Intergranular Magnetic Interaction Research Articles

Four-fold multifunctional properties in self-organized layered ferrite

Four different functional properties at room temperature were found in Ba12Fe28Ti15O84 quaternary ferrite ceramics: a thermoelectric character with a prominent Seebeck coefficient of -400 μV/K, a strong ferrimagnetic character and weak magnetoresistivity, non-linear dielectric character (tunability) and ferro/piezoelectric response. The complex crystalline structure with a naturally occurring self-organized layered structure was mapped by High Resolution Transmission Electron Microscopy. A n-type conduction was found for optimized samples, consistent with the negative Seebeck coefficient. The presence of magnetic domains with a lateral size ranging from few hundreds of nanometers, up to few microns, extended over several grains indicates a strong intergranular magnetic interaction. Local ferroelectric switching as revealed by piezoresponse force microscopy showed a relatively weak effective piezoelectric coefficient with a remnant value of 0.5 p.m./V and a clockwise cycling direction, which was attributed to an effective negative piezoelectric coefficient. The switching behavior was described as originating from the material’s nanoscale layered complexity, with a strong anisotropy of its dielectric and ferro/piezoelectric local properties.

Sandwiched structure of hot-deformed Nd-Fe-B permanent magnets processed by Nd-Cu eutectic alloys diffusion

A series of sandwiched structures with different near-surface mass fractions x (x = 3 wt%, 4 wt%, 5 wt%) was employed to develop high-coercivity hot-deformed Nd-Fe-B magnets by the addition of 2 wt% Nd-Cu eutectic alloys via adjusting the middle thickness and near-surface thickness. The designed magnet with a pronounced composite structure shows a 23% increase in coercivity with a 6% loss of remanence by adjusting the sandwiched structure at 4 wt% Nd-Cu eutectic alloys in the near-surface regions. The results indicate that the near-surface Nd-Cu-rich “shell” structure can effectively suppress the magnetization reversal of overall magnets, enhancing the coercivity. With the help of loading stress, Nd-Cu liquid enriched at the near-surface regions of the sample is infiltrated into the Nd-Cu-lean middle region, resulting in a concentration gradient. Microstructure characterizations further demonstrate that the infiltrated Nd–Cu eutectic plays a critical role in inhibiting grain growth and intergranular magnetic interaction. The optimized microstructure features suppress the reversed magnetization process, which makes a positive contribution to coercivity.

Microstructure Characterization and Magnetic Characteristics of Ce–Fe–B Based Spark Plasma Sintered Magnets

Nanocrystalline Ce–Fe–B based rare earth permanent magnets are prepared by spark plasma sintering (SPS) method using grounded ribbons as raw material. The phase compositions, microstructure, magnetic properties, intergranular magnetic interactions, recoil loops, and corrosion resistance of the magnets are investigated systemically. The relationship among sintering processes, microstructures, and magnetic characteristics are discussed in depth by mainly using transmission electron microscope (TEM) and Physical Property Measurement System (PPMS) techniques. It is demonstrated that the sintering temperature has a significant influence on the average grain size and width of coarse grain areas. The optimized magnetic properties of remanent magnetization $J_{r}= 0.37$ T, coercivity $H_{\mathrm {ci}} = 227$ kA/m, and maximum energy product $(BH)_{\mathrm {max}}= 16$ kJ/m3 are obtained at 650 °C under 50 MPa for 2 min. Their exchanging coupling interactions are quite weak compared with those of the counterpart alloys. The exchanged decoupled of soft magnetic materials and inhomogeneous magnetic anisotropy result in open and steep recoil loops. The electrochemical experiment results display that the corrosion resistance of the magnets is improved by sintering at high temperature. This paper plays a role in guiding the future research and development of nanocrystalline Ce–Fe–B based permanent magnetic materials.

