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Laura J. Heyderman

Publications and source records attributed to Laura J. Heyderman.

At least 19 recordsLinked to original sources
arXiv2026

Generating unconventional spin-orbit torques with patterned phase gradients in tungsten thin films

A key aim in spintronics is to achieve current-induced magnetization switching via spin-orbit torques without external magnetic fields. For this, the focus of recent work has been on introducing controlled lateral gradients across ferromagnet/heavy-metal devices, giving variations in thickness, composition, or interface quality. However, the small gradients achievable with common growth techniques limit both the impact of this approach and understanding of the underlying physical mechanisms. Here, spin-orbit torques are patterned on a mesoscopic length scale in tungsten thin films using direct-write laser annealing. Through transmission electron microscopy, resistivity, and second harmonic measurements, the continuous transformation of the crystalline phase of W films from the highly spin-orbit coupled, high resistivity $β$ phase to the minimally spin-orbit coupled, low resistivity $α$ phase is tracked with increasing laser fluence. Gradients with different steepness are patterned in the tungsten phase to create spin-orbit torque channels and, when interfaced with CoFeB, tungsten wires with a sufficiently strong gradient can switch the magnetization without an applied magnetic field. Therefore, exploiting the unique microstructure of mixed-phase W allows precise control of the local electronic current density and direction, as well as local spin-orbit torque efficiency, providing a new avenue for the design of efficient spintronic devices.

cond-mat.mes-hall↗
arXiv2025

Magnetic-Field Control of Emergent Order in a 3D Dipolar Pyramid Artificial Spin Ice

We realize a three-dimensional artificial spin ice of disconnected nanomagnets interacting solely via dipolar coupling, patterned on square pyramids. This Pyramid artificial spin ice, with both tilted and in-plane nanomagnets, supports tunable states. Monte Carlo simulations reveal a rich phase diagram and an emergent square ice of vertex-level effective spins. Tailored demagnetization protocols and magnetic force microscopy allow experimental access to low-energy states, establishing a platform for exploring three-dimensional artificial spin ices.

cond-mat.mes-hall↗
arXiv2025

Realizing Blume-Capel Degrees of Freedom with Toroidal Moments in a Ruby Artificial Spin Ice

Realizing exotic Hamiltonians beyond the Ising model is a key pursuit in experimental statistical physics. One such example is the Blume-Capel model, a three-state spin model, whose phase diagram features a tricritical point where second-order and first-order transition lines converge, leading to a coexistence of paramagnetic, ferromagnetic, and disordered phases. Here, we realize an artificial crystal of single-domain nanomagnets, placed on the links of the Ruby lattice, enabling real-space observation of the Blume-Capel degrees of freedom. These Blume-Capel degrees of freedom are represented by the presence, sign and interactions of the toroidal moments that emerge naturally in plaquettes of nanomagnets in the Ruby artificial spin ice. By precisely tuning the lattice parameters of the Ruby artificial spin ice, we demonstrate control over the two-step ordering process of the toroidal moments, whereby there is a high-temperature crossover from a paramagnetic phase to an intermediate paratoroidic regime, followed by a second-order phase transition to a ferrotoroidic ground state. This sequence of toroidal phases and transitions is accurately captured by the Blume-Capel framework and provides a direct realization of a substantial portion of the phase diagram associated with the model. This establishes a new platform for exploring exotic Hamiltonians in terms of artificial spin ice superstructures, here with groups of nanomagnets forming toroidal moments. The success of this mapping paves the way for an entirely new frontier in artificial spin ice: intentionally engineering lattice designs whose effective Hamiltonians mediate unconventional forms of magnetic order, with new behaviors and functionalities.

cond-mat.mes-hall↗
arXiv2025

Engineering and exploiting self-driven domain wall motion in ferrimagnets for neuromorphic computing applications

