Domain Wall Coercivity Research Articles

Modulation method of studying two-phase antiferromagnetic domain structures created by a pulsed magnetic field

It is shown that studies of the coercivity of AFM domain walls that arise in antiferromagnets during the first-order spin-orientation transition can be used as a tool for detecting, identifying, and studying spin structures realized in a crystal under the influence of an external magnetic field. The use of high-frequency modulation of the magnetic field made it possible to expand the capabilities of the standard induction method of studying magnetic phase transitions in antiferromagnets. The proposed measurement method was tested on well-known MnF2 and Cr2O3 crystals.

The Effect of Temperature and Film Thickness on Domain Wall Energy Density, Coercivity Force, and Magnetostriction of Galfenol Films

The Effect of Temperature and Film Thickness on Domain Wall Energy Density, Coercivity Force, and Magnetostriction of Galfenol Films

Spacer Thickness and Temperature Dependences of the Interlayer Exchange Coupling in a Co/Pt/Co Three-Layer Structure

Domain wall mobility as a function of nonmagnetic interlayer thickness and temperature was studied in ultrathin exchange-coupled ferromagnetic layers using magneto-optic Kerr microscopy. The system under study is a Pt/Co/Pt/Co/Pt heterostructure having perpendicular magnetic anisotropy and a middle Pt layer with spatially variable thickness. The ferromagnetic interaction between the Co layers is observed when the Pt interlayer thickness varies from 5 to 6 nm in a temperature range of 200–300 K. There is a certain interval of Pt layer thickness where domain walls in both ferromagnetic layers move independently. Nonlinear dependence of the domain wall displacement on the applied field was measured. It is shown that an equilibrium position of the relaxed domain wall depends on field, temperature, and the nonmagnetic interlayer thickness. This position is determined by the energy balance: (i) domain wall displacement provided by the applied field, (ii) interlayer exchange interaction in the area swept by the domain wall, and (iii) domain wall coercivity. The mechanism of domain wall stabilization in terms of independent wall motion near critical thickness was considered. It is found that both the coercivity of the Co layer and the critical thickness decrease at higher temperature, while the interlayer exchange constant J is changed weakly.

Coercive Properties of Magnetic Garnet Films

Magnetic garnet films represent a wide family of materials. By the proper choice of chemical composition and growth parameters, their magnetic behavior can be tuned in a very wide range. On one side, they are suitable for many different applications; on the other side, they are optimal model materials for studying the basic magnetization processes. Many assumptions of the existing theories can be checked or validated by magnetic garnet film investigation. Their production technology was developed many decades ago, but even nowadays, magnetic garnet films have been intensively studied, and newer and newer application possibilities have been found. In this review paper, those results are summarized, which are connected with their coercive properties. Coercivity, or coercive force, is a frequently used magnetic characteristic, but usually, it is considered rather a technical parameter. It is shown that there is no correlation between the so-called “technical coercive force” (which is the half-width of a major hysteresis loop) and the domain wall coercivity (this is frequently called a domain wall pinning field). This latter parameter is considered a real characteristic of domain wall movement. If magnetic garnet films are investigated, the correlation between moving domain wall and material defect structure can be studied. In this paper, the very complex feature of coercivity is shown. It is demonstrated that the domain structure, the properties of domain walls, the existence of mechanical stresses, the temperature, the size of the sample and many other parameters have an influence on the measured coercivity.

Nanostructure refinement and phase formation of flash annealed FeNi-based soft magnetic alloys

The resulting nanocomposite microstructures of FeNi nanocrystallites under different heating and cooling rates (5 °C/min vs 400–500 °C/s) is investigated. Conventional furnace annealing under low heating rates and slow cooling resulted in both BCC αFeNi and FCC γFeNi nanocrystallites with an average grain size on the order of 25–27 nm whereas high heating rates achieved via flash annealing techniques have enabled a dramatically refined microstructure consisting of 5–7 nm grains with FCC γFeNi phase and found to be the dominant phase following primary crystallization. Grain size refinement and phase identity optimization yielded low values of coercivities ∼17 A/m and high permeability ∼11 × 103 measured at 400 Hz/1 kA/m in flash annealed samples at 450 °C for 5 s. The magnetic behavior and the underlying mechanism of optimal soft magnetic properties are discussed in terms of the critical role of the grain size in domain wall pinning and coercivity.