Concurrent Core Loss Suppression and High Permeability by Introduction of Highly Insulating Intergranular Magnetic Inclusions to MnZn Ferrite

A novel approach entailing the introduction of incongruent insulating magnetic inclusions to the grain boundary region of polycrystalline ferrite cores has been proposed to target the eddy current loss as the major source of core loss at high operating frequency. To demonstrate the efficacy of this approach, highly resistive ferrimagnetic nanoparticles (NPs) of yttrium iron garnet (YIG) were introduced to the grain boundaries of MnZn ferrite. Diamagnetic NPs of barium titanate (BTO) were similarly added as a nonmagnetic insulating grain boundary control additive. A profound decrease was observed in the eddy current loss (up to 79%) by using this approach. For the case of magnetic inclusions, a retained high permeability (24% reduction) was achieved, whereas the samples with nonmagnetic additives experienced a considerable sacrifice to permeability (64% reduction). The sustained high permeability was attributed to the ferrimagnetic YIG NPs maintaining long-range magnetic interactions, whereas the BTO NPs both frustrated the eddy current and long-range intergranular magnetic interactions. These results indicate that low permeability is no longer a necessary constraint for suppression of core power loss.

Microstructural evolution and magnetization reversal mechanism of CoPt films with perpendicular magnetic anisotropy

Microstructural evolution and magnetization reversal mechanisms of perpendicular magnetic anisotropic Co72Pt28 films were investigated. Results showed that in the initial stage of film growth, the Co72Pt28 film was continuous and followed a dome-shaped structure with increasing film thickness. When the film thickness further increased above a certain critical value, the film growth acquired an inverted frustum shape structure and increased the intergranular magnetic interaction. The magnetization reversal mechanism showed a strong dependence on microstructures. The magnetization reversal followed the domain wall motion behaviour when the film was continuous and deviated towards the Stoner–Wohlfarth (S–W) model in the dome-shaped regime. A further deviation away from the S–W model was observed, when the film acquired an inverted frustum shape structure in order to minimize the surface energy.

Effects of Ru underlayer on microstructures and magnetic properties of Co72Pt28 thin films

The effects of Ru underlayer on crystallographic structure, microstructures, and magnetic properties of Co72Pt28 films were investigated. It was observed that Ru deposition pressure played a more important role in determining the crystallographic texture compared to Ru thickness. Ru had a preferred (0002) texture when deposited at low Ar pressure and induced perpendicular magnetic anisotropy for Co72Pt28. However, at high Ar pressure, Ru (101¯0) and Ru (101¯1) peaks appeared that changed the easy axes of magnetization of Co72Pt28 film from out-of-plane to in-plane directions. With increasing thickness of Ru top layer in the dual-layer structure of Ru underlayer, coercivities increased due to increased magnetic anisotropy and reduced intergranular magnetic interaction.

Transverse susceptibility of nanoparticle systems: The effect of interaction, dispersion, and texture

The effect of texture, dispersion, and interaction on transverse susceptibility has been studied. We have developed a model based on the well-known Stoner-Wohlfarth model, by taking into account the texture, the anisotropy field value distribution, and the intergranular magnetic interactions. This model shows a good agreement with experimental measurements on granular FeCoV thin films and allows us to determine the mean value of the local magnetic anisotropy, intergrain interaction, and texture of these alloys.

Structure characteristics and magnetization reversal of high-coercivity cobalt-based alloy films

The structure of “continuous” Co-W and Co-Ni-W alloy films, as well as Co-containing metallic oxide heterogeneous films, covering the aluminum surface is studied in dependence of intergranular magnetic interaction and magnetization reversal processes by taking the angular dependences of the coercive force and irreversible susceptibility and also from δM curves.

Correlation between activation volume and intergranular magnetic interaction in longitudinal media

We have investigated the correlation between the activation grain diameter derived from activation volume and the intergranular magnetic interaction, in CoCrPt-based thin film media fabricated by an ultra clean sputtering process. We found that the activation grain diameter depends on the physical grain diameter and intergranular exchange interaction, but is independent of intergranular magnetostatic interaction (4/spl pi/M/sub s//H/sub k//sup grain/). The activation grain diameter normalized by the physical grain diameter, which corresponds with the normalized coercivity (H/sub c//H/sub k//sup grain/) and microstructure, indicate the character of the intergranular exchange interaction in the media with various compositions. It is also clarified that the media noise can be characterized by the activation grain diameter together with the intergranular magnetostatic interaction.

