arXiv subjects

Dieter Suess

Publications and source records attributed to Dieter Suess.

At least 73 records · Page 4Linked to original sources

arXiv2021

Tension-free Dirac strings and steered magnetic charges in 3D artificial spin ice

3D nano-architectures present a new paradigm in modern condensed matter physics with numerous applications in photonics, biomedicine, and spintronics. They are promising for the realisation of 3D magnetic nano-networks for ultra-fast and low-energy data storage. Frustration in these systems can lead to magnetic charges or magnetic monopoles, which can function as mobile, binary information carriers. However, Dirac strings in 2D artificial spin ices bind magnetic charges, while 3D dipolar counterparts require cryogenic temperatures for their stability. Here, we present a micromagnetic study of a highly-frustrated 3D artificial spin ice harboring tension-free Dirac strings with unbound magnetic charges at room temperature. We use micromagnetic simulations to demonstrate that the mobility threshold for magnetic charges is by $\SI{2}{eV}$ lower than their unbinding energy. By applying global magnetic fields, we steer magnetic charges in a given direction omitting unintended switchings. The introduced system paves a way towards 3D magnetic networks for data transport and storage

cond-mat.mes-hall↗

arXiv2020

Non-planar geometrical effects on the magnetoelectrical signal in a three-dimensional nanomagnetic circuit

Expanding nanomagnetism and spintronics into three dimensions (3D) offers great opportunities for both fundamental and technological studies. However, probing the influence of complex 3D geometries on magnetoelectrical phenomena poses important experimental and theoretical challenges. In this work, we investigate the magnetoelectrical signals of a ferromagnetic 3D nanodevice integrated into a microelectronic circuit using direct-write nanofabrication. Due to the 3D vectorial nature of both electrical current and magnetisation, a complex superposition of several magnetoelectrical effects takes place. By performing electrical measurements under the application of 3D magnetic fields, in combination with macrospin simulations and finite element modelling, we disentangle the superimposed effects, finding how a 3D geometry leads to unusual angular dependences of well-known magnetotransport effects such as the anomalous Hall effect. Crucially, our analysis also reveals a strong role of the noncollinear demagnetising fields intrinsic to 3D nanostructures, which results in an angular dependent magnon magnetoresistance contributing strongly to the total magnetoelectrical signal. These findings are key to the understanding of 3D spintronic systems and underpin further fundamental and device-based studies.

cond-mat.mes-hall↗

arXiv2020

Chiral switching and dynamic barrier reductions in artificial square ice

Collective dynamics in lithographically-defined artificial spin ices offer profound insights into emergent correlations and phase transitions of geometrically-frustrated Ising spin systems. Their temporal and spatial evolution are often simulated using kinetic Monte Carlo simulations, which rely on the precise knowledge of the switching barriers to obtain predictive results in agreement with experimental observations. In many cases, however, the barriers are derived from simplified assumptions only, and do not take into account the full physical picture of nanomagnetic switching. Here we describe how the immediate magnetic environment of a nanomagnet reversing via quasi-coherent rotation can induce clockwise and counter-clockwise switching channels with different barrier energies. This barrier splitting for chiral reversal channels can be sizeable and, as string-method micromagnetic simulations show, is relevant for artificial spin ice systems made of both exchange -- as well as magnetostatically --dominated units. Due to the barrier splitting (and further reductions due to non-uniform reversal) transition rates can be exponentially enhanced by several orders of magnitude compared to mean-field predictions, especially in the limit of rare switching events where thermal excitation is less likely. This leads to significantly faster relaxation time scales and modified spatial correlations. Our findings are thus of integral importance to achieve realistic kinetic Monte Carlo simulations of emergent correlations in artificial spin systems, magnonic crystals, or the evolution of nanomagnetic logic circuits.

cond-mat.mes-hall↗

arXiv2020

Coexistence of distinct skyrmion phases observed in hybrid ferromagnetic/ferrimagnetic multilayers

