Demagnetization Process Research Articles

Visualization of magnetization reversal processes by dynamic Kerr microscopy

Abstract Dynamic Kerr microscopy enables the analysis of structural defects in magnetic materials and their impact on the demagnetization behavior. Using new visualization techniques, it is possible to image the demagnetization process in the microstructure. It has been found that residual stresses and inclusions in soft magnets, for example, will impede magnetization. In permanent magnets, abnormally large grains, for example, will demagnetize first and then cause an accelerated demagnetization of adjacent grains.

In-vacuum Helmholtz coils for pulsed magnetic fields studying ultrafast demagnetization

The availability of sub 100 fs short and highly intense free-electron laser (FEL) pulses allows for new insights in laser-induced ultrafast demagnetization (LID) of ferromagnetic thin films on nanometer length scales. We designed a pair of in-vacuum Helmholtz coils, providing pulsed magnetic fields up to µ 0 Hz = ±45 mT, for time-resolved experiments at FEL sources in transmission geometry. We report on the implementation of the Helmholtz coils in an optical-pump–resonant-magnetic-scattering (tr-XRMS) experiment at the FEL FERMI (Elettra, Trieste) to study LID in different magnetic domain networks. We discuss the limitations for multi-shot measurements, that rely on the full reversibility of the demagnetization process in-between two pump–probe events, and emphasize the importance of reference–pump–probe schemes, especially in tr-XRMS experiments that employ external Hz fields.

Dynamic magnetomechanical behavior of sintered Nd2Fe14B under impact

As an important magnetic functional material, sintered NdFeB has gradually applied from mild places to environments that need to withstand impact. The reliability of materials to work in this environment has received extensive attention, but current research cannot provide corresponding support of experimental data and theoretical model for this problem yet. To tackle this, we carried out research on the magnetomechanical behavior of sintered NdFeB under dynamic impact environment. Specifically, dynamic mechanics and impact demagnetization experiments were conducted separately. Then, according to the characteristics of brittleness and strain rate dependence shown in dynamic mechanical experiments, a dynamic damage constitutive model was established. Additionally, a new magnetic degradation model was proposed in which the stress knee position existed and the relationship between impact load and irreversibility was established. The co-simulation model of impact demagnetization based on dynamic mechanical constitutive model and magnetic degradation model was designed to better display the demagnetization process of materials. Meanwhile, the parameter identification process was carried out to determine the unknown parameters in the model. Finally, the effectiveness of identified parameters were verified by comparing the simulation values with the experimental values.

On the electronic and magnetic properties of RVO3 (R = Er, Ho, Y, Lu) vanadates: first principles study

In this paper, we investigate the influence of the rare earth element (R) on the physical properties of RVO3 (R = Er, Ho, Y, Lu). For this purpose, a theoretical work is reported in this study using ab initio method with the intent of enhancing our knowledge of the electronic, and magnetic properties of RVO3 (R = Er, Ho, Y, Lu) compounds. The electronic properties of these materials show the presence of Jahn-Teller effect, with a band gap of approximately 0.58eV, 1.57eV, 1.12eV, 1.28eV, for ErVO3, HoVO3, YVO3, LuVO3, respectively. The magnetic contribution of Ho, Er, and V atoms in HoVO3, and ErVO3 compounds was disentangled using X-ray magnetic circular dichroism (XMCD). The magnetic anisotropies of RVO3 with (R = Er, Ho, Y, Lu) are studied and analyzed using spin orbit coupling. We noticed an easy axis of magnetization along the c-direction with strong magneto-crystalline anisotropy energies for the non-magnetic rare earth elements (R = Lu, Y)VO3. For the magnetic rare earth elements, the easy and hard magnetization direction were found to be along b-axis and c-axis, and a-axis and c-axis for HoVO3, and ErVO3, respectively. This means that instead of the standard magnetization demagnetization process, large thermal effects can be obtained by spinning their single crystals in constant magnetic fields. The exchange coupling between (Ho–V) is also found to be greater than the other exchange interactions in HoVO3. Accordingly, a large conventional magnetocaloric could be created in HoVO3, since the Ho Ising moments experience a metamagnetic transition. However, the magnetocaloric effect (MCE) is expected to be lower in ErVO3 due to the weak (Er–V) exchange coupling.

