Pinning Of Magnetic Domain Walls Research Articles

Unusual Magnetocaloric Effect Triggered by Spin Reorientation

AbstractAdvancements and utilization of magnetic refrigeration technology hinge on the ongoing enhancement and optimization of magnetic refrigeration material properties. Nevertheless, the intricacy of the magnetocaloric effect (MCE) mechanism has emerged as a bottleneck, constraining the progress and refinement of magnetic refrigeration materials. In this study, a classic magnetic system is chosen to investigate the mechanism of MCE across four different scales–macroscopic magnetism, micrometer‐scale magnetic domains, atomic magnetic moments, and electronic structure. It simultaneously exhibits two inverse MCEs and one direct MCE, with a working temperature span as wide as 125 K (most are <50 K) for the direct MCE. The measurements of the vibrating sample magnetometer, in situ Lorentz electron microscopy and variable‐temperature neutron powder diffraction directly reveal that the complex magnetic entropy changes arise from the magnetic domain wall pinning, the instability of Ho magnetic moments, and the spin rotation. First‐principles calculations elucidate the crucial role of strong hybridization between localized Ho and itinerant Co electrons in the spin reorientation of HoCo4Al. This study contributes significantly to comprehending the induction mechanism of the MCE and holds vital reference value for exploring new magnetic refrigeration materials and enhancing MCE performance.

Magnetic domain wall pinning and depinning in DyFeO3 single crystal

Several papers reported the experimental phenomena of single crystal orthoferrite DyFeO3 samples: the sample magnetization increases with increasing temperature, under a constant low magnetic field along the easy magnetizing direction of the samples, and then reaches a maximum value. These phenomena are better explained using magnetic domain wall pinning at low temperatures and depinning at high temperatures, which were neglected in previous papers. In this article, we argue that the magnetic domain wall motion must be taken into consideration in order to explain these experimental phenomena. Based on an O 2p itinerant electron model, we discussed the role of the magnetic domain and the characteristics of cation magnetic moments in relevant single crystal materials.

Role of heterophase interfaces on local coercivity mechanisms in the magnetic Al0.3CoFeNi complex concentrated alloy

Microstructural features across different length scales have a profound influence on the coercivity of magnetic alloys. Whereas the role of homophase boundaries on the pinning of magnetic domain walls is well established, the influence of heterophase interfaces on domain wall motion is complex and poorly understood. Here, we use state-of-the-art electron microscopy techniques to show that the magnetization reversal process in an Al0.3CoFeNi magnetic complex concentrated alloy (CCA), which is responsible for its coercivity, changes dramatically from a nucleation-type mechanism in the FCC+L12 state of the CCA, with a domain wall width of 171 nm, to a pinning type mechanism in the microstructure with colonies of FCC/L12 nanorods embedded in a BCC/B2 matrix, with a domain wall width of 35 nm. Our work reveals that heterophase FCC/BCC interfaces have a much stronger effect on coercivity than isostructural chemically ordered/disordered interfaces and provides a powerful guide to the rational design of microstructure to tune magnetic properties in both complex concentrated alloys and conventional magnetic alloys.

Enhanced magnetic properties of Fe-rich Sm-Co-Fe-Cu-Zr magnets by compressive stress-aging

Precipitation of Sm/Cu-enriched cell boundary phase during isothermal aging plays a crucial role on the hard magnetic properties of the cellular nanostructured 2:17-type Sm-Co-Fe-Cu-Zr permanent magnets. However, this method offers only limited magnetic hardening effect in Fe-rich magnets as conventional thermal aging usually leads to decreased density of cell boundary precipitates. Here we report a stress-aging approach to rectify this limitation. As exhibited by a model magnet Sm25Co44.9Fe21.5Cu5.6Zr3.0 (wt.%), aging under 50 MPa uniaxial compressive stress for the same time at the same temperature can simultaneously enhance the intrinsic coercivity from 17.51 to 25.68 kOe and maximum energy product from 28.39 to 31.39 MGOe when compared with conventional isothermal aging. Detailed microstructural and microchemical investigations revealed that the magnetic performance enhancement stems from the retained [001] texture, the increased number density of Cu-enriched cell boundary precipitates, and the reduced stacking faults density at cell edges, which can significantly strengthen the magnetic domain wall pinning. Such microstructural improvements are contributed from the increased density of defects that promote the precipitate nucleation at early stage and the stress-induced dislocation re-arrangements that accelerate defects dissociation and atomic diffusion at the following stage, i.e. optimizing the thermodynamic-kinetic compromise of precipitation. Further study revealed that aging under higher stress leads to abnormal cell growth and weakened [001] texture due to the dynamic recrystallization effect, deteriorating the magnetic properties. Consequently, stress-aging provides a fresh freedom to manipulate the microstructure of Sm-Co-Fe-Cu-Zr magnets, but the stress magnitude should be carefully controlled to improve the magnetic performance.

