Initial Magnetization Process Research Articles

Large room-temperature spontaneous exchange bias effect in LaFeO3 micro-polyhedrons

Single-phase materials with exchange bias (EB) effect at room temperature (RT) are very important for future applications in spintronic devices, magnetic storage, and sensors. To explore materials showing the EB effect at RT is an interesting and important issue. In this paper, LaFeO3 micro-polyhedrons composed of an average particle size of 66 nm nanoparticles have been synthesized by annealing LaFe(CN)6 precursors in air at 630 °C for 8 h. Magnetic hysteresis loops show that LaFeO3 micro-polyhedrons present a large spontaneous EB (SEB) effect (∼ 1.5 kOe) at RT after zero-field cooling (ZFC) and a gigant conventional EB (CEB) effect (∼ 5.6 kOe) after field cooling (FC) of 70 kOe. To the best of our knowledge, the current compound shows the largest negative EB field after ZFC at RT among SEB materials discovered so far. The successful fitting of training effect (TE) data with Binek’s model shows that the origin of SEB lies in the interface exchange coupling between different magnetic phases. The interface with the “pinning” effect of antiferromagnetic (AFM) matrix relative to ferromagnetic (FM) clusters exhibits unidirectional anisotropy, which is formed during the initial magnetization process. This interesting finding of the SEB effect at RT in single-phase perovskite LaFeO3 system may open a new direction for possible technological applications.

軟X線磁気円二色性顕微分光によるNd-Fe-B熱間加工磁石の初磁化過程および減磁過程における磁区観察

A large step on the initial magnetization curve has been widely observed in Nd-Fe-B hot-deformed magnets. This behavior has been considered as that the first- and latter-half parts of the initial magnetization curve correspond to the magnetization reversals of multi- and single-domain grains, respectively. In this study, the detailed magnetic domain structure of a Nd-Fe-B hot-deformed magnet has been studied for the initial magnetization and demagnetization processes by using a soft X-ray magnetic circular dichroism (XMCD) microscopy. The observed magnetization reversal behavior for the initial magnetization process is quite different from that previously considered. The multi-domain states move from grains to grains through the gradual displacement of domain wall passing through many grains, and then the massive domain wall displacement takes place. The latter part of domain wall displacement is similar with that found in the demagnetization process near the coercivity. Moreover, we found that there are several strong pinning sites which are identical for both the initial magnetization and the demagnetization processes.

Imaging Domain Reversal in an Ultrathin Van der Waals Ferromagnet

The recent isolation of 2D van der Waals magnetic materials has uncovered rich physics that often differs from the magnetic behavior of their bulk counterparts. However, the microscopic details of fundamental processes such as the initial magnetization or domain reversal, which govern the magnetic hysteresis, remain largely unknown in the ultrathin limit. Here a widefield nitrogen-vacancy (NV) microscope is employed to directly image these processes in few-layer flakes of the magnetic semiconductor vanadium triiodide (VI3 ). Complete and abrupt switching of most flakes is observed at fields Hc ≈ 0.5-1T (at 5 K) independent of thickness. The coercive field decreases as the temperature approaches the Curie temperature (Tc ≈ 50K); however, the switching remains abrupt. The initial magnetization process is then imaged, which reveals thickness-dependent domain wall depinning fields well below Hc . These results point to ultrathin VI3 being a nucleation-type hard ferromagnet, where the coercive field is set by the anisotropy-limited domain wall nucleation field. This work illustrates the power of widefield NV microscopy to investigate magnetization processes in van der Waals ferromagnets, which can be used to elucidate the origin of the hard ferromagnetic properties of other materials and explore field- and current-driven domain wall dynamics.

Unusual exchange bias in Sr2FeIrO6/La0.67Sr0.33MnO3 multilayer

Here, we study interface induced magnetic properties in a 3d-5d based multilayer made of La0.67Sr0.33MnO3 and double perovskite Sr2FeIrO6, respectively. Bulk La0.67Sr0.33MnO3 is metallic and shows ferromagnetic (FM) ordering above room temperature. In contrast, bulk Sr2FeIrO6, is an antiferromagnet (AFM) with a Néel temperature around 45 K () and exhibits an insulating behavior. Two set of multilayers have been grown on SrTiO3 (1 0 0) crystal with varying thickness of FM layer. A multilayer with equal thickness of La0.67Sr0.33MnO3 and Sr2FeIrO6 (∼10 nm) shows exchange bias (EB) effect both in conventionally field cooled (FC) as well as in zero field cooled (ZFC) magnetic hysteresis measurements which is rather unusual. The ZFC EB effect is weakened both with increasing maximum field during initial magnetization process at low temperature and with increasing temperature. Interestingly, a multilayer with reduced thickness of La0.67Sr0.33MnO3 (∼5 nm) does not exhibit ZFC EB phenomenon, however, the FC EB effect is strengthened showing much higher value. We believe that an AFM type exchange coupling at the interface and its evolution during initial application of magnetic field causes this unusual EB in present multilayers.

