Stable Spin Configurations Research Articles

The martensitic structures, exchange bias effect and training effect in Ni-Mn-Ga ribbons

The exchange bias effect is the physical basics of spin valves and magnetic recording devices. A stable exchange bias effect would be suitable for practical application. In this work, Ni50Mn50-xGax (11≤ x ≤16) ribbons were obtained by melt spinning. The grain size and microstructure have been tuned through melt spinning, which changes the exchange interaction in ribbons and thus gets a giant and stable exchange bias effect. As the increase of x, the structure of Ni50Mn50-xGax (11≤ x ≤16) ribbons undergoes a transition from non-modulated martensite to non-modulated martensite &10 layers modulated martensite to 10 layers modulated martensite. The average grain size of all ribbons is less than 10 μm, and the martensitic plate widths are smaller than 200 nm. With the increasing x or the cooling fields, the magnetization of the ribbons has been enhanced. As a result, the competition between ferromagnetic and antiferromagnetic interactions has been changed and thus influencing spin glass behavior. Furthermore, a low degree of attenuation for the exchange bias effect after continuous cycles has been achieved which arises from the stable spin configuration with a larger contribution of frozen spin which has a slow relaxation rate. For example, the exchange bias field of 8.78 kOe in Ni50Mn38Ga12 ribbons can be achieved under the 40 kOe field cooling at 10 K, and the exchange bias field after infinite cycles can be kept at 8.58 kOe, the degree of attenuation is only 3.4 %. This work enriches the research on the exchange bias effect of Heusler alloys and provides the experimental basis and theoretical support for designing a stable exchange bias effect.

Exploring dielectric phenomena in sulflower-like nanostructures via Monte Carlo technique

This research focuses on the electric behavior of a mixed ferrielectric sulflower-like nanostructure. The structure includes a core with spin SiZ−1 atoms and a shell with spin σjZ−5/2 atoms. The Blume–Capel model and the Monte Carlo technique (MCt) with the Metropolis algorithm are employed. Diagrams are established for absolute zero, investigating stable spin configurations correlated with various physical parameters. The MCt method explores phase transition behavior and electric hysteresis cycles under different physical parameters.

Predicting magnetization plateaus in hexagonal boron nitride-like system: A theoretical study

This research employs Monte Carlo simulations to explore the ground state phase diagrams, hysteresis cycles, and magnetization plateaus of hexagonal boron nitride-like system. Specifically, the ground state phase diagrams are meticulously analyzed with respect to the crystalline field (D), exchange coupling parameter ([Formula: see text]), and external magnetic field (H), and are presented on the ([Formula: see text]), ([Formula: see text]), and ([Formula: see text]) planes. The study observes various stable spin configurations for different parameter values. Furthermore, this investigation delves into the behavior of sublattices of the magnetization plateaus, under the influence of exchange coupling parameter ([Formula: see text]), temperature (T), and the crystalline field (D). The outcomes of this study hold significant implications for the development of magnetic materials for diverse applications.

Antiferromagnetic Spin-Ordered Topology of Surface Functionalized Sb and Bi Monolayer Nanoflakes

Abstract Scale effect and topological frustration have been found to induce magnetic ordering in graphene-like nanoflakes. Based on triangular nanoflakes, the chemically surface-modified Sb and Bi monolayer nanotopological structures are proposed as spintronic logic gates, and their first-principles spin-polarized electron-structure calculations are performed to provide a theoretical basis for developing a new generation of ultrafast spin devices. The net spin of surface-functionalized Sb and Bi monolayer triangular nanoflakes mainly originates from the topological edge-states, exhibiting a sufficiently large spin bandgap, thus enabling stable large spin magnetic moments even under conditions of internal or edge defects and room-temperature electron excitation. The double-triangular nanoflakes of surface-functionalized Sb and Bi monolayers exhibit two stable spin configurations: ferromagnetic coupling and antiferromagnetic coupling. These configurations can serve as elementary variables for spin logic gates. Even at a linear scale as small as 2 nm, the spin coupling energy remains greater than 120 meV, and the error rate decreases below 1×10-5 at room temperature, demonstrating the spin logic gates of triple triangular nanoflakes protected by scattering-prohibited topology.