Magnetic domain wall motion has recently garnered significant interest as a physical mechanism to enable energy-efficient, next-generation brain-inspired computing architectures. However, realizing all behaviors required for neuromorphic computing within standard material systems remains a significant challenge, as these functionalities often rely on competing interactions. Here, we demonstrate how spontaneous domain wall motion in response to locally engineered lateral exchange coupling in transition metal-rare earth ferrimagnets can be leveraged to achieve numerous neuromorphic computing functionalities in devices with minimal complexity. Through experiments and micromagnetic simulations, we show how tuning the feature size, material composition, and chiral interaction strength controls the speed of self-driven domain wall motion. When integrated with spin-orbit torque, this control gives rise to behaviors essential for neuromorphic computing, including leaky integration and passive resetting of artificial neuron potential. These results establish locally engineered ferrimagnets as a tunable, scalable, and straightforward platform for domain wall-based computing architectures.

cond-mat.mtrl-sci↗
arXiv2025

X-ray linear dichroic orientation tomography: reconstruction of nanoscale three-dimensional orientation fields

Properties in crystalline and ordered materials tend to be anisotropic, with their orientation affecting the macroscopic behavior and functionality of materials. The ability to image the orientation of anisotropic material properties in three dimensions (3D) is fundamental for the understanding and functionality-driven development of novel materials. With the development of X ray linear dichroic orientation tomography (XL DOT), it is now possible to non-destructively map three-dimensional (3D) orientation fields in micrometer-sized samples. In this work, we present the iterative, gradient-based reconstruction algorithm behind XL DOT that can be used to map orientations based on linear dichroism in 3D. As linear dichroism can be exhibited by a broad spectrum of materials, XL DOT can be used to map, for example, crystal orientations as well as ferroic alignment, such as ferroelectric and antiferromagnetic order. We demonstrate the robustness of this technique for orientation fields that exhibit smoothly varying and granular configurations, and subsequently identify and discuss optimal geometries for experimental data acquisition and optimal conditions for the reconstruction. We anticipate that this technique will be instrumental in enabling a deeper understanding of the relationship between material structures and their functionality, quantifying, for example, the orientation of charge distributions and magnetic anisotropies at the nanoscale in a wide variety of systems - from functional to energy materials.

cond-mat.mtrl-sci↗
arXiv2025

Magnetic ordering in out-of-plane artificial spin systems based on the Archimedean lattices

Artificial spin systems, sometimes referred to as artificial spin ices, are arrays of coupled nanoscale magnets that order according to the lattice geometry, nanomagnet shape and magnetic anisotropy. Here we characterize a family of artificial spin systems that are formed by placing arrays of out-of-plane nanomagnets on the vertices of the Archimedean lattices. On demagnetizing these nanomagnet arrays using a magnetic field protocol and subsequently imaging the magnetic configuration using magnetic force microscopy, we observe different types of magnetic order. We compare our experimental results with those predicted by Monte Carlo simulations to assign an effective temperature to each lattice. We find that, for all of the lattices, the assigned effective temperature is above the transition temperature. This reflects the difficulty of obtaining system-spanning order in lattices with out-of-plane nanomagnets. We consider to what extent further-neighbor interactions affect the phase diagram and spin-spin correlations in each lattice, illustrating our results with four example lattices. We can divide the lattices into three main categories: bipartite lattices that admit a perfect antiferromagnetic ground state, frustrated lattices where ordering proceeds via a single step, and frustrated lattices with two-step-ordering. Our work highlights the diversity of magnetic ordering that can be hosted in two-dimensional artificial spin systems with out-of-plane nanomagnets, and demonstrates the importance of including long-range interactions to explain the magnetic ordering. Such insights will be important for incorporating artificial spin systems into novel computing applications.

cond-mat.mes-hall↗
arXiv2024

Control of spin-orbit torque-driven domain nucleation through geometry in chirally coupled magnetic tracks