Materials hosting magnetic skyrmions at room temperature could enable new computing architectures as well as compact and energetically efficient magnetic storage such as racetrack memories. In a racetrack device, information is coded by the presence/absence of magnetic skyrmions forming a chain that is moved through the device. The skyrmion Hall effect that would eventually lead to an annihilation of the skyrmions at the edges of the racetrack can be suppressed for example by anti-ferromagnetically-coupled skyrmions. However, avoiding modifications of the inter-skyrmion distances in the racetrack remains challenging. As a solution to this issue, a chain of bits could also be encoded by two different solitons such as a skyrmion and a chiral bobber. The major limitation of this approach is that it has solely been realized in B20-type single crystalline material systems that support skyrmions only at low temperatures, thus hindering the efficacy for future applications. Here we demonstrate that a hybrid ferro/ferri/ferromagnetic multilayer system can host two distinct skyrmion phases at room temperature. By matching quantitative magnetic force microscopy data with micromagnetic simulations, we reveal that the two phases represent tubular skyrmions and partial skyrmions (similar to skyrmion bobbers). Furthermore, the tubular skyrmion can be converted into a partial skyrmion. Such multilayer systems may thus serve as a platform for designing skyrmion memory applications using distinct types of skyrmions and potentially for storing information using the vertical dimension in a thin film device.

cond-mat.mtrl-sci↗

arXiv2020

Spin-canting effects in GMR sensors with wide dynamic field range

Magnetoresistive (xMR) sensors find extensive application in science and industry, replacing Hall sensors in various low field environments. While there have been some efforts in increasing the dynamic field range of xMR sensors, Hall sensors remain to dominate high field applications due to their wide linear range. Using a perpendicular magnetized reference system and an in-plane free layer allows us to overcome this disadvantage of xMR sensors, and, furthermore, investigate spin-canting effects in interlayer exchange coupled perpendicular synthetic antiferromagnets (p-SAF). We created p-SAFs with exchange coupling fields of up to 10 kOe, based on magnetic Co/Pt multilayer systems. The p-SAFs are either designed as "single" p-SAFs, where two Co/Pt multilayers are interlayer exchange coupled via a 4 Å thick Ru spacer, or as "double" p-SAFs, where an additional Co layer is interlayer exchange coupled to the top multilayer. These p-SAFs are used for giant magnetoresistance (GMR) sensors with wide dynamic field range. By using a p-SAF as the reference system and employing an in-plane magnetic layer as the GMR's free layer, the linear range can be effectively increased limited only by the p-SAF's switching fields. Additionally, the magnetic anisotropy of the in-plane free layer is fully controlled, which allows saturation fields by design. Different configurations were investigated, ranging from free layer magnetic saturation at lower to far higher fields than the p-SAF's switching fields. We can show through micromagnetic simulations that certain GMR transfer curves are dominated by spin-canting effects in the interlayer exchange coupled reference system. Finally, our simulation results lay out the correlation of the p-SAF's design parameters and its magnetization reversal behavior.

cond-mat.mtrl-sci↗

arXiv2020

Microscopic origin of magnetization reversal in exchange-coupled ferro-/ferrimagnetic bilayers

In this study, the magnetic reversal process of exchange-coupled bilayer systems, consisting of a ferrimagnetic TbFeCo alloy layer and a ferromagnetic [Co/Ni/Pt]N multilayer, was investigated. In particular, minor loop studies, probing solely the reversal characteristics of the softer ferromagnetic layer, reveal two distinct reversal mechanisms, which depend strongly on the thickness of the ferromagnetic layer. For thick layers, irreversible switching of the macroscopic minor loop is observed. The underlying microscopic origin of this reversal process was studied in detail by high-resolution magnetic force microscopy, showing that the reversal is triggered by in-plane domain walls propagating through the ferromagnetic layer. In contrast, thin ferromagnetic layers show a hysteresis-free reversal, which is nucleation-dominated due to grain-to-grain variations in magnetic anisotropy of the Co/Ni/Pt multilayer and an inhomogeneous exchange coupling with the magnetically hard TbFeCo layer, as confirmed by micromagnetic simulations.

cond-mat.mtrl-sci↗

arXiv2020

Dependence of energy barrier reduction on collective excitations in square artificial spin ice: A comprehensive comparison of simulation techniques

