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We present flatspin, a novel simulator for systems of interacting mesoscopic spins on a lattice, also known as artificial spin ice (ASI). A generalization of the Stoner-Wohlfarth model is introduced, and combined with a well-defined switching protocol to capture realistic ASI dynamics using a point-dipole approximation. Temperature is modelled as an effective thermal field, based on the Arrhenius-Néel equation. Through GPU acceleration, flatspin can simulate the dynamics of millions of magnets within practical time frames, enabling exploration of large-scale emergent phenomena at unprecedented speeds. We demonstrate flatspin's versatility through the reproduction of a diverse set of established experimental results from literature. In particular, the field-driven magnetization reversal of “pinwheel” ASI is reproduced, for the first time, in a dipole model. Finally, we use flatspin to explore aspects of “square” ASI by introducing dilution defects and measuring the effect on the vertex population. Flatspin: A large-scale artificial spin ice simulator

When thin films are patterned to realize nanoscale device geometries, maintaining their structural integrity is key to the quality of their functional properties. The introduction of new surfaces and interfaces by lateral modifications may alter material properties as well as the expected device functionality. In this study, two different techniques for nanoscale patterning of epitaxial thin films of La0.7Sr0.3MnO3 are used to investigate the effects on their ferromagnetic properties and film crystalline structure. Nanomagnets are realized as free- standing structures and embedded ferromagnets in a paramagnetic matrix, respectively. We find that the magnetic dichroism signal in x-ray spectomicroscopy is stronger along the edges of the embedded magnets close to TC. X-ray-diffraction measurements reveal a reduction of their in-plane lattice parameters. We discuss how in- plane stress from the nanomagnet surroundings can affect the magnetic properties in these structures. © 2020 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Long-range magnetic ordering can be stabilized in arrays of single-domain nanomagnets through dipolar interactions. In these metamaterials, the magnetic properties are determined by geometric parameters such as the nanomagnet shape and lattice symmetry. Here, we demonstrate engineering of the anisotropy in a dipolar magnetic metamaterial by tuning of the lattice parameters. Furthermore, we show how a modified Kittel's law explains the resulting domain configurations of the dipolar ferromagnetic arrays.

Two-dimensional lattices of dipolar-coupled thin film ferromagnetic nanodisks give rise to emergent superferromagnetic (SFM) order when the spacing between dots becomes sufficiently small. In this paper, we define micron-sized arrays of permalloy nanodisks arranged on a hexagonal lattice. The arrays were shaped as hexagons, squares, and rectangles to investigate finite-size effects in the SFM domain structure for such arrays. The resulting domain patterns were examined using x-ray magnetic circular dichroism photoemission electron microscopy. At room temperature, we find these SFM metamaterials to be below their blocking temperature. Distinct differences were found in the magnetic switching characteristics of horizontally and vertically oriented rectangular arrays. The results are corroborated by micromagnetic simulations.

Although it is well documented that the vortex core in a micromagnet can switch polarization in response to magnetic field pulses, the temperature dependence of this process has not yet been addressed by experiments. Using scanning transmission x-raymicroscopy, we investigate the magnetic vortex dynamics in a La0.7Sr0.3MnO3 microplatelet at temperatures ranging from 150K up to T-C at 350K. The time-resolved images reveal qualitatively different dynamic regimes as a function of temperature and applied field, as the relative strengths of the micromagnetic energy terms strongly vary over this temperature range. By explicitly accounting for the temperature dependence of the magnetic parameters in our micromagnetic simulations, we found excellent agreement with the experiments over the full measurement range. In line with previous models, the simulations reveal that the vortex core switches polarization when it reaches a critical velocity that mainly depends on the strength of the exchange interaction. It is therefore strongly temperature dependent, thus explaining our observation of the different dynamical regimes.

