Domain Wall Motion Research Articles

Effect of acceptor–donor co‐doping on the microstructure and electrical properties of BiScO3–PbTiO3‐based ceramics

AbstractDesigning BiScO3‐PbTiO3 (BS–PT) ceramics that merge high Curie temperature (TC) and large piezoelectric coefficient (d33) has always been a significant challenge for high‐temperature applications. Doping is a commonly used and effective method for enhancing the electrical performances of piezoelectric ceramics. In this study, the Li+/W6+ acceptor–donor co‐doping method was used to enhance the piezoelectric properties of 0.36BiSc1‐xLixO3–0.64PbTi1‐yWyO3 (0.36x = 0.64y) ceramics. The optimal electrical performance was observed at x = 0.01 near morphotropic phase boundaries, with a large d33 (505 pC/N), high TC (407°C), huge remnant polarization (Pr ∼ 40.9 μC/cm2), and inverse piezoelectric coefficient (d33* = 601 pm/V). Rietveld refinement and piezoelectric force microscopy results revealed that incorporating Li+/W6+ into the BS–PT ceramics decreased the T phase proportion and domain size and increased the domain wall density. The enhancement in piezoelectricity can be attributed to the lowered energy barrier resulting from the reduced T‐phase proportion and complex domain structure, which promotes polarization rotation and facilitates domain wall motion. This work demonstrates that acceptor–donor co‐doping is a promising strategy for optimizing the electrical performance of the BS–PT ceramics.

Unusual domain wall motion in the vicinity of the depinning field in a Pt/CoFeB/MgO film

Unusual domain wall motion in the vicinity of the depinning field in a Pt/CoFeB/MgO film

Direct observation and stochastic analysis on thermally activated nucleation and growth of individual magnetic domain

The nucleation and subsequent growth of the reversed magnetic domain at a finite temperature are stochastic processes, which are often analyzed using an ensemble of magnetic elements. In this study, we investigate the stochastics of magnetization reversal in multiple trials for the sole magnetic domain. Specifically, we utilize the robust magnetic domain structure formed in a dual-exchange-biased Pt/Co/Au/Cr2O3/Pt thin film. Our investigation encompasses the latency of reversed domain nucleation and the subsequent domain wall motion based on time-lapse magnetic domain observations. The time evolution of the magnetic domain is observed after applying a pulsed magnetic field superimposed on a DC field. Magnetization reversal is triggered by the nucleation of a small embryo with finite latency, followed by domain wall propagation. The nucleation probability of the embryo increases exponentially with the DC field, thus indicating that the nucleation process obeys the Poisson process. An analysis of the relaxation time for nucleation provides a suitable expression for the energy barrier. The nucleated domain wall propagates with temporal stops, thus indicating creep motion. The temperature dependence of the relaxation time and domain wall creep motion reveal that the magnetic anisotropy in the antiferromagnetic layer significantly affect both the energy barrier for nucleation and the depinning potential of domain wall propagation. This study provides comprehensive understanding into the coercivity mechanism and contributes to the thermal stability of magnetic/spintronic devices.

Multi-instrumental approach to domain walls and their movement in ferromagnetic steels – Origin of Barkhausen noise studied by microscopy techniques

Two steels, ferrite and ferrite-pearlite were thoroughly characterized by a multi-instrumental microscopy techniques to get detailed information about their microstructure and magnetic structure. Microstructural features act as pinning sites for the motion of magnetic domain walls (DWs) leading to changes in the magnetization of the sample. This phenomenon is the basis for industrially relevant non-destructive Barkhausen noise (BN) technique. With magnetic force microscopy (MFM), using bulk samples, and Lorentz microscopy, using thin films, we noticed that bulk and thin samples have similar domain structure still giving different BN signal amplitudes. We could explain an in-plane DW movement under out-of-plane applied magnetic field using anisotropy energetics. In-situ transmission electron microscopy (TEM) in Lorentz mode was used to visualize the motion of DWs and their interactions with different pinning sites. To help the interpretation of DW motions, alignment and denoising processes were tailored for in-situ TEM studies. Multi-instrumental and multidimensional structural analysis enabled us to visualize and verify many theoretical hypotheses related to the origin of BN signal in ferrite and ferrite-pearlite steels.

Magnetic Properties of the Soft Magnetic Composites Prepared Using Mixtures of Carbonyl Iron, FeSiCr, and FeSiAl Alloy Powders.

