Magnetization Reversal Mechanism Research Articles

Magnetic properties of bamboo-like Ni-Zn-in nanowires using 3D-AAO templates

In this paper, using three-dimensional porous alumina (3D-AAO) as a template, Ni-Zn-In nanowires array with periodically changing diameters was successfully prepared by direct current electrodeposition method. The effect of deposition voltage on the morphology, crystal structure and magnetic properties of Ni-Zn-In ternary alloy nanowires was studied. The experimental results show that the diameters of the nanowires are 212 nm and 151 nm, and the periodic length is 800 nm, which matches the pore size of the phosphoric acid-etched 3D-AAO template. XRD results indicate that the crystal structure of Ni-Zn-In nanowires can be adjusted by the deposition voltage. The VSM results reveal that the Ni-Zn-In nanowires possess soft magnetic properties and significant magnetic anisotropy, with the easy magnetization direction parallel to the film. The nanowires deposited at −1.5 V exhibit the highest coercivity of 161 Oe and squareness of 0.045. Micromagnetic simulations show that the magnetization reversal mechanism of the Ni-Zn-In nanowires is localized nucleation mode.

Using magneto-optical effects in soft X-ray reflectivity to study current driven magnetization reversal

The soft X-ray reflectivity technique is frequently utilized for studying magnetization reversal in thin films due to its elemental and depth sensitivity. The characteristic hysteresis loops measured with this technique are dependent on both the magnetization direction in magnetic materials and the incident soft X-ray polarization. In this note, we have discussed these magneto-optical effects in soft X-ray reflectivity measurements. These effects can be exploited to probe magnetization reversal mechanisms driven by stimuli beyond conventional means of magnetic field. To demonstrate this, we have presented our investigations on current-induced magnetization switching in ferromagnet (FM)/heavy metal(HM) heterostructures.

Investigation of transverse exchange-springs in electrodeposited nano-heterostructured films through first-order reversal curve analysis

The prerequisite of efficient exchange-spring nano-heterostructures, i.e., tuning both hard and soft phases at a nanometer level, has posed significant preparation challenges to ensure effective exchange-coupling. Here, we present a novel approach to fabricate transverse exchange-spring nano-heterostructures using single starting material through an “in situ” electrodeposition technique at room temperature. Utilizing modified acidic bath chemistry and controlled hydrogen evolution, we successfully prepared stress-free, shiny, fine-grained amorphous, and nanocrystalline Co-rich cobalt phosphorus films. These nano-heterostructured films exhibit a unique non-collinear anisotropy-driven transverse exchange-spring behavior, investigated systematically under ambient conditions. The comprehensive functional analyses reveal that intricate interplay between in-plane (IP) anisotropy of amorphous phase and out-of-plane (OOP) anisotropy generating from a nanocrystalline structure compete with each other, while producing characteristic stripe domain structures to novel corrugated stripe domain shapes. The angle-dependent first-order reversal curve distributions demonstrate new insights into the magnetic reversal mechanisms, further confirming the non-exchange-spring and exchange-spring nature of the films depending on the prevalent interfacial exchange coupling. Formation of anisotropy-driven metastable-state due to competition between IP and OOP anisotropy at a particular OOP orientation has led the normal exchange-spring structures to a transverse exchange-spring structure. Micromagnetic simulations, in excellent agreement with experimental data, further elucidate the formation of characteristic stripe domain patterns and the influence of anisotropy on the magnetic properties. The innovative methodology and detailed functional analysis presented here offer significant understanding to the field of exchange-spring magnetic materials, including anisotropy-driven metastable states, demonstrating the potential for scalable and cost-effective fabrication of advanced nano-heterostructures with tailored magnetic properties.

