arXiv subjects

Bert Koopmans

Publications and source records attributed to Bert Koopmans.

71 records · Page 4Linked to original sources

arXiv2018

Anomalous direction for skyrmion bubble motion

Magnetic skyrmions are localized topological excitations that behave as particles and can be mobile, with great potential for novel data storage devices. In this work, the current-induced dynamics of large skyrmion bubbles is studied. When skyrmion motion in the direction opposite to the electron flow is observed, this is usually interpreted as a perpendicular spin current generated by the spin Hall effect exerting a torque on the chiral Néel skyrmion. By designing samples in which the direction of the net generated spin current can be carefully controlled, we surprisingly show that skyrmion motion is always against the electron flow, irrespective of the net vertical spin-current direction. We find that a negative bulk spin-transfer torque is the most plausible explanation for the observed results, which is qualitatively justified by a simple model that captures the essential behaviour. These findings demonstrate that claims about the skyrmion chirality based on their current-induced motion should be taken with great caution.

cond-mat.mtrl-sci↗

arXiv2018

Tuning the Dzyaloshinskii-Moriya Interaction in Pt/Co/MgO heterostructures through MgO thickness

The interfacial Dzyaloshinskii-Moriya interaction (DMI) in the ferromagnetic/heavy metal ultra-thin film structures , has attracted a lot of attention thanks to its capability to stabilize Neel-type domain walls (DWs) and magnetic skyrmions for the realization of non-volatile memory and logic devices. In this study, we demonstrate that magnetic properties in perpendicularly magnetized Ta/Pt/Co/MgO/Pt heterostructures, such as magnetization and DMI, can be significantly influenced through both the MgO and the Co ultrathin film thickness. By using a field-driven creep regime domain expansion technique, we find that non-monotonic tendencies of DMI field appear when changing the thickness of MgO and the MgO thickness corresponding to the largest DMI field varies as a function of the Co thicknesses. We interpret this efficient control of DMI as subtle changes of both Pt/Co and Co/MgO interfaces, which provide a method to investigate ultra-thin structures design to achieve skyrmion electronics.

physics.app-ph↗

arXiv2017

Thickness dependence of unidirectional spin-Hall magnetoresistance in metallic bilayers

A nonlinear magnetoresistance - called unidirectional spin-Hall magnetoresistance - is recently experimentally discovered in metallic bilayers consisting of a heavy metal and a ferromagnetic metal. To study the fundamental mechanism of the USMR, both ferromagnetic and heavy metallic layer thickness dependence of the USMR are presented in a Pt/Co/AlOx trilayer at room temperature. To avoid ambiguities, second harmonic Hall measurements are used for separating spin-Hall and thermal contributions to the non-linear magnetoresistance. The experimental results are fitted by using a drift-diffusion theory, with parameters extracted from an analysis of longitudinal resistivity of the Co layer within the framework of the Fuchs-Sondheimer model. A good agreement with the theory is found, demonstrating that the USMR is governed by both the spin-Hall effect in the heavy metallic layer and the metallic diffusion process in the ferromagnetic layer.

cond-mat.mtrl-sci↗

arXiv2017

Scanning electron microscopy with polarization analysis for multilayered chiral spin textures

We show that scanning electron microscopy with polarization analysis (SEMPA) that is sensitive to both in-plane magnetization components can be used to image the out-of-plane magnetized multi-domain state in multilayered chiral spin textures. By depositing a thin layer of Fe on top of the multilayer we image the underlying out-of-plane domain state through the mapping of its stray fields in the Fe. We also demonstrate that SEMPA can be used to image the domain wall chirality in these systems after milling away the capping layer and imaging the topmost magnetic layer directly.

