Nucleation Field Research Articles

Controlling Magnetic Properties in Bulk Ferromagnetic Metals By Electrochemical Charging and Discharging of Hydrogen Atoms

Controlling magnetic properties of materials with external voltages has attracted much interest owing to its promising applications in various areas like actuation. However, the voltage effect in metals is usually restricted to few atomic layers as well as too small for real use due to strong electric field screening. Here, for the first time, we show that bulk magnetic properties of ferromagnetic metal can be tuned significantly through charging and discharging of hydrogen atoms. We manipulated the coercivity of micrometer-sized SmCo5 grains by a fraction of tesla with a voltage of only around 1 V. Employing this approach, voltage-assisted magnetization reversal is realized at room temperature. Through both experiments and simulation, we found that the huge voltage effect originates from the significant change of magnetocrystaline anisotropy in surface regions of SmCo5 grains due to hydrogen insertion and removal, which modifies the nucleation field during magnetization reversal. This work opens up a route for overcoming the screening length and has implication, for example, for using SmCo5 in sensors and actuators.

Electric-field control of the remanent-magnetic-state relaxation in a piezoelectric-ferromagnetic PZT-5%Fe3O4 composite

Magnetoelectric (ME) composites that exhibit both ferroelectric and ferromagnetic properties have attracted significant attention, thanks to their potential applications, e.g., low-energy-consumption storage devices. Here, we study bulk composites based on Pb(Zr0.52Ti0.48)O3 (PZT) as a piezoelectric (PE) matrix and Fe3O4 nanoparticles (NPs) as soft ferromagnetic (FM) and magnetostrictive additives, in the form PZT-xFe3O4 with 0% ≤ x ≤ 50 wt. %, all sintered at T = 1000 °C for 2 h in air. We focus our study on a completely insulating sample x = 5% and measure its properties at room temperature upon an out-of-plane external electric field, Eex: namely, piezoelectric response [in-plane strain, S(Eex)], polarization [P(Eex)], and relaxation of the remanent magnetization, [mrem(t,Eex)], prepared upon application and removal of an external magnetic field. The peaks observed in the butterflylike S(Eex) curves at E±peak = ±6 kV/cm and the nucleation field recorded in the P(Eex) loops at the same range around E±nuc = ±6 kV/cm (both referring to the PZT PE matrix) are clearly imprinted on the relaxation behavior of the mrem(t,Eex) data (referring to the Fe3O4 FM NPs). This experimental fact proves the ME coupling between the PZT matrix and the embedded Fe3O4 NPs. We ascribe this feature to the comparable piezoelectricity of the PZT matrix and the magnetostriction of the Fe3O4 NPs that probably motivate and/or promote a strain transfer mechanism occurring at the PZT matrix-Fe3O4 NP interfaces. Our work proves that the low cost PZT-xFe3O4 composite is a promising candidate ME material for future studies, aiming to potential applications.

Bistability of magnetic states in Fe-Pd nanocap arrays

Magnetic bistability between vortex and single domain states in nanostructures are of great interest from both fundamental and technological perspectives. In soft magnetic nanostructures, the transition from a uniform collinear magnetic state to a vortex state (or vice versa) induced by a magnetic field involves an energy barrier. If the thermal energy is large enough for overcoming this energy barrier, magnetic bistability with a hysteresis-free switching occurs between the two magnetic states. In this work, we tune this energy barrier by tailoring the composition of FePd alloys, which were deposited onto self-assembled particle arrays forming magnetic vortex structures on top of the particles. The bifurcation temperature, where a hysteresis-free transition occurs, was extracted from the temperature dependence of the annihilation and nucleation field which increases almost linearly with Fe content of the magnetic alloy. This study provides insights into the magnetization reversal process associated with magnetic bistability, which allows adjusting the bifurcation temperature range by the material properties of the nanosystem.