Contrasting shell thickness-dependent magnetic behaviors of CoFe2O4@Fe3O4 and Fe3O4@CoFe2O4 core/shell nanoparticles

The magnetic properties of bi-magnetic core/shell nanoparticles (NPs) have been a subject of extensive investigation. Yet, the role of shell thickness in hard/soft and soft/hard core/shell NPs, corresponding to conventional and inverse structures, remains incompletely elucidated. In this study, we synthesized two sets of core/shell NPs, namely Fe3O4@CoFe2O4 (IF@CF) and CoFe2O4@Fe3O4 (CF@IF), featuring a fixed core diameter of approximately 11 nm and varying shell thicknesses up to 5 nm. These NPs were synthesized via a seed-mediated particle growth approach. The formation of the core/shell configuration of typical NPs in both sets was directly verified through scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDS) imaging. Notably, the saturation magnetization (MS) of IF@CF NPs exhibited a significant increase compared to that of the IF core across all shell thickness variations, whereas it remained largely unchanged for CF core and CF@IF NPs. Conversely, the coercivity (HC) showed an opposite trend in the two structures with increasing shell thickness. We observed the emergence of a transition from constricted to smooth hysteresis loops in the conventional structure when the shell thickness reached 5 nm. The systematic evolution of hysteresis loops with escalating shell thickness in both structures closely correlates with distinct magnetic reversal mechanisms, influenced significantly by interparticle interactions. Our study presents a pathway for modulating the magnetic properties of bi-magnetic core/shell NPs, thereby opening avenues for advanced applications in magnetic imaging, sensing, drug delivery, and magnetic hyperthermia.

Micromagnetic study of exchange bias effect in sub-micron dots of Co2MnSi interfaced with uncompensated IrMn

Owing to its crucial role in spintronics devices, the exchange bias (EB) phenomenon has been extensively investigated in various ferromagnet (FM) and antiferromagnet (AFM) bilayers since its discovery in Co/CoO core–shell nanoparticles. In this study, we present the emergence of negative EB for the first time in the Co2MnSi Heusler alloy interfacing with an uncompensated AFM, exhibiting analogous anisotropy to the IrMn. Due to the high pinning and IrMn anisotropy values, EB is stronger here. Investigation into the influence of ferromagnetic layer thickness (tFM) on exchange bias reveals an inverse relationship, while coercivity displays a non-monotonic increase. The analysis of spin canting angles suggests the presence of a maximum canting angle in the Co2MnSi layer close to the interface. We thoroughly analyze the spin configurations at the interface as well as away from it in the Co2MnSi (25 nm)/IrMn (5 nm) bilayer to better understand the mechanism of magnetization reversal. Interestingly, our findings unveiled distinct spin behaviors for the first and second reversals. In cases of small AFM thicknesses (tAFM), the exchange field is proportionate to the tAFM, contrasting with large tAFM, where it scales as 1/tAFM. Notably, coercivity demonstrates an increasing behavior across all tAFM variations. The angular dependence of the Heusler alloy revealed a four-fold symmetry indicative of cubic anisotropy and a two-fold symmetry representative of uniaxial anisotropy. The angular dependency study of exchange bias indicated similar clockwise (CW) and counterclockwise (CCW) rotations, with cos (θ) unidirectional dependence. However, loop shifting revealed that the lower pinning ability at 0° was due to a low Meiklejohn-Bean parameter (R) value. Additionally, through the manipulation of the R-parameter, we can tune the magnitude of the coercive field and EB. All these results are crucial for the utilization of the Co2MnSi/IrMn heterostructures for various applications in spintronics-based devices.

Discerning the magnetization reversal mechanism and magnetic interactions in arrays of FeNi nanowires

FeNi nanowires (NWs), 40 nm in diameter, were grown by electrodeposition. Compositional analysis showed Fe anomalous co-deposition, with different trends across the different potentials and electrolytes used. Those trends were explained within a modified Bocris-Drazic-Despic chemical reactions model, highlighting the importance of the electrochemical theory in understanding Fe-Ni electrodeposited systems. Structural characterization showed a change from a body-centred cubic structure for Fe-rich NWs to face-centred cubic at mid and low Fe content. Magnetic hysteresis measurements showed a range of coercivities from 20 mT for Fe-rich to 100 mT for Ni-rich NWs. Angular measurements allowed to determine that the magnetization reversal mechanism was dominated by transverse domain wall reversal. The analysis also hinted at the non-shape anisotropy acting as a demagnetizing force. First-order reversal curves (FORCs) showed that arrays with a high absolute value for the non-shape anisotropy had higher magnetic interactions, and vice versa. The non-shape anisotropy is dominated by the magnetostatic interactions between NWs, causing a weakening of the magnetic hardness. In combination with the coercivity angular measurements, the FORC analysis explained the shift in easy axis preference depending on NW chemical composition observed in the hysteresis loops of the samples.

