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Magnetic charge propagation1-3 in the bulk frustrated materials known as spin-ice has yielded a paradigm shift in science, allowing the symmetry between electricity and magnetism to be studied in condensed matter systems. Artificial spin-ice materials4, arrays of frustrated nanostructured magnetic islands, have been produced in a range of sophisticated geometries5. These systems provide a powerful insight into the physics of frustration but do not capture the exact 3D geometry of the spin-ice lattice, meaning it is not possible to explore monopole transport upon the surface, where the lattice symmetry is broken. In this talk we will detail the direct imaging of magnetic charge propagation upon a 3D nanostructured diamond-bond lattice (Fig 1), capturing the geometry of bulk systems. Vastly different magnetic charge dynamics are observed along two principle lattice directions. Applying a field along the [110] direction leads to a small number of distant magnetic charges within the measured area. In contrast, a field along [-110] leads to a larger number of closely spaced monopole-antimonopole pairs. We will discuss these results by referring to detailed Monte-Carlo simulations and calculations which illuminate the importance of the surface coordination in determining the monopole effective chemical potential.References: 1. Castelnovo, C., Moessner, R. &amp; Sondhi, Nature 451, 42-45 (2008) 2. Jaubert, L. D. C. &amp; Holdsworth, P. C. W., Nat Phys 5, 258-261 (2009) 3. Morris, D. et al., Science 326, 411-414 (2009) 4. Wang, R. F. et al., Nature 446, 102-102 (2007) 5. S. H. Skjærvø, C. M., R. L. Stamps and L.J. Heyderman, Nature Physics Reviews 2, 13-28 (2020)

MMM 2020

Magnetic Charge Propagation upon a 3D Artificial Spin-ice

The 65th Annual Conference on Magnetism and Magnetic Materials (MMM 2020) was held in a virtual form. The COVID-19 pandemic, with its associated uncertainty about international travel and the ability to gather in large groups, meant that the Steering Committee decided to cancel the face-to-face meeting that had been planned in Palm Beach, Florida November 2-6, 2020.

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Magnetic charge propagation1-3 in the bulk frustrated materials known as spin-ice has yielded a paradigm shift in science, allowing the symmetry between electricity and magnetism to be studied in condensed matter systems. Artificial spin-ice materials4, arrays of frustrated nanostructured magnetic islands, have been produced in a range of sophisticated geometries5. These systems provide a powerful insight into the physics of frustration but do not capture the exact 3D geometry of the spin-ice lattice, meaning it is not possible to explore monopole transport upon the surface, where the lattice symmetry is broken. In this talk we will detail the direct imaging of magnetic charge propagation upon a 3D nanostructured diamond-bond lattice (Fig 1), capturing the geometry of bulk systems. Vastly different magnetic charge dynamics are observed along two principle lattice directions. Applying a field along the [110] direction leads to a small number of distant magnetic charges within the measured area. In contrast, a field along [-110] leads to a larger number of closely spaced monopole-antimonopole pairs. We will discuss these results by referring to detailed Monte-Carlo simulations and calculations which illuminate the importance of the surface coordination in determining the monopole effective chemical potential.References: 1. Castelnovo, C., Moessner, R. & Sondhi, Nature 451, 42-45 (2008) 2. Jaubert, L. D. C. & Holdsworth, P. C. W., Nat Phys 5, 258-261 (2009) 3. Morris, D. et al., Science 326, 411-414 (2009) 4. Wang, R. F. et al., Nature 446, 102-102 (2007) 5. S. H. Skjærvø, C. M., R. L. Stamps and L.J. Heyderman, Nature Physics Reviews 2, 13-28 (2020)

technical paper

The relaxation kinetics and emerging correlations of arrays of interacting Ising-like nanomagnets are governed by the switching rates of the individual magnets. These rates depend, via the Arrhenius law, on the energy barrier for moment reversal in the nanomagnets. In this work, we consider how the switching behaviour of a nanomagnet in archetypical artificial square ice is modified by the interaction with its six nearest neighbours. We find that due to two effects the transition barriers can be significantly lowered compared to an approximation commonly used for kinetic Monte Carlo simulations: (1) Based on a simplified point-dipole model, we find that the barriers for clockwise and counter-clockwise uniform rotation can differ significantly due to the interactions with neighbours. For a subset of environmental microstates this leads to a significant enhancement of the switching rate, which is especially marked in the limit of rare events. (2) Micromagnetic simulations employing the string method to model the moment reversal in an extended nanomagnets reveal that, in addition to the clockwise/counter-clockwise splitting, the environments promote non-uniform reversal leading to further reductions of the switching barriers. These results highlight that local microstates can significantly enhance moment reversals already in ideal systems, without defects or additional anisotropies, and are particularly pronounced for fully-magnetised configurations. Thus, we expect that the initial stages of demagnetisation will proceed faster than assumed from previous models. These findings are of integral importance to achieve realistic kinetic Monte Carlo simulations of emergent correlations in extended artificial spin systems or the evolution of small-scale nanomagnetic logic circuits.