Magnetic metamaterials such as artificial spin ice offer a route to tailor magnetic properties. Such materials can be fabricated by lithographically defining arrays of nanoscale magnetic islands. The magnetostatic interactions between the elements are influenced by their shape and geometric arrangement and can lead to long-range ordering. We demonstrate how the magnetic order in a two-dimensional periodic array of circular disks is controlled by the lattice symmetry. Antiferromagnetic and ferromagnetic order extending through the entire array is observed for the square and hexagonal lattice, respectively. Furthermore, we show that a minute deviation from perfect circularity of the elements along a preferred direction results in room-temperature blocking and favors collinear spin textures. Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

The magnetodynamic properties of a square thin-film micromagnet ($2\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}\ifmmode\times\else\texttimes\fi{}2\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}\ifmmode\times\else\texttimes\fi{}16\phantom{\rule{4pt}{0ex}}\mathrm{nm}$) with a flux-closure magnetic ground state were investigated by micromagnetic simulations. The system was excited with an applied magnetic field, displacing the vortex core sufficiently far from its equilibrium position to result in a nonlinear relaxation upon removal of the field. We find that creation of edge topological defects leads to exchange explosions and periodic emission of short-wavelength spin waves emanating from these defects. The exchange explosions of the edge topological defects are investigated and explained in terms of vortex/antivortex creation and the formation of a Bloch point. This finding could prove useful to the development of nanoscale devices for periodic generation of high-frequency microwave radiation.

Exchange bias occurs in field-cooled antiferromagnet/ferromagnet systems and can most often be explained in terms of uncompensated magnetic moments at the interface that are pinned in their orientation during field cooling. The presence of spin-flop coupling is often associated with spin-compensated interfaces. Here, we report exchange bias in complex oxide heterostructures of antiferromagnetic ${\mathrm{LaFeO}}_{3}$ and thin layers of ferromagnetic $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ with several intriguing features. The exchange bias does not require field cooling but can also be obtained by applying a setting field at elevated temperature. Furthermore, the exchange bias is positive for setting fields up to 3 T, and its magnitude is strongly dependent on the setting-field strength. X-ray magnetic linear dichroism measurements show a predominantly perpendicular spin configuration at the interface. We discuss the possibility of the exchange bias being driven by a net moment from spin canting in the antiferromagnet due to Dzyaloshinkii-Moriya interactions.

We have studied the impact of lattice geometry on the dynamic properties of close-spaced arrays of circular nanomagnets, also known as magnonic crystals. To this end, we prepared 2D nanomagnet arrays with both square and hexagonal lattice symmetries (300-nm disk diameter, 400-nm center-to-center distance) and performed broadband ferromagnetic resonance (FMR) measurements. Micromagnetic simulations were used to interpret distinct features of the measured resonance spectra. The FMR bias field was applied along two distinct principal directions for each lattice, and a sample with well-separated, decoupled disks was measured for reference. We found that the interdisk dipolar coupling has a strong impact on the FMR for these 2D magnonic crystals. Distinctly different oscillation modes were found for the individual nanomagnets, dependent on lattice symmetry and direction of the bias field. Moreover, we find that spectral peak splitting from excitation of edge and center modes, as well as the damping, depends on the lattice symmetry and the orientation of the bias field. These findings demonstrate that lattice geometry has a strong influence on the excited spin-wave spectrum and is a relevant design parameter for spintronic devices. © American Physical Society 2019. This is the authors accepted and refereed manuscript to the article.

In this study, we report on a shape-imposed magnetic anisotropy in micro- and nanostructures defined in antiferromagnetic (AF) LaFeO3 (LFO) thin films. Two distinct types of structures are investigated: embedded magnets created via ion implantation and free-standing magnets created via ion milling. Using a combination of x-ray photoemission electron microscopy and x-ray absorption spectroscopy, we examine the impact of the structure type, AF layer thickness, and crystal geometry on the Néel vector orientation in these structures. We demonstrate a distinct shape-imposed anisotropy in embedded and free-standing structures alike and show that both parallel and perpendicular alignments of the AF spin axis with respect to structure edges can be achieved by variation of the AF layer thickness and the orientation of the structure edges with respect to the LFO crystalline axes. This work demonstrates how the fabrication procedure affects the magnetic order in thin film AF nanostructures and shows how nanoscale patterning can be used to control the orientation of the Néel vector in epitaxial oxide thin films. This is the authors’ accepted and refereed manuscript to the article. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in (citation of published article) and may be found at https://doi.org/10.1063/1.5116806

