Magnetic Domains and Surface Effects in Hollow Maghemite Nanoparticles

In the present work, we investigate the magnetic properties of ferrimagnetic and noninteracting maghemite (g-Fe2O3) hollow nanoparticles obtained by the Kirkendall effect. From the experimental characterization of their magnetic behavior, we find that polycrystalline hollow maghemite nanoparticles are characterized by low superparamagnetic-to-ferromagnetic transition temperatures, small magnetic moments, significant coercivities and irreversibility fields, and no magnetic saturation on external magnetic fields up to 5 T. These results are interpreted in terms of the microstructural parameters characterizing the maghemite shells by means of an atomistic Monte Carlo simulation of an individual spherical shell model. The model comprises strongly interacting crystallographic domains arranged in a spherical shell with random orientations and anisotropy axis. The Monte Carlo simulation allows discernment between the influence of the structure polycrystalline and its hollow geometry, while revealing the magnetic domain arrangement in the different temperature regimes.Comment: 26 pages, 8 figures. In press in Phys. Rev.

Strain Induced Self-Assembly in Complex Oxide Thin Films

Trabajo presentado a la Conferencia "BNC-b Research Meeting" celebrada en Barcelona el 14 de julio de 2011.We acknowledge financial support from Spanish MICINN (MAT2009-08024), CONSOLIDER (CSD2007-00041) and FEDER program. ZK thanks the Spanish MICINN for the financial support through the RyC program.Peer reviewe

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

Interfacialeffects in sputtered La2/3Sr1/3MnO3thin 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 surfacemagnetization.We acknowledge financial support from the Spanish MEC (MAT2009-08024 and MAT2008-04931/NAN), CONSOLIDER (CSD2007-00041) and FEDER program. The research leading to these results has received funding from the European Community’s Seventh Framework Program (FP7/2007-2013) under Grant No. 226716. Z.K. thanks the Spanish MEC for the financial support through the RyC program.Peer reviewe

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

Under the terms of the Creative Commons Attribution (CC BY) license to their work.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.We acknowledge financial support from the Spanish MEC (MAT2011-29081 and MAT2012-33207), CONSOLIDER (CSD2007-00041), and FEDER program. Z.K. thanks the Spanish MINECO for the financial support through the RyC program. We thank Helmholtz-Zentrum Berlin for the allocation of neutron/synchrotron radiation beamtime. The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement No. 312284.Peer Reviewe

Self-arranged misfit dislocation network formation upon strain release in La0.7Sr0.3MnO3/LaAlO3(100) epitaxial films under compressive strain

Santiso, José et al.Lattice-mismatched epitaxial films of LaSrMnO (LSMO) on LaAlO (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 T). 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.This research was funded by the Spanish MINECO (projects: MAT2011-29081-C02, MAT2012-33207 and MAT2013-47869-C4-1-P, Consolider-Ingenio CSD2008-00023) and the European Union Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 645658. We also acknowledge financial aid from the Generalitat de Catalunya (2014 SGR 501 and 2014 SGR 1216). J. S. thanks the support of Ministry of Education and Science through program “Salvador de Madariaga” for a grant (ref: PRX14/00297) to perform a stay at MIT. Z.K. is grateful for the support from the Ministry of Education, Science, and Technological Development of the Republic of Serbia through Project III45018. Q.L. and B.Y. thank the National Science Foundation for support through the MIT Center of Materials Science and Engineering MRSEC under grant no. DMR-1419807. This work made use of the Shared Experimental Facilities supported in part by the MRSEC Program of the National Science Foundation under award number DMR – 1419807. We thank Dr. Belén Ballesteros (ICN2, Barcelona) and Prof. Marie-Jo Casanova (CEMES, Toulouse) for their assistance with transmission electron microscopy experiments. We are grateful to J. Rubio-Zuazo and the ESRF for providing assistance in using beamline BM25. We also thank HZB for the allocation of synchrotron radiation beamtime at Bessy.Peer Reviewe

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 0.7Sr0.3MnO3 epitaxial thin films grown on SrTiO3 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 orderin