Proposal of Granular-Type Cap Layer with High-$K_{u}$ CoPt Grains and Ferromagnetic Oxide Grain Boundaries for Full Granular Media

Investigation of magnetic properties and microstructure for the stacked media with Co80Pt20-30 vol% rare earth (RE) oxide (RE oxide: Gd2O3, Nd2O3, and Sm2O3) cap layers (CLs) deposited on a Co80Pt20-30 vol% B2O3 granular recording layer (GRL) was conducted. It was found that the employment of a granular-type CL with ferromagnetic oxide is effective to control intergranular exchange coupling of the magnetic grains in CoPt–B2O3 GRL while maintaining the granular structure of the CL. In this full granular media, continuous structure of columnar magnetic grains and grain boundaries from the GRL up to the CL can be observed. When a typical ferromagnetic oxide CL, such as CoPt–Gd2O3 (9 nm), is deposited on CoPt–B2O3 GRL (16 nm), the nucleation field is increased from 0.7 to 1.1 kOe, and the saturation field is reduced from 21.5 to 16 kOe which suggest that CoPt-RE oxide is promising for a CL.

Thermally Activated Magnetic Switching Mode for Various Thicknesses of Perpendicularly Ferromagnetic Nano-dot

Background:Even applying thermal pulse has been succeeded to reduce the coercivity through randomization the magnetization in such a way stimulate the magnetic reversion, the efficiency of magnetic switching field consumption in writing process still turns out to be an exciting research field to implement the HAMR technology. One of the remarkable geometric properties of HAMR storage media that can be correlated to the writing field reduction issue is the nano-dot thickness. Furthermore, thermal fluctuation causes the magnetization switching process to be probabilistic. This magnetic switching probability determines the magnitude of the writing field. This paper aims to investigate the impact of changes in media thickness on the magnetization process in particular at high temperatures numerically.Methods:Nano-dot was modeled as a parallelepiped with uniaxial anisotropy which was regarded as a magnetically isolated system where no disturbance field of neighboring nano-dots. Simulation arrangements were implemented to evaluate the two viewpoints in the current heat-assisted magnetic recording, either coercivity, as well as writing field consume. Coercivity was gauged by inducing a magnetic field which linearly increased up to 2 Tesla for 2.5 ns at thermal equilibrium to the surrounding. In evaluating writing field consume, thermal field pulse which just below the Curie temperature was generated while the magnetic field inducing the nano-dot. These schemes investigations were based on the Landau-Lifshift- Gilbert equation which accommodates the fluctuation-dissipation theorem in calculating thermal fluctuation effect. Also, temperature dependent material parameters such as magnetic saturation, magnetic anisotropy, and exchange interaction, were taken into account.Results:At room temperature, the coercive and nucleation fields are highly sensitive to the nano-dot thickness. Under thermal assistance, the writing field for 10 nm and 100 nm of the chosen thicknesses are 0.110 T and 0.125 T respectively. These writing grades are significantly lower than the coercivity of the media. For both thicknesses, zero field magnetization reversal phenomena are observed as indicated by the existences of the switching probabilities at H = 0.Conclusion:This numerical study showed that using the heating assistance close to the Curie point, nanodots with the chosen thicknesses and magnetic parameters were probably to be magnetized even no driven magnetic field. Along with this result, magnetic field induction which required to utterly magnetizing was only in the sub-Tesla - about a tenth of the coercive field. During magnetization processes under thermal assistance, randomization of magnetic moments initiated the switching dynamic before the domain wall was nucleated and propagated to reach a single magnetized domain.

Effect of anisotropy distribution on local nucleation field in bistable ferromagnetic microwires

Critical parameters defining the local nucleation field in amorphous ferromagnetic microwires with positive magnetostriction are obtained analytically through scaling procedures. An exact value of the nucleation field is obtained numerically as a function of geometric parameters of anisotropy distribution, which is fully taken into account instead of being averaged out. It is established that the value of the nucleation field depends predominantly on the steepness of the anisotropy change within the boundary between axial and radial domains, while the maximal value of anisotropy inside the wire or an overall average is not relevant.

