arXiv subjects

Sebastiaan van Dijken

Publications and source records attributed to Sebastiaan van Dijken.

51 records · Page 3Linked to original sources

arXiv2016

Temperature dependence of spin-orbit torques in W/CoFeB bilayers

We report on the temperature and layer thickness variation of spin-orbit torques in perpendicularly magnetized W/CoFeB bilayers. Harmonic Hall voltage measurements reveal dissimilar temperature evolutions of longitudinal and transverse effective magnetic field components. The transverse effective field changes sign at 250 K for a 2 nm thick W buffer layer, indicating a much stronger contribution from interface spin-orbit interactions compared to, for example, Ta. Transmission electron microscopy measurements reveal that considerable interface mixing between W and CoFeB is primarily responsible for this effect.

cond-mat.mes-hall↗

arXiv2015

Influence of elastically pinned magnetic domain walls on magnetization reversal in multiferroic heterostructures

In elastically coupled multiferroic heterostructures that exhibit full domain correlations between ferroelectric and ferromagnetic sub-systems, magnetic domain walls are firmly pinned on top of ferroelectric domain boundaries. In this work we investigate the influence of pinned magnetic domain walls on the magnetization reversal process in a Co40Fe40B20 wedge film that is coupled to a ferroelectric BaTiO3 substrate via interface strain transfer. We show that the magnetic field direction can be used to select between two distinct magnetization reversal mechanisms, namely (1) double switching events involving alternate stripe domains at a time or (2) synchronized switching of all domains. Furthermore, scaling of the switching fields with domain width and film thickness is also found to depend on field orientation. These results are explained by considering the dissimilar energies of the two types of pinned magnetic domain walls that are formed in the system.

cond-mat.mtrl-sci↗

arXiv2014

Reversible Electric-Field Driven Magnetic Domain Wall Motion

Control of magnetic domain wall motion by electric fields has recently attracted scientific attention because of its potential for magnetic logic and memory devices. Here, we report on a new driving mechanism that allows for magnetic domain wall motion in an applied electric field without the concurrent use of a magnetic field or spin-polarized electric current. The mechanism is based on elastic coupling between magnetic and ferroelectric domain walls in multiferroic heterostructures. Pure electric-field driven magnetic domain wall motion is demonstrated for epitaxial Fe films on BaTiO$_3$ with in-plane and out-of-plane polarized domains. In this system, magnetic domain wall motion is fully reversible and the velocity of the walls varies exponentially as a function of out-of-plane electric field strength.

cond-mat.mtrl-sci↗

arXiv2014

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 magnetoelectric coupling mechanisms. We directly observe, for the first time, in situ voltage driven O$^{2-}$ migration in a Co/metal-oxide bilayer, which we use to toggle the interfacial magnetic anisotropy energy by >0.6 erg/cm$^2$. We exploit the thermally-activated nature of ion migration to dramatically 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.

cond-mat.mtrl-sci↗

arXiv2014

Electric field driven magnetic domain wall motion in ferromagnetic-ferroelectric heterostructures

We investigate magnetic domain wall (MDW) dynamics induced by applied electric fields in ferromagnetic-ferroelectric thin-film heterostructures. In contrast to conventional driving mechanisms where MDW motion is induced directly by magnetic fields or electric currents, MDW motion arises here as a result of strong pinning of MDWs onto ferroelectric domain walls (FDWs) via local strain coupling. By performing extensive micromagnetic simulations, we find several dynamical regimes, including instabilities such as spin wave emission and complex transformations of the MDW structure. In all cases, the time-averaged MDW velocity equals that of the FDW, indicating the absence of Walker breakdown.

cond-mat.mtrl-sci↗

arXiv2014

Size Dependence of Domain Pattern Transfer in Multiferroic Heterostructures

Magnetoelectric coupling in multiferroic heterostructures can produce large lateral modulations of magnetic anisotropy enabling the imprinting of ferroelectric domains into ferromagnetic films. Exchange and magnetostatic interactions within ferromagnetic films oppose the formation of such domains. Using micromagnetic simulations and a 1-D model, we demonstrate that competing energies lead to the breakdown of domain pattern transfer below a critical domain width. Moreover, rotation of the magnetic field results in abrupt transitions between two scaling regimes with different magnetic anisotropy.

cond-mat.mtrl-sci↗

arXiv2013

Backhopping effect in magnetic tunnel junctions: comparison between theory and experiment

