown that under specific growth conditions, a particular strained state, substantially different from that of bulk-like materials and standard films, can be quenched up to film thicknesses around 60 nm. This strained state exhibits the same structural fingerprints of the interfacial dead layer in standard films and promotes surface morphology instabilities, which end up with the formation of self-organized nanopits array. At the same time, it has profound effects on the intrinsic magnetoelectronic properties of the films that exhibit an enhanced intrinsic CMR response.

Electroresistance and Joule heating effects in manganite thin films

Electroresistance phenomena have been investigated in La2/3Sr1/3MnO3 (LSMO) microbridges through the analysis of I-V characteristic curves. I-V curves are nonlinear in all the range of temperatures explored, but are almost reversible and exhibit origin symmetry which suggest the existence of Joule self-heating effects. However, comparing the resistance determined from the I-V curves (R = V/I) at the actual sample temperature, measured through a Pt thermometer microfabricated on top of the LSMO microbridges, with the resistance measured in thermal equilibrium, i.e., in the low current regime, it is evident that Joule self-heating effects alone cannot account for the experimental results. We demonstrate that oxygen depletion is the cause for the observed increase of resistance and the reduction of TC.

Kinetic versus strain formation of self-organized nanoholes in manganite thin films

We report on the formation of self-organized rows of pits in highly epitaxial La(2/3)Sr(1/3)MnO(3) thin films on top of substrates having different structural misfits by rf magnetron sputtering. The best-defined pits form in coherently grown films at a low misfit irrespective of its nature (tensile or compressive stress). It is also found that the pit rows align along the step edges, which indicates in-phase growth instability with the step edges, irrespective of the misfit. However, out-of-phase pit rows are also found when the terrace width increases due to a decrease of the miscut angle. Pits volume scales inversely with the lattice mismatch suggesting that structural strain alone does not favor the formation of pits. The formation of pits is analyzed within a thermodynamic model.

Tunneling anisotropic magnetoresistance in La2/3Sr1/3MnO3/LaAlO3/Pt tunnel junctions

The magnetotransport properties of La2/3Sr1/3MnO3(LSMO)/ LaAlO3(LAO)/Pt tunneling junctions have been analyzed as a function of temperature and magnetic field. The junctions exhibit magnetoresistance (MR) values of about 37%, at H=90 kOe at low temperature. However, the temperature dependence of MR indicates a clear distinct origin than that of conventional colossal MR. In addition, tunneling anisotropic MR (TAMR) values around 4% are found at low temperature and its angular dependence reflects the expected uniaxial anisotropy. The use of TAMR response could be an alternative of much easier technological implementation than conventional MTJs since only one magnetic electrode is required, thus opening the door to the implementation of more versatile devices. However, further studies are required in order to improve the strong temperature dependence at the present stage.

Macroscopic evidence of nanoscale resistive switching in La2/3Sr1/3MnO3 micro-fabricated bridges

In this work we report on a combined macro, micro and nanoscale investigation where electronic transport properties through La⅔Sr⅓MnO3 (LSMO) microfabricated bridges, in which nano-sized resistive states are induced by using a conducting scanning probe microscope (C-SPM), are analyzed. The strategy intentionally avoids the standard capacitor-like geometry, thus allowing the study of the electronic transport properties of the locally modified region, and approaches the integration of functional oxides in low dimensional devices while providing macroscopic evidence of nanoscale resistive switching (RS). The metallic and ferromagnetic LSMO is locally modified from its low resistance state (LRS) to a high resistance state (HRS) when a bias voltage is applied on its surface through the conducting tip, which acts as a mobile electrode. Starting from a metallic oxide the electroforming process is not required, thus avoiding one of the major drawbacks for the implementation of memory devices based on RS phenomena. The application of a bias voltage generates an electric field that promotes charge depletion, leading to a strong increase of the resistance, i.e. to the HRS. This effect is not only confined to the outermost surface layer, its spatial extension and final HRS condition can be modulated by the magnitude and duration of the potential applied, opening the door to the implementation of multilevel devices. In addition, the half-metallic character, i.e. total spin polarization, of LSMO might allow the implementation of memory elements and active spintronic devices in the very same material. The stability of the HRS and LRS as a function of temperature, magnetic field and compliance current is also analyzed, allowing the characterization of the nature of the switching process and the active material.

Interfacial effects in La2/3Sr1/3MnO3 thin films with different complex oxide capping layers

Interfacial effects in sputtered La2/3Sr1/3MnO3 thin films with different capping layers (MgO, LaAlO3, SrTiO3, NdGaO3, and Au) have been locally investigated by means of x-ray absorption spectroscopy and x-ray magnetic circular dichroism at the Mn L3,2-edge. Data were acquired by using the total electron yield detection mode thus guaranteeing maximum sensitivity to the interface. The data show that LaAlO3 capping almost does not modify the bulklike Mn valence at the interface. In case of SrTiO3 and Au, the presence of divalent Mn is detected, whereas MgO and NdGaO3 capping lead to an increase of the Mn valence oxidation state. The modification of the nominal Mn valence state leads to depressed surface magnetization.