Switching barriers in artificial square ice

The dynamics involving directed separation of a monopole-antimonopole pair in two- or three-dimensional spin ices is conventionally defined as magnetic current or magnetricity. While the feasibility of magnetricity in artificial spin ices has been shown theoretically and experimentally [1,2], the controllability of magnetic currents is still under debate. Particularly, it has been theoretically shown that maximization of Dirac string’s tension is a necessary condition for stabilization of directed magnetic currents [1]. Experimentally, however, magnetic currents are often of only short duration, because magnetic monopoles created at one sample edge do not reach the other sample edge, but rather stick in the middle of the sample. Here, we explore theoretically the role of the termination of arrays and the higher order magnetic interactions for the directionality of magnetic currents in two-dimensional artificial spin-ice. Our Monte-Carlo calculations show that arrays of point dipoles and identical array with realistic multipole moments give qualitatively similar results with respect to the end states and with respect to the averaged propagation of the monopoles. However, the Monte-Carlo calculations using the higher order contributions reflect much better formation of domains with phase shifted Dirac strings, found in recent experiments [3]. Our calculations permit us to elaborate a phase diagram predicting the probability of directed propagation of monopole current as a function of termination type of a realistic artificial spin-ice array.References: [1] E. Y. Vedmedenko, Dynamics of Bound Monopoles in Artificial Spin Ice: How to Store Energy in Dirac Strings. Physical Review Letters 116, 077202 (2016). [2] A. Farhan et al., Direct Observation of Thermal Relaxation in Artificial Spin Ice. Phys. Rev. Lett. 111, 057204 (2013). [3] A. Hanu, E. Y. Vedmedenko, J. Cuia, J. Vijayakumard, A. Kleibertd, L.J. Heyderman, Control of emergent magnetic monopole currents in artificial spin ice, submitted.

Role of Higher-Order Magnetostatic Interactions for the Directional Injection of Monopole Currents

1D and 2D arrays of coupled magnetic nanoislands, also known as mesospins, have recently attracted much attention for offering the possibility to mimic exotic physical systems [1]. In analyzing these coupled mesospin arrays, the individual building blocks are often assumed to consist of simple dipoles, where any inner structure is ignored. Such an approximation of a mesospin however, leads to discrepancies when temperature becomes an important parameter [2]. Temperature excites thermal magnons in a ferromagnet, which is the basis for temperature dependence of the magnetization as expressed by the Bloch law. Here, we present a framework for studying temperature induced magnetization fluctuations in terms of magnonic excitations. We use MuMax3 to numerically subject a single stadium shaped mesospin (450 nm x 150 nm x 5 nm) to a highly inhomogeneous stochastic thermal field [3], and study the emerging spectrum by Fourier transforming the magnetization. The length, width and thickness of the mesospin element are varied to study the resulting effect of size variation on the spectra. The spectra are found to be gapped, and the spectral peaks pertain to specific edge- and center oscillation modes. Finally, we study the effect of temperature variation and we find that the magnon density of states plays an important role on the overall shape of the spectra. The results of this study are not only useful from the perspective of thermal switching of nanoislands, but also for the design of magnonic crystals based on mesospins.References: 1. S. H. Skjærvø, et al., Nature Reviews Physics, Vol. 2, p. 13–28 (2020) 2. M. Pohlit, G. Muscas, I. A. Chioar, et al., Physical Review B, Vol. 101, p. 134404 (2020) 3. J. Leliaert, J. Mulkers, J. De Clercq, et al., AIP Advances, Vol. 7, p. 125010 (2017)