Electric field tuning of ultrafast demagnetization in a magnetoelectric heterostructure

Voltage/electric-field control of ultrafast magnetization dynamics in magnetoelectrics provides a novel avenue for electronic tuning with orders of magnitude less power consumption, improved tuning response time, and more compact form factor, compared with conventional magnetic field control of magnetization dynamics. Magnetoelectrically tuned laser-driven magnetization dynamics has the potential to enable the next generation of optomagnetic devices from THz communication to optical magnetic recording. In this study, we fabricated a magnetoelectric heterostructure, specifically ferromagnetic ${({\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19})}_{88}{\mathrm{B}}_{12}$ (FeGaB) thin film deposited on a $\mathrm{Pb}({\mathrm{Mg}}_{1/3}\phantom{\rule{0.16em}{0ex}}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{PbTi}{\mathrm{O}}_{3}$ ferroelectric substrate, to explore the electric field tuning of ultrafast demagnetization and to understand how magnetic anisotropy changes postdemagnetization. We characterized the magnetoelectric coupling of our heterostructure to demonstrate the static and dynamic magnetization tunability with an applied electric field. We utilized the time-resolved magneto-optic Kerr effect to understand the ultrafast demagnetization process under different applied electric fields and magnetic fields. The typically observed strain-induced magnetic easy-axis rotation in a ferromagnetic/ferroelectric heterostructure was also observed to tune the ultrafast demagnetization of FeGaB in our experiment. Additionally, we found that the magnetization rotation can be achieved with a lower electric field compared with static tuning without laser heating. Furthermore, we observed the hysteresis loops postultrafast demagnetization and found that the magnetoelectric heterostructure exhibits a mixture of volatile and nonvolatile behaviors. These findings shed light on the potential of our magnetoelectric heterostructure for ultrafast optomagnetic devices and electric field tuning of spintronic THz emitters.

Reply to the comment on “A remarkable last glacial loess sedimentation at Roxolany in the Dniester Liman (Southern Ukraine)” by

Bakhmutov and Hlavatskyi (2022) (BH) in their comment on our paper (Łanczont et al., 2022) focus on issue of the magnetostratigraphy of the whole loess-palaeosol sequence at Roxolany. However, we analysed only the formation conditions of the upper part of this sequence, which we believe to be of the Late Pleistocene. Its exceptionally large thickness is a very local phenomenon and we consider this site as an interesting case study, atypical for other areas of Ukraine and south-eastern Europe. In our paper, we described the formation of the loess cover in a close location in relation to the palaeorelief of the region referring to the Black Sea level oscillation. Unfortunately, BH do not address our multi-directional interpretation of the S2-L2-S1-L1-S0 sequence based on the multi-proxy results. They only refer to stratigraphic background outlined very briefly in the introduction. This outline was necessary before the discussion our main research topic. BH noted that our stratigraphy did not take into account their latest published data regarding magnetostratigraphy and magnetic susceptibility, and in particular, the position of the Matuyama/Brunhes boundary. However, their magnetostratigraphic interpretation of this profile is questionable due to the lack of analysis of the magnetic carriers while detecting their differentiation during the demagnetization process.

All-optical control of spin in a 2D van der Waals magnet

Two-dimensional (2D) van der Waals magnets provide new opportunities for control of magnetism at the nanometre scale via mechanisms such as strain, voltage and the photovoltaic effect. Ultrafast laser pulses promise the fastest and most energy efficient means of manipulating electron spin and can be utilized for information storage. However, little is known about how laser pulses influence the spins in 2D magnets. Here we demonstrate laser-induced magnetic domain formation and all-optical switching in the recently discovered 2D van der Waals ferromagnet CrI3. While the magnetism of bare CrI3 layers can be manipulated with single laser pulses through thermal demagnetization processes, all-optical switching is achieved in nanostructures that combine ultrathin CrI3 with a monolayer of WSe2. The out-of-plane magnetization is switched with multiple femtosecond pulses of either circular or linear polarization, while single pulses result in less reproducible and partial switching. Our results imply that spin-dependent interfacial charge transfer between the WSe2 and CrI3 is the underpinning mechanism for the switching, paving the way towards ultrafast optical control of 2D van der Waals magnets for future photomagnetic recording and device technology.

