Magnetization Reversal Research Articles

Direct Observation of Magnetic Domain and Magnetization Reversal on Prussian Blue-Based Magnetic Films.

Knowledge of the magnetic domain is indispensable for understanding the magnetostatic properties of magnets. However, to date, the magnetic domain has not yet been reported in the field of molecule-based magnets. Herein, we study the magnetic domains of molecule-based magnets. Two magnetic films of iron/chromium hexacyanidochromate FexCr1-x[Cr(CN)6]2/3·5H2O (x = 0; Film 1 and x = 0.2; Film 2) were prepared for investigation. The temperature evolution of surface magnetization was measured using magnetic force microscopy. Film 1 showed a magnetic domain below Curie temperature (TC) and its positive-magnetic polarization increased monotonously with decreasing temperature, while Film 2 showed positive magnetic polarization below TC and switches from positive to negative magnetization through a demagnetization state at 146 K. This study originally reports the temperature variation of the magnetization state at the magnetization reversal. The magnetic domains appeared as a maze pattern with an approximate domain size of one-to-several micrometers. This work shows that research on molecule-based magnets can be expanded from magnetochemistry to the magnetostatic engineering of bulk magnets, molecule-based magnetostatic engineering.

Slow Magnetization Relaxation in a Family of Triangular {CoIII 2 LnIII } Clusters: The Effect of Diamagnetic CoIII Ions on the LnIII Magnetic Dynamics.

The first use of the Schiff base chelate N-naphthalidene-o-aminophenol (naphH2 ) in Co/Ln chemistry has afforded a family of isostructural [CoIII 2 LnIII (OMe)2 (naph)2 (O2 CMe)3 (MeOH)2 ] (Ln=Tb, Dy and Er) complexes, revealing a rare {CoIII 2 Ln(μ3 -OMe)}8+ triangular core composed of two diamagnetic CoIII ions and a 4f-ion with slightly distorted square antiprismatic geometry. Alternating current (ac) magnetic susceptibility studies revealed that {Co2 Dy}, and its magnetic diluted analogue {Co2 Dy0.05 Y0.95 }, behave as mononuclear single-molecule magnets (SMMs) with similar energy barriers for the magnetization reversal, Ueff , of ~85-90 K. SMM properties were also detected for {Co2 Er}, with the compound exhibiting a Ueff of 18.7 K under an applied magnetic field of 800 Oe. To interpret the experimental magnetic results, ab initio CASSCF/RASSI-SO and DFT calculations were performed as a means of exploring the single-ion characteristics of LnIII ions and comprehend the role of the diamagnetic CoIII ions in the magnetization relaxation of the three heterometallic compounds.

Electric field-controlled deterministic magnetization reversal in nanomagnet Fe3Si/PMN-PT multiferroic heterostructures

Pure electric field-controlled 180° magnetization switching plays a vital role in low-power magnetoelectric memory devices. Using micromagnetic simulation, we engineered a square-shaped epitaxial Fe3Si nanomagnet on a PMN-PT piezoelectric substrate to make the magnetic easy axis slightly deviate 18° from the piezostrain axis, aiming to break the symmetry of the magnetization distribution and achieve deterministic magnetization reversal paths. Under the coaction of a magnetic field and an electric field, the simulated magnetic hysteresis loops and magnetic domain patterns reveal a fourfold to twofold magnetic anisotropy transition and magnetization reversal paths. Stimulated by pure electric field-induced piezoelectric strain, deterministic 180° magnetization reversals are accomplished by the two successive clockwise 90° switching process. The results help to comprehend electrically regulated deterministic magnetization reversal and pave an avenue for designing multistate spintronics devices.