Experimental verification of the Thiele equation for skyrmion Hall angle

Magnetic skyrmions are a topologically stable spin configurations that are currently extensively studied as promising information carriers in spintronic devices. When an electric current is injected, the skyrmions exhibit the skyrmion Hall effect. This skyrmion Hall effect can be described by the Thiele equation. Here, we experimentally examined the validity of the Thiele equation. For this study, a series of Pt/Co/W films with a wide range of Co layer thicknesses were examined. The results verified the Thiele equation of the skyrmion Hall angle as a function of the magnetic anisotropy and Gilbert damping constants.

Magnetic properties and thermal behavior of the monolayer Rubrene-like nano-island: Monte Carlo simulations

The magnetic properties of a mixed Rubrene-like nano-island have been extensively investigated using Monte Carlo simulations (MCs) using the Metropolis algorithm in the context of the Blume Capel model. The ground state phase diagrams have been established to display the more stable spin configurations. The effects of various physical parameters on compensation and transition temperatures have been studied. Additionally, triple loops have been found by exploring the hysteresis cycles, which is useful in many applications in multistate memory systems.

The stability of 3D skyrmions under mechanical stress studied via Monte Carlo calculations

Using Monte Carlo (MC) simulations, we study the skyrmion stability/instability as a response to uniaxial mechanical stresses. Skyrmions emerge in chiral magnetic materials as a stable spin configuration under external magnetic field B→ with the competition of ferromagnetic interaction and Dzyaloshinskii–Moriya interaction (DMI) at low temperature T. Skyrmion configurations are also known to be stable (unstable) under a compressive stress applied parallel (perpendicular) to B→. To understand the origin of such experimentally confirmed stability/instability, we use the Finsler geometry modeling technique with a new degree of freedom for strains, which plays an essential role in DMI being anisotropic. We find from MC data that the area of the skyrmion state on the B−T phase diagram increases (decreases) depending on the direction of applied stresses, in agreement with reported experimental results. This change in the area of the skyrmion state indicates that skyrmions become more (less) stable if the tensile strain direction is parallel (perpendicular) to B→. From the numerical data in this paper, we find that the so-called magneto-elastic effect is suitably implemented in the effective DMI theory with the strain degree of freedom without complex magneto-elastic coupling terms for chiral magnetic materials. This result confirms that experimentally-observed skyrmion stability and instability are caused by DMI anisotropy.

Anisotropic magnetostructural transition in epitaxial Mn–Ni–Co–Ti Heusler alloy thin film

Heusler alloys are distinctive functional materials related to the phase transitions due to the strong magnetic and structural coupling. By the epitaxial strain from the rigid substrates, anisotropic magnetostructural transition (MST) might be observed in different crystalline orientations, leading to novel properties and functions. In this work, 62 nm thick (001)-oriented Mn49.1Ni34.0Co9.6Ti7.3 films are epitaxially grown on (001) MgO substrates. Clear MST is observed accompanied by a broad transition hysteresis from the thermomagnetic (M–T) curves with an out-of-plane (OP) magnetic field, while the transition hysteresis is absent under an in-plane (IP) magnetic field. It is related to the main lattice distortion occurring in OP orientation during the martensitic transformation because of the kinetic arrest of the IP structure by the MgO substrate. The spin glass (SG) phase is observed in IP orientation, which is lacking in OP orientation and the bulk. Clear exchange bias (EB) is observed in both the IP and OP directions. OP EB originates from exchange coupling between the ferromagnetic (FM) and antiferromagnetic phases, while IP EB is partially contributed by the exchange coupling between the SG and FM phases. It is ascribed to the establishment of metastable spin configuration in IP orientation with sufficient surrounding spins, but stable spin configuration in OP orientation due to the limited number of surrounding spins, which has been confirmed by Monte-Carlo simulation results. Our results provide a novel strategy for the modification of the physical properties of Heusler alloys and the design of novel magnetic devices.

Commensurate vortex-core switching in magnetic nanodisks at gigahertz frequencies

The development of future spintronic applications requires a thorough and fundamental understanding of the magnetisation dynamics. Of particular interest are magnetic nanodisks, in which the vortex state emerges as a stable spin configuration. Here, we focus on how the vortex core polarisation can be reversed periodically by an oscillating magnetic field, applied perpendicularly to the disk's surface. By means of micromagnetic simulations, we demonstrate the presence of several subharmonic switching modes, i.e., the commensurate ratio between the switching frequency of the core and the driving frequency. The underlying mechanism of this periodic behaviour depends on the disk thickness. For thin disks, the core switches periodically due to resonant excitation of radial spin wave modes, while it is due to the breathing mode in the case of thick disks. However, overlap of both modes impedes periodic vortex core switching. For thin disks, the threshold field amplitude required for periodic switching can be lowered to about 30~mT by increasing the disk diameter. For thick disks, in contrast, the minimal field is largely unaffected by the disk diameter, as only the energy density of a central region around the vortex core is relevant to excite the breathing mode. Our results contribute to the understanding of the switching mechanisms in magnetic nanodisks, which are of technological interest due to their potential in non-volatile memory devices.

