Search

Vortex domain walls (DWs) are characterized by their chirality, an important property that needs to be controlled for the use of such walls in potential technological applications. In this work we explore a wire-ring structure in which we have alternate hard and soft magnetic materials. Our results evidence that, depending on the materials, it is possible to control the DW chirality when it goes through the ring section. Therefore, this system can be used as a device that controls domain wall chirality.

In this work we performed a detailed numerical analysis on the static and dynamic properties of magnetic antidot arrays as a function of their geometry. In particular, we explored how by varying the shape of these antidot arrays from circular holes to stadium-shaped holes, we can effectively control the magnetic properties of the array. Using micromagnetic simulations we evidenced that coercivity is very sensitive to the shape of antidots, while the remanence is more robust to these changes. Furthermore, we studied the dynamic susceptibility of these systems, finding that it is possible to control both the position and the number of resonance peaks simply by changing the geometry of the holes. Thus, this work provides useful insights on the behavior of antidot arrays for different geometries, opening routes for the design and improvement of two-dimensional technologies.

The manifestations of the dipolar interaction in magnetic nano-devices can take subtle forms. In this work, we address the effects of dipolar interactions on skyrmions textures. The so-called twisted skyrmion is a natural state between the Bloch skyrmion, that arises from the bulk-like Dzyalonshinskii-Moriya (DM) interaction, and the Neel-skyrmion, that results from the interfacial form of the DM interaction. Often neglected, or approximated by local interactions, we show how, when explicitly included, dipolar interactions generate gaps in the phase diagram for certain helicities. Besides, this interaction allows the existence of skyrmions with two chiralities, which are unstable when the dipolar interaction is approximated by a shape anisotropy.

Magnetic nanorings have been widely studied due to their potential applications in spintronic and magnonic devices. In this work, by means of analytical calculations and micromagnetic simulations we have analyzed the magnetic energy of nanorings with variable anisotropy along their radius. Four magnetic states, including two new magnetic configurations, here called meron and knot-like states, are considered, looking to the relative lower energy states as a function of anisotropy. Phase diagrams with this states are presented., 6 pages, 4 figures

The study of patterned magnetic elements that can sustain more than one bit of the information is an important research line for developing new routes in magnetic storage and magnetic logic devices. Previous Monte Carlo studies of T-shaped magnetic nanostructures revealed the equilibrium and evolution of magnetic states that could be found as a result of the strong configurational anisotropy of these systems. In this work, for the first time, such behavior of T-shaped magnetic nanostructures is experimentally studied. In particular, T-shaped Co nanostructures have been produced by electron beam lithography using a single step lift-off process over Si substrates. The existence of four magnetic stable states has been proven by Magnetic Force Microscopy (MFM) and the analysis was complemented by Micromagnetic Simulations. The results confirmed that even for what can be considered large structures, with μm sizes, such four stable magnetic states can be achieved, and therefore two magnetic bits of information can be stored. We also addressed how to write and read those bits., This research was funded by Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro—FAPERJ, grants E26/210.903/2014, E26/210.183/2018 and E-26/010.000980/ 2019, Conselho Nacional de Desenvolvimento Científico e Tecnológico—CNPq—Brasil, grants 312275/2018-8 and 424022/2018-4, and by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior—CAPES—Brasil, grant PROEX/88882.332427/2010-01.

The study of curvature-induced effects on the properties of nanostructures has become a cornerstone of magnetism. However, several methodologies usually used for studying nanoscale magnetic systems present difficulties for adequately describing curvature. In this work, we present a method that allows studying, under specific conditions, curved dome/antidome surfaces using an equivalent system without curvature. From the described methodology we obtain the phase diagram between easy-normal and skyrmionic magnetization configurations, as a function of spin-orbit coupling, Dzyaloshinskii-Moriya interaction (DMI), and curvature. The effective DMI of the dome structure increases with the curvature. Nevertheless, the effective anisotropy presents the opposite behavior, decreasing with curvature. These results allow us to conclude that an increase in the skyrmion stability is observed in nanostructures having positive curvature. The presented results propose a route that could facilitate the study of curved nanofilms with intrinsic DMI from comparing them with their planar counterparts.