The interfacial Dzyaloshinskii-Moriya interaction (DMI) can be exploited in magnetic thin films to realize lateral chirally coupled systems, providing a way to couple different sections of a magnetic racetrack and realize interconnected networks of magnetic logic gates. Here, we systematically investigate the interplay between spin-orbit torques, chiral coupling and the device design in domain wall racetracks. We show that the current-induced domain nucleation process can be tuned between single-domain nucleation and repeated nucleation of alternate domains by changing the orientation of an in-plane patterned magnetic region within an out-of-plane magnetic racetrack. Furthermore, by combining experiments and micromagnetic simulations, we show that the combination of damping-like and field-like spin-orbit torques with DMI results in selective domain wall injection in one of two arms of a Y-shaped devices depending on the current density. Such an element constitutes the basis of domain wall based demultiplexer, which is essential for distributing a single input to any one of the multiple outputs in logic circuits. Our results provide input for the design of reliable and multifunctional domain-wall circuits based on chirally coupled interfaces.

cond-mat.mes-hall↗
arXiv2024

Five-stage ordering to a topological-defect-mediated ground state in a buckyball artificial spin ice

Artificial spin ices are arrays of coupled nanomagnets, which exhibit a variety of fascinating collective behaviour including emergent magnetic monopoles, charge screening, and novel phase transitions. However, they have mainly been confined to two dimensions due to the challenges inherent to their fabrication and characterisation in three dimensions. Exploiting the third dimension offers new degrees of freedom leading to, for example, topological effects that arise from the curvature. Here, using numerical simulations, we uncover the low-temperature magnetic behaviour of a finite three-dimensional spin lattice: the buckyball artificial spin ice, where the spins are located on the edges of a regular buckyball. This frustrated system has a non-trivial structural topology that results in a rich spectrum of thermal magnetic behaviour, beginning with a crossover from paramagnetism to a Spin-Ice sector followed by the formation of an imperfect charge crystal, before partial spin order is established in three separate steps. The final ground state configuration is described by a pair of robust topological magnetic defects that arise because of the finite curved nature of the spatial-spin interaction. Our work uncovers the intricate thermodynamics of the buckyball artificial spin ice. In doing so, we pave the way to designing unusual magnetic textures in other curved three-dimensional nanomagnetic systems by exploiting the interplay between structural topology and the dipolar interaction.

cond-mat.mes-hall↗
arXiv2024

X-ray Linear Dichroic Tomography of Crystallographic and Topological Defects

The functionality of materials is determined by their composition and microstructure, that is, the distribution and orientation of crystalline grains, grain boundaries and the defects within them. The characterisation of the material's microstructure is therefore critical for materials applications such as catalysis, energy storage and buildings. Until now, characterization techniques that map the distribution of grains, their orientation, and the presence of defects have either been limited to surface investigations, to spatial resolutions of a few hundred nanometres, or to systems of thickness around one hundred nanometres, thus requiring destructive sample preparation for measurements and preventing the study of system-representative volumes or the investigation of materials under operational conditions. Here, we present X-ray linear dichroic orientation tomography, a quantitative, non-invasive technique that allows for an intra- and inter-granular characterisation of extended polycrystalline and amorphous materials in three dimensions (3D). We present the detailed characterisation of a polycrystalline sample of vanadium pentoxide (V2O5), a key catalyst in the production of sulfuric acid. In addition to determining the nanoscale composition, we map the crystal orientation throughout the polycrystalline sample with 73 nm spatial resolution. We identify grains, as well as twist, tilt, and twin grain boundaries. We further observe the creation and annihilation of topological defects promoted by the presence of volume crystallographic defects in 3D. Our method's non-destructive and spectroscopic nature opens the door to in-operando combined chemical and microstructural investigations of functional materials, including energy and mechanical materials in existing industries, as well as quantum materials for future technologies.