First-principles study of electronic structure and magnetic properties of L10-ordered FeNi, FePd, and FePt alloys

The structural, electronic, and magnetic properties of three spin configurations of L10-ordered FeM alloys (M = Ni, Pd, or Pt) were studied using the first-principles method. The calculations were carried out using Quantum ESPRESSO package within the framework of Density Functional Theory (DFT). The exchange-correlation functional potentials were studied using local density approximation (LDA) of Perdew-Zunger (PZ), the generalized gradient approximation (GGA) of Perdew–Burke–Ernzerhof (PBE), Perdew and Wang 91 (PW91), and Perdew-Burke-Ernzerhof revised for solids (PBEsol). We found that the PBE approximation has the most accurate results for lattice parameters compared to the experimental values. Furthermore, our results reveal that the most stable spin configuration for the considered alloys is the ferromagnetic configuration, where all spins are aligned perpendicular to the (001) plane. However, in FePd and FePt alloys, a small variation in the tetragonality ratio c/a (from 0.98 to 0.92) can transform them from ferromagnetic to antiferromagnetic state. In an antiferromagnetic state, a pseudogap is observed just below fermi energy for each alloy. Moreover, our calculations reveal large magnetocrystalline anisotropies for FePt alloy in the order of 3 meV/f.u. On the other hand, FePd and FeNi show relatively lower values in the range of 0.18–0.42 meV/f.u. Finally, Heisenberg exchange interactions are calculated from first-principles and Green's functions formalism.

Controllable interskyrmion attractive interactions and resulting skyrmion-lattice structures in two-dimensional chiral magnets with in-plane anisotropy

We study inter-skyrmion interactions and stable spin configurations in a 2D chiral magnet with in-plane anisotropies of a tilted magnetic field and the magneto-crystalline anisotropy on a (011) thin film. We find that in both cases a small deformation of a skyrmion shape makes the inter-skyrmion interaction anisotropic, and that the skyrmions are weakly bounded along a certain direction due to an emergent attractive interaction. Furthermore, when the magneto-crystalline anisotropy is comparable to the Zeeman energy, skyrmions embedded in a uniform magnetization are tightly bound by creating a magnetic domain between them. The formation of the magnetic domain, and thus the strength of the inter-skyrmion interaction, can be controlled by the direction of an external magnetic field. The anisotropic interaction also affects the skyrmion alignment in the skyrmion crystal (SkX) phase. By employing the Monte Carlo simulation and the micromagnetic simulation, we obtain an elongated triangular lattice structure in the SkX phase. In particular, in the presence of a strong magneto-crystalline anisotropy, magnetic domains appear in the background of the lattice structure, and bimerons aligned on the domain walls form an elongated triangular lattice. We also find a parameter region that the SkX phase is stabilized due to the inter-skyrmion attraction.

Ab-initio and Monte Carlo studies of the multiferroic double perovskite Ba[formula omitted]FeMnO[formula omitted

The double perovskite Ba2FeMnO6 is studied by using density functional theory calculations and Monte Carlo simulation within the framework of the Ising model. Based on the minimum energy, the structure was optimized and the stable configuration of spins in Ba2FeMnO6 double perovskite was determined. The magnetic moments, the band structure and the electronic density of states (DOS) were analyzed in details. The oxide Ba2FeMnO6 exhibits a half-metallic behavior with a band gap close to the value 0.3 (eV). The stable spin configuration is considered for the Monte Carlo simulation. A first order transition and a compensation behavior have been found.

Weakly spin-dependent band structures of antiferromagnetic perovskite LaMO3 (M = Cr, Mn, Fe)

We investigate the spin-dependent electronic states of antiferromagnetic (AFM) lanthanum chromite (LaCrO3), lanthanum manganite (LaMnO3), and lanthanum ferrite (LaFeO3) using spin-polarized first-principles density functional theory with Hubbard U correction. The band structures are calculated for 15 types of their different AFM structures. It is verified for these structures that there is a very simple rule to identify which wave number exhibits spin splitting or degeneracy in the band structure. This rule uses the symmetry operations that map the up-spin atoms onto the down-spin atoms. The resulting spin splitting is very small for the most stable spin configuration of the most stable experimental structure. We discuss a plausible benefit of this characteristic, i.e. the direction-independence of the spin current, in electrode applications.