cond-mat.mes-hall↗

arXiv2017

Interface-Induced Phenomena in Magnetism

This article reviews static and dynamic interfacial effects in magnetism, focusing on interfacially-driven magnetic effects and phenomena associated with spin-orbit coupling and intrinsic symmetry breaking at interfaces. It provides a historical background and literature survey, but focuses on recent progress, identifying the most exciting new scientific results and pointing to promising future research directions. It starts with an introduction and overview of how basic magnetic properties are affected by interfaces, then turns to a discussion of charge and spin transport through and near interfaces and how these can be used to control the properties of the magnetic layer. Important concepts include spin accumulation, spin currents, spin transfer torque, and spin pumping. An overview is provided to the current state of knowledge and existing review literature on interfacial effects such as exchange bias, exchange spring magnets, spin Hall effect, oxide heterostructures, and topological insulators. The article highlights recent discoveries of interface-induced magnetism and non-collinear spin textures, non-linear dynamics including spin torque transfer and magnetization reversal induced by interfaces, and interfacial effects in ultrafast magnetization processes.

cond-mat.mtrl-sci↗

arXiv2017

Absorption and generation of femtosecond laser-pulse excited spin currents in non-collinear magnetic bilayers

Spin currents can be generated on an ultrafast timescale by excitation of a ferromagnetic (FM) thin film with a femtosecond laser-pulse. Recently, it has been demonstrated that these ultrafast spin currents can transport angular momentum to neighbouring FM layers, being able to change both the magnitude and orientation of the magnetization in the adjacent layer. In this work, both the generation and absorption of these optically excited spin currents are investigated. This is done using non-collinear magnetic bilayers, i.e. two FM layers separated by a conductive spacer. Spin currents are generated in a Co/Ni multilayer with out-of-plane (OOP) anisotropy, and absorbed by a Co layer with an in-plane (IP) anisotropy. This behaviour is confirmed by careful analysis of the laser-pulse induced magnetization dynamics, whereafter it is demonstrated that the transverse spin current is absorbed very locally near the injection interface of the IP layer (90% within the first approx. 2 nm). Moreover, it will also be shown that this local absorption results in the excitation of THz standing spin waves within the IP layer. The dispersion measured for these high frequency spin waves shows a discrepancy with respect to the theoretical predictions, for which a first explanation involving intermixed interface regions is proposed. Lastly, the spin current generation is investigated using different number of repeats for the Co/Ni multilayer, which proves to be of great relevance for identifying the optical spin current generation mechanism.

cond-mat.mes-hall↗

arXiv2017

Chiral magnetoresistance in Pt/Co/Pt zigzag wires

The Rashba effect leads to a chiral precession of the spins of moving electrons while the Dzyaloshinskii-Moriya interaction (DMI) generates preference towards a chiral profile of local spins. We predict that the exchange interaction between these two spin systems results in a 'chiral' magnetoresistance depending on the chirality of the local spin texture. We observe this magnetoresistance by measuring the domain wall (DW) resistance in a uniquely designed Pt/Co/Pt zigzag wire, and by changing the chirality of the DW with applying an in-plane magnetic field. A chirality-dependent DW resistance is found, and a quantitative analysis shows a good agreement with a theory based on the Rashba model. Moreover, the DW resistance measurement allows us to independently determine the strength of the Rashba effect and the DMI simultaneously, and the result implies a possible correlation between the Rashba effect, the DMI, and the symmetric Heisenberg exchange.

cond-mat.mes-hall↗

arXiv2017

Magnetic properties in ultra-thin 3d transition metal alloys II: Experimental verification of quantitative theories of damping and spin-pumping

A systematic experimental study of Gilbert damping is performed via ferromagnetic resonance for the disordered crystalline binary 3d transition metal alloys Ni-Co, Ni-Fe and Co-Fe over the full range of alloy compositions. After accounting for inhomogeneous linewidth broadening, the damping shows clear evidence of both interfacial damping enhancement (by spin pumping) and radiative damping. We quantify these two extrinsic contributions and thereby determine the intrinsic damping. The comparison of the intrinsic damping to multiple theoretical calculations yields good qualitative and quantitative agreement in most cases. Furthermore, the values of the damping obtained in this study are in good agreement with a wide range of published experimental and theoretical values. Additionally, we find a compositional dependence of the spin mixing conductance.