A Magnon Approach to Thermal Fluctuations in Ising-like Mesospins

The extent of magnetic ordering in highly-frustrated thermally-active artificial kagome spin ice is limited by the lowest achievable blocking temperature, as the moments freeze in before ordering is achieved [1]. A more substantial degree of magnetic ordering can be achieved by lowering the blocking temperature of the individual nanomagnets or by increasing the critical transition temperature for the system. We pursue an original approach of introducing interfacial Dzyaloshinskii-Moriya interactions, which lowers the blocking temperature while keeping the transition temperature unchanged [2]. Using this approach, we demonstrate that a seven-ring kagome structure consisting of 30 nanomagnets can be thermally annealed into its ground state. Furthermore, the spin-ice correlations extracted from extended kagome lattices are found to exhibit the quantitative signatures of long-range charge-order, thereby giving experimental evidence for the theoretically predicted continuous transition to a charge-ordered state. We also find that slight modifications of the nanomagnets at a vertex in the artificial kagome ice lead to an increase in the critical temperatures, and allow us to control the energy landscape. Our results provide the foundations for the tuning of magnetic ordering in several different artificial spin systems.References: [1] S.H. Skjærvø, C.H. Marrows, R.L. Stamps, L.J. Heyderman. “Advances in artificial spin ice.” Nature Reviews Physics 2, 13-28 (2020). [2] K. Hofhuis et al., under review.

Long-range Magnetic Ordering in Artificial Kagome Spin Ice

Artificial spin ices (ASIs) are lithographically patterned arrays of single domain magnetic nanoislands where the dipolar interactions between these Ising-like macrospins have served as the basis for investigating phenomena such as geometric frustration and emergent magnetic monopoles [1]. While the majority of ASI studies have utilized nanoislands composed of soft magnetic metals (e.g. permalloy), the use of complex oxides, such as La0.7Sr0.3MnO3 (LSMO), could permit room temperature studies of thermally-active ASIs (TC ~ 360 K) and serve as a platform for coupling the rich physics of both complex oxide heterostructures and ASI structures. In this work, we utilize x-ray photoemission electron microscopy (X-PEEM) to image LSMO-based brickwork ASI structures where a breakdown of the Ising model of individual nanoislands occurs at a critical coupling strength which balances intra- and inter-island magnetic interactions. For ASI structures with nanoislands of identical dimensions, the ASI lattice parameter controls the inter-island coupling strength, resulting in systems composed solely of single-domain magnetic nanoislands, or a mixture of single-domain and complex spin textures (CST, i.e. single and double vortex states). Analysis of magnetic ordering in X-PEEM images reveals that nucleation of CST in frustrated nanoislands is accompanied by a disruption in long-range ordering. While it is generally assumed that inter-island interactions in ASI systems only influence the ordering of the nanomagnetic ensemble, in this LSMO-based ASI system inter-island coupling can dictate both the spin texture within individual nanoislands and long-range magnetic ordering. These results suggest the potential to create tunable nanoisland domain states, providing a new direction in developing ASI systems with exotic phase transitions as well as novel nanomagnetic logic architectures.References: 1. Skjærvø, S.H., et al., Nature Reviews Physics., Vol. 2, p.13 (2020)

The Formation of Complex Spin Textures in La0.7Sr0.3MnO3 Artificial Spin Ice Arrays

