Sergio Valencia

Helmholtz-Zentrum Berlin | HZB · Department Materials for green Spintronics

Multiferroic materials possess two or more ferroic orders but have not been exploited in devices owing to the scarcity of room-temperature examples. Those that are ferromagnetic and ferroelectric have potential applications in multi-state data storage if the ferroic orders switch independently, or in electric-field controlled spintronics if the mag...

In this work we investigate interfacial effects in bilayer systems integrated by La2/3Sr1/3MnO3 (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. Ho...

A large manifold of nontrivial spin textures, including the stabilization of monopole‐like fields, are generated by using a completely new and versatile approach based on the combination of superconductivity and magnetism. Robust, stable, and easily controllable complex spin structures are encoded, modified, and annihilated in a continuous magnetic...

Spin-orbit coupling in magnetic systems lacking inversion symmetry can give rise to nontrivial spin textures. Magnetic thin films and heterostructures are potential candidates for the formation of skyrmions and other noncollinear spin configurations as inversion symmetry is inherently lost at their surfaces and interfaces. However, manganites, in s...

Magnetic monolayers show great promise for future applications in nanoelectronics, data storage, and sensing. The research in magnetic two‐dimensional (2D) materials focuses on synthetic iodides and tellurides, which suffer from a lack of ambient stability. So far, naturally occurring layered magnetic materials have been overlooked. These minerals...

The generation, manipulation and sensing of magnetic domain walls is a cornerstone in the design of more efficient spintronic devices. Half-metals are amenable for this purpose as large low field magnetoresistance signals can be expected from spin accumulation at spin textures. However, half metals are scarce and in many case present chemical stabi...

Magnetotactic bacteria are envisaged as potential theranostic agents. Their internal magnetic compass, chemical environment specificity and natural motility enable these microorganisms to behave as nanorobots, as they can be tracked and guided towards specific regions in the body and activated to generate a therapeutic response. Here we provide add...

Magnetic monolayers show great promise for the future applications in nanoelectronics, data storage, and sensing. The research in magnetic two-dimensional materials focuses on synthetic iodides and tellurides, which suffer from the lack of ambient stability. So far, naturally occurring layered magnetic materials are overlooked. These minerals offer...

Magnetotactic bacteria Magnetospirillum magneticum AMB-1 have been cultured using three different media: magnetic spirillum growth medium with Wolfe's mineral solution (MSGM + W), magnetic spirillum growth medium without Wolfe's mineral solution (MSGM - W), and flask standard medium (FSM). The influence of the culture medium on the structural, morp...

We demonstrate how shape-dependent strain can be used to control antiferromagnetic order in NiO/Pt thin films. For rectangular elements patterned along the easy and hard magnetocrystalline anisotropy axes of our film, we observe different domain structures and we identify magnetoelastic interactions that are distinct for different domain configurat...

Epitaxial strain is a useful handle to engineer the physical properties of perovskite oxide materials. Here, we apply it to orthorhombic chromites that are a family of antiferromagnets showing fruitful functionalities as well as strong spin–lattice coupling via antisymmetric exchange interaction along Cr–O–Cr bonds. Using pulsed laser deposition, w...

We demonstrate how shape-induced strain can be used to control antiferromagnetic order in NiO/Pt thin films. For rectangular elements patterned along the easy and hard magnetocrystalline anisotropy axes of our film, we observe different domain structures and we identify magnetoelastic interactions that are distinct for different domain configuratio...

In prototype ferromagnet-antiferromagnet interfaces we demonstrate that surface acoustic waves can be used to identify complex magnetic phases arising upon evolution of exchange springs in an applied field. Applying sub-GHz surface acoustic waves to study the domain structure of the ferromagnetic layer in exchange-biased bilayers of Ir20Mn80−Co60Fe...

The Josephson effect results from the coupling of two superconductors across a spacer such as an insulator, a normal metal or a ferromagnet to yield a phase coherent quantum state. However, in junctions with ferromagnetic spacers, very long-range Josephson effects have remained elusive. Here we demonstrate extremely long-range (micrometric) high-te...