cond-mat.mtrl-sci↗

arXiv2017

Magnetic properties of ultra-thin 3d transition-metal binary alloys I: spin and orbital moments, anisotropy, and confirmation of Slater-Pauling behavior

The structure and static magnetic properties - saturation magnetization, perpendicular anisotropy, spectroscopic g-factor, and orbital magnetization - of thin-film 3d transition metal alloys are determined over the full range of alloy compositions via X-ray diffraction, magnetometry, and ferromagnetic resonance measurements. We determine the interfacial perpendicular magnetic anisotropy by use of samples sets with varying thickness for specific alloy concentrations. The results agree with prior published data and theoretical predictions. They provide a comprehensive compilation of the magnetic properties of thin-film Ni-Co, Ni-Fe and Co-Fe alloys that goes well beyond the often-cited Slater-Pauling dependence of magnetic moment on alloy concentration.

cond-mat.mtrl-sci↗

arXiv2016

Asymmetric hysteresis for probing Dzyaloshinskii-Moriya interaction

The interfacial Dzyaloshinskii-Moriya interaction (DMI) is intimately related to the prospect of superior domain-wall dynamics and the formation of magnetic skyrmions. Although some experimental efforts have been recently proposed to quantify these interactions and the underlying physics, it is still far from trivial to address the interfacial DMI. Inspired by the reported tilt of the magnetization of the side edge of a thin film structure, we here present a quasi-static, straightforward measurement tool. By using laterally asymmetric triangular-shaped microstructures, it is demonstrated that interfacial DMI combined with an in-plane magnetic field yields a unique and significant shift in magnetic hysteresis. By systematic variation of the shape of the triangular objects combined with a droplet model for domain nucleation, a robust value for the strength and sign of interfacial DMI is obtained. This method gives immediate and quantitative access to DMI, enabling a much faster exploration of new DMI systems for future nanotechnology.

cond-mat.mes-hall↗

arXiv2015

Field-free magnetization reversal by spin-Hall effect and exchange bias

Magnetic random-access memory (MRAM) driven by spin-transfer torque (STT) is a major contender for future memory applications. The energy dissipation involved in writing remains problematic, even with the advent of more efficient perpendicular magnetic anisotropy (PMA) devices. A promising alternative switching mechanism employs spin-orbit torques and the spin-Hall effect (SHE) in particular, but additional symmetry breaking is required to achieve deterministic switching in PMA devices. Currently used methods rely on in-plane magnetic fields or anisotropy gradients, which are not suitable for practical applications. Here, we interface the magnetic layer with an anti-ferromagnetic material. An in-plane exchange bias (EB) is created, and shown to enable field-free SHE-driven magnetization reversal of a perpendicularly magnetized Pt/Co/IrMn structure. Aside from the potential technological implications, our experiment provides additional insight into the local spin structure at the ferromagnetic/anti-ferromagnetic interface.

cond-mat.mtrl-sci↗

arXiv2015

Thickness dependence of the interfacial Dzyaloshinskii-Moriya interaction in inversion symmetry broken systems

In magnetic multilayer systems, a large spin-orbit coupling at the interface between heavy metals and ferromagnets can lead to intriguing phenomena such as the perpendicular magnetic anisotropy, the spin Hall effect, the Rashba effect, and especially the interfacial Dzyaloshinskii-Moriya (IDM) interaction. This interfacial nature of IDM interaction has been recently revisited because of its scientific and technological potential. Here, we demonstrate an experimental technique to straightforwardly observe the IDM interaction, namely Brillouin light scattering. The non-reciprocal spin wave dispersions, systematically measured by Brillouin light scattering, allow not only the determination of the IDM energy densities beyond the regime of perpendicular magnetization but also the revelation of the inverse proportionality with the thickness of the magnetic layer, which is a clear signature of the interfacial nature. All together, our experimental and theoretical approaches involving double time Green,s function methods open up possibilities for exploring magnetic hybrid structures for engineering the IDM interaction.