Magnetic properties improvement of hot-deformed Nd–Fe–B permanent magnets by Pr–Cu eutectic pre-diffusion process

Pre-diffusion grain boundary strategy was proposed to fabricate die-upset Nd-Fe-B magnets with excellent magnetic properties. Microstructure modification of die-upset magnets played a key role in determining the magnetic properties. The effective coercivity enhancement of die-upset magnets through pre-diffusion process was enhanced from 0.076 T/wt%Pr to 0.114 T/wt%Pr with a slight remanence loss. Driven by heat, the Pr–Cu eutectic alloys were diffused into melt-spun ribbons with weakened exchange couple of matrix phase, which led to enhanced coercivity. The uniform distribution of intergranular phase was obtained in die-upset magnet with pre-diffusion process and the remanence reduction was limited with a high squareness factor. Microstructure analysis confirmed that the pre-diffusion process suppressed the longitudinal/lateral ratio of platelet-shaped grains and grain growth in die-upset process. Simultaneously, the pre-diffusion process facilitated the formation of continuous and uniform grain boundaries (GBs). The grain size was distributed over a narrow range of value with similar local critical nucleation field, which resulted in the improved squareness. The thick and continuous intergranular phase strengthened the magnetic isolation between neighboring Nd2Fe14B phases and offered more “pinning” sites for domain wall shift. The modified structure hindered the nucleation and spread of reverse domains in a low magnetic field in favor of the coercivity enhancement. The view-direct time-dependent behavior of reverse magnetic domains indicated that the coercivity mechanism for die-upset magnet was a combination of “pinning” effect and nucleation model.

Influence of strain and substitution on magnetocrystalline anisotropy of R2Fe14B (R=Pr, Dy and Y)

Abstract The intermetallic rare earth-transition metal ( R - T ) compound Nd 2 Fe 14 B has a large energy density and is an industrially important permanent magnet material. Its low Curie Temperature ( T C ) though limits its use for electric motor applications. To improve its high temperature permanent magnetic properties, the coercive field ( H C ) should be enhanced, which can be achieved by influencing intrinsic atomistic properties such as saturation magnetization ( M S ) and magnetocrystalline anisotropy energy (MAE). Strain effects and substitution are two important methods for influencing M S and MAE. Using numerical methods based on density functional theory, firstly M S and MAE of three R 2 Fe 14 B compounds with R = P r , D y and Y are calculated at 0 K. Further, the influence of changing the lattice parameter ratio c / a on MAE is studied. For increasing c / a , MAE of Y 2 F e 14 B and Pr 2 F e 14 B remains nearly constant, while MAE of Dy 2 F e 14 B decreases monotonously, up to 40 % per 1 % change in c / a . In the next step, 50 % of the R - atoms are substituted. For Pr 2 F e 14 B , MAE decreases with substitution of Dy and Y ; for Dy 2 F e 14 B it increases with Pr but decreases with Y ; for Y 2 F e 14 B it increases with Pr and Dy . The nucleation field ( H N ) of the substituted phases are estimated for different microstructures and visually compared to the non-substituted compounds, showing a large H N for P r D y F e 14 B . Further, based on DOS plots of the Y - D y F e 14 B system, the contribution of different atoms and atomic sites to MAE is discussed, and by visualizing the electronic density contours, the influence of substitution on Fe - atoms in Dy 2 F e 14 B and D y Y F e 14 B is studied.

Thickness-dependent coercivity mechanism and hysteresis loops in hard/soft magnets

Two models are established to reveal the underlying coercivity mechanism for SmCo/Fe films, where one model considers a transition layer between hard and soft layers, while the other model does not consider this layer. Based on the two models, hysteresis loops, nucleation fields and coercivity are obtained by one-dimensional (1D) and three-dimensional (3D) micromagnetic methods. In particular, the calculated nucleation fields (HN) and coercivity (HC) match very well with the experimental data. It is found that the increase in the soft phase thickness (Ls) leads to a transition of the coercivity mechanism from nucleation to pinning. Such a pinning is inherently related to nucleation and has both attributes of traditional nucleation and pinning, called as a hybrid coercivity mechanism here. It is general for all hard/soft composites and can be extended to single-phased permanent magnets where defects are inevitable.

