Remanent State Research Articles

Engineering Spin Configurations of Synthetic Antiferromagnet by Controlling Long-Range Oscillatory Interlayer Coupling and Neighboring Ferrimagnetic Coupling.

Controllable manipulation of specific spin configurations of magnetic materials is the key to constructing functional spintronic devices. Here, it is demonstrated by integrating the merits of ferromagnetic, ferrimagnetic, and antiferromagnetic spin configurations into one synthetic antiferromagnetic (SAF) heterostructure by controlling both long-range oscillatory interlayer coupling and neighboring ferrimagnetic coupling. A controllable manipulation of four types of spin configurations of the Pt/[Co/Pt/Co]/Ru/CoTb SAF heterostructures composed of ferromagnetic Co/Pt/Co and ferrimagnetic CoTb layers is successfully achieved. In particular, the compensated magnetization, enhanced anomalous Hall resistance in the remanence state, wide-temperature spin-orbit torque switching of magnetization, and high immunity to the external magnetic field are simultaneously obtained in one of the SAF heterojunctions with macroscopic interlayer antiferromagnetic coupling. This design concept of engineering spin configurations may enable efficient spin manipulation for customized memory and logic applications.

Conducting ferroelectric domain walls emulating aspects of neurological behavior

The electrical conductivity of lithium niobate thin film capacitor structures depends on the density of conducting 180° domain walls, which traverse the interelectrode gap, and on their inclination angle with respect to the polarization axis. Both microstructural characteristics can be altered by applying electric fields, but changes are time-dependent and relax, upon field removal, into a diverse range of remanent states. As a result, the measured conductance is a complex history-dependent function of electric field and time. Here, we show that complexity in the kinetics of microstructural change, in this ferroelectric system, can generate transport behavior that is strongly reminiscent of that seen in key neurological building blocks, such as synapses. Successive voltage pulses, of positive and negative polarity, progressively enhance or suppress domain wall related conductance (analogous to synaptic potentiation and depression), in a way that depends on both the pulse voltage magnitude and frequency. Synaptic spike-rate-dependent plasticity and even Ebbinghaus forgetting behavior, characteristic of learning and memory in the brain, can be emulated as a result. Conductance can also be changed according to the time difference between designed identical voltage pulse waveforms, applied to top and bottom contact electrodes, in a way that can mimic both Hebbian and anti-Hebbian spike-timing-dependent plasticity in synapses. While such features have been seen in, and developed for, other kinds of memristors, few have previously been realized through the manipulation of conducting ferroelectric domain walls.

In plane magnetization reversal in nanosized thin truncated conical double-disks of permalloy

In plane magnetization reversal of permalloy thin truncated conical double-disk as a function of the is investigated using micromagnetic simulations with fixed base radius (R) of 100 nm. When external magnetic field is applied along the longer axis of the double-disk, the remanent states change gradually from vortex state to S state and then to buckled magnetization state with reduction of σ. When σ ≈ 1 and the conical nanodisk resembles to a regular cylindrical nanodisk, incoherent magnetization reversal is dominant whereas tapering of conical disk reduces extent of incoherence in magnetization reversal. As tapering of nanodisk goes extreme so that σ ≈ 0.1, the magnetization reversal is governed only by coherent rotation. Correspondingly, coercive field reduces monotonically as σ increases. On the other hand, when a field is applied in plane but perpendicular to the long axis, almost zero coercivity is discovered. These variations are explained using a analytical calculation of demagnetization factors which quantifies shape anisotropy as well as the consideration of incoherence in magnetization reversal.

Functional nanostructures for bias-magnet-free and reconfigurable microwave magnetic devices

Recent demonstrations of the reconfigurable microwave properties based on patterned magnetic nanostructures without any external bias magnetic field have been reviewed. Two main design strategies for the nanostructures have been discussed. Firstly, self-biased nanomagnetic networks and multilayer structures that possess two different remanent magnetic states are exploited. Different remanent states are associated with distinct microwave properties and they are within a nanosecond time scale by using a simple field initialization scheme. The dipolar coupling field and the demagnetization field variations have been attributed to the origin of the tunable microwave responses. Secondly, magnetic skyrmions have been explored for tunable microwave properties. In this regard, skyrmion size which is directly related to its resonant modes has been controlled by placing it at different positions in an engineered nanostructure with varying edge repulsions. Finally, an outlook on the future directions and scopes of bias-free microwave devices have been discussed. It is also outlined that such devices have potential implications for the logic and magnonic technologies beyond their applications for microwave magnetic devices.