FexCo1-x alloy nanoparticles: Synthesis, structure, magnetic characterization and magnetorheological application

We present results concerning the synthesis of FexCo1-x (0 < x < 1) alloy nanoparticles (NPs) with different compositions by the chemical reduction technique and subsequent structural (XRD patterns), morphological (TEM images) and magnetic (VSM magnetometry) characterization. We have got excellent quality NPs in the cubic bcc structure showing a room temperature magnetization as high as 235 emu/g for the Fe66Co34 composition alloy. Powder of composition Fe47Co53 was used to fabricate a magnetorheological fluid (MRF) by using mineral oil as carrier liquid and Aerosil 300 as additive to control the viscosity of the fluid. This MRF showed a strong magnetorheological response with a superior performance under applied magnetic field up to 616.7 kA/m and good reversibility after demagnetization process. At that highest applied magnetic field, we determined a yield stress value of 2729 Pa, that competes well with the best ones reported in the most recent literature.

Quantitative analyses of surface and bulk magnetization in Nd2Fe14B and SmCo5 single crystals: towards understanding the large N eff in nucleation-type magnets

The performance of sintered permanent magnets with nucleation-type coercivity mechanism is largely governed by the magnetic state of thin surface layers of constituent grains, and a deeper insight into magnetization–demagnetization processes occurring in the shell part of the grains is very important for further improvement of hard magnetic materials. In this work, we used Nd2Fe14B and SmCo5 single crystals as model objects. By applying magneto-optical Kerr microscopy and conventional magnetometry, we compare the magnetization–demagnetization processes occurring in the thin surface layer and in the volume of both single crystals. We show that upon magnetization along the c-axis, the volume of the single crystals saturates in the field, rigorously determined by demagnetization factor of the bulk sample, whereas in the surface layer a magnetic domain structure exists up to 1.88 T in Nd2Fe14B and 1.19 T in SmCo5 regardless of their bulk demagnetization factors. This means that the surface layer with orientation perpendicular to c-axis magnetizes as a thin magnetic film and has an effective demagnetization factor N eff ∼ 1. We also show that this effect can be reproduced in the framework of conventional finite element method modeling but the analytical solution of this problem still needs to be found. We believe that our findings can be useful for understanding of the formation of a high coercive state in nucleation-type permanent magnets, where the phenomenological concept of the large effective demagnetization factor N eff plays an important role.

Analysis and control of ultrafast demagnetization dynamics in ferrimagnetic Gdx(CoFe)1-x alloys

Deep understanding and powerful control of the magnetization dynamics is of great importance for developing high speed spintronic devices. In this work, laser-induced ultrafast demagnetization process of Gdx(CoFe)1-x alloy films has been extensively studied by using the fs-laser pump-probe technique. Interestingly, it is found that by adjusting the Gd content x and hence the intersublattice 3d-4f exchange coupling strength, the demagnetization dynamics varies from a fast one-stage decay to a continuous or a discrete two-stage decay, which is mainly resulted from the different energy transfer rates via the intersublattice exchange coupling and has been well simulated by considering the Elliott-Yafet type spin-flip scattering theory and the 4-Temperature model. Moreover, we observed that the second-stage decay time is strongly dependent on the external magnetic field, which is analyzed theoretically and suggests an origin of ultrafast giant magnetic cooling effect via the modulation of lattice-spin coupling. These studies are quite helpful for achieving the efficient manipulation of ultrafast magnetization behaviors.