The effect of carbonyl iron powder, FeSiCr alloy powder, and annealed FeSiAl alloy powder, both individually and in binary combinations, on the density, microstructure, and magnetic properties (including permeability and power loss) of inductors manufactured by molding compaction was investigated in this study. The investigation demonstrates that hysteresis loss dominates power loss in the tested frequency range. Due to higher compacted density and reduced coercivity, adding 50% carbonyl iron powder to annealed powder resulted in the lowest hysteresis loss, allowing for domain wall movement. On the other hand, adding 50% FeSiCr alloy powder to annealed powder resulted in higher hysteresis loss due to impurity components hindering domain wall motion. Due to extreme plastic deformation, the carbonyl iron powder and FeSiCr alloy powder combinations displayed the most significant hysteresis loss. Eddy current loss followed the same trends as hysteresis loss in the mixtures. This study provides important insights for refining the soft magnetic composite design to obtain higher magnetic performance, while minimizing power loss.

Electrical control of 180° domain walls in an antiferromagnet

We demonstrate the reversible current-induced motion of 180° antiferromagnetic domain walls in a CuMnAs device. By controlling the magnitude and direction of the current pulse, the position of a domain wall can be switched between three distinct pinning sites. The domain wall motion is attributed to a field-like spin–orbit torque that induces the same sense of rotation on each magnetic sublattice, owing to the crystal symmetry of CuMnAs. Domain wall motion is observed for current densities down to ≈2.5×1010 A/m2 at room temperature.

Ultrafast stroboscopic time-resolved magneto-optical imaging of domain wall motion in Pt/GdFeCo wires induced by a current pulse

Ultrafast stroboscopic time-resolved magneto-optical imaging of domain wall motion in Pt/GdFeCo wires induced by a current pulse

Magnetic non-destructive evaluation of microstructural and mechanical characteristics of hardened AISI H13 die steel upon sustainable grinding

This paper describes the potential of non-destructive techniques such as micro-magnetic Barkhausen noise (MBN) to assess the grinding burn flaws in terms of microstructural and mechanical indices of hardened AISI H13 die steel. To overcome this grinding burn issue, minimum quantity lubrication grinding with eco-friendly machining fluids like paraffin oil (PO) and castor oil (CO) was adopted and compared its performance with well-established conventional grinding like wet and dry. Ground surface integrity with castor oil (CO) was found to be significantly improvement in surface quality including lower surface roughness (Rα: 0.23 μm and Rz: 1.83 μm) with uniform surface topography, absence of oxidization and carbonization film, and negligible change in microhardness and microstructure. The reason is strong and uniform anti-friction and anti-wear tribo-film was produced by castor oil, which minimizes heat penetration through limited ploughing and rubbing action. The MBN outcome i.e., root mean square (RMS) has followed a linear relation with microhardness and temperature of the ground sample with a correlation coefficient of 0.993 and 0.981. Besides, lower amplitude and wide peak envelope get shifted towards a higher magnetic field in the MQL-CO grinding caused by poor thermal softening effect on the ground sample, which restricts magnetic domain wall movement and rotation. These outcomes could guide tooling industries for efficient sustainable grinding with faster thermal damage assessment, which helps to minimize production cycle time and back to clean environment.

Stress dependent impedance spectroscopy of niobium based ceramics

In this study, stress-dependent impedance spectra were characterized as a function of uniaxial compressive stress up to –300 MPa for Zr-doped (Na0.5K0.5)(Nb1−x/125Zr x/100)O3−δ (x = 5, 10 mol%). This allowed for the evaluation of the stress-induced changes in the grain and grain boundary capacitance. The grain capacitance from equivalent circuit fitting exhibited decreasing behavior with increasing uniaxial compressive stress increased, which is attributed to hindered domain wall movement reducing the extrinsic contributions. Interestingly, NKNZ10 showed a more significant degradation in grain capacitance than NKNZ5. One possible explanation is the difference in oxygen vacancies, and by extension defect dipoles, when introducing Zr into the lattice. As such, this study reveals the amplifying effect of oxygen vacancies to the mechanical suppression of domain wall movement.