Tailoring of effective biaxial anisotropy in a 2-D array of thin truncated conical nanodisk

In the present study, the magnetization reversal of 2-D rectangular array of soft magnetic permalloy thin truncated conical disks is simulated by using micromagnetic modeling. The base radius (R) of the conical disk is varied between 20–100nm and upper radius (r) varied between 10nm to R−10nm such that the ratio σ=r/R lies between 0 to 1. The analysis of angular dependence (θ=0° to 90°) of hysteresis loops for the case of R=100nm is done in particular and its comparison with Stoner–Wohlfarth (SW) model reveals the presence of a bi-axial anisotropy which is attributed to the dipolar interaction between the nanodisks in the array. When the nanodisks are close to regular cylindrical shape (for σ∼0.9), the orthogonal axes of the facet of the array acts as magnetic easy axes. As the nanodisks are tapered to σ∼0.5 and 0.1, the same set of axes turns as hard axis of the bi-axial anisotropic system. Corresponding bi-axial anisotropy constant is estimated from the coercivity of the hysteresis loop along the easy axis as Keff=13.508kJ/m3, −5.403kJ/m3 and −4.322kJ/m3 respectively for σ=0.9,0.5 and 0.1 with R=100nm. In addition to this, an analytical expression is also formulated to calculate the total demagnetization energy of the 2-D array as a function of the magnetization orientation. This demagnetization energy is fitted with the effective bi-axial anisotropy energy expression considering an angular dispersion of the bi-axial anisotropy. The fitting suggests that the extent of angular dispersion of bi-axial anisotropy reduces as the conical disks are tapered (σ decreased). Irrespective of the magnetic field orientation, the remanent state exhibits a s-state of spatial magnetization profile when σ is large whereas a buckled magnetization state symmetric to body diagonal is exhibited for lower σ.

In-situ investigation of exchange bias property in FePt (L10)/FeCo bilayer with increasing FeCo thickness

Magnetization reversal of soft ferromagnetic (FM) FeCo layer coupled to hard FM FePt (L10) layer in FePt (L10)/FeCo) bilayer has been studied in-situ with increasing thickness of FeCo layer. During FePt(L10) phase formation, an in-plane magnetic saturation field (HMAX) is applied to keep the hard FePt(L10) layer in its remanent state. Magneto-optical Kerr effect measurements reveal the absence of hysteresis loops initially up to 4.8 nm thickness of FeCo layer while loops with significant shift, i.e. exchange bias (EB) opposite to the HMAX direction, were observed on further deposition. The EB is found to decrease with the increasing thickness of the FeCo layer. It is due to the combined effect of both magnetostatic coupling and interfacial exchange interaction, where the contribution of magnetostatic interaction to EB is almost constant, and interfacial exchange coupling exhibits an inverse relation. Furthermore, different nucleation mechanisms during magnetization reversal in the descending and ascending branches of hysteresis loops lead to asymmetry in magnetization reversal at the lower thickness of the FeCo layer. This asymmetry decreases with the decrease in the interfacial coupling as the thickness increases. Azimuthal angle dependence of exchange shift with respect to the direction of HMAX further revealed the unidirectional character of the EB.