Magnetism of antiphase boundaries in ordered alloys studied using electron holography

Magnetic anomalies in antiphase boundaries (APBs), which are a type of planar defect produced in ordered crystals, have been studied using electron holography. For many ordered crystals, including a Ni50Mn25Al12.5Ga12.5 alloy with a Heusler-type structure, atomic disordering in the APB region (showing a finite thickness) deteriorates the ferromagnetic spin order. This relationship has several functions, such as the pinning of magnetic domain walls by APBs. However, in an Fe70A130 alloy with a B2-type (CsCl-type) ordered structure, ferromagnetism can be developed by APBs. The unusual relationship observed in the Fe70A130 alloy can provide useful information for engineering APB-induced phenomena. This paper reviews recent electron holography observations from these two types of alloys, and technical advancements in the APB studies.

Hysteretic response of bulk magnetostrictive material employing a novel hyperbolic vector generalized magneto-thermoelastic constitutive model

Taylor series expansion of Gibbs free energy density function for elastic deformation, the physics of demagnetization and the Weiss molecular field coupled with the thermodynamic relations have been used to develop a generalized nonlinear hysteretic thermo-magneto-elastic vector model with potential application for the development of sensors and actuators. The nonlinear elastic strains in the magnetostrictive material are induced due to the magnetic domain rotation caused by tensile and compressive stresses in giant magnetostrictive materials. A vector function of the hyperbolic tangent is defined to take into account this nonlinear characteristic validating the elastic stress field boundary condition. The vector generalized Jiles-Atherton hysteresis model with modified Langevin equation is employed to incorporate the pinning of magnetic domain walls in the mathematical model. Thus, derived macroscopic model is put to the test for 1D, 2D and 3D nonlinear magnetostriction, magnetization and elastic constitutive relation with Terfenol-D (giant magnetostrictive materials) as a potential candid material. Finite element analyses for rods and films have been conducted using the general H(B) relation coded in the C programming language, as shown in the solution algorithm (Fig. 2). Employing the calibrated physical and experimental parameters (Tables 2 and 3), the confidence of the model is checked with existing analytical and experimental models. The magnetization and magnetostriction hysteresis responses have been evaluated considering the variation of a broad range of the prestress (up to 110 MPa), temperature (0 °C–80 °C) and applied magnetic field (0 kA/m–193.2 kA/m). The numerical simulation of this fully coupled phenomenological model demonstrates good agreement with the experimental data. The theoretical modelling error obtained from averaging the normalized root mean square error for each experimental data set is found to be maximum of 2.8 % and hence vindicates the efficacy of the present model.

Nanoscale Mapping of Heterogeneous Strain and Defects in Individual Magnetic Nanocrystals

We map the three-dimensional strain heterogeneity within a single core-shell Ni nanoparticle using Bragg coherent diffractive imaging. We report the direct observation of both uniform displacements and strain within the crystalline core Ni region. We identify non-uniform displacements and dislocation morphologies across the core–shell interface, and within the outer shell at the nanoscale. By tracking individual dislocation lines in the outer shell region, and comparing the relative orientation between the Burgers vector and dislocation lines, we identify full and partial dislocations. The full dislocations are consistent with elasticity theory in the vicinity of a dislocation while the partial dislocations deviate from this theory. We utilize atomistic computations and Landau–Lifshitz–Gilbert simulation and density functional theory to confirm the equilibrium shape of the particle and the nature of the (111) displacement field obtained from Bragg coherent diffraction imaging (BCDI) experiments. This displacement field distribution within the core-region of the Ni nanoparticle provides a uniform distribution of magnetization in the core region. We observe that the absence of dislocations within the core-regions correlates with a uniform distribution of magnetization projections. Our findings suggest that the imaging of defects using BCDI could be of significant importance for giant magnetoresistance devices, like hard disk-drive read heads, where the presence of dislocations can affect magnetic domain wall pinning and coercivity.