Control of skyrmion movement in nanotrack by using periodic strain

Magnetic skyrmions are a topologically stable and particle-like chiral spin configuration. They are appealing because of their potential applications in racetrack memory and other spintronic devices. These applications are strongly dependent on the skyrmion motion in confined geometry. Therefore, it is important to study the moving behaviors of skyrmions in a nanotrack to make them have more practical applications. Mechanical strain and stress have been demonstrated theoretically and experimentally to be able to effectively control the skyrmion phase. It can stabilize the skyrmion lattice in a broad range, and change the shape of the skyrmion crystal. In this paper, we study the moving behaviors of ferromagnetic skyrmions and antiferromagnetic skyrmions under the action of sinusoidally distributed strain in a nanotrack by using micromagnetic simulation. We assume that strain is uniaxial and perpendicular to the plane of the nanotrack. Its strength varies sinusoidally along the x-axis. Meanwhile, we apply an in-pane current along the nanotrack to drive the skyrmion moving towards the right side. We first find that there is a threshold current density that is defined as the minimum current that can drive skyrmion moving continuously. When the current density is larger than the threshold current density, the skyrmion can move continuously in the nanotrack. The threshold current density increases with the amplitude of strain increasing, but decreases with the period of strain increasing. Second, we find that the trajectory of skyrmion changes under the action of the sinusoidal distributed strains. For ferromagnetic skyrmion, its trajectory changes from straight line to periodic wavy line. Also, we find that the longitudinal velocity of skyrmion is affected by the boundary of the nanotrack. When the skyrmion is close to the upper boundary of the nanotrack, the longitudinal velocity increases sharply and it will form a peak in the velocity curve, but when the skyrmion is close to the lower boundary of the nanotrack, the longitudinal velocity decreases and it will form a valley in the velocity curve. The transverse velocity of skyrmion relates to the strain gradient. It is inversely proportional to the strain gradient. For antiferromagnetic skyrmion, we find that the movement trajectory of antiferromagnetic skyrmion does not change under the stress control. However, its diameter and velocity change periodically. Its velocity can vary between 103 m/s and 0. Our results demonstrate that the sinusoidal strain can control the skyrmion motion. This work may provide guidance in designing and developing of the spintronic devices based on magnetic skyrmions.

Skyrmions-based magnetic racetrack memory

Magnetic skyrmions are topologically stable spin configurations with small size, which can be driven into motion by a small current. They are widely regarded as building blocks for next-generation magnetic storage. The main advantage of skyrmions lies in their particular dynamic behaviors, especially in their ability to move stably in racetrack under the action of small spin-polarized currents. The writing, driving and reading methods of skyrmions in racetrack are reviewed in detail in this paper, including the most recent research findings. The review focuses on the most commonly used driving method, i.e., driving skyrmions by applying spin-polarized currents. The clogging and annihilation of skyrmions in racetrack are analyzed, with the skyrmion Hall effect discussed which may lead skyrmion signals to lose. Methods to avoid skyrmion Hall effect are introduced and hence the optimized designs for skyrmion-based racetrack are also reviewed. Finally, some challenges of skyrmion-based racetrack memory are discussed.

Stabilization and current-induced motion of antiskyrmion in the presence of anisotropic Dzyaloshinskii-Moriya interaction

Topological defects in magnetism have attracted great attention due to fundamental research interests and potential novel spintronics applications. Rich examples of topological defects can be found in nanoscale non-uniform spin textures, such as monopoles, domain walls, vortices, and skyrmions. Recently, skyrmions stabilized by the Dzyaloshinskii-Moriya interaction have been studied extensively. However, the stabilization of antiskyrmions is less straightforward. Here, using numerical simulations we demonstrate that antiskyrmions can be a stable spin configuration in the presence of anisotropic Dzyaloshinskii-Moriya interaction. We find current-driven antiskyrmion motion that has a transverse component, namely antiskyrmion Hall effect. The antiskyrmion gyroconstant is opposite to that for skyrmion, which allows the current-driven propagation of coupled skyrmion-antiskyrmion pairs without apparent skyrmion Hall effect. The antiskyrmion Hall angle strongly depends on the current direction, and a zero antiskyrmion Hall angle can be achieved at a critic current direction. These results open up possibilities to tailor the spin topology in nanoscale magnetism, which may be useful in the emerging field of skyrmionics.