Research on proximity effects in superconductor/ferromagnetic hybrids has most often focused on how superconducting properties are affected—and can be controlled—by the effects of the ferromagnet’s exchange or magnetic fringe fields. The opposite, namely the possibility to craft, tailor and stabilize the magnetic texture in a ferromagnet by exploit...

Rashba interfaces yield efficient spin-charge interconversion and give rise to nonreciprocal transport phenomena. Here, we report magnetotransport experiments in few-nanometer-thick films of PdCoO$_2$, a delafossite oxide known to display a large Rashba splitting and surface ferromagnetism. By analyzing the angle dependence of the first- and second...

The formation of the crystalline organic polymer (OP) islands on top of double perovskite ferromagnetic La2Ni1–xMn1+xO6 (x=0.4) thin films by RF sputtering is reported. The presence of randomly ramified organic polymer islands for film grown on top of SrTiO3 and LaAlO3 substrates suggest the fractal growth. The stability of fractal OP islands is te...

Electric field-induced strain engineering of the magnetic anisotropy offers a highly attractive perspective for designing future generations of energy-efficient information technologies. In this work, we show using x-ray magnetic imaging and magneto-optic Kerr effect that the applicability of this approach is limited to systems with comparably low...

Understanding the electrical manipulation of the antiferromagnetic order is a crucial aspect to enable the design of antiferromagnetic devices working at THz frequencies. Focusing on collinear insulating antiferromagnetic NiO/Pt thin films as a materials platform, we identify the crystallographic orientation of the domains that can be switched by c...

Magnetotactic bacteria Magnetospirillum gryphiswaldense synthesize cubo-octahedral shaped magnetite nanoparticles, called magnetosomes, with a mean diameter of 40 nm. The high quality of the biosynthesized nanoparticles makes them suitable for numerous applications in fields like cancer therapy, among others. The magnetic properties of magnetite ma...

The dynamic of the magnetic structure in a well ordered ferromagnetic CoPd stripe domain pattern has been investigated upon excitation by femtosecond infrared laser pulses. Time-resolved X-ray magnetic circular dichroism in photoemission electron microscopy (TR-XMCD-PEEM) is used to perform real space magnetic imaging with 100 ps time resolution in...

The dynamic of the magnetic structure in a well ordered ferromagnetic CoPd stripe domain pattern has been investigated upon excitation by femtosecond infrared laser pulses. Time-resolved X-ray magnetic circular dichroism in photoemission electron microscopy (TR-XMCD-PEEM) is used to perform real space magnetic imaging with 100 ps time resolution in...

The Dzyaloshinskii-Moriya interaction gives rise to a chiral exchange between neighboring spins in the technologically relevant class of perpendicularly magnetized ultrathin film materials. In this paper, we study the temperature dependence of the Dzyaloshinskii-Moriya interaction based on extensive characterization of a thin film which hosts a sky...

While spintronics has traditionally relied on ferromagnetic metals as spin generators and detectors, spin–orbitronics exploits the efficient spin–charge interconversion enabled by spin–orbit coupling in non-magnetic systems. Although the Rashba picture of split parabolic bands is often used to interpret such experiments, it fails to explain the lar...

We probe the current-induced magnetic switching of insulating antiferromagnet–heavy-metal systems, by electrical spin Hall magnetoresistance measurements and direct imaging, identifying a reversal occurring by domain wall (DW) motion. We observe switching of more than one-third of the antiferromagnetic domains by the application of current pulses....

Competition between coexisting electronic phases in first-order phase transitions can lead to a sharp change in the resistivity as the material is subjected to small variations in the driving parameter, for example, the temperature. One example of this phenomenon is the metal–insulator transition (MIT) in perovskite rare-earth nickelates. In such s...

Strong electronic correlations can produce remarkable phenomena such as metal-insulator transitions and greatly enhance superconductivity, thermoelectricity, or optical non-linearity. In correlated systems, spatially varying charge textures also amplify magnetoelectric effects or electroresistance in mesostructures. However, how spatially varying s...

