Sebastiaan van Dijken

Magneto-ionic control of interfacial magnetism

In metal/oxide heterostructures, rich chemical, electronic, magnetic and mechanical properties can emerge from interfacial chemistry and structure. The possibility to dynamically control interface characteristics with an electric field paves the way towards voltage control of these properties in solid-state devices. Here, we show that electrical switching of the interfacial oxidation state allows for voltage control of magnetic properties to an extent never before achieved through conventional magneto-electric coupling mechanisms. We directly observe in situ voltage-driven O(2-) migration in a Co/metal-oxide bilayer, which we use to toggle the interfacial magnetic anisotropy energy by >0.75 erg cm(-2) at just 2 V. We exploit the thermally activated nature of ion migration to markedly increase the switching efficiency and to demonstrate reversible patterning of magnetic properties through local activation of ionic migration. These results suggest a path towards voltage-programmable materials based on solid-state switching of interface oxygen chemistry.

Electric-field control of magnetic domain wall motion and local magnetization reversal

Spintronic devices currently rely on magnetic switching or controlled motion of domain walls by an external magnetic field or spin-polarized current. Achieving the same degree of magnetic controllability using an electric field has potential advantages including enhanced functionality and low power consumption. Here we report on an approach to electrically control local magnetic properties, including the writing and erasure of regular ferromagnetic domain patterns and the motion of magnetic domain walls, in CoFe-BaTiO3 heterostructures. Our method is based on recurrent strain transfer from ferroelastic domains in ferroelectric media to continuous magnetostrictive films with negligible magnetocrystalline anisotropy. Optical polarization microscopy of both ferromagnetic and ferroelectric domain structures reveals that domain correlations and strong inter-ferroic domain wall pinning persist in an applied electric field. This leads to an unprecedented electric controllability over the ferromagnetic microstructure, an accomplishment that produces giant magnetoelectric coupling effects and opens the way to electric-field driven spintronics.

Pattern transfer and electric-field induced magnetic domain formation in multiferroic heterostructures

IO N The ability to tailor magnetic properties via coupling to ferroelectric domains is of great interest for the design of electric-fi eld-tunable magnetic devices. Imprinting of ferroelectric domains into continuous magnetic fi lms would enable local control over magnetization dynamics but requires interfacial coupling to overcome exchange and magnetostatic interactions within the ferromagnet. Here, we demonstrate full pattern transfer and electric-fi eld-induced magnetic domain formation in multiferroic heterostructures consisting of a ferroelectric substrate and a thin magnetic fi lm. Simultaneous imaging of ferroelectric and ferromagnetic domains and local magnetization reversal analysis using polarization microscopy reveals strong lateral modulations of magnetic hysteresis due to strain coupling to the underlying ferroelectric substrate. While the magnetic confi guration is an exact copy of the ferroelectric domain structure in the as-deposited state, new magnetic domains form when an external electric fi eld is applied. In fact, the electric-fi eld response of the magnetic fi lm is characterized by a superposition of two patterns, one that mirrors the current ferroelectric domain confi guration (electric-fi eld induced pattern) and another that refl ects the ferroelectric domain structure during deposition (growth-induced pattern). This ability to electrically write magnetic domains in continuous magnetic fi lms opens up new avenues for electric-fi eld control of magnetic functionalities and provides a framework for the exploration of ferroelectric, ferroelastic, and ferromagnetic domain interactions in multiferroic heterostructures. Domain formation and hysteresis in magnetic thin fi lms depend on internal material parameters (exchange stiffness, magnetization), fi lm thickness and shape (magnetostatics), and various other sources of magnetic anisotropy including crystal symmetry, lattice deformations (strain), and interfaces. [ 1 ] The interplay between these fi lm properties and an external magnetic fi eld usually results in a laterally uniform energy landscape with random dispersions due to defects and interface roughness. The magnetic hysteresis therefore depends little on sample position. Area-specifi c magnetic responses in continuous fi lms require a local modifi cation of the magnetic properties. This can be accomplished, for example, by the growth of ferromagnetic fi lms onto prepatterned substrates [ 2 ] or by local ion irradiation. [ 3–5 ] In these cases, the magnetic anisotropy remains fi xed after sample preparation. Interface strain

Ultrasensitive and label-free molecular level detection enabled by light phase control in magnetoplasmonic nanoantennas

Systems allowing label-free molecular detection are expected to have enormous impact on biochemical sciences. Research focuses on materials and technologies based on exploiting localized surface plasmon resonances in metallic nanostructures. The reason for this focused attention is their suitability for single molecule sensing, arising from intrinsically nanoscopic sensing volume, and the high sensitivity to the local environment. Here we propose an alternative route, which enables radically improved sensitivity compared torecently reported plasmon-based sensors. Such high sensitivity is achieved by exploiting the control of the phase of light in magnetoplasmonic nanoantennas. We demonstrate a manifold improvement of refractometric sensing figure-of-merit. Most remarkably, we show a raw surface sensitivity (i.e., without applying fitting procedures) of two orders of magnitude higher than the current values reported for nanoplasmonic sensors. Such sensitivity corresponds to a mass of ~0.8 ag per nanoantenna of polyamide-6.6 (n=1.51), which is representative for a large variety of polymers, peptides and proteins.