The magnetic behaviour of ferromagnetic antidot thin films can be tuned by engineering their geometry, such as the hole diameter, inter-hole distance and film thickness [1,2]. Although antidot patterned nanostructures are grown by template-assisted methods mostly using lithographical techniques [1,2], the use of anodic alumina templates has also been checked successfully [3]. However, this latter alternative shows some challenging aspects for the patterning of large areas with sufficiently high ordering and/or complex antidot geometries. These limitations are partly overcome by performing a mechanical nanoimprinting process before a single anodization process [4-6]. This work introduces an easy and less-expensive route to synthesize highly ordered nanoporous alumina templates in large areas and subsequently magnetic antidot arrays with different and complex geometries. The procedure is based on large-scale nanoimprint using patterned commercial disks as a 1-D grating stamp (e.g., CD, DVD and BR disks) followed by single anodization process which allows obtaining 1-D anodic alumina templates (Figure 1a). In addition, complex 2-D patterned nanoporous alumina templates, with square or triangular ordering (Figures 1b and c), were generated using the 1-D grating stamp by multiple imprinting steps at different angles. Afterwards, Co antidot thin films were sputtered onto these templates with different thickness (from 20 to 100 nm) and symmetries. Magnetic studies confirm the presence of in-plane anisotropy with easy axis parallel to the nanostripes in Co antidot films, while a modest in-plane bi-axial magnetic anisotropy with perpendicular magnetization easy axes is concluded in Co squared nanodot arrays. In summary, a non-expensive methodology, combining soft-imprint and single anodization, is introduced for massive mould production and complex pattern generation, avoiding standard lithographical techniques. The magnetic response of the final sputtered antidot films can be engineered by the appropriate nanoimprinted processes.References: 1. R. P. Cowburn, A. O. Adeyeye, and J. A. C. Bland, Appl. Phys. Lett. 70, 2309 (1997). 2. N. G. Deshpande, M. S. Seo, X. R. Jin, S. J. Lee, Y. P. Lee, J. Y. Rhee and K. W. Kim, Appl. Phys. Lett. 96, 122503 (2010). 3. K. J. Merazzo, C. Castán-Guerrero, J. Herrero-Albillos, F. Kronast, F. Bartolomé, J. Sesé, R.P. Del Real, L. M. García and M. Vázquez, Phys. Rev. B. 85, 184427 (2012). 4. W. Lee, R. Ji, C. A. Ross, U. Gosele and K. Nielsch, Small, 2, 978 (2006). 5. J. Choi, R.B. Wehrspohn, and A. Gösele, Adv. Mater., 15, 1531 (2003). 6. J. Um, M. R. Z. Kouhpanji, S. Liu, Z. N. Porshokouh, S.-Y. Sung, J. Kosel and B. Stadler, IEEE Trans. Magn. 56, 6701006 (2020).

Ferromagnetic antidot arrays with complex geometries

Artificial spin ice (ASI) structures, consisting of arrays of interacting, single domain nanomagnets (NMs) are promising model systems to explore microscopic features of geometrical frustration [1]. While the structural geometry is responsible for tuning the shape anisotropy and the configurational anisotropy of the system, the strength of dipolar coupling can be modified by the film thickness. In the present work, we achieved magnetic tunability for Ni80Fe20 (Py) ASI structures by three distinct methods namely, the geometrical arrangements of the nanomagnets, the variation in film thickness (20 nm, 30 nm and 50 nm) for each geometry and the applied field orientation [2]. The different ASI geometries such as square spin ice (SSI), kagome spin ice (KSI) and comparatively newer triangular spin ice (TSI) structures are shown in Fig. 1 (a), (b) and (c) respectively. The corresponding hysteresis loops in Fig. 1 (d) depict a marked difference in magnetization reversal mechanism with the variation in film thickness which indicates a possible transition from single domain nanostructure to the formation of vortex states with increasing thickness. The ferromagnetic resonance (FMR) spectra, shown in Fig. 2 explain the dependence of various spin wave modes and their spatial localization on the structural symmetry, film thickness and applied field (Happ) orientation. Micromagnetic simulations are in good agreement with the experimental data and shows the space-frequency resolved localization of the spin wave modes. The results show a great potential towards designing reconfigurable magnonic crystals for microwave filter applications.References: [1] R. F. Wang, C. Nisoli, R. S. Freitas, J. Li, W. McConville, B. J. Cooley, M. S. Lund, N. Samarth, C. Leighton, V. H. Crespi and P. Schiffer, Nature 439, 303-306 (2006). [2] A. Talapatra, N. Singh and A. O. Adeyeye, Physical Review Applied 13, 014034 (2020).

Magnetic Tunability of Permalloy Artificial Spin Ice Structures

Artificial spin ices (ASI) are magnetic metamaterials initially designed to mimic the frustrated behavior of crystalline spin ice systems such as pyrochlore crystals. They show complex magnetic ordering and can exhibit exotic phase diagrams. Unlike their crystalline counterparts, the ASI geometry can be designed and the state of their constituent elements can be directly probed. Recent studies have focused on their dynamic behavior at high frequencies (in the GHz and tens of GHz range), as they can be used as magnonic metamaterials to modify the spin-wave properties by creating band gaps in the resonance spectra [1]. Previous works have shown that different geometries produce unique dynamic spectra. Here, we performed angular-dependent broadband ferromagnetic resonance measurements on a square ASI composed of different materials for each sublattice (NiFe and CoFe). Our experiments show that the interaction between the sublattices results in unique resonance spectra attributed to the two sublattices (see Fig. 1). By performing micromagnetic simulations, we identify the modes observed in the experiment. Our results show that the interaction in the ASI can be tuned not only by the geometry of the lattice, but also by the proper choice of the materials.References: [1] S Lendinez, MB Jungfleisch, Journal of Physics: Condensed Matter, Vol. 32, p. 013001 (2019)