Fully understanding the performance of nanocrystalline ternary Pr-Fe-B alloys with three typical phase constitutions

Pr2Fe14B phase exhibit similar intrinsic magnetic properties as Nd2Fe14B with even higher magnetic anisotropy field. Here we investigated the magnetic properties, exchange interaction, and corrosion resistance of melt-spun ternary Pr-Fe-B alloys with three typical compositions including Pr10Fe84B6, Pr12Fe82B6, and Pr14Fe80B6. The hard magnetic Pr2Fe14B phase is present in all alloys, while the α-Fe and RE-rich phases are additionally present in Pr10Fe84B6 and Pr14Fe80B6 alloys, respectively. Under optimum wheel speed of melt spinning, uniform single-phase behavior has been obtained for all alloys, which gives optimal hard magnetic properties. Based on exchange interaction behavior analysis, the nanocomposite Pr10Fe84B6 alloy and single phase Pr12Fe82B6 alloy show more uniform reversal domain nucleation behavior during demagnetization process. Importantly, Pr-Fe-B alloys show significant advantages over Nd-Fe-B alloys at cryogenic temperature. The hard magnetic properties increase with reducing temperature and extremely high coercivity has been obtained at 100 K. Pr10Fe84B6 alloy exhibits not only better low temperature stability but also much higher corrosion resistance than the Pr14Fe80B6 and Pr12Fe82B6 alloys. The existence of soft phase can improve the anti-corrosion properties, while the RE-rich phase is not advantageous to corrosion resistance although it enhances the coercivity.

On the use of cross polarization in solid-state NMR: 1H spin-lock versus adiabatic demagnetization in the rotating frame

Cross polarization (CP) is a widely used solid-state nuclear magnetic resonance (NMR) technique for enhancing the polarization of dilute S spins from much larger polarization of abundant I spins such as 1H. To achieve such a polarization transfer, the I spin should either be spin-locked or be converted to the dipolar ordered state through adiabatic demagnetization in the rotating frame. In this work, we analyze the spin dynamics of the Hartmann-Hahn CP (HHCP) utilizing the 1H spin-locking, and the dipolar-order CP (DOCP) having the 1H adiabatic demagnetization. We further propose an adiabatic demagnetization CP (ADCP) where a constant radio-frequency pulse is applied on the S spin while 1H is adiabatically demagnetized. Our analyses indicate that ADCP utilizes the adiabatic passage to effectively achieve the polarization transfer from the 1H to S spins. In addition, the dipolar ordered state generated during the 1H demagnetization process could also be converted into the observable S polarization through DOCP, further enhancing the polarized signals. It is shown by both static and magic-angle-spinning (MAS) NMR experiments that ADCP has dramatically broadened the CP matching condition over the other CP schemes. Various samples have been used to demonstrate the polarization transfer efficiency of this newly proposed ADCP scheme.

Role of Spin–Orbit Coupling on Ultrafast Spin Dynamics in Nonmagnet/Ferromagnet Heterostructures

AbstractSpin–orbit coupling (SOC), the interaction between spin and orbital angular momentum of electrons, is imperative to control magnetic properties of nonmagnet (NM)/ferromagnet (FM) heterostructures and design energy‐efficient and faster spin‐based devices. Here, femtosecond pulsed laser‐induced time‐resolved magneto‐optical Kerr effect magnetometry is employed to investigate magnetization dynamics in different NM/Co20Fe60B20 heterostructures, where the NM layer varies as Cu, Ta, W, Pt, Ta/Ru/Ta, and Si/SiO2 (no underlayer) that differ in SOC strength. It is observed that there is a systematic variation in ultrafast demagnetization time (τm), fast remagnetization time (τr), and Gilbert damping parameter (α) with the SOC strength of the underlayer and an inverse relationship between α and τm, τr is established due to the dominant contribution of spin current transport in ultrafast demagnetization and fast remagnetization processes. The spin pumping formalism estimates the effective spin‐mixing conductance (Geff) for different interfaces, which signifies that the high SOC strength of underlayers results in high Geff indicating more efficient transport of spin current through it. This study suggests that the SOC strength of the NM underlayer plays a significant role in controlling the ultrafast demagnetization process through interfacial spin current transport in a NM/FM heterostructure which can be beneficial for the development of ultrafast spintronics devices.