Plasma-induced magnetic patterning of FePd thin films without and with exchange bias

We demonstrate control of magnetic domain structures in continuous FePd thin films by patterning their surfaces with plasma treatment. The Fe-oxide layer formed on the surface upon ambient exposure of the FePd alloy thin film grown on an Al2O3(0001) substrate was patterned into microstructures by e-beam lithography followed by O2- or Ar-plasma treatment. Microscopic pinning of magnetic domain walls in the thin films is then observed by magneto-optic Kerr effect microscopy, with the magnetic field needed to reverse the magnetization of the plasma-treated areas being larger than that for the untreated areas. An intriguing competition between the uniaxial anisotropy and the exchange bias is also observed in the system. This study demonstrates that patterning of the film surface with plasma treatment can be an easy and efficient method for sophisticated engineering of magnetic structures in thin films, and therefore has potential application in developing future data-storage and spintronic devices.

Metamagnetism and kinetic arrest in a long-range ferromagnetically ordered multicaloric double perovskite Y2CoMnO6

A systematic magnetic study of the double perovskite oxide Y2CoMnO6 (YCMO) has been performed. A monoclinic P21/n phase of YCMO was synthesized using a sol-gel method. Neutron diffraction (ND) measurements evidence the onset of long-range ferromagnetic (FM) ordering at TC ~ 76 K, which persists down to 5 K. The presence of 25% antisite disorder, estimated from the ND data, leads to the appearance of short-range antiferromagnetic (AFM) interactions. The existence of thermal hysteresis due to competing interactions between FM and AFM phases is observed in the M vs. T measurements. The pinning of magnetic domain walls at the Co/Mn antiphase boundaries results in a metamagnetic-like behavior. The field dependence of thermomagnetic irreversibility and the nature of virgin curves indicate the occurrence of a kinetic arrest phenomenon, which is further verified via cooling and heating of the system in an unequal fields (CHUF) protocol. In full agreement with the ND data, a detailed analysis of critical exponents near the paramagnetic (PM)-FM phase transition also establishes YCMO as a long-range interacting mean-field system. A careful examination of the temperature- and field-dependent magnetic entropy change yields an in-depth understanding of coexisting magnetically ordered and disordered phases in YCMO, leading to a comprehensive magnetic phase diagram of this multifunctional double perovskite system.

Domain wall pinning and hard magnetic phase in Co-doped bulk single crystalline Fe3GeTe2

We report the effects of cobalt doping on the magnetic properties of two-dimensional van der Waals ferromagnet Fe3GeTe2. Single crystals of (Fe{1-x}Cox)3GeTe2 with x=0-0.78 were successfully synthesized and characterized with x-ray diffraction, energy dispersive x-ray spectroscopy and magnetization measurements. Both the Curie-Weiss temperature and ferromagnetic (FM) ordered moment of Fe3GeTe2 are gradually suppressed upon Co doping. A kink in zero-field-cooling low field M(T) curve which is previously explained as an antiferromagnetic transition is observed for samples with x=0-0.58. Our detailed magnetization measurements and theoretical calculations strongly suggest that this kink is originated from the pinning of magnetic domain walls. The domain pinning effects are suddenly enhanced when the doping concentration of cobalt is around 50%, both the coercive field Hc and the magnetic remanence to saturated magnetization ratio MR/MS are largely improved and a hard magnetic phase emerges in bulk single crystal samples. The strong doping dependent magnetic properties suggest more spintronic applications of Fe3GeTe2.