In this study, we report on a temperature-driven antiferromagnetic (AF) spin reorientation transition in micro- and nanostructures of AF/ferromagnetic (FM) LaFeO3/La0.7Sr0.3MnO3 thin film bilayers. Using a combination of x-ray photoemission electron microscopy and x-ray absorption spectroscopy, the Néel vector is shown to reorient 90° as a result of the competition between a shape-imposed anisotropy in the AF layer and interface coupling to the adjacent FM layer. We demonstrate how a temperature dependence of the AF/FM spin configuration in line-shaped nanomagnets can be tuned by variation of their linewidth. This work provides insight into the AF/FM interface exchange coupling in complex oxide heterostructures and the possibilities of spin control by nanostructuring in thin film spintronics. This is the authors’ accepted and refereed manuscript to the article. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in (citation of published article) and may be found at https://doi.org/10.1063/1.5094604

In antiferromagnetic spintronics control of the domains and corresponding spin axis orientation is crucial for devices. Here we investigate the antiferromagnetic axis in (111)-oriented LaFeO3/SrTiO3, which is coupled to structural twin domains. The structural domains have either the orthorhombic a- or b-axis along the in-plane h110i cubic directions of the substrate, and the corresponding magnetic domains have the antiferromagnetic axis in the sample plane. Six degenerate antiferromagnetic axes are found corresponding to the h110i and h112i in-plane directions. This is in contrast to the biaxial anisotropy in (001)-oriented films and reflects how crystal orientation can be used to control magnetic anisotropy in antiferromagnets. © 2017 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). [http://dx.doi.org/10.1063/1.4986555]

The emergent magnetism in close-packed assemblies of interacting superparamagnetic particles is commonly referred to as supermagnetism. The magnetic characteristics of such systems are determined by the dipolar coupling between the nanomagnets, rather than the exchange interaction responsible for ferro- and antiferromagnetism in continuous material. The dipolar coupling facilitates tuning of the magnetism, which renders supermagnetic ensembles suitable model systems for exploration of new physics. In this work, we discuss micromagnetic simulations of regular arrays of thin film nanomagnets, with magnetic material parameters typical of the ferromagnetic oxide La0.7Sr0.3MnO3. The ground state supermagnetic order in these systems is primarily determined by the lattice configuration, in that a square lattice results in antiferromagnetic order, whereas a triangular lattice shows ferromagnetic order. We found that a square lattice of circular nanomagnets may be switched from superferromagnetic to superantiferromagnetic order by a small external field applied in the appropriate direction © 2017 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). [http://dx.doi.org/10.1063/1.4978319]

© 2016 American Chemical Society. Engineered topological spin textures with submicron dimensions in magnetic materials have emerged in recent years as the building blocks for various spin-based memory devices. Examples of these magnetic configurations include magnetic skyrmions, vortices, and domain walls. Here, we show the ability to control and characterize the evolution of spin textures in complex oxide micromagnets as a function of temperature through the delicate balance of fundamental materials parameters, micromagnet geometries, and epitaxial strain. These results demonstrate that in order to fully describe the observed spin textures, it is necessary to account for the spatial variation of the magnetic parameters within the micromagnet. This study provides the framework to accurately characterize such structures, leading to efficient design of spin-based memory devices based on complex oxide thin films.

The combination of soft X-ray absorption spectroscopy with photoemission electron microscopy (X-PEEM) provides a powerful tool for imaging the domain structure in magnetically ordered systems. Using linearly polarized X-rays in the PEEM-3 microscope of the Advanced Light Source, we have investigated the antiferromagnetic (AFM) domain patterns in LaFeO 3 thin films and nanostructures. Embedded nanostructures with AFM order were defined using a structuring technique which relies on local disruption of the magnetic order by Ar + ion implantation. We demonstrate that extended AFM domains can be stabilized along the edges of such embedded nanostructures, when the edges are aligned with AFM easy axes in the thin film material. Detailed analysis of the magnetic linear dichroism in polarization-dependent X-PEEM data from LaFeO 3 /La 0.7 Sr 0.3 MnO 3 bilayers shows stabilization of multiple degenerate easy axes of the LaFeO 3 layer. We argue that the extra set of easy axes, not present in LaFeO 3 grown directly on a cubic substrate (i.e., SrTiO 3 ), arises from structural coupling to the La 0.7 Sr 0.3 MnO 3 buffer layer, acting as the effective substrate. Finally, we demonstrate how the combination of this structural perturbation and nanoscale patterning may be exploited to selectively stabilize extended AFM domains along either of the available easy axes.

