Magnetocrystalline Anisotropy Research Articles

Phase Formation Mechanism and Anomalous Magnetic Variation of High-Performance La-Co-Doped Strontium Ferrites

La-Co-doped ferrite is widely used due to its excellent magnetic properties, but the mechanisms of La-Co doping on its phase formation and magnetic properties remain unclear. This study clarifies the phase formation mechanisms and reveals that La-Co doping reduces the formation temperatures of the intermediate phase SrFeO3−x and thus the final SrFe12O19 phase. This promotes complete formation of SrFe12O19, enhancing saturation magnetization. The unexpected change in coercivity after La-Co doping contradicts the variation in the determined magnetocrystalline anisotropy field. We identify that it arises from the La-Co doping lowering the formation temperature of SrFe12O19, leading to excessive particle growth.

Magnetic anisotropy of 4f atoms on a WSe2 monolayer: a DFT + U study

Inspired by recent advancements in the field of single-atom magnets, particularly those involving rare-earth (RE) elements, we present a theoretical exploration employing DFT+U calculations to investigate the magnetic properties of selected 4f atoms, specifically Eu, Gd, and Ho, on a monolayer of the transition-metal dichalcogenide WSe2 in the 1H-phase. This study comparatively examines RE with diverse 4f orbital fillings and valence chemistry, aiming to understand how different coverage densities atop WSe2 affect magnetocrystalline anisotropy. We observe that RE lacking 5d occupation exhibit larger magnetic anisotropy energies at high densities, while those with outer 5d electrons show larger anisotropies in dilute configurations. Additionally, even half-filled 4f shell atoms with small orbital magnetic moments can generate substantial energy barriers for magnetization rotation due to prominent orbital hybridizations with WSe2. Open 4f shell atoms further enhance anisotropy barriers through spin-orbit coupling effects. These aspects are crucial for realizing stable magnetic information units experimentally.

Tunable in-plane conductance anisotropy in 2D semiconductive AgCrP2S6 by ion-electron co-modulations.

In-plane anisotropic two-dimensional (2D) semiconductors have gained much interest due to their anisotropic properties, which opens avenues in designing functional electronics. Currently reported in-plane anisotropic semiconductors mainly rely on crystal lattice anisotropy. Herein, AgCrP2S6 (ACPS) is introduced as a promising member to the anisotropic 2D semiconductors, in which, both crystal structure and ion-electron co-modulations are used to achieve tunable in-plane conductance anisotropy. Scanning tunneling electron microscopy and polarized Raman spectroscopy show the structural anisotropy of ACPS. Electrical transport measurements show that its tunable in-plane conductance anisotropy is related to the ion-electron co-modulations, where Ag ion migration is anisotropic along a axis and b axis. Electrical transport measurements show the semiconducting properties of ACPS, as also supported by photoluminescence results. Moreover, the transfer curves of ACPS showcase large Vg-related hysteresis, which is directionally controlled by anisotropic Ag ion migration. This work offers a possibility of using anisotropic charge transport in functional electronics by ion-electron co-modulations.

Magnetic vortex: Fundamental physics, developments, and device applications.

A magnetic vortex (MV) is one of the fundamental and topologically nontrivial spin textures in condensed matter physics. Magnetic vortices are usually the ground states in geometrically restricted ferromagnets with zero magnetocrystalline anisotropy. Magnetic vortices have recently been proposed for use in a variety of spintronics applications due to their resistance to thermal perturbations, flexibility in changing core polarity, simple patterning procedure, and potential uses in magnetic data storage with substantial density, sensors for the magnetic field, devices for logic operations, and other related fields. The data storage and computing capabilities of vortex-based devices are highly integrated and energy-efficient, with low drive current requirements. Thus, a comprehensive understanding ranging from basic physics to real-world applications is necessary to realize these devices. This article provides an overview of the recent developments in our knowledge of magnetic vortices and computing and data storage technologies that are based on them. This thorough analysis aims to advance knowledge and awareness of the possibilities of vortex-based spintronic devices in modern technologies.&#xD.

Discovery of highly anisotropic dielectric crystals with equivariant graph neural networks.