Nanoscale Mapping of Heterogeneous Strain and Defects in Individual Magnetic Nanocrystals

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

Analysis of the magnetic properties of a silicate-coated spherical FeSiAl-based soft magnetic composite for high-frequency power-applications

In this paper, the relationship of the frequency-dependent magnetic permeability, magnetic loss, and DC-bias of a soft magnetic composite was analyzed. The permeability is divided into a domain wall movement component at low frequencies and a spin-rotation part at high frequencies. The resonance-frequency increases when the pressure decreases and, simultaneously, the permeability also decreases following the Snoek limit. Magnetic loss is separated into hysteresis loss, eddy-current loss, and excess loss, which is found to closely relate to the density of the powder core. The DC-bias properties of the composite tend to deteriorate slightly with the density increase. This is due to a decrease in the gap between the particles, which is associated with the pinning of the domain wall movement and coercivity. It is found that DC-bias is positively correlated with hysteresis loss and excess loss with respect to coercivity and pinning of the movable domain-wall. These results provide insights into the soft magnetic performance of soft magnetic composites, particularly with respect to compact, high-DC applications with high frequency and high magnetic saturation for high-power supply.

Database of novel magnetic materials for high-performance permanent magnet development

This paper describes the open Novamag database that has been developed for the design of novel Rare-Earth free/lean permanent magnets. Its main features as software technologies, friendly graphical user interface, advanced search mode, plotting tool and available data are explained in detail. Following the philosophy and standards of Materials Genome Initiative, it contains significant results of novel magnetic phases with high magnetocrystalline anisotropy obtained by three computational high-throughput screening approaches based on a crystal structure prediction method using an Adaptive Genetic Algorithm, tetragonally distortion of cubic phases and tuning known phases by doping. Additionally, it also includes theoretical and experimental data about fundamental magnetic material properties such as magnetic moments, magnetocrystalline anisotropy energy, exchange parameters, Curie temperature, domain wall width, exchange stiffness, coercivity and maximum energy product, that can be used in the study and design of new promising high-performance Rare-Earth free/lean permanent magnets. The results therein contained might provide some insights into the ongoing debate about the theoretical performance limits beyond Rare-Earth based magnets. Finally, some general strategies are discussed to design possible experimental routes for exploring most promising theoretical novel materials found in the database.

Influencing mechanisms of atomic diffusion and compositional distribution on the magnetic anisotropy of Cr/SmCo/(Cu)/Cr thin films

Rare earth (RE) permanent magnetic thin films, such as SmCo-based films, are promising candidates for future thermal-assisted magnetic recording media because of their supreme thermal stability and fine superparamagnetic critical size. Phase compositional distributions and crystallographic orientations can directly influence the magnetic domain evolutions and magnetic performance of SmCo films. However, these films often exist in multi-phases without well-defined distinct magnetic anisotropy, thereby causing difficulties in analyzing their functions. The effects of atomic diffusions induced by annealing on crystallization and magnetic anisotropy energy are poorly understood. In this work, the influence of doping Cu atoms via introducing Cu layer on magnetic anisotropy of Cr/SmCo/(Cu)/Cr films is investigated. Introducing a Cu layer and post-annealing lead to tunable magnetic anisotropy from isotropy to anisotropy and domain structure formation. Micromagnetic simulation is used to further analyze the effects of crystallization, anisotropy field and compositional distribution on the magnetization reversal behavior and coercivity variations in SmCo films. It reveals that the differences of anisotropy field of Co and amorphous phases could affect the domain wall motions and coercivity, which shows the analogous trend as an experimental result. Moreover, the elevated anisotropy constant of Sm(Co,Cu) alloy and the small fractions of SmCo5 boundary phases are also beneficial to the enhancement of coercivity derived by calculating domain wall energy. In-plane magnetic anisotropy is improved by the SmCo5 phase, which has an in-plane preferred orientation of the c axis. This work provides theoretical and experimental bases for the future fabrication of SmCo-based films by controlling atomic diffusion with optimized grain boundary phase distribution.