We report on the magnetic switching and backhopping effects due to spin-transfer-torque in magnetic tunnel junctions. Experimental data on the current-induced switching in junctions with MgO tunnel barrier reveal a random back-and-forth switching between the magnetization states, which appears when the current direction favors the parallel magnetic configuration. The effect depends on the barrier thickness $t_b$, and is not observed in tunnel junctions with very thin MgO tunnel barriers, $t_b$ $<$ 0.95 nm. Switching dependence on the bias voltage and barrier thickness is explained in terms of the macrospin model, with the magnetization dynamics described by the modified Landau-Lifshitz-Gilbert equation. Numerical simulations indicate that the competition between in-plane and out-of-plane torque components can result at high bias voltages in a non-deterministic switching behavior, in agreement with experimental observations. When the barrier thickness is reduced, the overall coupling between the magnetic layers across the barrier becomes ferromagnetic, which suppresses the backhopping effect.

cond-mat.mtrl-sci↗

arXiv2013

Temperature control of local magnetic anisotropy in multiferroic CoFe/BaTiO3

This paper reports on the temperature evolution of local elastic interactions between ferromagnetic CoFe films and ferroelectric BaTiO3 substrates. Polarization microscopy measurements indicate that growth-induced stripe domains in the CoFe films are preserved and strengthened during cooling and heating through the structural phase transitions of BaTiO3. Moreover, rotation of the magnetic easy axes at the tertragonal-to-orthorhombic transition (T = 278 K) and at T = 380 K simultaneously switches the local magnetization of both uniaxial domains by 90 degrees. Irreversible changes in the ferromagnetic domain pattern are induced when the room-temperature ferroelectric domain structure is altered after temperature cycling.

cond-mat.mtrl-sci↗

arXiv2013

Influence of MgO tunnel barrier thickness on spin-transfer ferromagnetic resonance and torque in magnetic tunnel junctions

Spin-transfer ferromagnetic resonance (ST-FMR) in symmetric magnetic tunnel junctions (MTJs) with a varied thickness of the MgO tunnel barrier (0.75 nm < $t_{MgO}$ < 1.05 nm) is studied using the spin-torque diode effect. The application of an RF current into nanosized MTJs generates a DC mixing voltage across the device when the frequency is in resonance with the resistance oscillations arising from the spin transfer torque. Magnetization precession in the free and reference layers of the MTJs is analyzed by comparing ST-FMR signals with macrospin and micromagnetic simulations. From ST-FMR spectra at different DC bias voltage, the in-plane and perpendicular torkances are derived. The experiments and free-electron model calculations show that the absolute torque values are independent of tunnel barrier thickness. The influence of coupling between the free and reference layer of the MTJs on the ST-FMR signals and the derived torkances are discussed.

cond-mat.mes-hall↗

arXiv2013

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.

cond-mat.mtrl-sci↗

arXiv2012

Magnetic field sensor with voltage-tunable sensing properties

We report on a magnetic field sensor based on CoFeB/MgO/CoFeB magnetic tunnel junctions. By taking advantage of the perpendicular magnetic anisotropy of the CoFeB/MgO interface, the magnetization of the sensing layer is tilted out-of-plane which results in a linear response to in-plane magnetic fields. The application of a bias voltage across the MgO tunnel barrier of the field sensor affects the magnetic anisotropy and thereby its sensing properties. An increase of the maximum sensitivity and simultaneous decrease of the magnetic field operating range by a factor of two is measured. Based on these results, we propose a voltage-tunable sensor design that allows for active control of the sensitivity and the operating filed range with the strength and polarity of the applied bias voltage.

cond-mat.mtrl-sci↗

arXiv2012

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%.

cond-mat.mtrl-sci↗

arXiv2011

Spin transfer torque oscillator based on asymmetric magnetic tunnel junctions

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^0.5 at 1.5 GHz.

cond-mat.mtrl-sci↗

arXiv2011

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 multiferroic 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 multiferroic spintronic devices.

cond-mat.mtrl-sci↗

arXiv2009

Influence of substrate bias on the structural and dielectrical properties of magnetron-sputtered BaxSr1-xTiO3 thin films

The application of a substrate bias during rf magnetron sputtering alters the crystalline structure, grain morphology, lattice strain and composition of BaxSr1-xTiO3 thin films. As a result, the dielectric properties of Pt/BaxSr1-xTiO3/Pt parallel-plate capacitors change significantly. With increasing substrate bias we observe a clear shift of the ferroelectric to paraelectric phase transition towards higher temperature, an increase of the dielectric permittivity and tunability at room temperature, and a deterioration of the dielectric loss. To a large extent these changes correlate to a gradual increase of the tensile in-plane film strain with substrate bias and an abrupt change in film composition.

cond-mat.mtrl-sci↗