[EN] Magnetic cylindrical nanowires are promising candidates for future three-dimensional nanotechnology. Domain walls (DWs) in magnetic nanowires play the role of information carriers, and the development of applications requires proper description of their dynamics. Here we perform a detailed analytical and numerical analysis of the DW motion along a bent magnetic nanowire under the action of tangential magnetic fields. Our results show that the DW velocity, precession, and oscillation frequencies can be controlled by the interplay between the curvature and the external magnetic field. Small magnetic fields induce a DW motion without precession and oscillatory behavior, while higher magnetic fields yield a Walker breakdown regime, in which an oscillatory forward and backward DW motion is observed. Controlled DW motion under the Walker breakdown regime makes magnetic nanowires potential candidates for nanoscale microwave generation and sensing., In Brazil, this study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior -Brasil (CAPES) - Finance Code 001. The authors also thank CNPq (Grants No. 401132/2016-1 and No. 302084/2019-3). In Scotland, we acknowledge the Natural Environment Research Council (Grant No. NE/S011978/1). In Chile, we acknowledge financial support from FONDECYT Grant No. 1160198 and CONICYT through Centro Basal FB0807. In Spain, we acknowledge the financial support from the Ministry of Economy and Competitivity under Grant No.MAT2016-76824-C3-1-R.

During the last years, topologically protected collective modes of the magnetization have called much attention. Among these, skyrmions and merons have been the object of intense study. In particular, topological skyrmions are objects with an integer skyrmion number $Q$ while merons have a half-integer skyrmion charge $q$. In this work, we consider a $Q=1$ skyrmion, composed by a meron and an antimeron (bimeron), displacing in a ferromagnetic racetrack, disputing a long-distance competition with its more famous counterpart, the typical $Q=1$ cylindrically symmetrical skyrmion. Both types of topological structures induce a Magnus force and then are subject to the Hall effect. The influence of the Dzyaloshinskii-Moriya interaction ($DMI$) present in certain materials and able to induces $DMI$-skyrmions is also analyzed. Our main aim is to compare the motions (induced by a spin-polarized current) of these objects along with their own specific racetracks. We also investigate some favorable factors which are able to give breath to the competitors, impelling them to remain in the race for longer distances before their annihilation at the racetrack lateral border. An interesting result is that the $DMI$-skyrmion loses this hypothetical race due to its larger rigidity., Comment: 6 pages, 5 figures

In this work, the current-induced inertial effects on skyrmions hosted in ferromagnetic systems are studied. {When the dynamics is considered beyond the particle-like description, magnetic skyrmions can deform due to a self-induced field. We perform Monte Carlo simulations to characterize the deformation of the skyrmion during its movement}. In the low-velocity regime, the deformation in the skyrmion shape is quantified by an effective inertial mass, which is related to the dissipative force. When skyrmions move faster, the large self-induced deformation triggers topological transitions. The transition is characterized by the proliferation of skyrmions and different total topological charge, which are obtained in terms of the skyrmion velocity. Our findings provide an alternative way to describe the skyrmion dynamics that take into account the deformations of its structure. Furthermore, the motion-induced topological phase transition brings the possibility to control the number of ferromagnetic skyrmions by velocity effects., Comment: 6 pages + 4 figures