Magnetization reversal and ground states in thin truncated conical nanodisks: Analytical and micromagnetic modelling approach

Simulation of magnetization reversal of soft permalloy thin truncated conical disks is done using micromagnetic modelling. The thickness of the disks are fixed at 20nm. Base radius (R) of the conical disk is varied between 50−100nm while the radius of upper surface (r) is varied such that the ratio σ=r/R lies between 0 to 1. As the conical disks are gradually tapered by reducing r, the remanent states change from vortex state to shifted vortex state to buckled magnetization state. In buckled state, the magnetic moments are essentially parallel except at the corners where they curl away from the applied field direction. The extent of curling away of magnetization in buckled state reduce as base radius R is decreased. Thus at smaller base radius, the buckled states converge to homogeneous magnetization state or so called ferromagnetic state. Analytical expressions for the total energy of these three remanent states are calculated using simple models. Analytical calculations reveal that vortex configuration is the ground state for a wide range of geometrical parameters and shifted vortex state is a metastable state which appears as remanent state in the in-plane magnetization reversal due to a possible existence of energy barrier. Other observations from micromagnetic simulations are well corroborated by the results of analytical calculations.

Control and tunability of magnetic bubble states in multilayers with strong perpendicular magnetic anisotropy at ambient conditions

The reversal of magnetic bubble helicity through topologically trivial transient states provides an additional degree of freedom that promises the development of multidimensional magnetic memories. A key requirement for this concept is the stabilization of bubble states at ambient conditions on application-compatible substrates. In the present work, we demonstrate a stabilization routine for remanent bubble states in high perpendicular magnetic anisotropy ${\text{[Co(0.44 nm)/Pt(0.7 nm)]}}_{X},$ $X=48$, 100, and 150 multilayers on $\mathrm{Si}/{\mathrm{SiO}}_{2}$ substrates by exploring the effect of external magnetic fields $({H}_{\mathrm{m}})$ of different strength and angles $(\ensuremath{\theta})$ with respect to the film surface normal. By systematic variation of these two parameters, we demonstrate that remanent bubble density and mean bubble diameter can be carefully tuned and optimized for each sample. Our protocol based on magnetometry only reveals the densest remanent bubble states at ${H}_{\mathrm{m}}=0.87{H}_{\mathrm{s}}$ (${H}_{\mathrm{s}}$ is the magnetic saturation field) and $\ensuremath{\theta}={60}^{\ensuremath{\circ}}$--${75}^{\ensuremath{\circ}}$ for all $X$ with a maximum of 3700 domains/100 $\ensuremath{\mu}{\mathrm{m}}^{2}$ for the $X=48$ sample. The experimental observations are supported by micromagnetic simulations, taking into account the nanoscale lateral grain structure of multilayers synthesized by magnetron sputter deposition, and thus helping to understand the different densities of the bubble states found in these systems.

Multilevel polarization switching in ferroelectric thin films

Ferroic order is characterized by hystereses with two remanent states and therefore inherently binary. The increasing interest in materials showing non-discrete responses, however, calls for a paradigm shift towards continuously tunable remanent ferroic states. Device integration for oxide nanoelectronics furthermore requires this tunability at the nanoscale. Here we demonstrate that we can arbitrarily set the remanent ferroelectric polarization at nanometric dimensions. We accomplish this in ultrathin epitaxial PbZr0.52Ti0.48O3 films featuring a dense pattern of decoupled nanometric 180° domains with a broad coercive-field distribution. This multilevel switching is achieved by driving the system towards the instability at the morphotropic phase boundary. The phase competition near this boundary in combination with epitaxial strain increases the responsiveness to external stimuli and unlocks new degrees of freedom to nano-control the polarization. We highlight the technological benefits of non-binary switching by demonstrating a quasi-continuous tunability of the non-linear optical response and of tunnel electroresistance.

Giant converse magnetoelectric effect in a multiferroic heterostructure with polycrystalline Co2FeSi

To overcome a bottleneck in spintronic applications such as those of ultralow-power magnetoresistive random-access memory devices, the electric-field control of magnetization vectors in ferromagnetic electrodes has shown much promise. Here, we show the giant converse magnetoelectric (CME) effect in a multiferroic heterostructure consisting of the ferromagnetic Heusler alloy Co2FeSi and ferroelectric-oxide Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) for electric-field control of magnetization vectors. Using an in-plane uniaxial magnetic anisotropy of polycrystalline Co2FeSi film grown on PMN-PT(011), the nonvolatile and repeatable magnetization vector switchings in remanent states are demonstrated. The CME coupling coefficient of the polycrystalline Co2FeSi/PMN-PT(011) is over 1.0 × 10−5 s/m at room temperature, comparable to those of single-crystalline Fe1-xGax/PMN-PT systems. The giant CME effect has been demonstrated by the strain-induced variation in the magnetic anisotropy energy of Co2FeSi with an L21-ordered structure. This approach can lead to a new solution to the reduction in the write power in spintronic memory architectures at room temperature.