We perform micromagnetic simulations to study the switching barriers in square artificial spin ice systems consisting of elongated single domain magnetic islands arranged on a square lattice. By considering a double vertex composed of one central island and six nearest neighbor islands, we calculate the energy barriers between two types of double vertices by applying the string method. We investigate by means of micromagnetic simulations the consequences of the neighboring islands, the inhomogeneities in the magnetization of the islands and the reversal mechanisms on the energy barrier by comparing three different approaches with increasing complexity. The micromagnetic models, where the string method is applied, are compared to the currently common method, the mean barrier approximation. Our investigations indicate that a proper micromagnetic modeling of the switching process leads to significantly lower energy barriers, by up to 35% compared to the mean-barrier approximation, so decreasing the expected average life time up to seven orders of magnitude. Hereby, we investigate the influence of parallel switching channels and the conceptional approach of using a mean-barrier to calculate the corresponding rates.

physics.comp-ph↗

arXiv2020

Hysteresis-free magnetization reversal of exchange-coupled bilayers with finite magnetic anisotropy

Exchange-coupled structures consisting of ferromagnetic and ferrimagnetic layers become technologically more and more important. We show experimentally the occurrence of completely reversible, hysteresis-free minor loops of [Co(0.2 nm)/Ni(0.4 nm)/Pt(0.6 nm)]$_N$ multilayers exchange-coupled to a 20 nm thick ferrimagnetic Tb$_{28}$Co$_{14}$Fe$_{58}$ layer, acting as hard magnetic pinning layer. Furthermore, we present detailed theoretical investigations by means of micromagnetic simulations and most important a purely analytical derivation for the condition of the occurrence of full reversibility in magnetization reversal. Hysteresis-free loops always occur if a domain wall is formed during the reversal of the ferromagnetic layer and generates an intrinsic hard-axis bias field that overcomes the magnetic anisotropy field of the ferromagnetic layer. The derived condition further reveals that the magnetic anisotropy and the bulk exchange of both layers, as well as the exchange coupling strength and the thickness of the ferromagnetic layer play an important role for its reversibility.

cond-mat.mtrl-sci↗

arXiv2020

Learning time-stepping by nonlinear dimensionality reduction to predict magnetization dynamics

We establish a time-stepping learning algorithm and apply it to predict the solution of the partial differential equation of motion in micromagnetism as a dynamical system depending on the external field as parameter. The data-driven approach is based on nonlinear model order reduction by use of kernel methods for unsupervised learning, yielding a predictor for the magnetization dynamics without any need for field evaluations after a data generation and training phase as precomputation. Magnetization states from simulated micromagnetic dynamics associated with different external fields are used as training data to learn a low-dimensional representation in so-called feature space and a map that predicts the time-evolution in reduced space. Remarkably, only two degrees of freedom in feature space were enough to describe the nonlinear dynamics of a thin-film element. The approach has no restrictions on the spatial discretization and might be useful for fast determination of the response to an external field.

physics.comp-ph↗

arXiv2019

3D printing of polymer-bonded anisotropic magnets in an external magnetic field and by a modified production process

The possibility of producing polymer-bonded magnets with the aid of additive processes, such as 3D printing, opens up a multitude of new areas of application. Almost any structures and prototypes can be produced cost-effectively in small quantities. Extending the 3D printing process allows the manufacturing of anisotropic magnetic structures by aligning the magnetic easy axis of ferromagnetic particles inside a paste-like compound material along an external magnetic field. This is achieved by two different approaches: First, the magnetic field for aligning the particles is provided by a permanent magnet. Secondly, the 3D printing process itselfs generates an anisotropic behavior of the structures. An inexpensive and customizable end-user fused filament fabrication 3D printer is used to print the magnetic samples. The magnetical properties of different magnetic anisotropic Sr ferrite and SmFeN materials are investigated and discussed.

cond-mat.mtrl-sci↗

arXiv2019

Statistical analysis of read-back signals in magnetic recording on granular media