The understanding of the domain wall (DW) dynamics along magnetic nanowires is crucial for spintronic applications. In this work, we perform a detailed analysis of the transverse DW motion along nanowires with polygonal cross-sections. If the DW displaces under a magnetic field above the Walker limit, the oscillatory motion of the DW is observed. The amplitude, the frequency of oscillations, and the DW velocity depend on the number of sides of the nanowire cross-section, being the DW velocity in a wire with a triangular cross-section one order of magnitude larger than that in a circular nanowire. The decrease in the nanowire cross-section area yields a DW behavior similar to the one presented in a cylindrical nanowire, which is explained using an analytical model based on the general kinetic momentum theorem. Micromagnetic simulations reveal that the oscillatory behavior of the DW comes from energy changes due to deformations of the DW shape during the rotation around the nanowire., We thank the fnancial support from Financiamiento Basal AFB 180001 para Centros Científcos y Tecnológicos de Excelencia and proyecto FONDECYT 1200867. In Brazil, we thank CNPq (Grant numbers 401132/2016-1 and 302084/2019-3) and Fapemig for fnantial support. YPI acknowledges the support from the state task of the Ministry of Science and Higher Education of the Russian Federation No. 0657-2020-0005. O.C.-F. thanks the support from the Spanish MINECO under Grant MAT2016-76824-C3-1-R. R.C. acknowledge University of Santiago de Chile through Grant Dicyt 041831AD.

Based on Monte Carlo numerical simulations: results for the magnetization reversal process in thin circular Ni dots connected by a bridge are presented. The dependence of the process on both the width of the bridge and the orientation of the applied magnetic field has been investigated. It was found that when the applied magnetic field is set parallel to the bridge, the hysteresis curves are weakly dependent on the width of the bridge, being rather close to that of a single dot of the same diameter. On the other hand, when the magnetic field is applied perpendicularly to the bridge, a significant reduction in the coercivity of the system is obtained, even in the case of narrower bridges.

The magnetic stability of a multilayer system formed by N slabs separated by nonmagnetic spacers has been studied, emphasizing the role of dipolar interactions among magnetic slabs. By means of numerical and analytical calculations, we have identified a regime in which the magnetic slabs behave independently, i.e., a critical thickness of the spacer at which dipolar interactions can be safely disregarded. The methodology used to establish this critical distance can be used for other geometries, as it is a useful tool to identify regimes of non-interacting magnetic slabs.

In the last years, electric fields have been used to control the magnetic exchange interactions and anisotropies in nanometric devices. In this paper, we study the spin-spin exchange interaction between two magnetic impurities embedded in a three-dimensional nonrelativistic electron gas, namely the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. The gas is confined in an insulating structure, and an applied voltage produces local changes in the electron density, which modulates the Fermi level of the system. Using a simple model, we demonstrate that this voltage modifies the strength and wavelength of the coupling between the impurities. Depending on the voltage, the effective RKKY exchange can change from a ferro- to an antiferromagnetic coupling, and vice versa. The spin-spin coupling can also be switched on and off by the voltage.

Skyrmions have become one of the most visited topics during the last decade in condensed matter physics. In this work, and by means of analytical calculations and micromagnetic simulations, we explore the effect of the magnetostatic field generated by a magnetic tip on the stability of skyrmions. Our results show that the interaction energy between the tip and the skyrmion plays a fundamental role in the stabilization of N\'eel skyrmions confined in nanodisks, allowing its nucleation and annihilation, and also providing precise control of its size and polarity. Based on our results, we propose a very simple and cyclic method to nucleate and annihilate skyrmions, as well as to control their polarity and chirality. This proposal could open new possibilities for logic devices taking advantage of all the degrees of freedom that skyrmionic textures have.

The magnetization dynamics of nano-oscillators may be excited by both magnetic fields and spin-polarized currents. While the dynamics of single oscillators has been well characterized, the synchronization of several ones is not fully understood yet. An analytical and numerical study of the nonlinear dynamics of two magnetostatically coupled spin valves driven by spin-transfer torques is presented under the macrospin approximation. The oscillators interact via magnetostatic fields and exhibit a robust synchronized magnetization motion. We describe the magnetization dynamics of the system using the Landau-Lifshitz-Gilbert-Slonczewski equation. Using a modal decomposition technique, we describe the dynamics, synchronization, and competition of oscillatory modes as a function of the current density, and the geometrical parameters of the setup. Simulations of the Landau-Lifshitz-Gilbert-Slonczewski equation show good agreement with an approximate analytic solution.