On the Localization of Persistent Currents Due to Trapped Magnetic Flux at the Stacking Faults of Graphite at Room Temperature

Granular superconductivity at high temperatures in graphite can emerge at certain two-dimensional (2D) stacking faults (SFs) between regions with twisted (around the c-axis) or untwisted crystalline regions with Bernal (ABA…) and/or rhombohedral (ABCABCA…) stacking order. One way to observe experimentally such 2D superconductivity is to measure the frozen magnetic flux produced by a permanent current loop that remains after removing an external magnetic field applied normal to the SFs. Magnetic force microscopy was used to localize and characterize such a permanent current path found in one natural graphite sample out of ∼50 measured graphite samples of different origins. The position of the current path drifts with time and roughly follows a logarithmic time dependence similar to the one for flux creep in type II superconductors. We demonstrate that a ≃10 nm deep scratch on the sample surface at the position of the current path causes a change in its location. A further scratch was enough to irreversibly destroy the remanent state of the sample at room temperature. Our studies clarify some of the reasons for the difficulties of finding a trapped flux in a remanent state at room temperature in graphite samples with SFs.

Cubic Anisotropy for a Reconfigurable Magnonic Crystal Based on Co2MnSi Heusler Alloy

We explore the possibility offered by magnetic materials with cubic anisotropy to realize reconfigurable magnonic crystals with a very simple geometry working at zero magnetic field. As a proof of concept, we use micromagnetic simulations to calculate the static and dynamic magnetic configurations of a squared antidot lattice made with ${\mathrm{Co}}_{2}\mathrm{Mn}\mathrm{Si}$ Heusler alloy having an anisotropy constant of about 17 \ifmmode\times\else\texttimes\fi{} ${10}^{3}\phantom{\rule{0.25em}{0ex}}{\mathrm{J}/\mathrm{m}}^{3}$. We show that the cubic anisotropy allows very different magnetic states to be obtained at remanence as a function of the direction of a saturation magnetic field, including quasiuniform remanent states that cannot be obtained in materials with the same magnetic parameters but without crystal anisotropy. This leads to the possibility to excite or extinct several quantized spin-wave modes whose frequencies can be tuned with the antidot dimensions. Reconfigurable magnetic states are demonstrated for antidot sizes in the range 300--350 nm for a ratio between the antidot size and spacing equal to 1/3. The transition between two different remanent states can be obtained with low-amplitude magnetic field (down to 3.5 mT) and with a switching time faster than 1 ns, which is of great interest for applications. Oppositely, for a particular orientation of the antidot lattice with respect to the cubic anisotropy axes, we also obtain a single stable remanent state independent of the saturation field direction. Finally, propagation properties and frequency band gaps in an antidot lattice with lateral antidot size of 100 nm are studied for frequency-filtering applications at remanence. Both magnetostatic surface waves and magnetostatic backward-volume wave configurations are explored for different positions of the microwave excitation with respect to the magnonic crystal.

Tailoring magnetic properties by tuning the interface in a Pt/Co/Pt/IrMn system with perpendicular and double-exchange biases

