摘要:

The room‐temperature coercivity of acicular &ggr;‐Fe2O3particles having cobalt ions adsorbed in thepH range from 5 to 13.5 and in the temperature range from 283 to 373 K was measured. Emission Mo¨ssbauer spectroscopy at room temperature was used to study cobalt adsorbed on the surface of acicular &ggr;‐Fe2O3particles. The coercivity of cobalt‐adsorbed &ggr;‐Fe2O3particles increased with increasing cobalt content up to about 2 wt %, above which it leveled off. The saturated values of coercivity increased with increasing adsorption temperature from 283 to 303 K, decreased with further increasing temperature up to 343 K, and again increased remarkably at 373 K. It was found from Mo¨ssbauer spectroscopy that the sample with high coercivity shows a strong magnetic interaction between the absorbed cobalt ions and the ordered magnetic ions of the substrate. Nonmagnetic components that do not contribute to the coercivity are probably due to the oxides and hydroxides of divalent and trivalent cobalts. The effect of pretreatment with Mg2+, Ca2+, and Sr2+ions on the adsorption of cobalt on the surface of &ggr;‐Fe2O3at 373 K was also studied by emission Mo¨ssbauer spectroscopy. It was found that the average internal magnetic field of &ggr;‐Fe2O3with both Mg2+and Co2+ions adsorbed was larger than that for Co2+ions alone, whereas in the case of Ca2+and Sr2+ions, the average internal magnetic field did not change. These phenomena are discussed on the basis of the site preference of cations in ferrites.

ISSN:0021-8979    

DOI:10.1063/1.348382

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The coercive force of acicular, nonstoichiometric (partially reduced) &ggr;‐Fe2O3particles is increased after surface treatment with solutions of sodium polyphosphate. The coercive force of 0.6×0.1 &mgr;m particles having Fe+2/Fe+3=0.134 increases from 450 to 670 Oe when treated with a 0.15 g/ml aqueous solution of (NaPO3)n≊14⋅Na2O. The magnitude of the change in the coercive force is dependent upon the relative amounts of polyphosphate and iron oxide present during treatment, but is independent of the polyphosphate solution concentration within the range 0.05–0.15 g/ml and the solutionpH within the range 7–9. The enhanced coercivity values decrease slowly with time when the treated particles are exposed to air. In contrast to the partially reduced oxide, enhanced coercive forces are not observed in stoichiometric &ggr;‐Fe2O3after similar treatment. These results are qualitatively consistent with earlier reports thatHccan be increased in nonstoichiometric &ggr;‐Fe2O3via surface treatment with chainlike sodium polyphosphates. Possible mechanisms for this behavior are discussed.

ISSN:0021-8979    

DOI:10.1063/1.348383

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Fe‐(SiO2) granular films were fabricated by sputtering with a mosaic target. The fractional area of iron on the target was varied from 0.29 to 0.42. The volume fraction of iron was found from the saturation magnetization to be within 0.03 of that predicted by the target composition. The sample coercivities ranged fromHc= 130 to 950 Oe at room temperature and 690 to 2390 Oe at 77 K. Characteristics relevant for recording media were investigated for the highest‐coercivity sample. The squareness values for hysteresis loops measured in and out of the plane of the film,Mr/Ms= 0.54 and 0.43, respectively, indicate that the granules have a uniaxial anisotropy and the easy axes tend to lie more in the plane of the film. The rotational hysteresis was measured at room temperature; the rotational hysteresis integral was calculated to beIr= 1.56. Because the magnetization reversal is assisted by thermal fluctuation, the low temperature value ofIrwas estimated by scaling the measured value by the ratio of the coercivities measured at 77 K and at room temperature. The value ofIr≊ 0.6 is closest to the theoretical value for the coherent rotation mechanism. The relative decay of magnetization between 30 and 3600 s had a maximum value of &Dgr;M/Ms= 0.10 for a reversal field nearHc. This value falls within the range of the values reported for conventional particulate media, which suggests that a stable transition can be recorded in this material.

ISSN:0021-8979    

DOI:10.1063/1.348956

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The interaction of commercially available, iron‐based magnetic particles with certain particulate media formulation components has been investigated using the technique of flow microcalorimetry. The heat of wetting of these particles by solvents has been found to be a function of the solvent dipole moment and particle coercivity and surface area. Di(2‐ethyl hexyl) phosphoric acid, a model dispersant, appears to interact chemically with a particle of high coercivity. Solvent basicity influences the heat and extent of interaction of the dispersant.<lz> <lz> <lz> <lz> <lz> <lz> <lz>

ISSN:0021-8979    

DOI:10.1063/1.348358

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Acicular &agr;‐FeOOH particles with particle length of about 0.2 &mgr;m and axial ratio of about 8 were used as the starting material to prepare acicular Fe‐Co alloy particles. They were made by adsorbing Co(OH)2on the surface of &agr;‐FeOOH particles, followed by dehydration, annealing, and reduction. In order to prevent sintering at high temperatures, the Co‐&agr;‐FeOOH particles were first dehydrated at 300 °C and then coated with a thin layer of silica before high‐temperature treatments. Furthermore, after reduction, the metallic powder was immersed in toluene to avoid oxidation. The particle structure of Fe‐Co was determined from x‐ray diffraction. It has been found that, up to 50 at. % Co, all the Fe‐Co particles have the same bcc structure and similar lattice constant. The magnetic properties of Fe‐Co alloy particles with different Co concentrations were investigated using a vibrating sample magnetometer. The specific saturation magnetizations &sgr;sare about 30% below those of bulk materials. The maximum value of &sgr;sat room temperature is obtained with 42 at. % Co. The coercivity that mainly originates from shape anisotropy is as high as 1250 Oe for the particles with 33 at. % Co. Furthermore, &sgr;sdrops sharply with Si content whileHcpeaks at 0.6% Si. The coercivities of the particles also depend on the crystallite size.Hcshows a maximum when the crystallite size is about 300 A˚.