We investigate the dynamics of a transverse domain wall (DW) in a bent nanostripe under an external field and spin-polarized current. Besides the standard Walker breakdown phenomenon, we show the emergence of a second Walker-like critical field, which depends on both the curvature of the nanostripe and its cross section geometry. At this field, DW can change its phase, i.e., can be re-oriented along another direction with respect to the nanostripe face. Additionally, we show that the amplitude and frequency of the DW oscillations above the Walker breakdown field also depend on the nanostripe geometry and can be controlled by external stimuli. Our results evidence that the inclusion of local curvatures in nanostripes is an important component for applications that demand an adequate control of the DW phase by the proper choice of external stimuli.

Mosaic nanocomposites composed of nickel and cobalt nanowires arranged in different configurations were investigated using Monte Carlo simulations and a simple model that considers single-domain structures including length corrections due to the shape anisotropy. Our results showed that for an ordered array both the coercivity and the remanence decrease linearly as a function of the concentration of nickel nanowires. Besides, we obtained that the magnetic properties of an array of a certain hard magnetic material (cobalt) will not change, unless we have more than 50% of nanowires of other soft magnetic material (nickel) in the array. In principle the second material could be other soft magnetic material, but could also be a nonmagnetic material or could even be a situation in which some of the pore arrays were not filled by electrodeposition. Therefore, our results allow us to predict the behavior of magnetic mosaic nanocomposites that are promising candidates for functional electrodes, sensors, and model catalysts.

The relative stability of out-of-plane ferromagnetic, vortex without core and vortex with core configurations in cylindrical nanostructures are examined in terms of their geometric and magnetic parameters. Numerical calculations are done for general geometrical structures, while analytical expressions are obtained in the limiting cases of very long nanostructures. A magnetic phase diagram that clearly identifies the parameters required to obtain a vortex with core as minimum energy configuration is obtained by computing the transition lines between these configurations. The results define the geometric and magnetic parameters for which the vortex state can be made more stable by removing the highly energetic vortex core. Finally, a phase diagram that may be useful for the potencial use of vortices in technological applications is obtained and discussed.

The magnetic properties of a thin film consisting of an exchange-coupled [Co/Ni](2)/NiFe multilayer have been studied as a function of the NiFe thickness by using Monte Carlo modeling and compared ...

In this paper, by means of analytical calculations and a simple model, we studied the effect of curvature in magnetic thin stripes. Three different domain walls were observed, depending on the slab geometry. A phase diagram that shows the geometric parameters at which each domain wall is preferred was obtained. The presented methodology can also be used to obtain the magnetization behavior in other curved structures with proper aspect ratio relations.

By means of Monte Carlo simulations we studied field driven nucleation and propagation of transverse domain walls (DWs) in magnetic nanowires subjected to temperature gradients. Simulations identified the existence of critical thermal gradients that allow the existence of reversal processes driven by a single DW. Critical thermal gradients depend on external parameters such as temperature, magnetic field and wire length, and can be experimentally obtained through the measurement of the mean velocity of the magnetization reversal as a function of the temperature gradient. Our results show that temperature gradients provide a high degree of control over DW propagation, which is of great importance for technological applications.

Curvature of nanomagnets can be used to induce chiral textures in the magnetization field. Here we perform analytical calculations and micromagnetic simulations aiming to analyze the stability of in-surface magnetization configurations in toroidal nanomagnets. We have obtained that despite toroidal vortex-like configurations are highly stable in magnetic nanotori, the interplay between geometry and magnetic properties promotes the competition between effective interactions yielding the development of a core in a vortex state when the aspect ratio between internal and external radii of nanoturus is ≥ 0.75 .