Tunability in the Dynamics of a Bicomponent Artificial Spin Ice

Magnetic nanoparticles operating in the magnetic vortex configuration are of interest in magnetic field sensors [1] and as magnetic nanoprobes for immunoassay diagnostics [2]. In both applications, vortex states are found to be more advantageous compared to single-/multi-domain magnetization states. We use thin film technologies and nanoimprint lithography to reliably produce and controllably stabilize magnetic spin-textures in large-area nanoparticle arrays. The periodically arranged nanoparticles are monodisperse, elliptically shaped cylinders (fig. 1). The ellipses have a size of 400 nm x 200 nm with a periodicity of 600 nm x 400 nm. The very narrow size distribution within 3% standard deviation results in highly uniform magnetic properties. The total number of ellipses is more than 108 on 1 cm2. We use this well-defined system as a model to study magnetic vortex behavior by first-order-reversal-curves (FORC). By increasing the film thickness of the Permalloy (Ni80Fe20) ellipses from 5 to 50 nm, the magnetization state changes from quasi-single domain to the vortex state. The system is anisotropic. FORC measurements provide insight into the relative proportions of reversible and irreversible magnetization processes. We interpret the FORC diagrams by a kind of Hysteron model (fig. 2) and micromagnetic simulations. From these, we can relate the FORC features to vortex properties like critical field distributions of nucleation and annihilation and interaction fields. This work helps to better interpret former FORC publications.References: [1] D. Suess, A. Bachleitner-Hofmann, A. Satz, H. Weitensfelder, C. Vogler, F. Bruckner, C. Abert, K. Prügl, J. Zimmer, C. Huber, S. Luber, W. Raberg, T. Schrefl, H. Brückl, Nature Electronics 1, 362–370 (2018) [2] S. Schrittwieser, F. Ludwig, J. Dieckhoff, K. Soulantica, G. Viau, L.-M. Lacroix, S.M. Lentijo, R. Boubekri, J. Maynadié, A. Huetten, H. Brueckl, J. Schotter, ACS Nano 6, 791 (2012)

FORC investigations of large-scale NiFe nano-ellipses arrays

In recent years, a great deal of interest has been attracted by the physics of designer materials called `Artificial Spin Ice' (ASI) systems [1-2]. One of the most promising aspects of ASI system is their high tunability of the array geometry, which has opened new avenues [2]. While most efforts have focused on study of array of artificial spin ice structures [3-4], little work has been done on understanding the switching behavior of individual nanoislands coupled by dipolar interactions in ASI systems [5-6]. These strong shape anisotropic nanomagnets exhibits Ising spin-like behavior and may get affected by the unintentional defects introduced during fabrication. Thus, it is essential to investigate how such defects modify the local magnetic behavior which in turn may play a significant role in the overall switching behavior of the corresponding nanostructures [7]. In this work, we have investigated the magnetization reversal of a special geometry of dipolar coupled system of Ti/Ni80Fe20/Al arranged in double square-ring geometry, which represents the building block of square ASI system. We employed two-dimensional electron gas (2-DEG) based micro-Hall magnetometry technique to measure the nanoislands’ stray field and study their switching behavior. Fig. 1(a) shows the SEM image of fabricated structure on top of GaAs/AlGaAs based Hall sensors. The changes in the Hall voltage reflect the changes in the magnetic state of the dipolarly coupled nanomagnets. We observe that although magnetic force microscopy images exhibit single domain like magnetic states for the nanostructures (shown in Fig. 1(b)), their reversal processes undergo complex behavior. The results suggest that local irregularities may play critical role in defining the exact micromagnetic state of the otherwise single domain nanomagnets leading to complex switching behavior, which is discussed in this work.References: [1] R. Wang, et al., Nature 439, 303 (2006). [2] N. Rougemaille and B. Canals, Eur. Phys. J. B 92, 62 (2019). [3] A. Farhan, et al., Science Adv. 5, eaav6380 (2019). [4] E. Östman, et al., Nature Phys. 14, 375 (2018). [5] F. Montoncello, et al., Phys. Rev. B 97, 014421 (2018). [6] N. Keswani and P. Das, J. Appl. Phys. 126, 214304 (2019). [7] N. Keswani, et al., Appl. Phys. Lett. 116, 102401 (2020).

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