Anisotropy in crystals plays a pivotal role in many technological applications. For example, anisotropic electronic and thermal transport are thought to be beneficial for thermoelectric applications, while anisotropic mechanical properties are of interest for emerging metamaterials, and anisotropic dielectric materials have been suggested as a novel platform for dark matter detection. Understanding and tailoring anisotropy in crystals is therefore essential for the design of next-generation functional materials. To date, however, most data-driven approaches have focused on the prediction of scalar crystal properties, such as the spherically averaged dielectric tensor or the bulk and shear elastic moduli. Here, we adopt the latest approaches in equivariant graph neural networks to develop a model that can predict the full dielectric tensor of crystals. Our model, trained on the Materials Project dataset of c.a. 6700 dielectric tensors, achieves state-of-the-art accuracy in scalar dielectric prediction in addition to capturing the directional response. We showcase the performance of the model by discovering crystals with almost isotropic connectivity but highly anisotropic dielectric tensors, thereby broadening our knowledge of the structure-property relationships in dielectric crystals.

Effect of pressure-induced lattice distortion on physical properties of L10-FeNi ordered alloy

The ground-state properties of tetragonal L10 type FeNi alloy have been thoroughly studied, while its physical properties under high pressure are poorly understood. Here, the effect of pressure on the structural, magnetic, mechanical and dynamical properties of L10-FeNi are systematically investigated from the first principles calculations. The critical pressure of ferromagnetic collapse is detected to be 250 GPa, and the system is mechanically and dynamically stable below the critical pressure. The pressure-induced lattice distortion is identified in the pressure range of 80-150 GPa. Within this pressure range, the magnetic moments of the system decrease dramatically, the elastic constants C12, C13, C33 and bulk modulus B show softening behavior, and consequently the ductility, longitudinal sound velocity, and acoustic Grüneisen constant exhibit softening behavior, while the elastic anisotropy increase sharply. Furthermore, there are significant variations in magnetocrystalline anisotropy, coercivity, maximum magnetic energy product and magnetic hardness parameters within the pressure range of lattice distortion. More interesting is the discovery that the pressure-induced lattice distortion triggers a transition from semi-hard to hard magnet near 130 GPa.

Impact of route of particle engineering on dissolution performance of posaconazole

Even when they have similar particle size, micron sized drug crystals prepared via different process routes may still exhibit considerable variability in pharmaceutical properties, due to the anisotropy of molecular crystals. This study aims to evaluate the dissolution performance of micronized posaconazole obtained through both milling and precipitation, with and without polymer coating. To overcome the problem of pressure-induced amorphization of posaconazole, powder dissolution was performed instead of intrinsic dissolution, which requires compressing powder into pellets. However, direct powder dissolution was challenged by the poor dispersibility of micronized posaconazole powders because of their extremely poor wettability. To solve this problem, we pretreated powders by dispersing them in an aqueous solution with a surfactant. Despite posaconazole forming a hydrate after pretreatment, differences in measured powder dissolution rates are meaningful in predicting impact of routes of API engineering on biopharmaceutical performance since hydration of posaconazole also occurs in vivo. This case study presents a systematic approach in addressing challenges when characterizing dissolution performance of drug powders.

Pressure-driven magnetic phase change in the CrI3/Br3Cr2I3 heterostructure.

Vertically stacked van der Waals (vdW) heterostructures not only provide a promising platform in terms of band alignment, but also constitute fertile ground for fundamental science and attract tremendous practical interest towards their use in various device applications. Beyond most two-dimensional (2D) materials, which are intrinsically non-magnetic, CrI3 is a novel material with magnetism dependent on its vdW-bonded layers, promising potential spintronics applications. However, for particular device applications, a heterostructure is commonly fabricated and it is necessary to examine the effect of the interface or contact atoms on the magnetic properties of the heterostructure. Most importantly, the effect of assembly stress on the electronic and magnetic properties remains unclear. In this study, we design a vdW heterostructure from two-chromium tri-halides, namely the CrI3/Br3Cr2I3 heterostructure, where the Janus equivalent of the CrI3 monolayer, Br3Cr2I3, is also an intrinsically magnetic 2D material. Using state-of-the-art first-principles calculations, we uncover the effects of the contact atoms, as well as external pressure, on the electronic and magnetic properties of the CrI3/Br3Cr2I3 heterostructure. It is found that the heterostructure transitions from an antiferromagnetic (AFM) to ferromagnetic (FM) ground state with pressure larger than certain threshold. We also investigate the magneto-crystalline anisotropy energy (MAE) of the CrI3/Br3Cr2I3 heterostructure. Remarkably, it is found that the MAE is significantly influenced by both the stacking and the contact atoms, varying abruptly and inconsistently with the contact atoms and external pressure. Further, we also reveal a correlation between the MAE and the polar angle. The pressure-regulated magnetic properties of the CrI3/Br3Cr2I3 heterostructure as revealed in this study highlight its potential applications in spintronic devices.