High-frequency magnetic properties of FeB nanochains based soft magnetic composites for potential antenna applications up to GHz

Soft magnetic materials are a kind of promising materials for antenna substrates because their permeability and permittivity are both greater than unity. However, their magnetic properties would deteriorate dramatically as the frequency reaches 1 GHz owing to Snoek limit and domain wall resonance, thus limiting their application at high frequency. Here, we report a low-dimensional soft magnetic composite (SMC) prepared by FeB nanochains, achieving the optimized magnetic loss tangent (tanδμ) of 0.09 and the real part of permeability (μ′) of 3.3 at 1 GHz, which is more competitive than the conventional spherical FeB SMCs. Such an optimization is ascribed to the change in the dominant mechanism of the magnetization process from domain wall motion to magnetic moment precession, associated with the transition of the magnetic domain structure of the particles from multi- to mono-domain. The SMC proposed in this work provides a significantly potential candidate for high-frequency and miniaturized 5G antennas. Furthermore, the design concept and preparation method can be extended to other SMCs, providing a guidance for the development of high-performance antenna substrate materials.

In Situ Observation of Domain Wall Lateral Creeping in a Ferroelectric Capacitor

AbstractAs a promising candidate for next‐generation nonvolatile memory devices, ferroelectric oxide films exhibit many emergent phenomena with functional applications, making understanding polarization switching and domain evolution behaviors of fundamental importance. However, tracking domain wall motion in ferroelectric oxide films with high spatial resolution remains challenging. Here, an in situ biasing approach for direct atomic‐scale observations of domain nucleation and sideways motion is presented. By accurately controlling the applied electric field, the lateral translational speed of the domain wall can decrease to less than 2.2 Å s−1, which is observable with atomic resolution STEM imaging. In situ observations on a capacitor structured PbZr0.1Ti0.9O3/La0.7Sr0.3MnO3 heterojunction demonstrate the unique creeping behavior of a domain wall under a critical electric field, with the atomic structure of the creeping domain wall revealed. Moreover, the evolution of the metastable domain wall forms an elongated morphology, which contains a large proportion of charged segments. Phase‐field simulations unveil the competition between gradient, elastic, and electrostatic energies that decide this unique domain wall creeping and morphology variation. This work paves the way toward a complete fundamental understanding of domain wall physics and potential modulations of domain wall properties in real devices.

Enhanced piezoelectricity in Pb(Zr0.48Ti0.52)O3-Pb(Mn1/3Sb2/3)O3-Pb(Mg1/3Ta2/3)O3 ceramics through the synergistic effect of defect engineering and lattice distortion

With the increasing demand for industrial applications, it has become crucial to develop special high-power devices that can achieve both high piezoelectric performance and large mechanical quality factor. In this work, we prepared quaternary piezoelectric ceramics, namely (0.95-x)Pb(Zr0.48Ti0.52)O3-xPb(Mg1/3Ta2/3)O3-0.05Pb(Mn1/3Sb2/3)O3 (PZT-PMT-PMS), using a traditional solid-state route. These ceramics exhibit an excellent mechanical quality factor and low loss. By adding an appropriate amount of PMT (x = 0.09), the Mg2+ ions were homogeneously incorporated into the crystal lattice, resulting in a reduction in the content of the tetragonal phase. The formation of (MgZr,Ti′′−VO..) defect dipoles has been studied through Rietveld refinement, which indicates that defect dipoles can hinder domain-wall motion, strengthen the pinning effect, and result in a “harder” performance. Moreover, the high asymmetry caused by large lattice distortion, regulates the piezoelectric properties. The sample shows the optimal performance when the PMT content is 0.09, with d33 = 430 pC/N, Qm = 1740, TC = 258 °C, εr = 1740, tanδ = 0.16 %, Pr = 44.5μC/cm2, and kp = 0.569.

Ultralow Power and Shifting-Discretized Magnetic Racetrack Memory Device Driven by Chirality Switching and Spin Current.

Magnetic racetrack memory has significantly evolved and developed since its first experimental verification and is considered one of the most promising candidates for future high-density on-chip solid-state memory. However, both the lack of a fast and precise magnetic domain wall (DW) shifting mechanism and the required extremely high DW motion (DWM) driving current make the racetrack difficult to commercialize. Here, we propose a method for coherent DWM that is free from the above issues, which is driven by chirality switching (CS) and an ultralow spin-orbit-torque (SOT) current. The CS, as the driving force of DWM, is achieved by the sign change of the Dzyaloshinskii-Moriya interaction, which is further induced by a ferroelectric switching voltage. The SOT is used to break the symmetry when the magnetic moment is rotated in the Bloch direction. We numerically investigate the underlying principle and the effect of key parameters on the DWM by micromagnetic simulations. Under the CS mechanism, a fast (∼102 m/s), ultralow energy (∼5 attoJoule), and precisely discretized DWM can be achieved. Considering that skyrmions with topological protection and smaller size are also promising for future racetracks, we similarly evaluate the feasibility of applying such a CS mechanism to a skyrmion. However, we find that the CS causes it to "breathe" instead of moving. Our results demonstrate that the CS strategy is suitable for future DW racetrack memory with ultralow power consumption and discretized DWM.