Influence of magnetic anisotropy on the vortex stability in circular Permalloy nanodots using energy analysis

We thoroughly investigated the stability of vortex magnetization in Permalloy (Py) circular nanodots with and without magnetocrystalline anisotropy using micromagnetic simulations. This study explores a wide-ranging understanding of influence of anisotropy on hysteresis loops, and spin-configurations, and provides a detailed analysis of the energy profile. The ground state vortex configuration considered of size 64 × 20-nm2 Py-nanodot at zero out-of-plane anisotropy had been modulated till the anisotropy 250 kJ/m3. We analyzed different energy terms at nucleation field, annihilation field and remanent states to gain deeper insights into the magnetization reversal mechanism. The energy analysis revealed an interplay between exchange and demagnetization energies, resulting in the stability of vortex up to a critical anisotropy (CK) 170 kJ/m3. Beyond this CK, the vortex destabilizes, switching the magnetization to a single-domain state. Additionally, we estimated the energy barrier involved in single domain to vortex state transformation. These findings provide valuable insights for designing vortex-based magnetic data storage and memory devices.

Temperature dependent structural and magnetic properties of permalloy (Ni80Fe20) nanotubes

One dimensional Permalloy (Ni80Fe20) nanotubes (NTs) were successfully fabricated using Anodic Aluminum Oxide (AAO) templates with 100 nm diameter by employing a low-cost electrodeposition route at room temperature. As prepared Ni80Fe20 NTs morphology, elemental analysis and structural details have been investigated by using field-emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS) and X-Ray diffraction (XRD) respectively. The structure of prepared Permalloy nanostructures has face centered cubic (FCC) phase with polycrystalline in nature. The Ni80Fe20 NTs were annealed at 200 °C, 400 °C and 600 °C. The vibrating sample magnetometer (VSM) measurements showed that all the samples exhibit ferromagnetic nature. Furthermore, VSM has been employed to study the saturation magnetization (MS), Squareness (SQ) and Coercivity (HC) as the function of annealing temperature. A comparative study of magnetization reversal mechanism has also been conducted. The easy axis lies in the direction perpendicular to the NTS axis due to the dominance in shape anisotropy. Annealing temperature improved the magnetic properties of Permalloy (PA) NTs. This work will provide significant applications in the field of spintronics and high-density data storage devices.

Temperature dependent interfacial-coupling and magnetization reversal of La2/3Sr1/3MnO3 film with inverted hysteresis features

Temperature-dependent magnetic anisotropy and interfacial coupling of La2/3Sr1/3MnO3 (LSMO) epitaxial films with inverted hysteresis (IHLs) features are investigated using a broadband ferromagnetic resonance (FMR) spectrometer. Through the dispersion relation obtained along [100], [110] and [010] axes, a simplified method is proposed to investigation in-plane magnetic anisotropy by broadband FMR in a cryostat. According to this method, the perpendicular uniaxial anisotropy, magneto-crystalline anisotropy, and interfacial coupling between the LSMO film and hard-magnetic interface layer are investigated as a whole. The measured value of interfacial coupling is −17.6 Oe at 300 K, close to the bias field found in minor loop, i.e., −18.4 Oe. The interfacial coupling is enhanced with the dropping of temperatures, and the value of interfacial coupling changed to −112 Oe at 10 K. A Stoner-Wohlfarth-like model is employed to calculate the hysteresis loops of the films based on the measured values of in-plane magnetic anisotropy and interfacial-coupling. The calculation reveals the mechanism of magnetization reversal and the origin of IHLs found in LSMO films.