The comprehensive simulation of magnetic recording, including the write and read-back process, on granular media becomes computationally expensive if the magnetization dynamics of each grain are explicitly computed. In addition, in heat-assisted magnetic recording, the writing of a single track becomes a random process since the temperature must be considered and thermal noise is involved. Further, varying grain structures of various granular media must also be taken into account to obtain correct statistics for the final read-back signal. Hence, it requires many repetitions of the write process to investigate the mean signal as well as the noise. This work presents a method that improves the statistical evaluation of the whole recording process. The idea is to avoid writing the magnetization to one of its binary states. Instead, we assign each grain its probability of occupying one of its stable states, which can be calculated in advance in terms of a switching probability phase diagram. In the read-back process, we combine the probabilities to calculate a mean signal and its variance. Afterwards, repetitions on different media lead to the final read-back signal. Using a recording example, we show that the statistical behavior of the evaluated signal-to-noise ratio can be significantly improved by applying this probability mapping method, while the computational effort remains low.

physics.app-ph↗

arXiv2019

Polymer-bonded anisotropic SrFe$_\text{12}$O$_\text{19}$ filaments for fused filament fabrication

In this publication we describe the extrusion process and the properties of polymer-bonded anisotropic SrFe$_\text{12}$O$_\text{19}$ filaments for fused filament fabrication (FFF). Highly filled polyamide 12 filaments with a filling fraction from 40 vol.% to 55 vol.% are mixed and extruded into filaments with a diameter of 1.75 mm. Such filaments are processable with a conventional FFF 3D printer. No modifications of the 3D printer are necessary. Detailed mechanical and magnetic investigations of printed samples are performed and discussed. In the presence of an external alignment field, the Sr ferrite particles inside the PA12 matrix can be aligned along an external magnetic field. The remanence can be increased by 40 % by printing anisotropic structures. For the 55 vol.% filled filament, a remanence of 212.8 mT and a coercivity of 307.4 mT are measured. The capabilities of printing magnetic anisotropic structures in a complex external field are presented with a Halbach-array arrangement. By the aim of an inverse field model, based on a finite element method, the orientation of the particles and the quality of the print can be estimated by a nondestructive method.

physics.app-ph↗

arXiv2019

Additive manufactured isotropic NdFeB magnets by stereolithography, fused filament fabrication, and selective laser sintering

Magnetic isotropic NdFeB powder is processed by the following additive manufacturing methods: (i) stereolithography (SLA), (ii) fused filament fabrication (FFF), and (iii) selective laser sintering (SLS). For the first time, a stereolithography based method is used to 3D print hard magnetic materials. FFF and SLA use a polymer matrix material as binder, SLS sinters the powder directly. All methods use the same hard magnetic NdFeB powder material. Complex magnets with small feature sizes in a superior surface quality can be printed with SLA. The magnetic properties for the processed samples are investigated and compared. SLA can print magnets with a remanence of 388 mT and a coercivity of 0.923 T. A complex magnetic design for speed wheel sensing applications is presented and printed with all methods.

physics.app-ph↗

arXiv2019

Hybrid FFT algorithm for fast demagnetization field calculations on non-equidistant magnetic layers

In micromagnetic simulations, the demagnetization field is by far the computationally most expensive field component and often a limiting factor in large multilayer systems. We present an exact method to calculate the demagnetization field of magnetic layers with arbitrary thicknesses. In this approach we combine the widely used fast-Fourier-transform based circular convolution method with an explicit convolution using a generalized form of the Newell formulas. We implement the method both for central processors and graphics processors and find that significant speedups for irregular multilayer geometries can be achieved. Using this method we optimize the geometry of a magnetic random-access memory cell by varying a single specific layer thickness and simulate a hysteresis curve to determine the resulting switching field.

physics.comp-ph↗

arXiv2019

The superior role of the Gilbert damping on the signal-to-noise ratio in heat-assisted magnetic recording

In magnetic recording the signal-to-noise ratio (SNR) is a good indicator for the quality of written bits. However, a priori it is not clear which parameters have the strongest influence on the SNR. In this work, we investigate the role of the Gilbert damping on the SNR. Grains consisting of FePt like hard magnetic material with two different grain sizes $d_1=5\,$nm and $d_2=7\,$nm are considered and simulations of heat-assisted magnetic recording (HAMR) are performed with the atomistic simulation program VAMPIRE. The simulations display that the SNR saturates for damping constants larger or equal than 0.1. Additionally, we can show that the Gilbert damping together with the bit length have a major effect on the SNR whereas other write head and material parameters only have a minor relevance on the SNR.