ISSN:0021-8979    

DOI:10.1063/1.348955

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Initial results of a micromagnetic study on elongated iron particles are presented. A numerical micromagnetic model is utilized to compute the magnetization configurations and switching fields for both the homogeneous metal particle and the inhomogeneous passivated particle from first principles. The volume of the passivated particle is divided into a metallic core and a passivated shell with significantly reduced magnetic‐moment density. The calculations show that the topology of the magnetization configurations is similar to those of homogeneous &ggr;‐Fe2O3particles. The switching field is higher for the passivated particles as compared to the homogeneous particle depending on the exchange interaction in the passivation layer. The enhancement of the switching field is explained in terms of reduced torques and reduced demagnetization at the passivated particle ends.

ISSN:0021-8979    

DOI:10.1063/1.348333

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

This paper discusses the relationship between the noise characteristics and magnetic properties of longitudinal barium ferrite (Ba‐F) rigid disks with different switching field distributions (SFD). The magnetomotive force dependencies of reverse dc‐erase (RDC) noise are measured and compared with SFD values. Coated disks with acicular magnetic particles have dips and thin‐film disks peaks in the RDC. In Ba‐F disks, both cases are observed depending on the SFD values, though the depths or heights of the RDC noise are much smaller than those of coated disks with acicular particles or thin‐film disks. Disks with small SFD values have peaks, and disks with large SFD values have dips. In order to find the relationship between noise properties and magnetic properties, interparticle interactions in Ba‐F disks are investigated. Reverse dc remanenceId(H) and ac‐demagnetized isothermal remanenceIr(H) are measured. Both are normalized by the saturation remanence. The deviation from the noninteracting system, &Dgr;M=Id(H) − [1&Dgr;M=Id(H)−[1− 2Ir(H)] and an interaction field factor (IFF) given by (H’r−Hr)/Hc, are derived from these remanent properties. Here,H’ris the field corresponding to 50% of the remanent magnetization,Hris remanence coercivity. In Ba‐F disks, &Dgr;Mshows positive interactions, and the peak heights of &Dgr;Mincrease and IFF decrease with decreasing SFD values. Positive interactions between Ba‐F particles seem to be caused by particle stacking. Therefore, particle stacking results in small SFD values and peak‐type RDC noise.

ISSN:0021-8979    

DOI:10.1063/1.348334

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

AM‐Hloop model for Ba‐ferrite particulate media is introduced where a modified Stoner–Wohlfarth model is used as a magnetization reversal model. It is confirmed that this model can be applied for read/write simulation of Ba ferrite. Using this model, the read/write performance of Ba‐ferrite media is investigated and compared with that of acicular particulate media. The results indicate that: (i) For high‐density recording, longitudinally oriented Ba ferrite is superior to acicular particulate media because of its high coercive squarenessS*; (ii) to improve read/write performance of the Ba‐ferrite in use today, narrowing of anisotropy field distribution is effective; and (iii) perpendicularly oriented Ba ferrite has good read/write performance only at narrow head‐medium spacings.

ISSN:0021-8979    

DOI:10.1063/1.348335

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Magnetic interactions between particles or crystallites in recording media can have an important effect on their switching behavior and on their recording performance. If the types of interactions that tend to stabilize the saturated state are defined as positive and those that tend to destabilize it as negative, it can be argued that positive interactions would reduce demagnetization losses and lead to improved recording performance. Furthermore, positive interactions would be expected to give rise to cooperative switching of groups of particles or crystallites that, on the one hand, would tend to reduce the switching field distribution and, on the other hand, would tend to increase noise. Remanence curves, being strongly influenced by interparticle interactions, provide a sensitive way to investigate the magnitude and sign of such interactions. In this paper, the interactions of different particulate and thin‐film media have been studied by comparing their demagnetization remanence curves with their isothermal remanence curves. Recording media employing oriented acicular particulate assemblies exhibit negative interactions. Nonoriented particulate media with either acicular or platelet‐type particles exhibit very small and slightly negative interactions. Thin‐film media and oriented Ba‐ferrite media with platelet‐type particles usually exhibit strong positive interactions. These large differences in interaction fields are discussed on the basis of the type of coupling (magnetostatic and/or exchange), the particle morphologies, and the anisotropy fields in these media. The polarity, magnitude, and shape of the interaction curves seem to be qualitatively correlated with the recording media noise.

ISSN:0021-8979    

DOI:10.1063/1.348336

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

In an assembly of particles, as used in magnetic recording media, there are always magnetic interactions. The magnetic properties of the media are modified as a consequence of the effect of such interactions on the magnetic behavior of the particles. In this work an attempt is made to find an experimental correlation between the presence and strength of the interactions and the magnetization reversal of the particles. The reversal mode is investigated by rotational hysteresis measurements and by the evolution of the angular dependence of the coercivity in samples of iron oxides at different dilution degrees. The magnetic interactions are evaluated with measurements of the remanence curves. It is found that when the magnetic interactions among the particles increase, the parameters used to study the reversal mode evolve from the behavior typical of the incoherent magnetization processes to behaviors which can be considered as intermediate between coherent and incoherent reversal modes. This evolution can be attributed to the tendency of the magnetic interactions to increase the percentage of particles in the aggregate, which reverse with coherent rotation, particularly in the agglomerates often present in closely packed assemblies.

ISSN:0021-8979    

DOI:10.1063/1.348337

出版商:AIP    

年代:1991

数据来源: AIP