Pulsed laser deposition of La1-xSrxMnO3 : thin-film properties and spintronic applications

Materials engineering on the nanoscale by precise control of growth parameters can lead to many unusual and fascinating physical properties. The development of pulsed laser deposition (PLD) 25 years ago has enabled atomistic control of thin films and interfaces and as such it has contributed significantly to advances in fundamental material science. One application area is the research field of spintronics, which requires optimized nanomaterials for the generation and transport of spin-polarized carriers. The mixed-valence manganite La1−xSrxMnO3 (LSMO) is an interesting material for spintronics due to its intrinsic magnetoresistance properties, electric-field tunable metal–insulator transitions, and half-metallic band structure. Studies on LSMO thin-film growth by PLD show that the deposition temperature, oxygen pressure, laser fluence, strain due to substrate–film lattice mismatch and post-deposition annealing conditions significantly influence the magnetic and electrical transport properties of LSMO. For spintronic structures, robust ferromagnetic exchange interactions and metallic conductivity are desirable properties. In this paper, we review the physics of LSMO thin films and the important role that PLD played in advancing the field of LSMO-based spintronics. Some specific application areas including magnetic tunnel junctions, multiferroic tunnel junctions and organic spintronic devices are highlighted, and the advantages, drawbacks and opportunities of PLD-grown LSMO for next-generation spintronic devices are discussed.

Electron‐Beam‐Induced Perovskite–Brownmillerite–Perovskite Structural Phase Transitions in Epitaxial La2/3Sr1/3MnO3 Films

Structural phase transitions driven by oxygen-vacancy ordering can drastically affect the properties of transition metal oxides. The focused electron beam of a transmission electron microscope (TEM) can be used to control structural phase transitions in epitaxial La2/3Sr1/3MnO3. The ability to induce and characterize oxygen-deficient structural phases simultaneously in a continuous and controllable manner opens up new pathways for atomic-scale studies of transition metal oxides and other complex materials.

Alternating domains with uniaxial and biaxial magnetic anisotropy in epitaxial Fe films on BaTiO3

We report on domain formation and magnetization reversal in epitaxial Fe films on ferroelectric BaTiO3 substrates with ferroelastic a–c stripe domains. The Fe films exhibit biaxial magnetic anisotropy on top of c domains with out-of-plane polarization, whereas the in-plane lattice elongation of a domains induces uniaxial magnetoelastic anisotropy via inverse magnetostriction. The strong modulation of magnetic anisotropy symmetry results in full imprinting of the a–c domain pattern in the Fe films. Exchange and magnetostatic interactions between neighboring magnetic stripes further influence magnetization reversal and pattern formation within the a and c domains.

Field tuning of ferromagnetic domain walls on elastically coupled ferroelectric domain boundaries

We report on the evolution of ferromagnetic domain walls during magnetization reversal in elastically coupled ferromagnetic-ferroelectric heterostructures. Using optical polarization microscopy and micromagnetic simulations, we demonstrate that the spin rotation and width of ferromagnetic domain walls can be accurately controlled by the strength of the applied magnetic field if the ferromagnetic walls are pinned onto 90 degrees ferroelectric domain boundaries. Moreover, reversible switching between magnetically charged and uncharged domain walls is initiated by magnetic field rotation. Switching between both wall types reverses the wall chirality and abruptly changes the width of the ferromagnetic domain walls by up to 1000%.

Zero-Field Spin Torque Oscillator Based on Magnetic Tunnel Junctions with a Tilted CoFeB Free Layer

We present a study of the spin transfer torque oscillator based on CoFeB/MgO/CoFeB asymmetric magnetic tunnel junctions. We observe microwave precession in junctions with different thickness of the free magnetization layer. Taking advantage of the ferromagnetic interlayer exchange coupling between the free and reference layer in the MTJ and perpendicular interface anisotropy in thin CoFeB electrode we demonstrate the nanometer scale device that can generate high frequency signal without external magnetic field applied. The amplitude of the oscillation exceeds 10 nV/ √ Hz at 1.5 GHz.Microwave emission from spin torque oscillators based on CoFeB/MgO/CoFeB magnetic tunnel junctions is analyzed with respect to the thickness of the magnetically free electrode. Taking advantage of the ferromagnetic interlayer exchange coupling between the free and reference layers and the perpendicular interface anisotropy of thin CoFeB electrodes on MgO, we demonstrate that large-amplitude oscillations of the tilted CoFeB free layer can be generated in zero applied magnetic field.

Resistive Switching in All-Oxide Ferroelectric Tunnel Junctions with Ionic Interfaces

Universal, giant and nonvolatile resistive switching is demonstrated for oxide tunnel junctions with ferroelectric PbZr0.2 Ti0.8 O3 , ferroelectric BaTiO3, and paraelectric SrTiO3 tunnel barriers. The effects are caused by reversible migration of oxygen vacancies between the tunnel barrier and bottom La2/3 Sr1/3 MnO3 electrode. The switching process, which is driven by large electric fields, is efficient down to a temperature of 5 K.