Temperature Dependence of Magnetization Dynamics in Co/IrMn and Co/FeMn Exchange Biased Structures

Thin film ferromagnet/antiferromagnet (F/AF) exchange biased structures that are widely used in GMR spin valves are considered nowadays as promising systems for antiferromagnetic spintronic and spin-orbitronic devices. Here, the temperature dependences of magnetization dynamics in Co/IrMn and Co/FeMn F/AF structures are investigated using ferromagnetic resonance (FMR) in comparison to a free Co layer. A strong additional decrease in the resonance field was observed in Co/IrMn with a temperature decrease attributed to the rotatable anisotropy increase, which almost vanished at room temperature. In contrast to Co/IrMn, the contribution of the rotatable anisotropy in Co/FeMn is much weaker, even though it exists at RT, it is negative, and slightly varies with the temperature and resonance field shift in Co/FeMn. This is mainly due to unidirectional exchange anisotropy. FMR linewidth for the free Co layer increases with decreasing temperature and is accompanied with a slow relaxation process, while the additional contribution to FMR line broadening in Co/IrMn and Co/FeMn structures is correlated with variation in the exchange anisotropy. The observed results are discussed based on structural and surface morphology and magnetization reversal characterization using X-ray diffraction, atomic force microscopy, and vibrating sample magnetometry data.

Effect of thermal annealing on the magnetization reversal and spin dynamics in ferrimagnetic TbCo thin films

“Ferrimagnetic rare earth-transition metal alloys are promising materials for high-speed and low-power spintronic device applications due to their wide tunability in magnetic properties as a function of temperature and composition. Here, we report the influence of thermal annealing on the magnetization reversal and spin dynamics of ferrimagnetic Tb10Co90 thin films. We have used the co-sputtering technique for the deposition of two different thicknesses (20 nm and 25 nm) of the TbCo films. Annealing of the samples has been carried out in the vacuum at 350 °C for 1 h. The magnetization reversal and dynamic properties of the as-deposited and annealed films are studied using the magneto-optical Kerr effect and broadband ferromagnetic resonance measurements at room temperature, respectively. The hysteresis loops reveal an anisotropic reversal process. The anisotropy field reduces from 66 Oe to 38 Oe for 20-nm-thick films and 98 Oe to 31 Oe for 25-nm-thick films upon annealing. Consequently, thermal annealing has led to a large reduction (∼70 %) in inhomogeneous linewidth broadening of the ferromagnetic resonance modes. Gilbert damping constant is found to decrease from 0.027 to 0.021 for 20-nm-thick films, whereas it decreases from 0.043 to 0.019 for 25-nm-thick films. The results have been attributed to the enhanced crystallinity of the films as observed in the structural analysis using grazing incidence X-ray diffraction.”

Growth rate and substrate temperature dependent magnetic and magneto-microstructural properties of sputtered Fe-Co-Al thin films

This work reports the effect of growth rate and substrate temperature on the structural, magnetic and magneto-microstructural properties of Fe-Co-Al alloy thin films. 75 nm thick Fe-Co-Al films were deposited on Si 〈100〉 substrates at room temperature by high power impulse magnetron sputtering at different growth rates viz., 0.67–1.6 Å/s. All the films exhibited a uniform composition of about Fe60Co25Al15 irrespective of growth rates. In addition to this, all the films crystallized in single phase with bcc type structure. Growth rate was found to have a considerable impact on the magnetic properties of Fe-Co-Al thin films. Film grown at 1.3 Å/s exhibited an in-plane magnetic anisotropy with large saturation magnetization combined with low coercivity when compared with other films. To improve the magnetic properties further, Fe-Co-Al films were also processed at different substrate temperatures (Ts) viz., 300, 400 and 500 °C. Substrate temperature of about 300 °C was found to have beneficial effect on the soft magnetic characteristics of Fe-Co-Al films. While the crystal structure did not show any perceptible change with processing conditions, magnetic microstructures traced from magneto optic Kerr microscopy displayed substantial change. Upon increase in substrate temperature, during magnetization reversal process, magnetic domain images traced along the easy magnetization direction magnetization showed transformation from branched band domains (as-deposited) → wide band domains (Ts = 300 °C) → less periodic branched band domains (Ts = 400 °C) → ripple domains (Ts = 500 °C). Attempts were made to correlate the achieved magnetic properties in Fe-Co-Al with magnetic microstructures.