Mechanical and magnetic responses of sintered NdFeB under impact

Sintered neodymium iron boron (NdFeB) has been widely applied in various industrial fields thanks to its excellent magnetic properties. However, the impact loads in increasing number of applications challenged its mechanical safety and magnetic field reliability. This is a blind spot in the existing researches on the properties of sintered NdFeB. To tackle this, this article proposes a dynamic damage constitutive model and impact demagnetization model respectively to illustrate the dynamic mechanics and impact demagnetization process of sintered NdFeB. Specifically, Split Hopkinson Pressure Bar (SHPB) experiments are conducted to obtain the dynamic response of sintered NdFeB at different strain rates. The dynamic fracture process of NdFeB is also analyzed by combining the damage constitutive model based on experimental data with the SHPB dynamic simulation model. This is the foundation of the demagnetization model in subsequent. Different from the previous researches on complete demagnetization caused by intensive impacts at the magnitude of GPa, the demagnetization study in this article is aimed at the more regular impacts below 100 MPa. Based on the stress knee hypothesis and irreversible aggravation hypothesis respectively, the reversible and irreversible processes during the demagnetization are individually demonstrated in the form of magnetic domain flip. The specific comparison between simulation and experimental results verified the effectiveness of the demagnetization model. • The dynamic stress-strain curve of NdFeB is obtained through the SHPB experiments. • The damage constitutive model of brittle materials is constructed. • A new impact demagnetization model is proposed to describe the reversible and irreversible demagnetization phenomena.

Distinguishing Local Demagnetization Contribution to the Magnetization Process in Multisegmented Nanowires.

Cylindrical magnetic nanowires are promising materials that have the potential to be used in a wide range of applications. The versatility of these nanostructures is based on the tunability of their magnetic properties, which is achieved by appropriately selecting their composition and morphology. In addition, stochastic behavior has attracted attention in the development of neuromorphic devices relying on probabilistic magnetization switching. Here, we present a study of the magnetization reversal process in multisegmented CoNi/Cu nanowires. Nonstandard 2D magnetic maps, recorded under an in-plane magnetic field, produce datasets that correlate with magnetoresistance measurements and micromagnetic simulations. From this process, the contribution of the individual segments to the demagnetization process can be distinguished. The results show that the magnetization reversal in these nanowires does not occur through a single Barkhausen jump, but rather by multistep switching, as individual CoNi segments in the NW undergo a magnetization reversal. The existence of vortex states is confirmed by their footprint in the magnetoresistance and 2D MFM maps. In addition, the stochasticity of the magnetization reversal is analysed. On the one hand, we observe different switching fields among the segments due to a slight variation in geometrical parameters or magnetic anisotropy. On the other hand, the stochasticity is observed in a series of repetitions of the magnetization reversal processes for the same NW under the same conditions.

Magnetization reversal mechanism for core-shell grains in GBD Nd-Fe-B sintered magnets based on in-situ domain observation

Understanding the magnetization reversal mechanism of a core-shell structure grains formed in the grain boundary diffused Nd-Fe-B magnet can further improve the utilization efficiency of heavy rare earth resources to more effectively improve the coercivity of Nd-Fe-B sintered permanent magnet. In this paper, magnetic domain structure and its dynamic evolution of core-shell structural grains in grain-boundary-diffused Nd-Fe-B magnet were in-situ observed. Wider domains are found in shell regions due to higher anisotropy constant and lower saturation magnetization. The domain motion revealed that the demagnetization process of core-shell structure has a multi-step nucleation process. The nucleation of the core-shell structural grain firstly occurs at the core regions, while another reverse nucleation appears at the outmost portion in shell-region at a larger reverse field. The micromagnetic simulation results confirm the possibility of the nucleation and reversal process observed in the experiment. Our results can help understanding the demagnetization process and the coercivity enhancement mechanism of grain-boundary-diffused Nd-Fe-B sintered magnet.

Magnetic properties and coercivity mechanism of high Ce-content CeNdFeB film with Tb diffusion

Magnetic properties and coercivity mechanism of an anisotropic Ce–Nd–Fe–B film with a high Ce content (70 at. % Nd is replaced by Ce) have been investigated. After grain boundary diffusion with Tb layers, an enhancement of coercivity from 4150 to 9250 Oe is observed. Combining the initial magnetization curves, micromagnetic theory, and in-siut observation of magnetic domains in the demagnetization process, it is confirmed that the coercivity mechanism for the high-Ce-content magnets is the mixed type dominated by the pinning mechanism. Moreover, as the thickness of the Tb diffusion layer increases, the pinning center of domain walls changes from narrow planar inhomogeneities to wide planar inhomogeneities. A growing role of pinning plays in determining the coercivity of samples with increasing the thickness of Tb layer due to the increase in magnetocrystalline anisotropy after Tb substitution in the RE2Fe14B phase. In addition, the CeFe2 intergranular phase leads to the enhancement of coercivity due to decoupling the hard magnetic phase grains. Our results provide an insight into the coercivity mechanism of high-Ce-concentration Ce–Nd–Fe–B magnetic materials and promote the comprehension of the effect of Tb diffusion in the magnetization reversal process.