Recording performance of CoCrPt-(Ta, B)/TiCr perpendicular recording media

The magnetic properties, microstructure and the recording performances of CoCrPt-(Ta, B)/TiCr perpendicular magnetic recording media were studied in conjunction with the effect of thermal agitation. The addition of Ta or B to CoCrPt media significantly improves its low noise performance, mainly due to the reduction of grain size and intergranular magnetic interactions. However, adding Ta or B also reduces the value of perpendicular magnetic anisotropy, K/sub u/, especially in the initial growth layer, resulting in the degradation of thermal stability. In order to achieve high thermal stability without degrading the low noise performance of these media, an M/sub r//M/sub g/ value of nearly 1.0 is required. This may be achieved by suppressing of the reduction of K/sub u/ and the reducing intergranular magnetic interactions, whilst maintaining their small grain size.

Compensation for the Demagnetizing Field in Remanence Measurements of Perpendicular Magnetic Recording Films.

Remanence measurements are very useful for estimating the characteristics of intergranular magnetic interactions in recording layers. However, it is necessary to compensate for the demagnetizing field in remanence measurements of perpendicular magnetic recording films. In this study, a new method of compensating for the demagnetizing field is proposed and an effective reverse field Heff is used to evaluate the dc demagnetization measurements, isothermal remanent magnetization measurements, and ΔM for perpendicular magnetic recording films.

Novel sputtering process to reduce the grain size and its distribution in Co-based longitudinal thin film media-new seedlayer and high K/sub u//sup Grain/ material

To reduce the grain size and its distribution, two kinds of new techniques have been introduced. In the first one, a new WCr seedlayer combined with dry-etching process was found to be effective in reducing the grain size of the magnetic layer while maintaining the intergranular magnetic interactions. As a result, the media signal to noise ratio is significantly increased. In the second one, the usage of a Co co-precipitated NiP/Al plated layer combined with dry-etching and oxygen exposure processes was found to be effective to increase the media signal to noise ratio while maintaining the resolution and coercivity. A new ternary magnetic alloy of Co-Ge-Cr (85 at.% Co, 10 at.% Ge, 5 at.% Cr) was found to have the following magnetic properties: K/sub u//sup Grain/=3/spl times/10/sup 6/ erg/cm/sup 3/, H/sub k//sup grain/=10 kOe and 4/spl pi/M/sub k//H/sub k//sup grain/<1.0. This new magnetic material is a very promising candidate for use in high-density magnetic recording media.

Magnetic cluster and medium noise in CoCrTa/Cr longitudinal film media studied by magnetic force microscopy

The two-dimensional analysis method of magnetic force microscopy is proposed and applied to investigate magnetic cluster and medium noise in CoCrTa/Cr longitudinal film media. It is found that both the magnetic cluster size and root mean square (rms) noise intensity are dependent on recording density and the rms noise intensity strongly enhances with the increase of magnetic cluster size. To understand the factors that influence magnetic cluster formation, the magnetic cluster size, rms noise intensity and half-amplitude pulse width are plotted against tBS/Hc and δM/MS, respectively. It reveals that the increase of coercive force Hc and the reduction of intergranular magnetic interaction could control magnetic cluster size and decrease medium noise. The relationship between medium noise and residual magnetization fluctuation has been discussed.

Influence of the distribution of magnetic moments on the magnetization and magnetoresistance in granular alloys

In granular solids, the magnetoresistance is directly related to the macroscopic magnetization, but this relationship is extremelly complex due to the distribution of grain sizes and the intergranular magnetic interactions. The dependence of the magnetoresistance on the magnetization is here investigated by means of a theoretical model that is developed taking explicitly into account the magnetic moment distribution and the spin-dependent electron-impurity scattering within magnetic grains and at the interface between the grains and the metallic matrix. Using this model, one can explain large experimental deviations from the parabolic behavior of the magnetoresistance vs magnetization curves that are typically expected for equal noninteracting superparamagnetic grains. The expressions for the magnetization and magnetoresistance, obtained for general distribution funtions, are tested considering a log-normal-type distribution function by fitting on data obtained from melt-spun Cu${}_{90}$Co${}_{10}$ ribbons after annealing by dc Joule heating. The experimental data are well traced using just three parameters that determine the particle size distribution, the particle density, and the ratio of the scattering cross section at the boundaries of the grains to the scattering cross section within the grains.