cond-mat.mtrl-sci↗

arXiv2015

Asymmetric magnetic bubble expansion under in-plane field in Pt/Co/Pt: effect of interface engineering

We analyse the impact of growth conditions on asymmetric magnetic bubble expansion under in-plane field in ultrathin Pt / Co / Pt films. Specifically, using sputter deposition we vary the Ar pressure during the growth of the top Pt layer. This induces a large change in the interfacial structure as evidenced by a factor three change in the effective perpendicular magnetic anisotropy. Strikingly, a discrepancy between the current theory for domain-wall propagation based on a simple domain-wall energy density and our experimental results is found. This calls for further theoretical development of domain-wall creep under in-plane fields and varying structural asymmetry.

cond-mat.mes-hall↗

arXiv2014

Large time-dependent coercivity and resistivity modification under sustained voltage application in a Pt/Co/AlOx/Pt junction

The coercivity and resistivity of a Pt/Co/AlOx/Pt junction are measured under sustained voltage application. High bias voltages of either polarity are determined to cause a strongly enhanced, reversible coercivity modification compared to low voltages. Time-resolved measurements show a logarithmic development of the coercive field in this regime, which continues over a period as long as thirty minutes. Furthermore, the resistance of the dielectric barrier is found to change strongly and reversibly on the same time scale, suggesting an electrochemical process is taking place within the dielectric. It is argued that the migration of oxygen vacancies at the magnet/oxide interface could explain both the resistance variation and the enhanced electric field effect at high voltages. A thermal fluctuation aftereffect model is applied to account for the observed logarithmic dependence.

cond-mat.mes-hall↗

arXiv2014

Tunable chiral spin texture in magnetic domain-walls

Magnetic domain-walls (DWs) with a preferred chirality exhibit very efficient current-driven motion. Since structural inversion asymmetry (SIA) is required for their stability, the observation of chiral domain walls in highly symmetric Pt/Co/Pt is intriguing. Here, we tune the layer asymmetry in this system and observe, by current-assisted DW depinning experiments, a small chiral field which sensitively changes. Moreover, we convincingly link the observed efficiency of DW motion to the DW texture, using DW resistance as a direct probe for the internal orientation of the DW under the influence of in-plane fields. The very delicate effect of capping layer thickness on the chiral field allows for its accurate control, which is important in designing novel materials for optimal spin-orbit-torque-driven DW motion.

cond-mat.mes-hall↗

arXiv2014

Voltage controlled propagating spin waves on a perpendicularly magnetized nanowire

We numerically and analytically investigate the voltage controlled spin wave (SW) propagations in a nanowire with locally manipulated perpendicular magnetic anisotropy (PMA) by applying an electric field. It is shown that the velocity and wavelength of the propagating SWs are tailored by the modified PMA, which can be locally controlled by the external electric field. First, we observe a phase shift when the propagating SWs pass through the area of locally modified PMA. By introducing phase control of the SWs, we finally propose a three terminal SW device. Constructive/destructive interferences of two propagating SWs are controlled at the detecting area by the voltage controlled phase shift.

cond-mat.mes-hall↗

arXiv2011

Tunable resistivity of individual magnetic domain walls

Despite the relevance of current-induced magnetic domain wall (DW) motion for new spintronics applications, the exact details of the current-domain wall interaction are not yet understood. A property intimately related to this interaction is the intrinsic DW resistivity. Here, we investigate experimentally how the resistivity inside a DW depends on the wall width D, which is tuned using focused ion beam irradiation of Pt/Co/Pt strips. We observe the nucleation of individual DWs with Kerr microscopy, and measure resistance changes in real-time. A 1/D^2 dependence of DW resistivity is found, compatible with Levy-Zhang theory. Also quantitative agreement with theory is found by taking full account of the current flowing through each individual layer inside the multilayer stack.

cond-mat.mes-hall↗