Conservation of angular momentum in ultrafast spin dynamics

The total angular momentum of a close system is a conserved quantity, which should remain constant in time for any excitation experiment once the pumping signal has extinguished. Such conservation, however, is never satisfied in practice in any real-time first principles description of the demagnetization process. Furthermore, there is a growing experimental evidence that the same takes place in experiments. The missing angular momentum is usually associated to lattice vibrations, which are not measured experimentally and are never considered in real-time simulations. Here we critically analyse the issue and conclude that current state-of-the-art simulations violate angular momentum conservation already at the electronic level of description. This shortcoming originates from an oversimplified description of the spin-orbit coupling, which includes atomic contributions but neglects completely that of itinerant electrons. We corroborate our findings with time-dependent simulations using model tight-binding Hamiltonians, and show that indeed such conservation can be re-introduced by an appropriate choice of spin-orbit coupling. The consequences of our findings on recent experiments are also discussed.

Experimental observation of transition from type I to type II ultrafast demagnetization dynamics in chemically disordered Fe60Al40 thin film, driven by laser fluence

Understanding of the laser-induced ultrafast demagnetization dynamics is one of the most challenging and hot topics in magnetism research due to its potential applications in magnetic storage devices and the field of spintronics. Recently, a laser-induced switching of ferromagnetism, driven by a disorder–order transition on FeAl thin films, has been experimentally demonstrated. The switching of ferromagnetic ordering by ultrafast laser pulses in FeAl thin films may open new possible applications of this material such as magnetic data storage and manipulation. Since the speed of the magnetic switching of magnetic states in thin films is one of the critical parameters for these applications, here we used time resolved magneto-optical Kerr measurements to investigate the demagnetization dynamics of Fe60Al40 thin films at room temperature. We have for the first time observed a clear transition from one-step dynamics (type I) to two-step (type II) dynamics in the same material by increasing pump laser fluence. This experimental observation may give a strong confirmation that the ultrafast demagnetization process can be treated as a thermal process and is driven by the difference between temperatures of the electron and spin systems.

Spin-orbit coupling induced demagnetization in Ni: Ab initio nonadiabatic molecular dynamics perspective

Spin-orbit coupling (SOC), which can induce spin flip during the relaxation of photoexcited charge carrier, plays a crucial role in spin dynamics. In this work, we have used time-domain ab initio nonadiabatic molecular dynamics (NAMD) method to study the SOC induced ultrafast demagnetization in Ni at $300\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The spin-diabatic representation using spin-polarized Kohn-Sham (KS) basis sets and spin-adiabatic representation using spinor basis sets have been applied, and both of them achieve demagnetization in Ni with a timescale around $100\phantom{\rule{0.16em}{0ex}}\mathrm{fs}$. The spin-diabatic representation suggests a picture that the electron-phonon coupling (EPC) provides direct energy relaxation channel among the same-spin states, while the SOC can induce spin flip. After photoexcitation, it is found the spin-minority electrons relax to the same-spin states rather than the opposite-spin states, since EPC is larger than SOC by one order of magnitude. By contrast, for the spin-majority electrons, spin flip occurs since there are no empty same-spin states as electron acceptor above the Fermi level. The different relaxation pathways for spin-majority and spin-minority electrons induce the demagnetization. The spin-adiabatic representation provides an Elliott-Yafet spin-phonon scattering picture. The SOC induced reduction of magnetic moment in Ni may induce magnon to drive further demagnetization. The ab initio NAMD simulation provides a critical angle to understand how the SOC and EPC affect demagnetization process in Ni.