A novel analytical model for hysteresis loops of exchange-coupled hard-soft magnets

Hard/soft permanent magnets have attracted a lot of attention because of their rich magnetic properties and their potential for realizing giant energy products. However, energy products obtained by scientists in experiments are much smaller than the theoretical values, which has been studied by various analytical and numerical methods. The famous Stoner-Wohlfarth model (S-W model) is too simple to give the hysteresis loops whereas the intensively used variational method is too complicated to reveal the underlying mechanism in a simple form. The analytical model proposed in this paper maintains a balance between simplicity and precision, where the spins in the soft layer rotate fast and coherently with the applied field while those in the hard layer response to the applied field much slower but also coherent. An exchange coupling is provided to maintain the exchange spring which drags the spins in the hard layer to follow those in the soft layer. Similar to the more sophisticated model, the calculated hysteresis loops display three typical magnetic phases, i.e., the rigid composite magnet, the exchange spring and decoupled magnet, whereas the simple SW model can only give one single phase, i.e., the rigid composite one. In addition to the hysteresis loop, the energy product and the nucleation fields have been calculated and compared with those calculated by other methods, which justifies our model. Careful comparisons show that our calculations are in good agreement with the experimental results and other theoretical results, especially for the important coercivity value and the related mechanism.

Study on the high temperature magnetization reversal mechanism of grain boundary diffusion Nd–Fe–B sintered magnet by magnetic domain evolution observation

Nd–Fe–B sintered magnets with ultrahigh coericivity of 47.1 kOe and maximum energy product of 38.5 MGOe have been successfully prepared by TbH3 nanoparticles grain boundary diffusion (GBD) process. Microstructural observations show that Tb enriches in the outer regions of 2:14:1 matrix phase grains, forming a core–shell structure. High temperature magnetic domain evolution observations indicate that reversed domains tend to nucleate at the outer regions of 2:14:1 matrix phase grains at 200 °C, while the Tb-rich shells show high magnetocrystalline anisotropy improving the nucleation field at outer regions, leading to enhancement of high temperature magnetic performance.

Micromagnetic Study on the Magnetization Reversal of Barium Hexaferrite (BaFe12O19) Thin Film

This study investigates a magnetization reversal mechanism based on the hysteresis curve of Barium Hexaferrite (BFO) thin film by micromagnetic simulation through parallel and perpendicular magnetization directions along the axes. The hexagonal shape of the BFO film was modeled with thicknesses of 5, 10, and 15 nm and a diameter size ranging from 50 to 100 nm. It was found that the coercivity field H C and the saturation field H S of the BFO film decreased as the diameter size increased and thickness decreased. It was observed that the nucleation field H N increased as the diameter size increased. An analysis of energies showed that the demagnetization energy was dominantly influenced by the diameter and thickness in comparison to the anisotropic energy. From the hysteresis curve, the switching time was also investigated. Interestingly, the switching time was faster for the thinner BFOs with a diameter under 70 nm. For particles larger than 70 nm in diameter, the switching time showed fluctuation irrespective of the BFO thickness. Based on these results, a diameter size of 70 nm is proposed as the critical size for producing the equal time for switching domain polarity.

Pt\Co\W as a candidate for low power nanomagnetic logic

perpendicular nanomagnetic logic (pNML) applications. Starting from single and bilayer stacks, the material system is explored for maximum Co thicknesses while preserving perpendicular magnetic anisotropy (PMA) as well as single-domain behavior for micrometer-sized structures. Results for single trilayers show significantly reduced saturation magnetizations when compared to Pt\Co\Pt. Anisotropy measurements reveal a comparable volume component together with substantially reduced surface contributions. The subsequent analysis of [Co1.22\Pt1.45\WX]2 bilayer stacks indicates the emergence of weak interlayer coupling superposed by anisotropy differences, for Tungsten thicknesses , not exceeding the low mT range. The impact of local anisotropy engineering on micro-magnetic test structures by means of ion beam irradiation for different stack compositions is investigated, targeting low nucleation fields and shaping hysteresis behaviors for single step switching. First operational dipole-coupled inverter structures based on a Pt\Co\Heavy-Metal system are demonstrated achieving clocking fields down to 50mT.