Size effects and magnetoelastic couplings: a link between Hall–Petch behaviour and coercive field in soft ferromagnetic metals

ABSTRACTSize effects regarding Hall–Petch (HP) relation are studied in this work for cobalt, nickel and Fe–3wt.%Si (FeSi), from polycrystalline to multicrystalline states. The materials show a breakdown in HP plot for thickness (t) to grain size (d) ratio less than a critical value. This appears in the beginning of plasticity for cobalt and FeSi whereas a plastic strain threshold must be overcome for nickel. Measurements of the coercive field on strained samples are able to depict such modification for low t/d ratio. Values of the coercive field in the polycrystalline domain allow an estimation of the magnetocrystalline anisotropy energy, related to the grain volume fraction concerned by reversal mechanisms for magnetic domains. Multicrystalline samples of cobalt and FeSi becomes magnetically softer at the yield stress. This is linked to a delay of the maximum intergranular stress towards higher strains for FeSi. For cobalt, non-linear elasticity and strong basal texture modify the magnetoelastic effects in coarse grain samples. For nickel, size effect on the coercive field appears after a few per cent of plastic strain as for HP relationship. A mean internal stress can be captured by magnetic measurements on polycrystals, related to the intragranular part of the kinematic stress. The softening of the magnetic properties for strained nickel multicrystals is due to a competition between the apparition of dislocation cells, which increases the coercive field by mechanisms of magnetic domain wall pinning, and surface softening of multicrystals, which tends to decrease the value of Hc.

Pinning of domain walls in thin ferromagnetic films

We present a quantitative investigation of magnetic domain wall pinning in thin magnets with perpendicular anisotropy. A self-consistent description exploiting the universal features of the depinning and thermally activated sub-threshold creep regimes observed in the field driven domain wall velocity, is used to determine the effective pinning parameters controlling the domain wall dynamics: the effective height of pinning barriers, the depinning threshold, and the velocity at depinning. Within this framework, the analysis of results published in the literature allows for a quantitative comparison of pinning properties for a set of magnetic materials in a wide temperature range. On the basis of scaling arguments, the microscopic parameters controlling the pinning: the correlation length of pinning, the collectively pinned domain wall length (Larkin length) and the strength of pinning disorder, are estimated from the effective pinning and the micromagnetic parameters. The analysis of thermal effects reveals a crossover between different pinning length scales and strengths at low reduced temperature.

Influence of precipitates on the magnetic properties of Fe–Cr–Mo alloys studied by X-ray diffraction, Mössbauer spectroscopy and vibrating sample magnetometry

Abstract

Foucault imaging and small-angle electron diffraction in controlled external magnetic fields.

We report a method for acquiring Foucault images and small-angle electron diffraction patterns in external magnetic fields using a conventional transmission electron microscope without any modification. In the electron optical system that we have constructed, external magnetic fields parallel to the optical axis can be controlled using the objective lens pole piece under weak excitation conditions in the Foucault mode and the diffraction mode. We observe two ferromagnetic perovskite-type manganese oxides, La0.7Sr0.3MnO3 (LSMO) and Nd0.5Sr0.5MnO3, in order to visualize magnetic domains and their magnetic responses to external magnetic fields. In rhombohedral-structured LSMO, pinning of magnetic domain walls at crystallographic twin boundaries was found to have a strong influence on the generation of new magnetic domains in external applied magnetic fields.

Magnetic hysteresis loss crossover in Ni0.4Zn0.6Fe1.95Ti0.05O4 ferrite

Ni0.4Zn0.6Fe2O4 and Ni0.4Zn0.6Fe1.95Ti0.05O4 ferrites were prepared by solid state reaction method and their magnetic properties were investigated. It was found that there is a magnetic hysteresis loss crossover around Bm = 4 mT, the hysteresis loss coefficient from 5.5 × 10−2 T−1 in Bm < 4 mT jump down to 4.0 × 10−2 T−1 in Bm > 4 mT in Ni0.4Zn0.6Fe1.95Ti0.05O4 sample by measuring field dependence of relative loss factor. This crossover was verified by measuring field dependence of real part permeability and magnetic loops. The possible mechanism of this crossover was discussed, and it was regarded that the crossover was due to magnetic domain wall pinning by Ti cation doped in ferrite.

Influence of Laser-Induced Surface Morphology on the Magnetic Domains of CoFeSiB Amorphous Ribbons

To investigate the relationship between magnetic domain structure and morphology variations induced by laser processing, Co 68.15Fe 4.35Si 12.5 B 15 amorphous ribbons were irradiated by a 1064-nm pulsed Nd:YAG laser in air. The illumination was performed at various pulse repetition rates which as a key parameter can effectively change the morphology of ribbons. The morphology and magnetic domain structure changes were correlated with magnetic force microscopy. It was shown that depending on the pulse repetition rate, various surface morphologies can be induced, which in turn may cause transformation of magnetic domains, anisotropy induction, and also pinning of magnetic domain walls.