physics.app-ph↗

arXiv2019

Write head design for effective curvature reduction in heat-assisted magnetic recording by topology optimization

The reduction of the transition curvature of written bits in heat-assisted magnetic recording (HAMR) is expected to play an important role for the future areal density increase of hard disk drives. Recently a write head design with flipped write and return poles was proposed. In this design a large spatial field gradient of the write head was the key to significantly reduce the transition curvature. In this work we optimized the write pole of a heat-assisted magnetic recording head in order to produce large field gradients as well as large fields in the region of the heat pulse. This is done by topology optimization. The simulations are performed with dolfin-adjoint. For the maximum field gradients of $8.1\,$mT/nm, $8.6\,$mT/nm and $11.8\,$mT/nm, locally resolved footprints of an FePt like hard magnetic recording medium are computed with a coarse-grained Landau-Lifshitz-Bloch (LLB) model and the resulting transition curvature is analysed. Additional simulations with a bilayer structure with $50\%$ hard and $50\%$ soft magnetic material are computed. The results show that for both recording media, the optimized head design does not lead to any significant improvement of the written track. Thus, we analyse the transition curvature for the optimized write heads theoretically with an effective recording time window (ERTW) model. Moreover, we check how higher field gradients influence the curvature reduction. The results show that a simple optimization of the conventional head design design is not sufficient for effective curvature reduction. Instead, new head concepts will be needed to reduce transition curvature.

physics.app-ph↗

arXiv2019

Improving the Signal-to-noise Ratio for Heat-Assisted Magnetic Recording by Optimizing a High/Low Tc bilayer structure

We optimize the recording medium for heat-assisted magnetic recording by using a high/low $T_{\mathrm{c}}$ bilayer structure to reduce AC and DC noise. Compared to a former work, small Gilbert damping $α=0.02$ is considered for the FePt like hard magnetic material. Atomistic simulations are performed for a cylindrical recording grain with diameter $d=5\,$nm and height $h=8\,$nm. Different soft magnetic material compositions are tested and the amount of hard and soft magnetic material is optimized. The results show that for a soft magnetic material with $α_{\mathrm{SM}}=0.1$ and $J_{ij,\mathrm{SM}}=7.72\times 10^{-21}\,$J/link a composition with $50\%$ hard and $50\%$ soft magnetic material leads to the best results. Additionally, we analyse how much the areal density can be improved by using the optimized bilayer structure compared to the pure hard magnetic recording material. It turns out that the optimized bilayer design allows an areal density that is $1\,$Tb/in$^2$ higher than that of the pure hard magnetic material while obtaining the same SNR.

physics.app-ph↗

arXiv2019

Systematic parameterization of heat-assisted magnetic recording switching probabilities and the consequences for the resulting SNR

The signal-to-noise ratio (SNR) of a bit series written with heat-assisted magnetic recording (HAMR) on granular media depends on a large number of different parameters. The choice of material properties is essential for the obtained switching probabilities of single grains and therefore for the written bits' quality in terms of SNR. Studies where the effects of different material compositions on transition jitter and the switching probability are evaluated were done, but it is not obvious, how significant those improvements will finally change the received SNR. To investigate that influence, we developed an analytical model of the switching probability phase diagram, which contains independent parameters for, inter alia, transition width, switching probability and curvature. Different values lead to corresponding bit patterns on granular media, where a reader model detects the resulting signal, which is finally converted to a parameter dependent SNR value. For grain diameters between 4 and 8nm, we show an increase of ~10dB for bit lengths between 4 and 12nm, an increase of ~9dB for maximum switching probabilities between 0.64 and 1.00, a decrease of ~5dB for down-track-jitter parameters between 0 and 4nm and an increase of ~1dB for reduced bit curvature. Those results are furthermore compared to the theoretical formulas for the SNR. We obtain a good agreement, even though we show slight deviations.

physics.app-ph↗