Chiral helimagnetic state in a Kondo lattice model with the Dzyaloshinskii–Moriya interaction

Monoaxial chiral magnets can form a noncollinear twisted spin structure called the chiral helimagnetic state. We study magnetic properties of such a chiral helimagnetic state, with emphasis on the effect of itinerant electrons. Modeling a monoaxial chiral helimagnet by a one-dimensional Kondo lattice model with the Dzyaloshinskii–Moriya interaction, we perform a variational calculation to elucidate the stable spin configuration in the ground state. We obtain a chiral helimagnetic state as a candidate for the ground state, whose helical pitch is modulated by the model parameters: the Kondo coupling, the Dzyaloshinski–Moriya interaction, and electron filling.

Voltage Controlled Magnetic Skyrmion Motion for Racetrack Memory.

Magnetic skyrmion, vortex-like swirling topologically stable spin configurations, is appealing as information carrier for future nanoelectronics, owing to the stability, small size and extremely low driving current density. One of the most promising applications of skyrmion is to build racetrack memory (RM). Compared to domain wall-based RM (DW-RM), skyrmion-based RM (Sky-RM) possesses quite a few benefits in terms of energy, density and speed etc. Until now, the fundamental behaviors, including nucleation/annihilation, motion and detection of skyrmion have been intensively investigated. However, one indispensable function, i.e., pinning/depinning of skyrmion still remains an open question and has to be addressed before applying skyrmion for RM. Furthermore, Current research mainly focuses on physical investigations, whereas the electrical design and evaluation are still lacking. In this work, we aim to promote the development of Sky-RM from fundamental physics to realistic electronics. First, we investigate the pinning/depinning characteristics of skyrmion in a nanotrack with the voltage-controlled magnetic anisotropy (VCMA) effect. Then, we propose a compact model and design framework of Sky-RM for electrical evaluation. This work completes the elementary memory functionality of Sky-RM and fills the technical gap between the physicists and electronic engineers, making a significant step forward for the development of Sky-RM.

Theoretical design of magnetic wires from acene and nanocorone derivatives.

Theoretical calculations on a series of molecular models based on amino derivatives of linear and cycled acenes acting as organic linkers between two copper(ii) ions have shown a wire-like magnetic behaviour, so that the intermetallic magnetic communication does not vanish when the linker becomes larger due to its polyradical nature. Hence, these models can be considered as molecular magnetic wires, which can be used as active components for molecular spintronics, where the information transport is based on spin carriers instead of the more conventional charge transport. The nature of the spin ground state along these two series of models is governed by the topology of the organic linker, in agreement with the spin polarization mechanism, allowing a wide diversity in the design of molecular spintronic circuits. Different approaches to search for the more stable spin configuration have been tested, the most efficient being the one that allows generating a guess function that agrees with the spin polarization mechanism.

Temperature and magnetic field-assisted switching of magnetization and observation of exchange bias in YbCrO3 nanocrystals.

In this paper, we demonstrate an interesting feature in YbCrO3 (YCO) nanocrystals, in which the material shows temperature and external magnetic field-assisted switching (a complete sign reversal) of zero field cooled magnetization (MZFC) and observation of exchange bias (EB) as a result of competing spin interaction at low temperature. This feature can be applied in nonvolatile memories, where, simply by changing the magnitude of the Hext and T, the polarity of the magnetization can be switched between negative and positive. We also observed negative magnetization in YCO. Our results showed that, below its Nèel temperature (TN ≈ 119 K), the MZFC crosses over to negative sign for H < 1000 Oe. At 60 K, YCO showed a significant negative MZFC ≈ -0.05 emu/g (at 100 Oe) due to the competing effects of Yb(3+), Cr(3+) spins, thermal activation energy, and Hext. At further lower temperatures, the MZFC showed a crossover to positive values, and the crossover temperature showed the dependence on Hext (∼19 K for 100 Oe curve). The YCO also showed Hext and T-dependent HEB, which changed its sign with T. The observed T-dependent sign reversal in the EB was closely associated with the sign reversal of MZFC. The symmetric shift in field-cooled isothermal hysteresis curves confirmed that the observed EB was not due to the unsaturated minor loop. The training cycle further confirmed that the HEB value decreased to ∼2% of the initial value of observed EB, which was very small compared to the observed HEB in YCO, which indicated stable spin configuration at the locally formed ferromagnetic/antiferromagnetic interface.