Geometry-induced Bloch point domain wall in Permalloy conical frustum nanowires for advanced spintronics applications

In this study, we investigate the pseudo-static magnetic properties of Permalloy conical frustum nanowires using micromagnetic simulations. We thoroughly examine how both the major and minor radii influence the magnetic reversal mechanism when an external magnetic field is applied parallel to the nanowire axis. The obtained results show that under specific geometrical conditions, magnetization reverts though a Bloch point-type domain wall. In these cases, hysteresis curves exhibit two Barkhausen jumps during magnetization reversal, forming a plateau field range in which a Bloch point domain wall nucleates and propagates until its annihilation after the second Barkhausen jump. The nucleation of a Bloch point domain wall in a frustum conical nanowire geometry is reported. These findings highlight the significance of this geometry in nucleating these attractive topological defects for promising applications.

Analyzing the mechanism of reducing B content on the coercivity of sintered (Nd, Ce)-Fe-B magnets

In recent years, the research focus has shifted towards the preparation of non-heavy rare earth (RE)-Fe-B magnets with high coercivity (Hcj) by reducing the content of B element. In this work, we achieved resource-saving sintered (Nd, Ce)-Fe-B magnets by adjusting the B element content, optimizing microstructure, and enhancement of Nd/Ce element distribution. We conducted an analysis on the magnetic characteristics, phase composition, microstructure, magnetization reversal mechanism, and economic viability of these sintered magnets. With a decrease in the B element content from 0.96 wt% to 0.90 wt%, there is a increase of approximately 1 kOe in Hcj.At the same time, the temperature coefficient of Hcj has been improved, resulting in a absolute value decrease from 0.67%/°C to 0.63%/°C. The findings suggested that reducing the mass fraction of B element could facilitate the formation of a thicker grain boundary phase between the grains. It could significantly weaken the ferromagnetic coupling effect, and enhance the magnetic isolation effect between grains, resulting in a higher Hcj. From an economic perspective, compared to the cost of magnet with similar coercivity prepared by increasing the RE content, the cost of magnet with low B element content was reduced by 554.4 US$/t, approximately saving 3.08%. This study provides a new theoretical foundation and experimental basis for resource-saving sintered (Nd,Ce)-Fe-B magnet preparation.

Magnetization reversal and coercivity mechanism in ferrimagnetic M-type hexaferrite by controlling phase evolution

The correlation of phase evolution (annealing at different temperatures) with magnetic properties like magnetization reversal, coercivity mechanism, and energy product in M-type hexaferrite by substituting Sr in place of Ba has been reported in the present article. The Ba1-xSrxFe12O19 (x = 0.0, 0.5, & 1.0) have been prepared by the sol–gel auto-combustion method followed by annealing at different temperatures from 800 °C to 1200 °C. The structural and magnetic properties have been characterized by X-ray diffraction, Field emission scanning electron microscopy, Raman spectroscopy, and Vibrating sample magnetometer. The Rietveld refinement confirms contraction in unit cell parameters by replacing Sr in place of Ba and lattice expansion with increasing annealing temperature. There is no significant effect of substitution on magnetic properties found rather heat treatment affects greatly. The transition from a single domain to multidomain particles and their contribution to initial magnetization curves are analyzed. The differential susceptibility (dM/dH) calculated from room temperature magnetization confirms the existence of a pinning effect during magnetization reversal for samples heat treated below 1200 °C. Secondary phases or grain boundaries are believed to act like pinning centers. This work opens insight into the domain wall motion by microstructure engineering. The coercivity is tuned to 6.09 kOe by controlling the annealing temperature for the Ba1-xSrxFe12O19 (x = 1.0) sample. The maximum energy product (BH)max in the range of 0.94–1.27 MGOe has been achieved.

Understanding the Effect of Curvature on the Magnetization Reversal of Three-Dimensional Nanohelices.