Concentration-driving pinning effect in lead-free Mn-substituted BCZT ferroelectric ceramics

Domain wall motion in ferroelectrics, closely related to electric characteristics and amenable to tuning through doping, remains incompletely understood, particularly with regard to the pinning defect in the (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 (BCZT) system, which has attracted significant interest. Here, we investigate the defect dipole-assisted pinning effect in Mn-substituted BCZT (abbreviated as BCZT-xMn) ceramics with Mn concentrations ranging from x = 0 to 0.04. As x increases, we observe a gradual decrease in the maximum permittivity, accompanied by a leftward shift in temperature, leading to a diffuse phase transition. Analysis of the dielectric spectrum reveals a consistent trend in the activation energy within a specific concentration range, suggesting the incorporation of oxygen vacancies to maintain electric neutrality. The resulting oxygen vacancies, combined with Mn dopants, form defect dipoles Mn4++e′− VO⋅⋅ −Mn4++e′, attaining sufficient concentration at x = 0.02, and enabling precise establishment of the pinning effect. This phenomenon is extensively documented by the observation of pinched polarization-electric field loops and double current peaks in the current-electric field curves, visually corroborated by experimental data and a 3D phase-field simulation model. Consequently, this study not only elucidates the concentration-driven pinning effect in the BCZT system but also provides valuable insights into doping effects in other lead-free ferroelectrics.

Tuning magnetoelectric effect in Bi6Fe1.6Co0.2Ni0.2Ti3O18/La0.7Sr0.3MnO3/Bi6Fe1.6Co0.2Ni0.2Ti3O18 sandwich films employing micromagnetic moments and force on dipole

Mutual modulating ferroelectric polarization and magnetization provides the potential for realizing new-type low-energy consumption, high-density memory, and logic processors. Despite extensive investigations, the underlying mechanism remains elusive. In this work, we employed Bi6Fe1.6Co0.2Ni0.2Ti3O18 (BFCNT) as the ferroelectric phase and La0.7Sr0.3MnO3 (LSMO) as the ferromagnetic phase to form environmentally friendly BFCNT/LSMO/BFCNT sandwich films fabricated on flexible FTO substrates using the all-solution chemical-solution deposition route. The ferromagnetism, ferroelectricity, and magnetoelectric coupling effect of the sandwich films were investigated by measuring at macro and micro scales. Then a dynamic micromagnetic moment model and a dipole force model were proposed to study the intermodulation of both ferromagnetism and ferroelectricity. It suggested that the combination of magnetic and electric moments makes the deflection of magnetization (or polarization) and a shift of the electron orbit, causing the movement of domain walls and the flip of magnetic domains, and thus a decrease of the saturation magnetization, offering new insights into the underlying physics of ME coupling in the model system.

Domain wall motion at low current density in a synthetic antiferromagnet nanowire

The current-driven motion of magnetic domain walls (DWs) is the working principle of magnetic racetrack memories. In this type of spintronic technology, high current densities are used to propel DW motion in magnetic nanowires, causing significant wire heating that corresponds to wasted energy. Synthetic antiferromagnets are known to show very fast DW motion at high current densities, but lower current densities around onset of motion have received less attention. Here we use scanning transmission x-ray microscopy to study the response of DWs in a SAF multilayer to short current pulses. We observe that the DWs depin at A m−2 and move more quickly in response to 5 ns duration current pulses than in comparable multilayers that lack antiferromagnetic coupling. The results suggest that DWs in SAF structures are superior to conventional Néel DWs for low energy consumption racetrack technologies.

Dynamic topological domain walls driven by lithium intercalation in graphene.

Stacking engineering in van der Waals (vdW) materials is a powerful method to control topological electronic phases for quantum device applications. Atomic intercalation into the vdW material can modulate the stacking structure at the atomic scale without a highly technical protocol. Here we report that lithium intercalation in a topologically structured graphene/buffer system on SiC(0001) drives dynamic topological domain wall (TDW) motions associated with stacking order change by using an in situ aberration-corrected low-energy electron microscope in combination with theoretical modelling. We observe sequential and selective lithium intercalation that starts at topological crossing points (AA stacking) and then selectively extends to AB stacking domains. Lithium intercalation locally changes the domain stacking order to AA and in turn alters the neighbouring TDW stacking orders, and continuous intercalation drives the evolution of the whole topological structure network. Our work reveals moving TDWs protected by the topology of stacking and lays the foundation for controlling the stacking structure via atomic intercalation. These findings open up new avenues to realize intercalation-driven vdW electronic devices.