cond-mat.mtrl-sci↗
arXiv2024

Grayscale control of local magnetic properties with direct-write laser annealing

Across the fields of magnetism, microelectronics, optics, and others, engineered local variations in physical properties can yield groundbreaking functionalities that play a crucial role in enabling future technologies. Beyond binary modifications, 1D lateral gradients in material properties (achieved by gradients in thickness, stoichiometry, temperature, or strain) give rise to a plethora of new effects in thin film magnetic systems. However, extending such gradient-induced behaviors to 2D is challenging to realize with existing methods, which are plagued by slow processing speeds, dose instabilities, or limitation to variation along one dimension. Here, we show for the first time how commonplace direct-write laser exposure techniques, initially developed for grayscale patterning of photoresist surfaces, can be repurposed to perform grayscale direct-write laser annealing. With this technique, we demonstrate the ease with which two-dimensional, continuous variations in magnetic properties can be created at the mesoscopic scale in numerous application-relevant materials, including ferromagnetic, ferrimagnetic, and synthetic antiferromagnetic thin-film systems. The speed, versatility, and new possibilities to create complex magnetic energy landscapes offered by direct-write laser annealing opens the door to the lateral modification of the magnetic, electronic, and structural properties of a variety of thin films with an abundance of applications.

physics.app-ph↗
arXiv2023

Electrically programmable magnetic coupling in an Ising network exploiting solid-state ionic gating

Two-dimensional arrays of magnetically coupled nanomagnets provide a mesoscopic platform for exploring collective phenomena as well as realizing a broad range of spintronic devices. In particular, the magnetic coupling plays a critical role in determining the nature of the cooperative behaviour and providing new functionalities in nanomagnet-based devices. Here, we create coupled Ising-like nanomagnets in which the coupling between adjacent nanomagnetic regions can be reversibly converted between parallel and antiparallel through solid-state ionic gating. This is achieved with the voltage-control of magnetic anisotropies in a nanosized region where the symmetric exchange interaction favours parallel alignment and the antisymmetric exchange interaction, namely the Dzyaloshinskii-Moriya interaction, favours antiparallel alignment. Applying this concept to a two-dimensional lattice, we demonstrate a voltage-controlled phase transition in artificial spin ices. Furthermore, we achieve an addressable control of the individual couplings and realize an electrically programmable Ising network, which opens up new avenues to design nanomagnet-based logic devices and neuromorphic computers

cond-mat.mes-hall↗
arXiv2023

Strong lateral exchange coupling and current-induced switching in single-layer ferrimagnetic films with patterned compensation temperature

Strong, adjustable magnetic couplings are of great importance to all devices based on magnetic materials. Controlling the coupling between adjacent regions of a single magnetic layer, however, is challenging. In this work, we demonstrate strong exchange-based coupling between arbitrarily shaped regions of a single ferrimagnetic layer. This is achieved by spatially patterning the compensation temperature of the ferrimagnet by either oxidation or He+ irradiation. The coupling originates at the lateral interface between regions with different compensation temperature and scales inversely with their width. We show that this coupling generates large lateral exchange coupling fields and we demonstrate its application to control the switching of magnetically compensated dots with an electric current.

cond-mat.mes-hall↗
arXiv2021

Engineering of Intrinsic Chiral Torques in Magnetic Thin Films Based on the Dzyaloshinskii-Moriya Interaction

The establishment of chiral coupling in thin magnetic films with inhomogeneous anisotropy has led to the development of artificial systems of fundamental and technological interest. The chiral coupling itself is enabled by the Dzyaloshinskii-Moriya interaction (DMI) enforced by the patterned noncollinear magnetization. Here, we create a domain wall track with out-of-plane magnetization coupled on each side to a narrow parallel strip with in-plane magnetization. With this we show that the chiral torques emerging from the DMI at the boundary between the regions of noncollinear magnetization in a single magnetic layer can be used to bias the domain wall velocity. To tune the chiral torques, the design of the magnetic racetracks can be modified by varying the width of the tracks or the width of the transition region between noncollinear magnetizations, reaching effective chiral magnetic fields of up to 7.8 mT. Furthermore, we show how the magnitude of the chiral torques can be estimated by measuring asymmetric domain wall velocities, and demonstrate spontaneous domain wall motion propelled by intrinsic torques even in the absence of any external driving force.

cond-mat.mes-hall↗
arXiv2021

Artificial out-of-plane Ising antiferromagnet on the kagome lattice with very small further neighbour couplings