The impact of the adatom energy on the stoichiometry, surface morphology, and crystalline twinning during pulsed laser deposition of La0.7Sr0.3MnO3 is studied. We show that although nonthermal growth using highly energetic adatoms results in very smooth ultrathin films, it also causes preferential resputtering of Mn and a surface roughening transition with increasing film thickness. This can be circumvented by carefully tuning the adatom energy into thermal growth, resulting in more Mn rich samples and a delayed roughening transition. Furthermore, we demonstrate that the crystalline twinning can be controlled by controlling the adatom energy. Hence, a detailed control of the adatom energy during growth opens for better stoichiometry control as well as surface quality.

In this study, we report on the magnetic domain formation in ultrathin blanket films and patterned micro- and nanostructures of ferromagnetic (FM) La0.7Sr0.3MnO3 single-layers and antiferromagnetic (AF)/ferromagnetic LaFeO3/La0.7Sr0.3MnO3 bilayers, as investigated by soft x-ray photoemission electron microscopy. In single-layer La0.7Sr0.3MnO3, the domain size is significantly reduced compared to that found in thicker layers, and rectangular micromagnets display metastable multidomain states distinctly different from the flux-closure ground states commonly found in thicker elements. In the LaFeO3/La0.7Sr0.3MnO3 bilayers, complex multidomain patterns are observed for blanket films and patterned magnets with robust perpendicular (spin-flop) coupling between spins in the AF and FM layers. By thermal cycling of the sample through the La0.7Sr0.3MnO3 Curie temperature, we find that the native antiferromagnetic domain pattern of LaFeO3 pins the location of domain boundaries in the adjacent La0.7Sr0.3MnO3 layer.In this study, we report on the magnetic domain formation in ultrathin blanket films and patterned micro- and nanostructures of ferromagnetic (FM) La0.7Sr0.3MnO3 single-layers and antiferromagnetic (AF)/ferromagnetic LaFeO3/La0.7Sr0.3MnO3 bilayers, as investigated by soft x-ray photoemission electron microscopy. In single-layer La0.7Sr0.3MnO3, the domain size is significantly reduced compared to that found in thicker layers, and rectangular micromagnets display metastable multidomain states distinctly different from the flux-closure ground states commonly found in thicker elements. In the LaFeO3/La0.7Sr0.3MnO3 bilayers, complex multidomain patterns are observed for blanket films and patterned magnets with robust perpendicular (spin-flop) coupling between spins in the AF and FM layers. By thermal cycling of the sample through the La0.7Sr0.3MnO3 Curie temperature, we find that the native antiferromagnetic domain pattern of LaFeO3 pins the location of domain boundaries in the adjacent La0.7Sr0.3MnO3 layer.

A field-driven transformation of a domain pattern in a square micromagnet, defined in a thin film of La0.7Sr0.3MnO3, is discussed in terms of creation and annihilation of bulk vortices and edge-bound topological defects with half-integer winding numbers. The evolution of the domain pattern was mapped with soft x-ray photoemission electron microscopy and magnetic force microscopy. Micromagnetic modeling, permitting detailed analysis of the spin texture, accurately reproduces the measured domain state transformation. The simulations also helped stipulate the energy barriers associated with the creation and annihilation of the topological charges and thus to assess the stability of the domain states in this magnetic microstructure. (C) 2018 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license.