Determination of interface layer effects on magnetic properties of nanocomposite magnets and exchange coupling between magnetic entities

The interlayer plays a crucial role in improving the properties of many extremely important magnetic materials and devices by regulating the magnetic exchange coupling, while it is still not fully understood in a quantitative way. The main obstacle is that there is still no effective way to characterize quantitatively the ferromagnetic exchange coupling (FEC) strength after inserting the interlayer. In this paper, a rationally designed hard/interlayer/soft layered film is used to study the interlayer effects. It is demonstrated that the magnetic properties of nanocomposite magnets can be effectively enhanced through the modification of interface. Here a new concept, using the nucleation field Hns of soft phase as a “detector” to directly and quantitatively characterize/measure the FEC strength across interface and interlayer, is put forward. It is applied to address the dependence of FEC strength on thickness of nonmagnetic Cr and rare earth-rich phase interlayers. The detailed, distinguishing and typical decay behavior and length of FEC strength for these interlayers are successfully resolved for the first time. The new conception and experimental results get also strongly supported by the theoretical calculation. The nature or mechanism of exchange coupling between ferromagnetic grains or layers after adding the nonmagnetic interlayer is identified by using the hard/interlayer/soft layered film as a model system. Our results demonstrate the validity for characterizing/measuring quantitatively the FEC strength across different interlayer materials via strategy established in this work. It opens up new perspectives to gain insight into the interlayer property, and mechanism comprehension and high-performance design for magnetic materials and devices, such as permanent magnets, high-density magnetic recording media, advanced spin-orbit torque devices, etc.

Effect of glass-removal on the magnetostriction and magnetic switching properties in amorphous FeSiB microwires

Abstract Results on the influence of glass-removal on the magnetostriction, magnetic switching properties and domain wall propagation in amorphous Fe77.5Si17.5B15 microwires are reported. Glass removal releases high thermoelastic stresses originated by the difference in thermal expansion coefficients of glass and metal, which reduces the coercivity, increases the domain wall mobility and decreases the local nucleation fields and their distributions. Evening the distribution of nucleation fields and eliminating the local surface defects due to glass removal increase the field range of single domain wall propagation regime. It was also found that the saturation magnetostriction coefficient significantly decreases after glass removal, which strengthens the impact of the reduced magnetoelastic anisotropy. The effect of glass removal on magnetostriction was previously observed only in Co-rich microwires with small negative magnetostriction. Understanding the factors that control the magnetic switching and domain wall dynamics is of interest for practical applications in sensing, storage and logic devices.

Effect of microstructure parameter on the energy product in two-phase permanent magnetic materials

Two-phase permanent magnets with soft and hard magnetic phases are suitable candidates for high energy product permanent magnets. To obtain enhanced energy product, the microstructure has to be optimum and the magnetization and nucleation field has to be as large as possible. The present studies suggest suitable combinations of soft–hard composites that could result in higher energy product. The role of microstructural parameter on the energy product is also presented.

Inserting a nonmagnetic spacer layer in Nd2Fe14B/α″-(FeCo)16N2 bilayers significantly improves their coercivity

The hysteresis loops, magnetic reversal processes, and energy variation of exchange-coupled Nd2Fe14B/α″-(FeCo)16N2 bilayers inserted with a nonmagnetic spacer layer were systematically investigated based on the object-oriented micromagnetic framework (OOMMF) software. The insertion of a nonmagnetic spacer layer can greatly improve the coercivity of the system. The coercivity of the system with different contact areas (CAs) between the Nd2Fe14B and α″-(FeCo)16N2 layers was discussed. It was calculated that the coercivity with 16% CA was about three times larger than that of soft/hard bilayers. The increased coercivity is attributed to the domain wall pinning. In addition, the effects of a deviation angle β between the easy axis and the applied field and different thicknesses of soft layer on the coercivity of the system were analyzed. The system possesses optimal magnetic properties and maximum coercivity when the applied field is applied along the easy axis. Furthermore, both nucleation fields and coercivity decrease monotonically with the increase of the soft layer thickness. Our results are useful in tuning the coercivity of the nanocomposite magnetic materials.