We prepared Pt/Co/Pt(t Pt)/IrMn heterostructures with perpendicular exchange bias (PEB) by inserting a Pt spacer between Co and IrMn. X-ray diffraction demonstrates that the IrMn and Pt layers exhibit a (111) texture promoting PEB. Here, the samples for various Pt spacer thicknesses exhibit double-shifted hysteresis loops with the coexistence of positive and negative exchange biases (EBs). Magnetic force microscopy measurement indicates that this behavior in the as-grown state results from the formation of an antiferromagnetic bidomain state with opposite signs. Also, the magnetic loop behaviors can be tailored by tuning the EB, coercive and switching fields which are sensitive to sub-nanometer changes in the spacer layer (Pt) thickness. It is found that the optimum thickness of the Pt layer is 0.8 nm by considering a well-defined single remanence state, where H EB is about seven times as large as the coercivity. Our results indicate that large EB and rather small coercivity necessary for the single-remanence state in the double-shifted Pt/Co/Pt/IrMn systems can be achieved by tuning the interface at atomic level. Moreover, magnetic properties were analyzed in detail depending on Pt space spacer layer thickness between Co and IrMn layers. These results may be useful for potential applications in future multilevel memory devices.

Self‐poling and electromechanical response of crystallographically textured PMN‐32PT prepared by templated grain growth

AbstractCrystallographic texturing of ferroelectrics is known to improve the piezoelectrics response due to the alignment of optimal grain orientations in polycrystalline materials. Using high‐energy x‐ray diffraction, a ferroelastic self‐poling effect was observed in crystallographically textured 0.68 Pb(Mg1/3Nb2/3)O3− 0.32PbTiO3 ceramic. It is shown that the BaTiO3 platelet templates used to induce crystallographic texture imposed a biaxial strain causing ferroelastic domains to re‐orient parallel to the template plate normal. In‐situ high‐energy x‐ray diffraction was then used to characterize the response mechanisms of the material with applied electric fields. The textured ceramic produced a (111) lattice strain of 0.13% in the remanent state, and a 0.16% (111) unipolar lattice strain at 2 kV/mm while the untextured ceramic had a higher (111) lattice strain of 0.18% in the remanent state and a smaller (111) unipolar lattice strain at 2 kV/mm of 0.096%. This contrast in the strain magnitudes can be linked to the self‐poling effect. A strain mechanism incorporating the self‐poling effect is proposed, furthering our understanding of how crystallographic texture impacts the piezoelectric properties and providing a pathway for engineering the self‐poling effect to further enhance material response.

Unexpected Magnetic Behavior of Natural Hematite‐Bearing Rocks at Low Temperatures

AbstractHematite is a commonly occurring magnetic mineral in nature that has numerous scientific and technological applications. A characteristic property of hematite is a low‐temperature spin‐flop transition called the Morin transition. Above the transition temperature, hematite is a canted antiferromagnet that can carry a remanent magnetization. Below this transition, spin canting disappears and hematite becomes a true antiferromagnet although a small defect moment is usually preserved. We observe Morin transition behavior in natural samples that has not been reported before for hematite. During repeated thermal cycling of a remanent magnetization acquired at room temperature, the remanence intensity at the end of the cycle oscillates between a high remanence state at the end of odd‐numbered cycles and a low remanence state (LRS) at the end of even‐numbered cycles. Alternation of the high and LRSs during repeated thermal cycling points to hysteretic behavior of the spin‐flop process, likely due to sublattice magnetization alignment switches along different easy magnetization axes in samples with preferred crystallographic orientations of hematite particles. We report these observations to seek to expand explanations of the magnetism of hematite.

Metastability and dynamic modes in magnetic island chains

The uniform states of a model for one-dimensional chains of thin magnetic islands on a nonmagnetic substrate coupled via dipolar interactions are described here. Magnetic islands oriented with their long axes perpendicular to the chain direction are assumed, whose shape anisotropy imposes a preference for the dipoles to point perpendicular to the chain. The competition between anisotropy and dipolar interactions leads to three types of uniform states of distinctly different symmetries, including metastable transverse or remanent states, transverse antiferromagnetic states, and longitudinal states where all dipoles align with the chain direction. The stability limits and normal modes of oscillation are found for all three types of states, even including infinite range dipole interactions. The normal mode frequencies are shown to be determined from the eigenvalues of the stability problem.

Transition Metal Synthetic Ferrimagnets: Tunable Media for All-Optical Switching Driven by Nanoscale Spin Current.