Formation of self-organized Mn3O4 nanoinclusions in LaMnO3 films

et al.We present a single-step route to generate ordered nanocomposite thin films of secondary phase inclusions (Mn3O4) in a pristine perovskite matrix (LaMnO3) by taking advantage of the complex phase diagram of manganese oxides. We observed that in samples grown under vacuum growth conditions from a single LaMnO3 stoichiometric target by Pulsed Laser Deposition, the most favorable mechanism to accommodate Mn2+ cations is the spontaneous segregation of self-assembled wedge-like Mn3O4 ferrimagnetic inclusions inside a LaMnO3 matrix that still preserves its orthorhombic structure and its antiferromagnetic bulk-like behavior. A detailed analysis on the formation of the self-assembled nanocomposite films evidences that Mn3O4 inclusions exhibit an epitaxial relationship with the surrounding matrix that it may be explained in terms of a distorted cubic spinel with slight (~9°) c-axis tilting. Furthermore, a Ruddlesden-Popper La2MnO4 phase, helping to the stoichiometry balance, has been identified close to the interface with the substrate. We show that ferrimagnetic Mn3O4 columns influence the magnetic and transport properties of the nanocomposite by increasing its coercive field and by creating local areas with enhanced conductivity in the vicinity of the inclusions.Financial support from the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Program for Centres of Excellence in R&D (SEV-2015-0496 and SEV 2013-0295), Projects MAT2011-29081 and MAT2015-71664-R and Ministry of Education and Science of Serbia (Grant—III45018) is acknowledged. This work has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 645658 (DAFNEOX Project). NB thanks the Spanish MINECO for financial support through the FPI program.Peer reviewedPeer Reviewe

Nonstoichiometry driven ferromagnetism in double perovskite La2Ni1-xMn1+ xO6 insulating thin films

This is an open access article published under an ACS AuthorChoice License. See Standard ACS AuthorChoice/Editors' Choice Usage AgreementIn this work we report on the epitaxial growth of LaNiMnO double perovskite thin films on top of (001) oriented SrTiO substrates by RF magnetron sputtering. The influence of oxygen pressure (P O) and growth temperature on the microstructure, stoichiometry of the films, and magnetic and transport properties is thoroughly investigated. It is found that high oxygen pressure promotes the growth of stoichiometric films, with a Ni/Mn ratio almost equal to 1. However, these films exhibit poor ferromagnetic properties with respect to the expected optimum values corresponding to ferromagnetic ordering mediated by superexchange interaction between Mn and Ni according to the Goodenough-Kanamori rules. Most interestingly, films grown at low P O exhibit Ni/Mn ratios below 1, but ferromagnetic properties close to the optimal ones. The valence balance between Ni and Mn ions in nonstoichiometric sample has been elucidated by X-ray absorption spectroscopy. The results indicate that Ni deficiency plays a crucial role in the puzzling insulating ferromagnetic behavior observed in nonstoichiometric samples

Engineering the microstructure and magnetism of La2CoMnO6-δ thin films by tailoring oxygen stoichiometry

Under the terms of the Creative Commons Attribution (CC BY) license to their work.We report on the magnetic and structural properties of ferromagnetic-insulating La2CoMnO6-δ thin films grown on top of (001) SrTiO3 substrates by means of RF sputtering technique. Careful structural analysis, by using synchrotron X-ray diffraction, allows identifying two different crystallographic orientations that are closely related to oxygen stoichiometry and to the features (coercive fields and remanence) of the hysteresis loops. Both Curie temperature and magnetic hysteresis turn out to be dependent on the oxygen stoichiometry. In situ annealing conditions allow tailoring the oxygen content of the films, therefore controlling their microstructure and magnetic properties.We acknowledge financial support from the Spanish MEC (MAT2011-29081 and MAT2012-33207), CONSOLIDER (CSD2007-00041), and FEDER program. R.G., L.L.-M. and N.B. thank the Spanish MINECO for the financial support through the FPI program. The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement No. 312284.Peer Reviewe

Interfacial effects on the tunneling magnetoresistance in L a0.7 S r0.3Mn O3/MgO/Fe tunneling junctions

Galceran, Regina et al.© 2015 American Physical Society. We report on magnetotransport properties on La0.7Sr0.3MnO3/MgO/Fe tunnel junctions grown epitaxially on top of (001)-oriented SrTiO3 substrates by sputtering. It is shown that the magnetoresistive response depends critically on the MgO/Fe interfacial properties. The appearance of an FeOX layer by the interface destroys the Δ1 symmetry filtering effect of the MgO/Fe system and only a small negative tunneling magnetoresistance (TMR) (∼-3%) is measured. However, in annealed samples a switchover from positive TMR (∼+25% at 70 K) to negative TMR (∼-1%) is observed around 120 K. This change is associated with the transition from semiconducting at high T to insulating at low T taking place at the Verwey transition (TV∼120K) in Fe3O4, thus suggesting the formation of a very thin slab of magnetite at the MgO/Fe interface during annealing treatments. These results highlight the relevance of interfacial properties on the tunneling conduction process and how it can be substantially modified through appropriate interface engineering.We acknowledge financial support from the Spanish MINECO through grants (MAT2012-33207, MAT2011-27470-C02, MAT2012-37638 and Consolider Ingenio 2010 - CSD2009-00013 (Imagine)), from CAM through Grant No. S2009/MAT-1756 (Phama) and Basque Government (PI2011-1). Financial support from EC through FEDER program and Marie Curie Actions (256470-ITAMOSCINOM) is also acknowledged. C.M.B. thanks the Spanish MINECO for the financial support through the RyC programPeer Reviewe