Magnetization reversal and coercivity in rare-earth permanent magnets and composite magnets

The coercivities in rare earth permanent magnets even with the same intrinsic properties may differ largely. In this paper, what determines the coercivity is discussed via the investigation of thermal activation in Pr-Fe-B ribbons prepared by melt-spinning method. The thermal activation, resulting from thermal fluctuation overcoming the energy barrier under the applied field, is the critical behavior of magnetization reversal. The activation size is comparable to the theoretical domain wall size, implying that the magnetization reversal undergoes the nucleation of revered domain wall at grain outer-layer in Pr-Fe-B ribbons, and the defects near the grain boundary are critical for the magnetization reversal and coercivity. The exchange coupling between the defect region at grain outer-layer and the perfect region in the inside of grain promotes the nucleation of reversed domain and the depinning of domain wall motion. The reduced anisotropy of the defect region also contributes to the overcoming of energy barrier of magneto crystallie anisotropy in the inside of Pr-Fe-B grains by the coupling effect, so the nucleation field of reversed domain and coercivity decrease largely, and the domain wall size is a little larger than the theoretical value due to the coupling between the defect region at grain outer-layer and the perfect region in the inside of grain in the critical magnetization reversal. In Pr<sub>2</sub>Fe<sub>14</sub>B/<i>α</i>-Fe composite magnets, the exchange coupling between the soft and hard magnetic phase leads the domain wall size to increase in the critical magnetization reversal of thermal activation, and so the exchange energy plays a role in overcoming the energy barrier, resulting in the further decrease of coercivity. Via the addition of Ti and Nb element, the coercivity increases significantly. Based on the investigation of thermal activation, the size of defect region involved in the domain wall decreases, and the contribution of the anisotropy in the defect region and interface to the overcoming of energy barrier is weakened, so the applied magnetic field should be increased in the magnetization reversal. The coercivity can be enhanced by reducing the size of defect region at grain outer-layer and by making the anisotropy change abruptly at the interface between the hard and soft magnetic phase.

Remanence enhancement for stray field-based applications in arrays of crystalline nanomagnets

With the aim of achieving stable, substantial remanences adequate for exploitation in stray field-based applications, we report on the hysteresis behavior occurring in arrays of single-crystal Fe motifs, e-beam lithographed into prisms with triangular bases and different orientations of their magnetocrystalline axes with respect to the morphological symmetry axes. From both experimental and simulational analyses we recognize the fact that the magnetization reversal processes of our samples were mediated by motif-sized vortices. Their nucleation and annihilation fields and sites within the motifs, and their field-induced displacements, are discussed in terms of the magnetocrystalline and configurational anisotropies and inter-motif dipolar interactions. From our data, we conclude that reduced remanences as large as 0.85 (sufficient for the application requirements), protected by nucleation fields of several tens of Oe, can be produced in arrays where magnetocrystalline easy axes reinforce and partly compensate the easiest and hardest configurational ones, respectively. The angular dependence of the reduced remanence associated with interplay of these anisotropies corresponds to a symmetry reduction from the triaxial one linked to the triangular morphology down to an effective uniaxial one. We also identify, for the particular case of inter-nanoprism distances that are short in comparison with the dimensions of the motif base, a contribution to the remanence enhancement originating from the dipolar interactions.

The influence of interface anisotropy on demagnetization progress and magnetic properties in parallelly oriented hard/soft exchange-coupled multilayers

Abstract The effect of the interface anisotropy on the demagnetization progress and magnetic properties in Nd2Fe14B/α-Fe exchange-coupled multilayers with parallel crystalline anisotropy has been investigated within a micromagnetic framework. A set of equations satisfied by the angle between the magnetization and the applied field, as well as the formula for the nucleation field, have been derived analytically. The nucleation field goes up linearly with the interface anisotropy constants varying from −1 erg/cm2 to 1 erg/cm2 in a wide thickness range, with an amplitude up to 13%. The microscopic hysteresis loops, the evolution of the domain walls, the macroscopic demagnetization curves and the energy products are given by numerical calculations. The results indicate that the interface anisotropy affects not only the nucleation field and the maximal magnetic energy product, but also the pinning field or the coercivity. Positive interface anisotropy evidently enhances the pinning field or the coercivity, however, negative interface anisotropy obviously deteriorates the maximal energy product. These results suggest that a positive interface anisotropy should be preferred experimentally.