All-optical switching of magnetization has great potential for use in future ultrafast and energy efficient nanoscale magnetic storage devices. So far, research has been almost exclusively focused on rare-earth based materials, which limits device tunability and scalability. Here, we show that a perpendicularly magnetized synthetic ferrimagnet composed of two distinct transition metal ferromagnetic layers, Ni3Pt and Co, can exhibit helicity independent magnetization switching. Switching occurs between two equivalent remanent states with antiparallel alignment of the Ni3Pt and Co magnetic moments and is observable over a broad temperature range. Time-resolved measurements indicate that the switching is driven by a spin-polarized current passing through the subnanometer Ir interlayer. The magnetic properties of this model system may be tuned continuously via subnanoscale changes in the constituent layer thicknesses as well as growth conditions, allowing the underlying mechanisms to be elucidated and paving the way to a new class of data storage devices.

Magnetic vortex structure for hollow Fe3O4 spherical submicron particles

Magnetic particles with a hollow structure have arisen as an important class of nanomagnets because of a large pore volume and higher surface-to-volume ratio compared with the same-sized solid particles. The hollow structure results in unique magnetic features such as enhanced surface exchange bias, spin freezing, and preferential stability of a magnetic vortex. Despite a recent growing understanding of sub-100 nm hollow spherical magnetic nanoparticles, magnetic properties of larger-sized hollow particles were not currently understood in detail. Here, we report results of observations of magnetic microstructures for 420 nm-sized hollow Fe3O4 spherical particles with an electron holography imaging technique, where a magnetic-vortex formation is inferred from bulk measurements. We directly observe a magnetic vortex in a remanence state with magnetization circularly oriented within the shell and the reduced stray field. Micromagnetic simulations demonstrate an increasing stability of a vortex for a hollow sphere and the formation of a field-induced curling double vortex with a pair of clockwise and counterclockwise vortices. This double vortex structure is not confirmed for the solid counterpart, and its stability enhances with decreasing the shell thickness. The present work provides useful knowledge in designing magnetic particles, where a hollow structure and a magnetic vortex are key factors for high-performance biomedical applications.

Reconfigurable microwave properties in trapezoid-shaped nanomagnets without bias magnetic field

Self-biased nanomagnetic structures with reconfigurable microwave properties are going to play a key role in the miniaturization of microwave devices. However, examples of such nanomagnets are very few. Here, we present a new nanostructure based on trapezoid shape which possesses two different remanent states through a simple sub-ns field initialization process. Different remanent states are associated with distinct microwave properties with a remarkably large shift between their microwave spectra. Other possible sample schemes based on networks, multilayers and arrays of trapezoid nanomagnets have also been discussed for a comprehensive report on this novel design architecture. The underlying physics of tunable microwave properties has been explained analytically using dipolar field driven variation of total effective field landscapes for different remanent states. The results offer a new route for the realization of the on-chip integration of microwave devices without any requirement of a bias magnetic field.

Field tunable three-dimensional magnetic nanotextures in cobalt-nickel nanowires

Cylindrical magnetic nanowires with large transversal magnetocrystalline anisotropy have been shown to sustain non-trivial magnetic configurations resulting from the interplay of spatial confinement, exchange, and anisotropies. Exploiting these peculiar 3D spin configurations and their solitonic inhomogeneities are prospected to improve magnetization switching in future spintronics, such as power-saving magnetic memory and logic applications. Here we employ holographic vector field electron tomography to reconstruct the remanent magnetic states in CoNi nanowires with 10 nm resolution in 3D, with a particular focus on domain walls between remanent states and ubiquitous real-structure effects stemming from irregular morphology and anisotropy variations. By tuning the applied magnetic field direction, both longitudinal and transverse multi-vortex states of different chiralities and peculiar 3D features such as shifted vortex cores are stabilized. The chiral domain wall between the longitudinal vortices of opposite chiralities exhibits a complex 3D shape characterized by a push out of the central vortex line and a gain in exchange and anisotropy energy. A similar complex 3D texture, including bent vortex lines, forms at the domain boundary between transverse-vortex states and longitudinal configurations. Micromagnetic simulations allow an understanding of the origin of the observed complex magnetic states.