Comprehending the interaction between geometry and magnetism in three-dimensional (3D) nanostructures is important to understand the fundamental physics of domain wall (DW) formation and pinning. Here, we use focused-electron-beam-induced deposition to fabricate magnetic nanohelices with increasing helical curvature with height. Using electron tomography and Lorentz transmission electron microscopy, we reconstruct the 3D structure and magnetization of the nanohelices. The surface curvature, helical curvature, and torsion of the nanohelices are then quantified from the tomographic reconstructions. Furthermore, by using the experimental 3D reconstructions as inputs for micromagnetic simulations, we can reveal the influence of surface and helical curvature on the magnetic reversal mechanism. Hence, we can directly correlate the magnetic behavior of a 3D nanohelix to its experimental structure. These results demonstrate how the control of geometry in nanohelices can be utilized in the stabilization of DWs and control of the response of the nanostructure to applied magnetic fields.

Computational modelling of a triaxial vibrating sample magnetometer

Magnetic Field Assisted Additive Manufacturing (MFAAM) enables 3D printing of magnetic materials of various shapes which exhibit a complex anisotropy energy surface containing contributions generated from different origins such as sample, particle, and agglomerate shape anisotropy, flow and field induced anisotropy, and particle crystal anisotropy. These novel magnet shapes require the need to measure the x, y, and z components of the magnetic dipole moment simultaneously to fully understand the magnetic reversal mechanism and unravel the complex magnetic anisotropy energy surface of 3D printed magnetic composites. This work aims to develop a triaxial vibrating sample magnetometer (VSM) by adding a z-coil set to a pre-existing biaxial VSM employing a modified Mallison coil set. The optimum size and location of the sensing coils were determined by modeling the sensitivity matrix of the z-coil set. The designed coil set was implemented using 3D printed spools, a manual coil winder, and gauge 38 copper wire. A 3D printed strontium ferrite nylon composite sample was used to estimate the sensitivity of the z-coils (50 mV/emu). The results herein are applicable for any VSM using a modified Mallison biaxial coil configuration allowing for a quick implementation on pre-existing systems.

Understanding of magnetization reversal mechanism of precipitation-hardening samarium-cobalt magnets from micromagnetic simulation

The precipitation-hardening Sm-Co magnets not only have irreplaceable applications under extreme conditions such as high temperatures, but are also the most typical magnets with pinning-controlled magnetization reversal mechanism. However, there have always been two views on this type of pinning, attractive or repulsive. In this article, the modification of the parallel interface to the titled interface, led to occurrence of quasi-repulsive and quasi-attractive pinning modes, by employing micromagnetic simulation methods. The corresponding domain wall energy relationship between the pinning phase and main phase was analyzed for different pinning modes, according to the morphology of domain wall from the simulation results. The analysis results indicated that relatively small difference in domain wall energy between the two phases led to quasi-repetitive and quasi-attractive pinning modes. Our findings can provide a reference for further understanding of the magnetization reversal mechanism of precipitation-hardening Sm-Co magnets. The emphasis on the influence of 1:5/2:17 interface orientation on pinning also provided a perspective on the magnetization reversal mechanism of other nanostructured permanent magnet materials.

“Tailoring thermal stability and in-plane magnetic anisotropy of CoFeB alloy thin films via growth conditions engineering”

The present work investigates the influence of growth conditions on the magneto-structural properties of magnetron-sputtered CoFeB alloy thin films. Films grown at room temperature (RT) are soft magnetic (coercivity <9 Oe), show amorphous microstructure, and possess a distinct uniaxial magnetic anisotropy (UMA) in the film plane. The amorphous microstructure and UMA persist up to a growth temperature (Ts) of 300 °C. CoFeB film grown at Si (100) substrate shows amorphous to crystalline phase transformation at Ts = 500 °C. In contrast, under the same growth conditions, the CoFeB film prepared on the SiO2 substrate maintains an amorphous structure up to Ts = 500 °C. The difference in the microstructures was explained in terms of the out-diffusion of B to the interfacial layer and segregation near the substrate for the films having crystalline and amorphous structure, respectively. On both the substrates Addition of W buffer layer facilitated the growth of smooth and nanocrystalline CoFeB films at Ts ∼300 °C. Upon crystallization, UMA evolution with a simultaneous increase in the coercive field is interpreted in terms of joint effect of random anisotropy model and surface/interface roughness-induced magnetization reversal mechanism. Unlike post-annealing, the CoFeB films prepared at high temperature shows superior thermal stability against crystallization. The relationship between the soft magnetic performance, UMA evolution, and thermal stability of CoFeB films with the growth conditions is established. The study reveals the role of growth condition engineering, to obtain CoFeB thin films with controlled microstructure and magnetic performance, which is crucial for designing the spintronics devices.