The single power law relationship between coercivity and width in pointed composite element magnetic barcodes

Pointed magnetic elements are introduced as an improvement upon rectangular strips currently employed in composite element magnetic barcodes. The coercivity of these elements, as measured using the magneto-optic Kerr effect, is found to strictly adhere to a single power law relationship with the element width, where the power law exponent is dependent on the length of the pointed region and takes values between −0.98 and −0.91. The steeper gradients here, along with the absence of the crossover region seen in rectangular devices, present these structures as a strict improvement in terms of potential device applications. These improvements are found to be present for all structures where the pointed region is as long as, or longer than, the magnetic element is wide. The remanent magnetization configuration, imaged using photo-emission microscopy with contrast from x-ray magnetic circular dichroism (XMCD-PEEM), is compared to the results of micromagnetic simulations. It is found to cant inward in the pointed section of the strip, aligning with the edges of the point, pinning the magnetization and giving a consistent magnetization reversal behavior for all element widths investigated.

Magnetic properties of Pt/Co/Pt trilayers with W insert layer

The results of magnetic investigations of epitaxial trilayers Pt/Co/Pt asymmetrically modified by inserting W layer at the bottom or top Co interfaces in the wide ranges of Co and W layer thicknesses are reported. The samples were epitaxially grown on Pt buffer in double wedge geometry: the dCo thickness gradient of the continuous Co wedge was orthogonal to the dW thickness gradient of non-magnetic W underlayer (overlayer) deposited either as step-like or continuous wedge. Resulting samples have Pt/W/Co/Pt (Pt/Co/W/Pt) stacking sequences. The influence of (dCo, dW) on static magnetization reversal processes were studied using magnetooptical Kerr effect; Brillouin light scattering (BLS) method was applied for dynamical characterization. The magnetic dead layer thickness d0 abruptly increases until dW reaches ∼ 0.5 nm for both orderings. Then its value saturates for Pt/Co/W/Pt, while for Pt/W/Co/Pt samples a slow growth is observed. For dCo corresponding to out-of-plane magnetization the inserting of W layer leads to the strong reduction in coercivity (two orders of magnitude) and transition to in-plane magnetization in the case of Pt/W/Co/Pt ordering, while the Pt/Co/W/Pt samples demonstrates weak changes of coercivity with small change in magnetic anisotropy. The strength of interfacial Dzyaloshinskii-Moriya interaction (iDMI) and spin wave damping for selected dCo thicknesses as a function of dW were derived from BLS measurements. Inserting W with thickness dW ∼ 0.1 nm induces significant iDMI changes, iDMI saturates for dW > 0.4 nm. Our findings demonstrate the efficiency of thin W interlayer on modification of magnetic parameters in Pt/Co/Pt trilayer.

Magnetization Reversal by Pulse of Magnetic Field in SQUID with Single φ0-Junction

Magnetization Reversal by Pulse of Magnetic Field in SQUID with Single φ0-Junction

Magneto-optical diffraction of visible light as a probe of nanoscale displacement of domain walls at femtosecond timescales.

Using diffraction of femtosecond laser pulses of visible light by a magnetic domain pattern in an iron garnet, we demonstrate a proof of concept of time-resolved measurements of domain pattern movements with nanometer spatial and femtosecond temporal resolution. In this method, a femtosecond laser (pump) pulse initiates magnetization dynamics in a sample that is initially in a labyrinth domain state, while an equally short linearly polarized laser pulse (probe) is diffracted by the domain pattern. The components of the diffracted light that are polarized orthogonally to the incident light generate several concentric diffraction rings. Nanometer small changes in the relative sizes of domains with opposite magnetization result in observable changes in the intensities of the rings. We demonstrate that the signal-to-noise ratio is high enough to detect a 6nm domain wall displacement with 100fs temporal resolution using visible light. We also discuss possible artifacts, such as pump-induced changes of optical properties, that can affect the measurements.

Interplay of spin–orbit and exchange interaction in a ferromagnet/heavy-metal hybrid system: Ni on W(110)

In a combined experimental and theoretical study, we investigate the interplay of spin–orbit interaction (SOI) and exchange interaction (XI) in the electronic structure of ultrathin Ni films on W(110). Using spin- and angle-resolved inverse photoemission, we observe that the size of the spin splitting of Ni-related exchange-split states differs for opposite magnetization directions. A quenched spin splitting for one of the magnetization directions reveals a contribution of SOI on an equal footing with XI. Using density-functional theory calculations, we explore the underlying mechanisms responsible for the experimentally observed coupling of SOI and XI. We find that a hybridization between adsorbate and substrate states, along with a high probability density of the respective states at the heavy W nuclei, cause the strong influence of SOI on the Ni-related exchange-split states.