Large-Scale Micromagnetics Simulation of Magnetization Dynamics in a Permanent Magnet during the Initial Magnetization Process

For better motors in electric vehicles, we had better understand magnetism at the microscale. Domain-wall motions and magnetization-reversal processes during the initial magnetization process in a hot-deformed Nd${}_{2}$Fe${}_{14}$B permanent magnet are simulated by solving the Landau-Lifshitz-Gilbert equation for a model ensemble of 5302 grains. The initial-magnetization curve reproduces the two-step structure seen in experiment. In the first step, the domain walls move inside the multidomain grains, and the magnetization reversal of the single domains occurs in the second step. The effects of networks among the grains through dipolar interaction are also estimated.

Strong room temperature spontaneous exchange bias in BiFeO3-CoFe2O4 nanocomposites

BiFeO3-CoFe2O4 nanocomposites were synthesized by hydrothermal method and an enormous spontaneous exchange bias (300Oe) observed during the measurement of our samples in an unmagnetized state without field cooling. The measuring field dependent Hc is monotonic increase, the exchange bias effect decreases as the sweeping range of the measuring field increases. Moreover as the CoFe2O4 proportion increases, the exchange bias effect first increases to the maximum and then decreases. This effect suggests that the magnetic unidirectional anisotropy can be set isothermally (room temperature) during the initial magnetization process due to the interface coupling between ferrimagnetic and antiferromagnetic phases in BiFeO3-CoFe2O4 nanocomposites. A possible model proposed to explain the mechanism underlying our experimental results reasonably.

Negative magnetization and zero-field cooled exchange bias effect in Eu0.9Pr0.1CrO3 ceramics

Interesting magnetic behaviors, including negative magnetization, zero field fooled and field cooled exchange bias effects, have been observed in Eu0.9Pr0.1CrO3. The negative magnetization at low temperature results from the antiparallel coupling between the Pr3+ moment and the canted moment of Cr3+ sublattice. Left shift of zero field cooled M-H loops, and right shift of field cooled M-H loops have been observed, due to the growth and competition of two types of magnetic structures with GxFz (EuCrO3) and GzFx (PrCrO3) orderings under magnetic field. When the cooling field is high enough, the Pr3+ moment is frozen antiparallel with the applied field during the cooling process, giving rise to a positive exchange bias (right shift of M-H). However, when the cooling field is near zero, magnetic clusters with GxFz (EuCrO3) or GzFx (PrCrO3) orderings are formed and distribute randomly during the cooling process, and the net Pr3+ moment get aligned along the applied field during the initial magnetization process to lower the Zeeman energy, and then negative exchange bias (left shift of M-H) appears.

Sign reversal of spontaneous exchange bias in Mn0.7Fe0.3NiGe alloy

Spontaneous exchange bias can be observed at 5K after zero-field cooling in Mn0.7Fe0.3NiGe alloy. More interestingly, its sign can reverse from positive to negative with the increase of maximum measurement field. Such results indicate a coexistence of ferromagnetic and antiferromagnetic coupling formed at the ferromagnetism/spin glass interface during the initial magnetization process. Hence, the sign reversal would be resulted from the relative strength change of these two types of exchange coupling, which can be tuned by measurement field.

Magnetic ground state and giant spontaneous exchange bias in Ni46Mn43In11 alloy

We report non-equilibrium dynamics and giant spontaneous exchange bias obtained in zero field cooled mode for Ni46Mn43In11 alloy. The dc magnetic measurements indicate a super spin glass type magnetic ground state in the system. This SSG state is formed when superparamagnetic domains are collectively frozen inside strong antiferromagnetic matrix at low temperature. The ZFCEB arises due to the development of strong superferromagnetic unidirectional anisotropy at the interface of SSG and AFM during the initial magnetization processes. The temperature variation of field cooled exchange bias and ZFCEB is almost similar whilst their field dependency is different. Possible reasons for observed behaviors are comprehensively discussed in this article.