Structurally and Electronically Anisotropic Nature of Bridgman-Grown Cs3Sb2Br9 Perovskite Single Crystal toward Efficient Photodetector.

Cs3Sb2Br9, as a sort of novel lead-free perovskite single crystal, has the merits of high carrier mobility and a long diffusion length. However, the large-sized and high-crystallized Cs3Sb2Br9 single crystals are not easily obtained. Herein, we apply the vertical Bridgman method to grow centimeter-sized Cs3Sb2Br9 single crystal. The temperature-dependent crystal structure of Cs3Sb2Br9 is in situ characterized in the temperature range of 100-400 K. A novel crystallographic and electronic structure anisotropy of the as-grown Cs3Sb2Br9 single crystal along the transmission directions of [100] and [001] is experimentally and theoretically proved. Owing to the layered two-dimensional (2D) structure of Cs3Sb2Br9, quantum confinement effects prolong the lifetime of hot carriers, leading to their accumulation within the Sb-Br plane along the [100] direction, thereby resulting in a higher density of electronic states. Accordingly, the [100] device exhibits a carrier mobility higher than that of the [001] device, with the [100] device mobility being 4 orders of magnitude higher than that of the [001] device at 423 K, showing a remarkable anisotropy. The [100] device also shows responsivity ∼10 times higher than that of the [001] device.

Modulation instability induced by quadratic nonlinearity in optically anisotropic medium

Abstract In anisotropic crystals, the inherent breakdown of circular symmetry due to anisotropy significantly impacts optical phenomena. This study theoretically investigates the effects of anisotropic diffraction on modulation instability (MI) spectra and subsequent optical filament formation. We model an anisotropic medium with quadratic nonlinearity, exhibiting noncritical phase-matched second-harmonic generation, using coupled nonlinear Schrödinger equations. Through linear stability analysis and numerical simulations, we examine MI in quasi-isotropic and anisotropic quadratic nonlinear media. Our results reveal that anisotropic diffraction induces asymmetry in the MI gain spectrum, leading to the formation of elliptical optical filaments for both fundamental and second harmonics. Notably, this asymmetry in the MI gain spectrum facilitates the generation of multiple filaments in high-gain MI regions.

Acoustic Phonon Scattering in Free‐Standing Anisotropic Silicon Plates

The electron–acoustic‐phonon interaction in a free‐standing anisotropic Si plate with (001) surface is studied taking into account the elastic anisotropy of the Si crystal and the modulated phonon modes. The interaction potential is derived from the modulated phonon modes and the anisotropic deformation potential constants. The effective deformation potential constant Dac is then calculated considering only the lowest electronic sub‐band. In the quantum well model for the electronic states, it is shown that the effective deformation potential for the modulated phonons in an anisotropic Si plate becomes Dac ≈ 13 eV in the thinner region of the plate thickness . It is also shown that both the phonon modulation and the crystal anisotropy have non‐negligible effects on the effective deformation potential and electron mobility.

Mechanical constraint to extend the operational range of (011)-oriented Mn-doped PIN–PMN–PT single crystals

AbstractWe investigated mechanical stress-dependent behaviors of (011)-oriented Mn-doped PIN–PMN–PT single crystals to further improve their stabilities for high-power piezoelectric applications. In contrast to the common belief that mechanical constraint induces depolarization of active piezoelectric materials, the (011)-oriented single crystals demonstrated up to 44% of enhancements in their coercive field when mechanical stress is loaded on the specific flank surfaces of the single crystals. Furthermore, the suggested strategy is also feasible over practical operational temperatures for electroacoustic sensors (5–60 °C), which is beneficial to high-temperature applications. Nevertheless, consistent with the generally accepted trade-off, their electromechanical responses are inevitably degraded at constrained states due to less shear deformation which is key in piezoelectricity of relaxor-PT single crystals. We hope that the current work will elucidate mechanical stress-dependent behaviors of domain-engineered anisotropic relaxor-PT single crystals and provide a clue for a proper selection of prestress levels to achieve better performances of electroacoustic transducers in a variety of environments.

Applications of Isosceles Triangular Coupling Structure in Optical Switching and Sensing.