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.

Multiferroics in Magneto Electric – Spin Orbital Devices

The development of new computing technologies has given a new stimulus in the study of multiferroics. The use of multiferroics allows the realization of competitive energy efficient scalable logic and storage devices. The low - power consumption in Magneto Electric – Spin Orbital logics [1] and Magnetic Random Access Memory components is provided by magnetoelectric switching in multiferroic – based systems using a low-energy electric field. Our work concerns the modelling of the Magneto Electric – Spin Orbital elements with an emphasis on the magnetoelectric component and simulation of magnetization reversal processes in a model system. The use of the proposed approach makes it possible to analyze the influence of dimensional factors (film thicknesses, transverse dimensions, sample shape) affecting the magnetic states of multiferroic nanoelements; taking into account interfacial interactions (magnetic anisotropy and interlayer exchange); energy-efficient external influences that allow switching magnetic states using magnetic and electric fields.

Numerical study of two-terminal SOT-MRAM

A fully three-dimensional model coupling charge, spin, magnetization, and temperature dynamics is employed to study the switching behavior of the two-terminal spin–orbit torque magnetoresistive random access memory (2T-SOT-MRAM). We demonstrated that free layer (FL) spin-transfer torque (STT) switching is strongly affected by the inclusion of the SOT. The nucleation of the FL magnetization reversal is similar to the conventional three-terminal (3T)-SOT-MRAM. In comparison to the traditional STT-MRAM, the switching times and critical switching voltages are considerably reduced. Furthermore, the full thermal simulations show the significance of the heating process and temperature modeling in MRAM devices as they demonstrate a significantly lower switching time than the one in a constant temperature model.

Magnetization reversal study of NiZn x Nb2O9 compounds using Monte Carlo simulations

The magnetization reversal (MR) of the layered NiZn x Nb2O9 ferrimagnetic compounds, with and 0.75, is studied in this work using Monte Carlo (MC) simulations and mean field (MF) calculations. First, we analyze the parent compound to set the parameters of our simulations; testing together MC simulations, MF calculations, and MR experiments reported by Bolletta et al (2022 J. Appl. Phys. 132 153901). Then using two different approaches we fit the MR curves of the series of compounds finding a quite good agreement between MC simulations and the experiments. According to these results, Zn substitutions change the relative contribution to the magnetization of the different layers. Here we present two possible hypotheses to explain this effect; one involving a heterogeneous distribution of Zn2+ among the layers, and the other related to distortions of the NiO6 octahedra.

Gain-assisted magnetoplasmonic switching in metal–dielectric–metal plasmonic waveguides

Optical switches in metal–dielectric–metal (MIM) structures attracted much interest for chip-scale plasmonic modulator merging nanoelectronics and ultrafast photonics. The absorption switches employed gain modulations; thus, their switching speed is limited to the sub-gigahertz range because of the spontaneous lifetime of the transition. In this paper, we theoretically predicted that the function of the absorption switches is achieved employing magnetic field reversal and constant gain instead of gain modulations. We investigated analytically and numerically transmission in the presence of both gain and gyration and revealed that the transmission shift by the external magnetic field is maximized at a resonance value of gain. The switching speed can reach the THz range thanks to the large optical bandwidth in the MIM stub structures and the advanced magnetization switching technology. The MIM structure with combined gain and magnetoplasmonic properties enhances interaction between light and magnetic field on nanoscale and enables the device footprint down to the deep subwavelength scale λ2/50.

Energy loss and hysteresis of reversible magnetization processes in iron-based soft magnetic composites

The paper focuses on investigation of the hysteresis originating from the reversible magnetization processes and the quantification of the corresponding energy dissipation, on soft magnetic composite materials differing in various parameters so exhibiting different proportions of magnetization processes types during quasi-static magnetizing reversal under different maximum inductions. It was confirmed that reversible magnetization processes show hysteresis too and generate energy losses. These losses come from frictional effects accompanying local rotations of spins at the microscopical view of all magnetization processes including the reversible ones. It was found that the area of a hysteresis loop obtained by the integration of reversible permeability reveals approximately the proportion of reversible magnetization vector rotations, which produce energy loss much more than reversible domain wall displacements.