Mechanical control of electrical properties in complex heterostructures, consisting of magnetic $\mathrm{FeO_x}$ nanoparticles on top of manganite films, is achieved using the atomic force microscope (AFM) based methods. Under applied pressure by AFM tip, the drop of the electrical conductivity is observed inducing an electrically insulating state...

Recent results in electric-field control of magnetism have paved the way for the design of alternative magnetic and spintronic devices with enhanced functionalities and low power consumption. Among the diversity of reported magnetoelectric effects, the possibility of switching on and off long-range ferromagnetic ordering close to room temperature s...

In various mineralizing marine organisms, calcite or aragonite crystals form through the initial deposition of amorphous calcium carbonate (ACC) phases with different hydration levels. Using X-ray PhotoEmission Electron spectroMicroscopy (X-PEEM), ACCs with varied spectroscopic signatures were previously identified. In particular, ACC type I and II...

The electronic reconstruction occurring at oxide interfaces may be the source of interesting device concepts for future oxide electronics. Among oxide devices, multiferroic tunnel junctions are being actively investigated as they offer the possibility to modulate the junction current by independently controlling the switching of the magnetization o...

Strong electronic correlations can produce remarkable phenomena such as metal–insulator transitions and greatly enhance superconductivity, thermoelectricity or optical nonlinearity. In correlated systems, spatially varying charge textures also amplify magnetoelectric effects or electroresistance in mesostructures. However, how spatially varying spi...

In the version of this Letter originally published, Hiroshi Kohno’s affiliation was incorrectly listed as Department of Earth and Space Science, Graduate School of Science, Osaka University, Osaka, Japan; it should have been Department of Physics, Nagoya University, Nagoya, Japan. This has been corrected in all versions of the Letter.

Spin-orbit coupling in magnetic systems lacking inversion symmetry can give rise to non trivial spin textures. Magnetic thin films and heterostructures are potential candidates for the formation of skyrmions and other non-collinear spin configurations as inversion symmetry is inherently lost at their surfaces and interfaces. However, manganites, in...

When ferromagnetic films become ultrathin, key properties such as the Curie temperature and the saturation magnetization are usually depressed. This effect is thoroughly investigated in magnetic oxides such as half‐metallic manganites, but much less in ferrimagnetic insulating perovskites such as rare‐earth titanates RTiO3, despite their appeal to...

Manipulation of magnetism using laser light is considered as a key to the advancement of data storage technologies. Until now, most approaches seek to optically switch the direction of magnetization rather than to reversibly manipulate the ferromagnetism itself. Here, we use ∼100 fs laser pulses to reversibly switch ferromagnetic ordering on and of...

Perovskite rare-earth nickelates RNiO3 are prototype correlated oxides displaying a metal–insulator transition (MIT) at a temperature tunable by the ionic radius of the rare-earth R. Although its precise origin remains a debated topic, the MIT can be exploited in various types of applications, notably for resistive switching and neuromorphic comput...

Magnetotactic bacteria synthesize a chain of magnetic nanoparticles, called magnetosome chain, used to align and swim along the geomagnetic field lines. In particular, Magnetospirillum gryphiswaldense biomineralize magnetite, Fe3O4. Growing this species in a Co-supplemented medium Co-doped magnetite is obtained, tailoring in this way the magnetic p...

Magnetotactic bacteria synthesize a chain of magnetic nanoparticles, called magnetosome chain, used to align and swim along the geomagnetic field lines. In particular, Magnetospirillum gryphiswaldense biomineralize magnetite, Fe 3 O 4. Growing this species in a Co-supplemented medium, Co-doped magnetite is obtained, tailoring in this way the magnet...

Magnetospirillum gryphiswaldense is a microorganism with the ability to biomineralize magnetite nanoparticles, called magnetosomes, and arrange them into a chain that behaves like a magnetic compass. Rather than straight lines, magnetosome chains are slightly bent, as evidenced by electron cryotomography. Our experimental and theoretical results su...