In the case of waveguide-based devices, once they are fabricated, their optical properties are already determined and cannot be dynamically controlled, which limits their applications in practice. In this paper, an isosceles triangular-coupling structure which consists of an isosceles triangle coupled with a two-bus waveguide is proposed and researched numerically and theoretically. The coupled mode theory (CMT) is introduced to verify the correctness of the simulation results, which are based on the finite difference time domain (FDTD). Due to the existence of the side mode and angular mode, the transmission spectrum presents two high transmittance peaks and two low transmittance peaks. In addition, the four transmission peaks exhibit different variation trends when the dimensions of the isosceles triangle are changed. The liquid crystal (LC) materials comprise anisotropic uniaxial crystal and exhibit a remarkable birefringence effect under the action of the external field. When the isosceles triangle coupling structure is filled with LC, the refractive index of the liquid crystal can be changed by changing the applied voltage, thereby achieving the function of an optical switch. Within a certain range, a linear relationship between refractive index and applied voltage can be obtained. Moreover, the proposed structure can be applied to biochemical sensing to detect glucose concentrations, and the sensitivity reaches as high as 0.283 nm·L/g, which is significantly higher than other values reported in the literature. The triangular coupling structure has advantages such as simple structure and ease of manufacturing, making it an ideal choice for the design of high-performance integrated plasmonic devices.

Magnetic properties of the Y<sub>2</sub> (Fe<sub>x</sub>Co<sub>1-x</sub>)<sub>17</sub> compounds

The results of magnetization curves analysis of Y2(FexCo1−x)17 compounds, measured in 300—923 K temperature interval along easy and hard magnetization direction, were represented. Magnetocrystalline anisotropy constants K1,2 of the samples were calculated. Temperature and compositional dependence of anisotropy constants K1,2 and saturation magnetization Ms were discussed. There is an increase of first anisotropy constant K1 value with the increase of relative samples iron concentration with maximum value of 5.1∙105 J·m-3 in x = 0.29 point, correspond to the middle of the easy-axis interval.

Stripe and bubble ratchets on asymmetric substrates

We show that various nonmonotonic ratchet effects can arise when mesophase pattern-forming systems, which exhibit anisotropic crystal, stripe, and bubble regimes, are coupled to one-dimensional asymmetric substrates under ac driving. The patterns emerge in the absence of a substrate when the ratio of attraction to repulsion is varied for particles with competing short-range attraction and long-range repulsion potentials. In the presence of the substrate, we find that the ratchet efficiency varies nonmonotonically with increasing attraction, depending upon how well the mesophase morphology matches the substrate spacing and periodicity. For strong repulsion, there is a weak but finite ratchet effect, while at intermediate attraction, there is a robust ratchet effect in which the system forms stripes aligned with the substrate symmetry direction. For strong attraction, large bubbles appear that have weak or no ratchet effects when the bubble width exceeds the substrate lattice spacing, causing the bubble to be only weakly coupled to the substrate. For very strong attraction, small bubbles form and undergo a strong ratchet effect with an efficiency that oscillates as a function of ac drive amplitude. Although the small bubble regime is a strongly correlated regime in which each bubble contains many particles, the system behaves as if it is in a single-particle regime since the bubble width is much smaller than the substrate lattice spacing. We map out the different rectification phases as a function of the pattern morphology, substrate strength, and ac drive amplitude. The pronounced ratchet effects that we observe in some regimes can be exploited for pattern sorting in hard and soft matter systems. Published by the American Physical Society 2024

Mechanical behavior of supersonic fine particle bombardment single crystal γ-TiAl alloys based on atomistic simulation: effects of velocity and crystal plane

Abstract The crystal orientation significantly affects the plastic deformation and mechanical properties of γ-TiAl alloys. However, there are few studies on the mechanical behavior of supersonic fine particle bombardment (SFPB) of single-crystal γ-TiAl alloys impacted with different crystal planes and velocities at the nanoscale. And the characterization of the mechanical properties in the presence of dislocation defects after impact is lacked. Therefore, in this paper, molecular dynamics is used to simulate the impact response and post-impact mechanical response of SFPB single-crystal γ-TiAl alloy. The results show that: at a certain impact velocity, the different ways of inducing dislocation slip due to the number of slip system initiations under crystal anisotropy lead to a smaller depth of subsurface damage on the (001) crystallographic plane compared to the other crystallographic planes; moreover, the yield strength and Young’s modulus of the (111) and (1 1 ¯ 0) crystallographic planes are the largest after the impact, respectively. Under the same crystal plane, the subsurface damage depths of all crystal planes decrease with the increase of impact velocity; the yield strength of the (111) crystal plane of γ-TiAl alloy gradually increases, the yield strength of the (001) and (1 1 ¯ 0) crystal planes decreases, and the modulus of elasticity of the (001) and (111) crystal planes after impact is less sensitive to velocity. The angle between the two slip surfaces formed by the L-C dislocation during impact is 70.24°, while the angle between the two slip surfaces formed by the Hirth dislocation is 109.76°. This will provide atomic-scale deformation details of the plastic deformation and mechanical response of single-crystal γ-TiAl alloys bombarded by supersonic fine particles.