A detailed analytical and numerical study of the spin wave modes of the free layer of a nanopillar spin torque nano-oscillator (STNO) has been performed as a function of the magnetostatic interaction between the free and the fixed magnetic layers. Results for higher frequency normal modes show that the magnetostatic interaction does not appreciably affect the spin wave frequencies and the critical current densities, due to more relevance of the exchange interaction in these modes. For lower frequency normal modes we observe a decrease in frequency and in the critical current density for auto-oscillations when the strength of the magnetostatic interaction between the layers increases, an effect that may be appreciable.

Understanding the domain wall dynamics is an important issue in modern magnetism. Here we present results of domain wall displacement in curved cylindrical nanowires at a constant magnetic field. We show that the average velocity of a transverse domain wall increases with curvature. Contrary to what it is observed in stripes, in a curved wire the transverse domain wall oscillates along and rotates around the nanowire with the same frequency. These results open the possibility of new oscillation-based applications., Comment: 6 Figures

The recently introduced area of topological magnetism searches for equilibrium structures stabilized by a combination of interactions and specific boundary conditions. Until now, the internal energy of open magnetic chains has been explored. Here, we study the energy landscape of closed magnetic chains with on-site anisotropy coupled with antiferromagnetic exchange and dipolar interactions analytically and numerically. We show that there are many stable stationary states in closed geometries. These states correspond to the noncollinear spin spirals for vanishing anisotropy or to kink solitons for high magnetic anisotropy. Particularly, the noncollinear M\"obius magnetic state can be stabilized at finite temperatures in nonfrustrated rings or other closed shapes with an even number of sites without the Dzyaloshinskii-Moriya interaction. We identify the described configurations with the stable stationary states, which appear due to the finite length of a ring.

In this work, we study nonlinear aspects of the deterministic spin dynamics of an anisotropic single-domain magnetic particle at finite temperature modeled by the Landau-Lifshitz-Bloch equation. The magnetic field has two components: a constant term and a term involving a harmonic time modulation. The dynamical behavior of the system is characterized with the Lyapunov exponents and by means of bifurcation diagrams and Fourier spectra. In particular, we explore the effects of the magnitude and frequency of the applied magnetic field, finding that the system presents multiple transitions between regular and chaotic states when varying the control parameters. We also address the temperature dependence and evidence that it plays an important role in these transitions, almost suppressing the chaotic behavior close to the Curie temperature. Finally, we find that the system has hyperchaotic states for specific values of field and temperature., In Chile, we acknowledge the partial support from a CONICYT-FONDECYT Postdoctoral fellowship, Grant No. 3130678, Fondecyt Grants No. 1120764 and No. 1160198, CONICYT-ANILLO ACT 1410, and the Basal Program through the Center for the Development to Nanoscience and Nanotechnology (Grant No. CEDENNA FB0807). We also acknowledge the support from the Spanish Ministry of Economy and Competitiveness under the Grant No. MAT2013-47078-C2-2-P. In Colombia, O.S. acknowledges the internal UniSucre Project No. 42-2015.

International audience; Segmented magnetic nanowires are a promising route for the development of three dimensional data storage techniques. Such devices require a control of the coercive field and the coupling mechanisms between individual magnetic elements. In our study, we investigate electrodeposited nanomagnets within host templates using vibrating sample magnetometry and observe a strong dependence between nanowire length and coercive field (25 nm to 5 µm) and diameter (25 nm to 45 nm). A transition from a magnetization reversal through coherent rotation to domain wall propagation is observed at an aspect ratio of approximately 2. Our results are further reinforced via micromagnetic simulations and angle dependent hysteresis loops. The found behavior is exploited to create nanowires consisting of a fixed and a free segment in a spin-valve like structure. The wires are released from the membrane and electrically contacted, displaying a giant magnetoresistance effect that is attributed to individual switching of the coupled nanomagnets. We develop a simple analytical model to describe the observed switching phenomena and to predict stable and unstable regimes in coupled nanomagnets of certain geometries.

