Wall Coercivity Research Articles

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.

DC magnetization processes in bistable glass-coated ferromagnetic microwires

Interaction of a single magnetic domain wall with an inhomogeneous magnetic field and distribution of local nucleation fields along glass-coated Fe77.5−xNixB15Si7.5 (x=0, 27.9, 34.9) microwires were experimentally studied. It was shown that the wall separating two axial domains and moving along the wire can be stopped by an inhomogeneous magnetic field. The wall remains stable and trapped in a local potential minimum after external fields are switched off. Wall coercivity increases with Ni content. For all samples the minimum of critical field for axial magnetization reversal was observed near the end of the wire. For samples with non-zero Ni content a distribution of nucleation fields lower than 950A/m was observed in regions far enough from the wire ends. In Ni-free samples the nucleation fields were higher than 950A/m.

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.

Design and performance of domain wall displacing-type field sensors using a magnetic tunnel junction and a giant magnetoresistive device

We report a new type of field sensor, which utilizes the oscillatory domain wall displacement in the ferromagnetic free layer to detect an external field. The sensor consists of a 1200 × 100 µm2 Al conducting wire placed on a spin-valve giant magnetoresistive (GMR) or magnetic tunnel junction. By transmitting an alternating current through the Al conducting wire, an ac field can be generated to oscillate the domain wall in the free layer. The oscillatory domain wall displacement reduces the influence of wall coercivity and Barkhausen effect, and enables the sensor to detect the magnetic field which is much smaller than the wall coercivity. The field sensitivities of the domain wall displacing-type GMR and tunnel magnetoresistive (TMR) sensors are 2.73 mV V−1 Oe−1 and 5.81 mV V−1 Oe−1, respectively.

High sensitivity giant magnetoresistance magnetic sensor using oscillatory domain wall displacement

A high sensitivity giant magnetoresistance (GMR) magnetic sensor has been demonstrated using the oscillatory domain wall displacing method. This sensor consists of a 30-μm-wide GMR element, CoFeB (10 nm)/Cu (2.2 nm)/CoFe (3 nm)/MnIr (10 nm), and a 100-μm-wide aluminum conducting wire placed onto the GMR element with an insulation layer between them. Domain walls in the free layer, whose easy axis is perpendicular to the length direction of GMR element, are oscillation-driven by a 140 kHz alternating magnetic field produced by an ac flowing through the aluminum conducting wire. The oscillatory wall displacement reduces the influence of wall coercivity and enables an output proportional to the external field. By applying a 1-kHz external field and separating the 1-kHz component from the amplified bridge signal using a band-pass filter, we successfully detected a magnetic field as small as 0.21 mOe (21 nT), which is much smaller than the wall coercivity of the free layer.

High-Sensitivity GMR Sensor Using Domain-Wall Oscillation

The high sensitivity detection of magnetic fields was demonstrated using the oscillatory domain-wall displacing method for a GMR sensor. A micro-wire with a width of 30 μm and a length of 200 μm of a spin valve film, Al (20 nm)/MnIr (20 nm)/CoFe (3 nm)/Cu (2.2 nm)/CoFeB (10 nm), was fabricated as a magnetic sensor device. The domain walls in the free layer, whose easy axis was perpendicular to the wire, were oscillatorily displaced by an alternating magnetic field produced by a current with 140 kHz through the micro-wire. The oscillatory wall displacement reduced the influence of wall coercivity and the linear relationship between the external field with 1 kHz and an output signal for the sensor circuit was obtained down to 10-3 Oe.

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.

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.

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.

Coercivity of TbFeCo/FePt Grain Composite Films

We report the simulated results for coupled granular and continuous (CGC) films for the thermo assisted magnetic recoding, consisting of FePt particles under layers that act as domain pinning sites of TbFeCo layers. As domain size should be smaller to further increase the recording density, the number of FePt grains has to be decreased in the domain. The simulated results demonstrated that they could be formed in a domain as small as 18.3 nm in diameter, which corresponds to a recording density of 1.8 Tbit/inch2. Here, three FePt grains were effective for pinning. We also estimated the effective wall coercivity, Hw, from the simulation varying the applied magnetic field until a domain had shrunk and disappeared. The Hw was over 12 [KOe] where there were only three grains for pinning. The Hw was a function of the radius of the curvature at the pinning point and also the inverse average wall length between the pinning points in the same composite films.

Continuous/Cluster-Pinned Recording Media

We propose and theoretically investigate a new class of nanostructured magnetic recording films, cluster-pinned recording media. The films consist of magnetic clusters exchange coupled to a continuous hard layer with perpendicular anisotropy and low coercivity. Our calculations yield the coercivity and the cross-track correlation length as a function of film thickness and pinning density and strength. The mechanism is very similar to the Gaunt-Friedel pinning in bulk magnets, which differs from ordinary strong pinning by the selfconsistent dependence of wall curvature and coercivity on defect concentration. The main difference is the exponent for the coercivity as a function of the pinning strength, which is equal to 2 in the bulk but equal to 3/2 in thin films. The pinning strength is estimated for various regimes, and it is shown that the diminished domain-wall curvature reduces jitter

Cu doping effect on FePt grains prepared by rapid thermal annealing on SiO2 substrate and wall structure in TbFeCo∕FePt CGC-like film

We report that Cu substitution is effective in raising the degree of (001) crystal orientation of FePt grains prepared by rapid thermal annealing on SiO2. We fabricated coupled granular and continuous (CGC) like films such as TbFeCo on FePt grains. The wall coercivity was about twice that of the TbFeCo single layer film. From three-dimensional micromagnetic simulations with Landau-Lifshitz-Gilbert (LLG) equation, it was confirmed that wall coercivity was enhanced and domain shapes made smooth in CGC film. When the thickness of the TbFeCo layer was thin, wall width was thinner than the theoretical value.

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.