Transformations of head-to-head domain walls in (La,Sr)MnO3 thin films

Kinetics of the in-plane magnetization reversal in a (La,Sr)MnO3 thin film has been studied at different temperatures using the magneto-optic visualization technique. In addition to changes of the domain wall coercivity and mobility, dramatic transformations of the domain wall structure have been observed. Motion of a single zig-zag domain wall is typical for high temperature magnetization reversal, whereas complicated diffuse transient structure between the two principal domains is observed at low temperature. Generation of secondary bubble domains in front of a moving wall has been observed for the first time in a quasistatic regime, contrary to previous superfast experiments.

Magnetic domain wall pinning and coercivity in FePt/MgO and FePt/Pt thin films

FePt thin layers have been epitaxied either on Pt(0 0 1) or on MgO(0 0 1) substrates, and magnetically characterized using extraordinary Hall effect magnetometry and magnetic force microscopy. The coercivity originates in both cases from the pinning of domain walls on structural defects. Whereas the coercivity increases with the FePt layer thickness in FePt/Pt samples, it decreases in FePt/MgO samples. This discrepancy is explained on the basis of structural observations, and of atomistic simulations of magnetic domain wall pinning.

STUDY OF DOMAIN WALL MOTION OF ROUND MARKS IN AN EXCHANGE-COUPLED CoNi/Pt FILM

We study the motion of domain wall in round marks formed in an exchange-coupled CoNi/Pt film, suitable for perpendicular magnetic recording. Marks were written by probe-based magnetic recording, with an average size of 180 nm. By applying external magnetic field on those marks, they begin to shrink. The minimum field required to move the domain wall of marks, i.e., domain wall coercivity, is 60% the nucleation coercivity of the medium. A model of dynamic domain motion was executed to study the stability of magnetic domains in exchange-coupled perpendicular media. It shows that the domain wall coercivity is a better fit than the nucleation coercivity to calculate the minimum stable domain size of the medium.

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.

LLG Simulations of TbFeCo/FeNiPt CGC-Like Film

Recently, much attention has been focused on so-called “hybrid recording” such as “optically assisted magnetic recording” or “heat assisted magnetic recording”. In other words, hybrid recording is expected to be a most promising storage device beyond 1 Tera bit/inch2, ultra high density region. When the domain radius becomes very small in magneto optical recording film, the wall energy is dominant in the total energy of the domain, and then it will shrink and disappear. To write a very small domain, it is necessary to increase the domain wall coercivity to freeze the written domain. We perform three-dimensional Landau-Lifshitz-Gilbert (LLG) simulations for estimating domain shapes and wall structures in coupled granular and continuous (CGC) like films of TbFeCo film/FeNiPt grains. In the results, even when using lower Tc FeNiPt (Tc= 307 °C, Ku= 1.8×107 [erg/cm3] at room temperature) grains than FePt (Tc = 497 °C, Ku = 7.0×107 [erg/cm3] at room temperature) grains, we find a minute magnetic domain of 26 nm diameter, could be made on TbFeCo /FeNiPt bi-layer film. Moreover, the domain wall width, the obstacle in the minute magnetic domain structure of bi-layer film, is narrow. This narrowness advantageously contributes to a high density recording.