Investigating a Solar Wind Stream Interaction Region using Interplanetary Spacecraft Radio Signals: A Magnetohydrodynamic Simulation Study

Stream interaction regions (SIRs) are spiral heliospheric structures that arise at the interface between fast and preceding slow solar wind regions. SIR enhancements of density and magnetic field intensity, often with magnetic polarity inversion, are potentially geoeffective and therefore important in the analysis of space weather. We studied an MHD heliospheric simulation containing a well-defined SIR using a new instrument concept based on trans-heliospheric radio sensing: Faraday Effect Tracker of Coronal and Heliospheric structures (FETCH). FETCH uses line-of-sight radio propagation techniques to measure Faraday rotation and electron column density. Analysis of the simulated FETCH observations clearly demonstrated density and magnetic field enhancements, and magnetic polarity reversal, all of which were confirmed in Wind spacecraft measurements at 1 au. FETCH provided 4.5–5.7 days lead times for predicting the arrival of SIR features at Earth. The SIR radial speed was estimated to be 350–390 km s−1. These initial results hold promise that FETCH will be valuable in detecting and characterizing the inner heliosphere SIR properties well ahead of their presentation in the local geospace environment.

Exchange coupling effect and magnetic properties in sintered RE-Fe-B magnets

Preparing the magnetic materials bearing both a high coercive field and a high energy product is a great challenge. In this paper the content of Nd was reduced in Nd-Fe-B for enhancing the interaction effect as well as for increasing the energy product, and Tb-Fe-B powders were mixed to enhance the coercivity. In the sintered RE-Fe-B magnets some of Tb atoms diffuse into the outer regions of Nd-Fe-B grains, and Tb-rich RE2Fe14B grains also exist with the crystal size at micrometer scale. The recoil loops bear the spring behaviors, and the reversal susceptibility implies the availability of exchange coupling effect among the micro grains. The activation volume also indicates that the exchange coupling is effective in magnetization reversal. So the magnetization reversal in Nd-Fe-B grains could be restrained by the high anisotropy phase of Tb-rich core. Therefore, besides the effects of Tb-diffusion into grain outer region and the intergranular phase, the exchange coupling effect could not be neglected in improving the coercivity in the Nd-content-reduced magnets, and both a high coercive field of 20.0 kOe and a high energy product of 50.1 MGOe are achieved in the micro-crystal composite magnets.

Magnetization reversals in core–shell sphere clusters: finite-element micromagnetic simulation and machine learning analysis

Recently developed permanent magnets, featuring specially engineered microstructures of inhomogeneous magnetic phases, are being considered as cost-effective alternatives to homogeneous single-main-phase hard magnets composed of Nd2Fe14B, without compromising performance. In this study, we conducted a comprehensive examination of a core–shell sphere cluster model of Ce-substituted inhomogeneous Nd2-δCeδFe14B phases versus homogeneous magnetic phases, utilizing finite-element micromagnetic simulation and machine learning methods. This involved a meticulous, sphere-by-sphere analysis of individual demagnetization curves calculated from the cluster model. The grain-by-grain analyses unveiled that these individual demagnetization curves can elucidate the overall magnetization reversal in terms of the nucleation and coercive fields for each sphere. Furthermore, it was observed that Nd-rich spheres exhibited much broader ranges of nucleation and coercive field distributions, while Nd-lean spheres showed relatively narrower ranges. To identify the key parameter responsible for the notable differences in the nucleation fields, we constructed a machine learning regression model. The model utilized numerous hyperparameter sets, optimized through the very fast simulated annealing algorithm, to ensure reliable training. Using the kernel SHapley Additive eXplanation (SHAP) technique, we inferred that stray fields among the 11 parameters were closely related to coercivity. We further substantiated the machine learning models’ inference by establishing an analytical model based on the eigenvalue problem in classical micromagnetic theory. Our grain-by-grain interpretation can guide the optimal design of granular hard magnets from Nd2Fe14B and other abundant rare earth transition elements, focusing on extraordinary performance through the careful adjustment of microstructures and elemental compositions.