Despite their simple formulation, short range classical antiferromagnetic Ising models on frustrated lattices give rise to exotic phases of matter, in particular due to their macroscopic ground state degeneracy. Recent experiments on artificial spin systems comprising arrays of chirally coupled nanomagnets provide a significant strengthening of the nearest neighbour couplings compared to systems with dipolar-coupled nanomagnets. This opens the way to design artificial spin systems emulating Ising models with nearest neighbour couplings. In this paper, we compare the results of an extensive investigation with tensor network and Monte Carlo simulations of the nearest- and further-neighbour ($J_1-J_2-J_{3||}$) kagome Ising antiferromagnet with the experimental spin-spin correlations of a kagome lattice of chirally coupled nanomagnets. Even though the ratios between the further neighbour couplings and the nearest neighbour coupling estimated from micromagnetic simulations are much smaller than for dipolar-coupled nanomagnets, we show that they still play an essential role in the selection of the correlations.

cond-mat.mes-hall↗
arXiv2021

Synchronization of chiral vortex nano-oscillators

The development of spintronic oscillators is driven by their potential applications in radio frequency telecommunication and neuromorphic computing. In this work, we propose a spintronic oscillator based on the chiral coupling in thin magnetic films with patterned anisotropy. With an in-plane magnetized disk imprinted on an out-of-plane magnetized slab, the oscillator takes a polar vortex-like magnetic structure in the disk stabilized by a strong Dzyaloshinskii-Moriya interaction. By means of micromagnetic simulations, we investigate its oscillatory properties under applied spin current and, by placing an ensemble of oscillators in the near vicinity, we demonstrate their synchronization with different resonant frequencies. Finally, we show their potential application in neuromorphic computing using a network with six oscillators.

cond-mat.mes-hall↗
arXiv2021

Field- and current-driven magnetic domain-wall inverter and diode

We investigate the inversion process of magnetic domain walls (DWs) propagating through synthetic noncollinear magnetic textures, whereby an up/down DW can be transformed into a down/up DW and vice versa. We exploit the lateral coupling between out-of-plane and in-plane magnetic regions induced by the interfacial Dzyaloshinskii-Moriya interaction in Pt/Co/AlOx trilayers to realize both field-driven and current-driven magnetic DW inverters. The inverters consist of narrow in-plane magnetic regions embedded in out-of-plane DW racetracks. Magnetic imaging and micromagnetic simulations provide insight into the DW inversion mechanism, showing that DW inversion proceeds by annihilation of the incoming domain on one side of the in-plane region and nucleation of a reverse domain on the opposite side. By changing the shape of the in-plane magnetic region, we show that the DW inversion efficiency can be tuned by adjusting the ratio between the chiral coupling energy at the inverter boundary and the energy cost of nucleating a reverse domain. Finally, we realize an asymmetric DW inverter that has nonreciprocal inversion properties and demonstrate that such a device can operate as a DW diode. Our results provide input for the versatile manipulation of DWs in magnetic racetracks and the design of efficient DW devices for nonvolatile magnetic logic schemes.

cond-mat.mes-hall↗
arXiv2021

Spin-wave dynamics and symmetry breaking in an artificial spin ice

Artificial spin ices are periodic arrangements of interacting nanomagnets successfully used to investigate emergent phenomena in the presence of geometric frustration. Recently, it has been shown that artificial spin ices can be used as building blocks for creating functional materials, such as magnonic crystals, and support a large number of programmable magnetic states. We investigate the magnetization dynamics in a system exhibiting anisotropic magnetostatic interactions owing to locally broken structural inversion symmetry. We find a rich spin-wave spectrum and investigate its evolution in an external magnetic field. We determine the evolution of individual modes, from building blocks up to larger arrays, highlighting the role of symmetry breaking in defining the mode profiles. Moreover, we demonstrate that the mode spectra exhibit signatures of long-range interactions in the system. These results contribute to the understanding of magnetization dynamics in spin ices beyond the kagome and square ice geometries and are relevant for the realization of reconfigurable magnonic crystals based on spin ices.

cond-mat.mes-hall↗