Aging and passivation of magnetic properties in Co/Gd bilayers

Synthetic ferrimagnets based on Co and Gd bear promise for directly bridging the gap between volatile information in the photonic domain and nonvolatile information in the magnetic domain, without the need for any intermediary electronic conversion. Specifically, these systems exhibit strong spin–orbit torque effects, fast domain wall motion, and single-pulse all-optical switching of the magnetization. An important open challenge to bring these materials to the brink of applications is to achieve long-term stability of their magnetic properties. In this work, we address the time-evolution of the magnetic moment and compensation temperature of magnetron sputter grown Pt/Co/Gd trilayers with various capping layers. Over the course of three months, the net magnetic moment and compensation temperature change significantly, which we attribute to quenching of the Gd magnetization. We identify that intermixing of the capping layer and Gd is primarily responsible for this effect, which can be alleviated by choosing nitrides for capping as long as reduction of nitride to oxide is properly addressed. In short, this work provides an overview of the relevant aging effects that should be taken into account when designing synthetic ferrimagnets based on Co and Gd for spintronic applications.

Additively manufactured high permeability Fe-Ni alloys and novel bi-metallic magnetic shields

The sensitivity and sophistication of spacecraft for Earth Science and Planetary Science missions are increasing with each successive mission. Advances in instrumentation, robotics, remote sensing, avionics and controls are allowing un-crewed missions to be incredibly sophisticated. Effective electromagnetic shielding is critical for high fidelity functioning of the spacecraft and onboard instrumentation. Moreover, the geometrical complexity of the shielding arrangement and the constraints of size and shape, make additive manufacturing (AM) a critical, enabling technology for current and future missions. Two AM approaches - directed energy deposition (DED) and laser powder bed fusion (LPBF) - were used to produce Fe-80Ni-5Mo alloy rings and shields. The microstructure and magnetic properties of the materials produced through these processes are reported in this paper. A mechanism relating the microstructure to the soft magnetic performance of the material is proposed. The AM material produced in this work demonstrated the highest reported magnetic permeability and lowest reported coercivity of any additively manufactured soft magnetic material. Two different combinations of bi-metallic shields, Fe-80Ni-5Mo/FeCo-2V and Fe-80Ni-5Mo/Fe-49Ni, were fabricated using the DED process. The creation of these shields as monoliths in general, and for space-related applications in particular, is a novel aspect of this work. These multi-alloy, multi-layer shields showed a nearly 10 dB increase in shielding performance over Fe-80Ni-5Mo single alloy shielding, a significant improvement. For the DED builds, the coercivity decreases and permeability increases (better soft magnetic performance) with increasing build power. With increasing build power in the DED process, the grain size in the printed part increases. The corresponding reduction in the grain boundary area results in fewer obstacles to the movement of magnetic domain walls.

Establishing a Relationship between the Piezoelectric Response and Oxygen Vacancies in Lead-Free Piezoelectrics.

BiFeO3-BaTiO3 (BF-BT)-based lead-free piezoceramics are desired materials for high-temperature applications of piezoelectric sensors with a high Curie temperature and good piezoelectric properties. Recent studies have shown that oxygen vacancies have a significant effect on electrostrain and piezoelectric properties. Interestingly, two different phenomena exist, i.e., the increase in piezoelectric properties is often associated with a decrease in the concentration of oxygen vacancies, while the increase in electrostrain is often associated with an increase in the concentration of oxygen vacancies. Especially, for BF-based ceramics, the physical mechanisms related to property differences caused by oxygen vacancies are rarely reported, which needs further exploration. Here, two ceramics with differences in their oxygen vacancy concentrations are designed. Based on Rayleigh analysis, thermal/electric field-induced domain response (ferroelectric scaling), and macro-microstructural characterization, we can conclude that the transient piezoelectric response and the aging process are significantly affected by the oxygen vacancy concentration. In other words, the increasing concentration of oxygen vacancies in BF-BT ceramics enhances the reversible piezoelectric response contributed by lattice distortion and strengthens the response of domain switching and domain wall motion to electric and thermal fields but deteriorates their aging behavior, which leads to the degradation of piezoelectric performance. Besides, polarization saturation and defect pegging significantly improve the temperature stability of the strain.