Magnetic Characteristics of TbFeCo Fabricated on Isolated FePt Grains

TbFeCo film is fabricated on isolated FePt grains to increase the domain wall coercivity (Hw) of TbFeCo film by exchange coupling effect between TbFeCo and FePt. The transition metal (TM) moment has a dominant charge in TbFeCo film ranging from room temperature (Troom) to Curie temperature. The FePt has average grain sizes such as 29.6, 16.6 and 7.0 nm in diameter, which are perpendicularly magnetized with a high coercivty (Hc), as high as 30-45 kOe at Troom. The TbFeCo/FePt composite film shows higher Hw, Hc, and MsHw than those of the single layer of TbFeCo film. We also estimate the amountof interfacial wall energy between TbFeCo and FePt.

Magnetization study of a UIrAl single crystal

The field and temperature dependence of the magnetization has been studied for the first time on a single crystal of the hexagonal intermetallic UIrAl. The compound is a ferromagnet below the Curie temperature TC=62 K with a ground-state spontaneous magnetic moment of Ms=0.96 μB. The huge uniaxial anisotropy (the anisotropy field Ba exceeds 200 T) leads to characteristic hysteresis properties which obey the model of high intrinsic coercivity of narrow domain walls

Magnetic Characteristics of TbFeCo/FePt Double Layer for MO Recording

To achieve ultra high-density Magneto-optical (MO) recording, it is necessary to increase the domain wall coercivity (HW) to freeze the shape of a written domain. TbFeCo film is fabricated on FePt crystalline film, which has large crystalline magnetic anisotropy, to increase the HW of TbFeCo film by the exchange coupling effect. The TbFeCo film is rare earth (RE) dominant ranging from room temperature (Troom) to Curie temperature (Tc). The FePt is deposited as multilayers and then it is subjected to vacuum annealing at 400°C for 2 hours. The TbFeCo/FePt film shows higher HW, coercivity (Hc), and the thermal stability than those characteristics of the single TbFeCo film. Those characteristics can be attributed to the exchange coupling between magnetic moments of Fe and Co in TbFeCo and FePt films.

Effect of surface roughness on magnetic domain wall thickness, domain size, and coercivity

We study the effect of surface roughness on magnetic domain wall thickness, domain size, and coercivity of thin magnetic films. We show that the roughness increases (decreases) the domain wall thickness and domain size for Bloch walls (Néel walls). The surface roughness affects the domain wall movement and causes the increase of coercivity for Néel walls. The coercivity due to domain rotation for Bloch walls decreases with the increase of roughness. The domain wall thickness, domain size, and coercivity are each related to the demagnetizing factor, which depends on the roughness and type of wall (Bloch wall or Néel wall). The calculated coercivity versus thickness is compared with experimental data of ultrathin Co films, where the thickness dependent roughness parameters are available.

Domain wall velocity in ultra-thin magnetic films with perpendicular anisotropy

Monte Carlo simulations were performed to study the effect of local variations of the domain wall coercivity on the rate of domain growth in ultra-thin magnetic films with strong perpendicular anisotropy. The analytic models used in previous studies are refined, compared with the Monte Carlo data and shown to be inadequate to describe realistically the domain growth process. When the field is increased a depinning transition occurs where the domain expands by a sequence of avalanches. If the avalanche size is small, the field dependence of the wall velocity is exponential, as in a thermo-activated process, so the full extent of the depinning transition may not be evident in experimental v(H) curves. The depinning field H0, when the avalanche size diverges, is weakly dependent on the width of the coercivity distribution. When H>H0, the phenomenological relation v = v0+µ´(H-H0) that is observed experimentally in Au/Co(8 Å)/Au films is shown to result from the temporary obstruction of the domain boundary by relatively hard pinning centres and µ´ is found to be larger than the wall mobility.

Coercivity of Ultrathin Magnetic Films with Perpendicular Anisotropy

The calculation of the temperature dependence of the domain wall coercivity of ultrathin magnetic films with perpendicular anisotropy is reported. In this case, the magnetization reversal is supposed to occur through domain wall motion. The proposed model takes into account thermal bending fluctuations of the domain wall , which is pinned by crystal defects such as grain boundaries. A fitting procedure, applied to recently published experimental data on the field induced domain wall velocity in Au/Co films, allows us to find out realistic values of the involved parameters.