A. Cecchetti

Influence of extreme dilutions on the magnetic properties of single-domain particle aggregates

Various magnetic properties of samples of single-domain particles dispersed in a nonmagnetic matrix are examined as functions of the packing fraction p . The range of variability of p is from 0.0003 to 0.20. The squareness ratio and the rotational and alternating hysteresis integrals change with p , while the coercive field, the initial anhysteretic susceptibility, and the areas between the remanence curves are not dependent on p . These results are interpretated as a consequence of the formation of agglomerates interacting with each other, rather than as a consequence of analogous interactions among the single-domain particles which are in the aggregate.

Print-through, erasability, playback losses: Different phenomena from the same roots

When testing some audio and video tapes made with different pigments, a strict correlation between print-through (in audio), variation of audio erasibility (in video tapes) and playback losses can be found. Our investigation on these phenomena lead us to correlate these properties with the defect structure of the oxides and with the amount and penetration of cobalt into the core of the particles. SIMS data on cobalt penetration, temperature dependence of coercivity/saturation and squareness confirmed the assumption that these performance parameters are strictly correlated to each other and such correlation is dependent on the mobility of Co2+and electron hopping of the Fe2+/Fe3+which is easier in particles bearing defects and large (cobalt-containing)/(cobalt-free) volume ratios.

Peculiar evolution of the magnetic properties with temperature for (Ti, Co)-substituted barium ferrites

The temperature dependence of coercivity for (Ti,Co)-substituted barium ferrites between 77 and 500 K is found to be substantially different from that of nonsubstituted material. This difference can be accounted for by a much steeper decrease of the magnetocrystalline anisotropy field component and by the possibility that the magnetization reversal mechanism varies from coherent to incoherent as the temperature is varied. >

Reversible coercivity vs. temperature losses in activated cobalt adsorbed iron oxides

Most properties of CrO 2 in magnetic recording media can be matched -and in some cases superseeded- by Surface-Cobalt Modified Iron Oxides. However, while the variation of coercivity with temperature for CrO 2 is a fully reversible process such variation is partially irreversible for Cobalt Iron Oxides. This relative weakness of Co-iron oxides could become a limiting factor in high density recording systems (such as the IBM 3480 and/or high density microdiskettes). This paper presents experimental evidence on a new Cobalt adsorption system which yealds products whose coercivity losses are fully and completely reversible in the whole range of temperature up to 100°C. In the process of comparing the properties of CrO 2 vs. Cobalt-Iron Oxides this paper also shows the equivalence of the applicative properties of the two products in terms of output vs. writing current and magnetostrictive effects.

An investigation of magnetic oxides' defects and conflict of anisotropies for diluted powders

Magnetostatic measurements and rotational hysteresis were carried out on extremely diluted samples of pure, surface modified, bulk doped iron oxides and CrO 2 . It is found that in all these materials but pure γ-Fe 2 O 3 a conflict of anisotropy is active. As a result, the values of remanence ratio j r can be either lower or higher than the theoretical value of ½ for strictly uniaxial particles depending on the angle between the two anisotropy axis and the relative value of the anisotropy constants. It is therefore found that while j_{r} for acicular magnetite and CrO 2 , the introduction of cobalt always produces an increase of j r : the extreme effect of such conflict can be found in high remanence isotropic particles. Such a conflict is minimized in Surface Modified Materials. From rotational hysteresis, H c vs. angle with applied field and, over all, from CF and IFF parameters, it is suggested that the postreatment of the iron oxides with the Co-containing solutions, beside the increase of H c , has a beneficial effect on the morphology of the particles.

Alternating and rotational hysteresis losses of γ-Fe 2 O 3 powders in a nonmagnetic matrix

Some aspects of the alternating and rotational hysteresis losses of very diluted samples of γ-Fe 2 O 3 particles are examined. The packing fraction p of the particles ranges from 0.001 to 0.120. The coercive force is found to be independent of p . The alternating and rotational hysteresis losses appear dependent upon p for p ; the rotational losses increase considerably as p approaches zero. The values of the rotational hysteresis integral are close to the theoretical ones corresponding to the model of magnetization reversal of chain of spheres with fanning, but they increase slightly with decreasing p for p . Generally, the ratio of rotational to alternating losses is about twice that for bulk materials. All these effects are ascribed to a weakening of the magnetic interactions among particles.

Influence of the Co-doping of iron oxides on magnetic particles interactions

The effects of magnetic interparticle interactions are investigated on tapes made with iron oxide, chromium dioxide, and Co-surface-modified and Co-bulk-doped iron oxides. The magnetic interactions were experimentally studied with measurements of (a) the magnetization and demagnetization remanence curves, (b) the initial anhysteretic remanent susceptibility, and (c) the reputation of the magnetization cycles. The latter two techniques give the most suitable results for estimating the particle interactions, and a possible correlation between the results obtained with these methods is proposed. The Co-surface modified iron oxide particles show the weakest interaction effects. This is ascribed to the different orientation of the magnetic moments at the Co-modified surface layer of the particle. >

A comparison between different methods for evaluating the particle interactions in magnetic recording systems

Two different methods for the determination of the magnetic interparticle interactions in recording systems are compared. The first one is that based on the evaluation of the deviation /spl Delta/m from the Wohlfarth equation for noninteracting particles which requires the measurement of the remanence curves. The other is an easier procedure which consists in the determination of the deviation /spl Delta/m* from an equation based on the initial magnetization curve and the hysteresis loop. For systems of oriented uniaxial anisotropy particles the solution is generally /spl Delta/m/spl cong/2/spl Delta/m*, while for systems of oriented multiaxial particles or for random oriented particles it is /spl Delta/m/spl cong/4/spl Delta/m*. The advantage of the second method is its applicability to systems where the initial curve and the hysteresis loop must be corrected by the demagnetizing field as for CoCr films magnetized perpendicularly to the film plane.

Exchange anisotropy in metal-evaporated tapes

The hysteretic properties of commercial Metal Evaporated tapes, consisting of one or three layers of Co-Ni-O, have been analysed at low temperature. When the tapes are cooled from room temperature in the saturation remanence, or in presence of a strong applied field, the hysteresis loops at low temperature are displaced, with dissymmetrical values of the coercive fields. Also the rotational hysteresis has an anomalous behaviour with a monotonic increase with the applied field. These effects are due to the interface action between the ferromagnetic Co and the antiferromagnetic CoO, which, at low temperatures, gives rise to the unidirectional exchange anisotropy. >

Magnetic viscosity and interaction effects of iron particles

Time-dependent magnetic phenomena are analyzed on aligned iron particle aggregates at different packing densities. The different roles of the interactions of positive and negative type, as deduced from the remanence curves analysis, are presented. The activation volume and the critical volume are calculated from the experimental data. The activation volume obtained by extrapolating to zero interactions is found to be equal to the volume of the spheres of the fanning model which governs the magnetization reversal of these particles. >