Mixed-valence manganite thin films are attractive for spintronic devices. Crystalline orientation is a promising route to tailor switching mechanisms, as magnetization reversal depends on the magnetic anisotropy. Here, magnetic properties of (111)-oriented La0.7Sr0.3MnO3 thin films are elucidated by correlating macroscopic and local properties. The coercive field is an order of magnitude lower than (001)-oriented La0.7Sr0.3MnO3. Locally, a 6-fold magnetic anisotropy is observed, while macroscopically, an isotropic response is prevailing. This local coupling between the symmetry of the (111)-facet and magnetization governs the domain reversal process, demonstrating that symmetry offers a route to control magnetic properties for spintronic devices. C 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

© 2015 American Physical Society. Using soft x-ray spectromicroscopy, we investigate the magnetic domain structure in embedded nanomagnets defined in La0.7Sr0.3MnO3 thin films and LaFeO3/La0.7Sr0.3MnO3 bilayers. We find that shape-controlled antiferromagnetic domain states give rise to a significant reduction of the switching field of the rectangular nanomagnets. This is discussed within the framework of competition between an intrinsic spin-flop coupling and shape anisotropy. The data demonstrates that shape effects in antiferromagnets may be used to control the magnetic properties in nanomagnets.

The magnetic domains of embedded micromagnets with 2 μm×2 μm dimensions defined in epitaxial La0.7Sr0.3MnO3 (LSMO) thin films and LaFeO3/LSMO bilayers were investigated using soft x-ray magnetic microscopy. Square micromagnets aligned with their edges parallel to the easy axes of LSMO provide an ideal experimental geometry for probing the influence of interface exchange coupling on the magnetic domain patterns. The observation of unique domain patterns not reported for ferromagnetic metal microstructures, namely divergent antiferromagnetic vortex domains and "Z"-type domains, suggests the simultaneous presence of spin-flop coupling and local exchange bias in this system. © 2013 American Physical Society.

The technologically important exchange coupling in antiferromagnetic/ferromagnetic bilayers is investigated for embedded nanostructures defined in a LaFeO(3)/La(0.7)Sr(0.3)MnO(3) bilayer. Exploiting the element specificity of soft X-ray spectromicroscopy, we selectively probe the magnetic order in the two layers. A transition from perpendicular to parallel spin alignment is observed for these nanostructures, dependent on size and crystalline orientation. The results show that shape-induced anisotropy in the antiferromagnet can override the interface exchange coupling in spin-flop coupled nanostructures.

The antiferromagnetic domain structure in LaFeO${}_{3}$ thin-film epilayers grown on La${}_{0.7}$Sr${}_{0.3}$MnO${}_{3}$ is imaged combining linearly polarized x-ray-absorption spectroscopy with photoemission electron microscopy. Detailed analysis of polarization-dependent domain contrast from magnetic dichroism reveals two sets of symmetrically different antiferromagnetic easy axes in the LaFeO${}_{3}$ layer along the in-plane $\ensuremath{\langle}110\ensuremath{\rangle}$ and $\ensuremath{\langle}100\ensuremath{\rangle}$ crystalline axes (pseudocubic notation), respectively. We show that extended antiferromagnetic domains can be stabilized selectively for nanowires defined in this perovskite thin-film system along either orientation of the easy axes. The results demonstrate how the equilibrium domain structure of thin-film nanoscale antiferromagnets depend on a combination of substrate symmetry, nanowire dimensions, and in-plane crystalline orientation.

J. E. Boschker,1 C. Folkman,2 C. W. Bark,2 A. F. Monsen,3 E. Folven,1 J. K. Grepstad,1 E. Wahlstrom,3 C. B. Eom,2 and T. Tybell1,* 1Department of Electronics and Telecommunications, Norwegian University of Science and Technology, 7491 Trondheim, Norway 2Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, WI 53706, USA 3Department of Physics, Norwegian University of Science and Technology, 7491 Trondheim, Norway (Received 9 August 2011; revised manuscript received 6 October 2011; published 14 November 2011)

We have studied spontaneous structural ordering in epitaxial barium stannate thin films, grown on SrTiO3(001) substrates by pulsed laser deposition. X-ray diffraction analysis revealed satellite reflections around the Bragg peaks, indicative of nanoscale periodic ordering in the out-of-plane direction. We find that mass transport strongly affects this spontaneous ordering, and that the thin film growth rate can be used to tune the periodicity of the superlattices, here between 4.3 and 57.7 nm.