Aging and rejuvenation in a ferromagnetic Ni3Al

Aging and rejuvenation phenomena, recently observed in ferromagnetic-like phases of complicated magnets, are studied using a simple itinerant-electron ferromagnet, Ni3Al. Results show that relaxation times of the magnetic response are widely distributed, hence, magnetic domain walls are collectively pinned in a multi-valley structured potential. The absorptive component of the ac-susceptibility gradually decreases with isothermal aging, and hence the domain-wall conformation becomes stabilized in following deeper valleys. If the temperature is shifted slightly after such isothermal aging, the absorptive component temporarily rises as if rejuvenated. This rejuvenation-like phenomenon is always accompanied by acceleration of the magnetic relaxations, regardless of cooling/heating. This thermal perturbation induced de-pinning indicates that the multi-valley structure itself significantly changes with temperature. In other words, the rejuvenation as well as aging can be explained using temperature-sensitive collective pinning of magnetic domain walls. We can hence say that they are intrinsic properties in actual ferromagnets with irregularly distributed pinning centers.

Domain wall pinning and dislocations: Investigating magnetite deformed under conditions analogous to nature using transmission electron microscopy

AbstractIn this study, we deformed samples cut from a single magnetite octahedron and used transmission electron microscopy (TEM) and magnetic measurements to experimentally verify earlier computational models of magnetic domain wall pinning by dislocations and to better understand the nature of dislocations in magnetite. Dislocations in magnetite have been of interest for many decades because they are often cited as a likely source of stable thermoremanent magnetizations in larger multidomain (MD) magnetite grains, so a better understanding of dislocation effects on coercivity in MD magnetite is crucial. TEM imaging shows, for the first time, domain walls sweeping through the magnetite sample and being pinned at dislocations. In agreement with theory, these findings demonstrate that domain walls are more strongly pinned at networks of dislocations than at single dislocations and that domain walls pinned at longer dislocations have higher microcoercivities than those pinned at shorter dislocations. This experimentally illustrates the ability of dislocations to increase the coercivity of larger multidomain magnetite grains. The observed values for microcoercivity and bulk coercivity are in reasonable agreement with theoretical calculations. Burgers vectors were determined for some dislocations to verify that they were in keeping with expected dislocation orientations. The dislocations were found to be primarily located on close‐packed {111} planes within the magnetite. Deformation caused only a minor change in bulk coercivity, but first‐order reversal curve diagrams show populations with increased coercivity not visible in hysteresis loops.

Modeling the effect of creep deterioration on magnetic properties in heat-resistant steels

Abstracts The hysteresis parameters of the Jiles–Atherton model are modified to elucidate the variation of magnetic properties with creep deterioration based on a consideration of the effect of pinning of magnetic domain walls on the grain boundaries, dislocations as well as precipitates in short-term creep process. Experiments are carried out to evaluate the magnetic hysteresis curves of 10CrMo910 specimens with controlled levels of creep-induced damage. An intelligent optimization algorithm is used to determine the hysteresis parameters of Jiles–Atherton model. The microstructure parameters of the crept specimens are determined by a quantitative metallographic analysis. The modified model is applied to correlate the experimental data of both 10CrMo910 and 410 stainless steel creep specimens. The calculated results are in good agreement with the measured data of the hysteresis parameters.

Control of domain wall pinning by switchable nanomagnet state

We report on a novel approach to establish switchable pinning of magnetic domain walls in a nanowire with perpendicular magnetic anisotropy by a single in-plane magnetized single-domain nanomagnet positioned on top of the wire. Devices were prepared by depositing a permalloy nanomagnet on top of a nanowire formed from a Co/Ni multilayer with their long axes parallel, separated by a nonmagnetic layer. We show by electrical measurements that the domain wall pinning strength depends critically on the state of the bistable nanomagnet and can differ by more than 10 mT. We also performed micromagnetic calculations that show that the difference in pinning strength is caused by the interaction of the forced Néel wall with the nanomagnet's magnetostatic field.