Coercivity, Activation Volumes, and Interactions in BaFe12O19

Coercivity and magnetic viscosity studies are presented in coprecipitated and ceramic BaFe12O19 hexaferrites. According to magnetic viscosity formalism, the activation volumes are obtained and for ceramic sample the temperature dependence of the activation volume is reported. Using remanence curves, interactions are evaluated by means of the ΔM technique. In both samples, interactions are evidenced, particularly demagnetizing‐like interactions. The activation volume values seem to indicate that coherent rotation is not the mechanism of magnetization reversal even in the sample with particle size below the critical single‐domain size. It is confirmed that the activation volume increases with temperature in 75–300 K temperature range, for both coprecipitated and ceramic samples.

Granular micromagnetic HAMR model: investigation of damping dependence and parametric optimization for high performance

Heat assisted magnetic recording (HAMR) is a novel high-density magnetic recording technology that relies on thermal assist from a laser during the writing process. To achieve high writing performance, it is important to study and optimize the crucial factors affecting the magnetization reversal mechanism at elevated temperature. In this work, we use a multiscale approach that combines atomistic and micromagnetic models to study the magnetization reversal behavior in the recording medium. The atomistic model allows to parameterize accurately the macroscopic approach, which is utilized to model the system and its dynamics. We perform a parametric investigation of the switching properties as a function of the HAMR setup characteristics as well as the material properties, such as magnetic damping. The results show that high damping and moderate external fields can achieve high-performance HAMR media characterized by high switching probability, short switching time and low peak temperature. We demonstrate that switching occurs via the linear reversal mechanism. By systematic variation of the longitudinal susceptibility we force a transition to coherent reversal and demonstrate that this reduces the switching probability, showing linear reversal to be an important component within the HAMR process.

Current-Induced Magnetization Switching in Light-Metal-Oxide/Ferromagnetic-Metal Bilayers via Orbital Rashba Effect.

The orbital angular momentum (OAM) generation as well as its associated orbital torque is currently a matter of great interest in spin-orbitronics and is receiving increasing attention. In particular, recent theoretical work predicts that the oxidized light metal Cu can serve as an efficient OAM generator through its surface orbital Rashba effect. Here, for the first time, the crucial current-induced magnetic-field-free in-plane magnetization reversal is experimentally demonstrated in CoFeB/CuOx bilayers without any heavy elements. We show that the critical current density can be comparable to that of strong spin-orbit coupling systems with heavy metals (Pt and Ta) and that the magnetization reversal mechanism is governed by coherent rotation in the grains through the second-harmonic and magneto-optical Kerr effect measurements. Our results indicate that light metal oxides can play an equally important role as heavy metals in magnetization reversal, broadening the choice of materials for engineering spintronic devices.

Angular dependence of the magnetization relaxation in Co/Pt multilayers

We study the influence of defects in Co/Pt multilayers on the room-temperature magnetization reversal and relaxation mechanisms via angle-dependent magnetic viscosity and coercive field measurements. The data reveal a transition from pinning-dominated domain wall propagation to a sequence of pinning-dominated and uniform switching, with increasing tilt away from the normal direction. The leading role of the dendritic domain wall propagation in the nanogranular exchange-coupled films is corroborated by the scaling of relaxation times, the angular dependence of the coercive field, and Kerr microscopy.