The Mg and Sr content of ostracod valves have been used to reconstruct past temperature and salinity, and their stable isotopes have been used to reveal aspects of marine, lake and estuary hydrology. However, significant uncertainties surround ostracod calcification processes, the incorporation mechanisms of trace elements, and the sensitivity of p...

Optical control of magnetization using femtosecond laser without applying any external magnetic field offers the advantage of switching magnetic states at ultrashort time scales. Recently, all-optical helicity-dependent switching (AO-HDS) has drawn a significant attention for potential information and data storage device applications. In this work,...

Implementation of antiferromagnetic compounds as active elements in spintronics has been hindered by their insensitive nature against external perturbations which causes difficulties in switching among different antiferromagnetic spin configurations. Electrically-controllable strain gradient can become a key parameter to tune the antiferromagnetic...

The antiferromagnetic to ferromagnetic phase transition in FeRh can be induced globally by either heating the material above its phase transition temperature or applying a combination of external stimuli (such as mechanical strain, electric/magnetic fields) on the material preheated close to its transition temperature. On the other hand, to locally...

The Mg and Sr content of ostracod valves have been used to reconstruct past temperature and salinity, and their stable isotopes have been used to reveal aspects of marine, lake and estuary hydrology. However, significant uncertainties surround the ostracod calcification processes, the incorporation mechanisms of trace elements, and the sensitivity...

In complex oxide heterostructures, the interplay between magnetic, electric and transport properties often results in novel functionalities [1‐4]. The introduction of controlled densities of oxygen vacancies may allow further tuning of the interface magnetic structure, providing a new path towards enhanced device functionalities. In this work, we h...

The UE49-PGMa beamline hosts a photoemission electron microscope (PEEM) dedicated to spectromicroscopy and element-selective magnetic imaging on the nanometer scale. The instrument is an Elmitec PEEM III equipped with energy filter and Helium cooled manipulator. Laser driven excitations can be studied using an attached Ti:Sa laser. A variety of cus...

We present a resonant x-ray diffraction study of the magnetic order in NdFe3(BO3)4 and its coupling to applied electric fields. Our high-resolution measurements reveal two different coexisting magnetic phases, which can directly be controlled and manipulated by external electric fields. More specifically, the volume fraction of the collinear magnet...

We present a microscopic investigation of how the magnetic domain structure in ultrathin films changes after direct excitation by single ultrashort laser pulses. Using photoelectron emission microscopy in combination with x-ray magnetic circular dichroism in the resonant absorption of soft x rays, we find that individual laser pulses of ≈60fs durat...

The spontaneous formation of double chemical terminated surfaces of La0.7Sr0.3MnO3 due to deviations from ideal epitaxial growth during its deposition on SrTiO3(001) is presented. The development of surface regions with differ-entiated topmost composition leads to outstanding surface nano-structuration presenting notably distinct local properties,...

Increasing the magnetic data recording density requires reducing the size of the individual memory elements of a recording layer as well as employing magnetic materials with temperature-dependent functionalities. Therefore, it is predicted that the near future of magnetic data storage technology involves a combination of energy-assisted recording o...

Low-field magnetotransport measurements of topological insulators such as Bi2Se3 are important for revealing the nature of topological surface states by quantum corrections to the conductivity, such as weak-antilocalization. Recently, a rich variety of high-field magnetotransport properties in the regime of high electron densities (∼1019 cm−3) were...

Magnetic doping is expected to open a band gap at the Dirac point of topological insulators by breaking time-reversal symmetry and to enable novel topological phases. Epitaxial (Bi1-xMnx)2Se3 is a prototypical magnetic topological insulator with a pronounced surface band gap of 1/4100 meV. We show that this gap is neither due to ferromagnetic order...

At interfaces between conventional materials, band bending and alignment are classically controlled by differences in electrochemical potential. Applying this concept to oxides in which interfaces can be polar and cations may adopt a mixed valence has led to the discovery of novel two-dimensional states between simple band insulators such as LaAlO3...