Direct measurements of the conventional and rotational magnetocaloric effects in Gd thick films

Abstract Magnetic refrigeration (MR) offers a sustainable and emission-free solution to the prevalent heat-pumping systems used worldwide. Typically, it utilizes the magnetocaloric effect (MCE) to achieve cooling by changing the external magnetic field intensity. However, an alternative approach involves maintaining a fixed field intensity while manipulating its orientation to induce temperature changes, in an effect known as the rotating MCE (RMCE). While the RMCE has been extensively studied in materials with magnetocrystalline anisotropy, its investigation in polycrystalline magnetocaloric samples with asymmetric shapes has been lacking until recently. In this case, the RMCE is induced by the demagnetizing effect, which becomes more pronounced in high aspect-ratio sample geometries exhibiting different effective demagnetizing factors at different orientations, such as in films. In this work, we characterize the conventional and rotational MCE of 40 μm-thick gadolinium films through magnetization and direct temperature measurements. The maximum adiabatic temperature change achieved under a 1 T magnetic field was 2.05 K when the film was oriented in plane with the field and 1.25 K when the film was perpendicular to the magnetic field, corresponding to an adiabatic temperature difference of around 0.8 K which may be induced through magnetic field rotation. Additionally, the maximum adiabatic temperature change upon rotation is shown to exhibit a non-monotonous behavior with field intensity, displaying a peak value for field intensities of around 0.8 T. The high aspect ratio of the Gd film has been demonstrated to considerably enhance the intensity of demagnetizing field-based RMCE compared to bulk samples, paving the way for future research in this emerging field of MR cooling.

Comparative view of coercivity mechanisms in soft and hard magnetic materials

The main differences between soft and hard magnetic materials are commented on. It is discussed how the coercivity mechanisms can be affected by the domain wall energy. A spherical cap nucleus is used for this analysis. There is a competition between magnetostatic energy and domain wall energy terms. As a consequence, for soft magnetic materials, the magnetostatic energy term is dominant over the domain wall energy term. An explanation for the dependence of the coercivity with grain size is presented. For grain size above the single domain size, in hard magnetic materials with high magnetocrystalline anisotropy, the coercivity decreases following a law proportional to the inverse of the square root of the grain size, whereas in soft magnetic materials, the coercivity reduces proportionally to the inverse of grain size.

Severe plastic deformation of Mn-Al permanent magnets

Manganese-aluminum permanent magnets are promising candidates to fill the cost and performance gap between lower-performance bonded ferrites and high-performance magnets based on rare-earth elements. This is due to the favorable combination of saturation magnetization and magnetocrystalline anisotropy in the Mn-Al τ phase combined with low raw material cost. However, the τ phase is metastable and prone to decomposition at high temperatures, making processing by conventional milling and sintering difficult. Severe plastic deformation (SPD) is an alternative processing route to control the microstructure of a material by applying very high amounts of strain. In this study, equal-channel angular extrusion (ECAE) and high-speed high-pressure torsion (HS-HPT) were both tested as SPD processing routes. ECAE improved magnetic energy product, (BH)max, by 220% by refining the grain size and imparting a high density of dislocations. HS-HPT enabled a rapid phase transformation from the high-temperature ε phase to the τ phase but lowered Hci, making it better suited to soft magnet processing.

Second-order anisotropy due to magnetostriction for L1[formula omitted]-FePt

The effective magnetocrystalline anisotropy energy associated with magnetostriction is studied for tetragonal L10 FePt by means of first-principles calculations, which is expressed in terms of the intrinsic anisotropy for an undeformed crystal, the magnetostrictive coefficients, and the elastic tensor. A very small correction is found for the first anisotropy constant ΔK1/K1=0.07%, while a much more significant contribution is obtained for the second one ΔK2/K2=21.86%. General analysis of this effect for tetragonal crystals is provided, finding that ΔK1 will be always positive for any stable phase with this symmetry. The potential implications and applications of these results are discussed.