A discussion on the interaction between skyrmions in a bi-layer system connected by a non-magnetic metal is presented. From considering a free charge carrier model, we have shown that the Ruderman-Kittel-Kasuya-Yosida (RKKY ) inter- action can induce attractive or repulsive interaction between the skyrmions depending on the spacer thickness. We have also shown that due to an increasing in RKKY energy when the skyrmions are far from each other, their widths are diminished. Finally, we have obtained analytical solutions to the skyrmion position when the in-plane distance between the skyrmions is small and it is shown that an attractive RKKY interaction yields a skyrmion precessory motion. This RKKY-induced coupling could be used as a skyrmion drag mechanism to displace skyrmions in multilayers., 5 Pages, 7 Figures

A detailed analytical and numerical study of the spin wave modes of two nanopillar spin torque nano-oscillators coupled by magnetostatic interactions is presented under the macrospin approximation. Results show that the normal modes of the system oscillate with the magnetizations in-phase or anti-phase in both disks. The frequencies and critical current densities necessary to induce auto-oscillations of the spin wave modes of the coupled system depend on the relative position of the nanopillars and the applied magnetic field. If the oscillators are identical, these modes are degenerate at a certain relative position of the nanopillars, while if the oscillators are non-identical, such degeneracy is removed. Then, we can conclude that the magnetostatic coupling between two spin transfer torque nano-oscillators is a powerful mechanism to control the spin wave modes of these systems.

In response to ultrafast laser pulses, single-phase metals have been classified as “fast” (with magnetization quenching on the time scale of the order of 100 fs and recovery in the time scale of several picoseconds and below) and “slow” (with longer characteristic time scales). Disordered ferrimagnetic alloys consisting of a combination of “fast” transition (TM) and “slow” rare-earth (RE) metals have been shown to exhibit an ultrafast all-optical switching mediated by the heat mechanism. The behavior of the characteristic time scales of coupled alloys is more complicated and is influenced by many parameters such as the intersublattice exchange, doping (RE) concentration, and the temperature. Here, the longitudinal relaxation times of each sublattice are analyzed within the Landau-Lifshitz-Bloch framework. We show that for moderate intersublattice coupling strength both materials slow down as a function of slow (RE) material concentration. For larger coupling, the fast (TM) material may become faster, while the slow (RE) one is still slower. These conclusions may have important implications in the switching time of disordered ferrimagnets such as GdFeCo with partial clustering. Using atomistic modeling, we show that in the moderately coupled case, the reversal would start in the Gd-rich region, while the situation may be reversed if the coupling strength is larger., This work was supported from CONICYT-FONDECYT Postdoctoral program fellowship under Grant No. 3130678 (Chile); Spanish Ministry of Economy and Competitiveness under the grants FIS2010-20979-C02-02, MAT2013-47078-C2-2-P and by the European Community's Seventh Framework Programme FP7/2007-2013 under Grant agreement No. 281043, FEMTOSPIN; FONDECYT under Grant No. 1120764 and 1120356; Basal Program Center for Development of Nanoscience and Nanotechnology (CEDENNA); Millennium Scientific Initiative (Chile) Grant No. P10-061-F, UTA-project No. 8750-12 (Chile), and Engineering and Physical Sciences Research Council (United Kingdom) Grant No. EP/L002922/1.

In this work, and by means of micromagnetic simulations, we study the magnetic properties of toroidal nanomagnets. The magnetization ground state for different values of the aspect ratio between the toroidal and polar radii of the nanotorus has been obtained. The hysteresis curves are also obtained, evidencing the existence of two reversal modes depending on the geometry: a vortex mode and a coherent rotation. A comparison between toroidal and cylindrical nanoparticles has been performed evidencing that nanotori can accommodate a vortex as the ground state for smaller volume than cylindrical nanorings. This is important because if vortices are used as bits of information, nanotori allow a higher density data storage., 19 pages, 14 Figures