Positive to negative zero-field cooled exchange bias in La0.5Sr0.5Mn0.8Co0.2O3 ceramics.

Exchange bias effect obtained after zero-field cooling from unmagnetized state usually exhibits a shift of hysteresis loop negative to the direction of the initial magnetic field, known as negative zero-field cooled exchange bias. Here, positive zero-field cooled exchange bias is reported in La0.5Sr0.5Mn0.8Co0.2O3 ceramics. In addition, a transition from positive to negative exchange bias has been observed with increasing initial magnetization field and measurement temperature. Based on a simple spin bidomain model with variable interface, two type of interfacial spin configuration formed during the initial magnetization process are proposed to interpret the observed phenomenon.

The magnetization behavior and open recoil loops of hot-deformed Nd-Fe-B magnets infiltrated by low melting point PrNd-Cu alloys

Ultrahigh coercivity of hot-deformed Nd-Fe-B magnets was obtained by low melting point PrNd-Cu alloys diffusion process. The coercivity was largely increased from 15.35 to 27.30 kOe. Magnetic isolation by nonmagnetic phases should be the primary explanation for the enhancement of coercivity. High susceptibility of the recoil loops at low field indicated that magnetic domain wall motion within grains was easy, while a near-zero reversible susceptibility of recoil loops implied that domain wall strong pinning at grain boundaries was dominant at higher field in initial magnetization process. Open recoil loops were also obtained in the infiltrated magnets and disappeared when the external field exceeded about 16 kOe lightly higher than the coercivity of the magnet without diffusion process. The inhomogeneous distribution of infiltrated phases that led to the grains aggregation and single grains with softer and harder magnetic characteristics, respectively, may be the primary reason for the phenomenon. The stray field produced by isolated single grains plays a critical role in the magnetization behavior. A phenomenological model for the magnetization reversal was built to interpret the open recoil loops in a recoil loop process.

Exchange bias induced after zero‐field cooling in antiperovskite compounds Ga1–xNMn3+x

AbstractZero‐field cooling (ZFC) exchange bias (EB) was observed in antiperovskite compounds Ga1–xNMn3+x (x = 0.06 and 0.1). The existence of ferromagnetic (FM) clusters has been demonstrated in a spin glass (SG) matrix. The ZFC‐EB is suggested to originate from the unidirectional anisotropy formed isothermally along the FM/SG interface. For this typical physical system, both ways including the initial magnetization process and FC process could result in the unidirectional anisotropy. In addition, the training effect after ZFC is similar to those observed in conventional FC‐EB systems.

Stacked Nanosheets of Pr1–xCaxMnO3 (x = 0.3 and 0.49): A Ferromagnetic Two-Dimensional Material with Spontaneous Exchange Bias

Two-dimensional (2D) planar architectures of inorganic materials have attracted great attention owing to their fascinating physical properties. However, nanosheets of the technologically relevant doped rare-earth manganites are still elusive. Stacked 10–14 nm thick nanosheets of phase pure Pr1–xCaxMnO3 (PCMO; x = 0.3 and 0.49) were obtained by decomposition of carbon coated CaCO3/MnCO3 microsheets and Pr2O2CO3 aggregates, the latter synthesized under pressure at 500–800 °C. The PCMO nanosheets had flatter Mn–O–Mn tilt angles in the MnO6 octahedra and ferromagnetic (FM) moments at 5 K. Spontaneous exchange bias (SEB) coupling between the antiferromagnetic (AF)/FM spins was observed at 5 K under zero-field cooling. High FM moments and SEB were possible due to the long-range magnetic interactions in the stacked 2D arrangement of Mn3+/Mn4+ d-electron spins, where charge ordering was completely suppressed. The SEB behavior was dependent on the initial magnetization process and the direction of rotation of the random spins at the AF/FM interface.

Mechanism of the giant irreversible positive magnetic entropy change in a Tb-based bulk metallic glass

A suitable coherent description of the reversible and irreversible magnetic entropy changes in a Tb-based metallic glass with strong random magnetic anisotropy (RMA) is presented. A giant irreversible positive magnetic entropy change is observed from magnetic measurements at low temperatures, which is found to arise from the entropy production, while a small negative one is obtained from heat capacity measurements. The temperature dependences of the irreversible positive magnetic entropy change under a field change of 5 T and the internal entropy production in the initial magnetization process can be described by an exponential law, which is determined by RMA.

Magnetization Process Simulation of Nd-Fe-B Magnets Taking the Demagnetization Phenomenon Into Account

This paper presents results obtained from a numerical simulation of the magnetization distribution of Nd-Fe-B magnets. We used the three-dimensional Variable Magnetization and the Stoner-Wohlfarth (VMSW) method in the initial magnetization process. In the residual-magnetization and demagnetization processes, we used the magnetization equation of motion, which can be described by the Landau-Lifshitz-Gilbert (LLG) equation. The calculation results were compared to the measured results to verify the accuracy of numerical simulation.

Large Exchange Bias after Zero-Field Cooling from an Unmagnetized State

Exchange bias (EB) is usually observed in systems with an interface between different magnetic phases after field cooling. Here we report an unusual phenomenon in which a large EB can be observed in Ni-Mn-In bulk alloys after zero-field cooling from an unmagnetized state. We propose that this is related to the newly formed interface between different magnetic phases during the initial magnetization process. The magnetic unidirectional anisotropy, which is the origin of the EB effect, can be created isothermally below the blocking temperature.

Dependence of equilibrium height of a magnet on initial magnetization process in a high T _c superconducting levitation system

The equilibrium height (EH) of a levitated permanent magnet (PM) is investigated in an oscillation system composed of the PM and a high temperature superconductor (HTS). Two kinds of initial quasi-static magnetization processes are performed respectively, and then the EH is achieved after a damped vibration of the PM. A peak value of the EH, associated with a transitional magnetization height (TMH), is found for the first initial process. When the magnetization height (MH) is less than the TMH, the EH increases with the increasing of the MH nonlinearly. Otherwise the EH decreases linearly with the MH. For the second initial magnetization process a minimum value of the EH is also presented. The magnetic energy of the system is believed to be responsible for these two different dependences.

Relation between domain wall motion and coercivity in Nd-Fe-B sintered magnets prepared in various conditions

For the study of the relationship between the domain wall motion (DWM) and the coercivity in the samples of Dy-free Nd-Fe-B sintered magnets, the DC susceptibility (• •DC) in the initial magnetization from the thermally demagnetized state and the DWM in the partially demagnetized state after the full magnetization in 5T-pulse field were measured. In the high coercivity (μ.Hc) samples of μ.Hc=1.35T and 1.62T, the stronger pinning in initial magnetization was detected as the• •DC compared to other samples of the smaller coercivities such as μ.Hc=0.80T. The former samples also showed little DWM in the partially demagnetized state even the domain structures were clearly detected by magneto-force microscopy (MFM). In the latter samples, however, the obvious DWM was measured as the differences (ΔM) between the magnetizations under applied field (Ma) and the remanences (Mr) in demagnetization process (ΔM = Mr - Ma). The pinning in DWM in initial magnetization process and the DWM in the partially demagnetized state are both originated from the interaction between the domain walls and grain boundary phases. The enhancement in coercivity, therefore, should come from the same interactions in the sintered samples.

Analysis of Magnetization Mechanism for NdFeB Magnet Using Spin-Polarized Scanning Electron Microscopy (Spin SEM)

The magnetization process of Nd-Fe-B bonded magnets prepared by the Hydrogenation-Disproportionation-Desorption-Recombination(HDDR) process was studied by observing magnetic domain structures using spin-polarized scanning electron microscopy (spin SEM). The positional relationship between the grain boundaries and the magnetic domain walls is clearly imaged. The result revealed that these magnets contain grains with a multi-domain structure in the thermally demagnetized state. The domain wall motions in grains with multi-domains are dominant in weak magnetic fields in the initial magnetization process, whereas fraction of the grains with a single domain increases with further increasing the magnetic field. The coercivity variations in these grains were also visualized.

Magnetization reversal mechanism of ultra thin Nd2Fe14B films with perpendicular magnetic anisotropy

The crystallographic and magnetic properties of sub-10-nm Nd2Fe14B films have been investigated. It is found that the magnetization reversal mechanism is changed in Nd–Fe–B films with the decrease of film thickness. Films with thickness greater than 10nm show domain-wall-motion type of behavior. Strong exchange coupling between the grains makes the magnetic domain wall easily propagate in the films. Films with thickness of 4nm show the Stoner-Wohlfarth particle behavior. The initial magnetization process is magnetization rotation. This is also verified by angular dependence of the coercivity.