Alternating and rotational hysteresis losses of γ-Fe2O3powders in a nonmagnetic matrix

Some aspects of the alternating and rotational hysteresis losses of very diluted samples of γ-Fe2O3particles are examined. The packing fractionpof the particles ranges from 0.001 to 0.120. The coercive force is found to be independent ofp. The alternating and rotational hysteresis losses appear dependent uponpforp < 0.04; the rotational losses increase considerably aspapproaches 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 decreasingpforp < 0.04. 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.     

On the behaviour of the rheological and magnetic parameters of densified γ-Fe2O3

Magnetic characteristics (coercivity, remanent coercivity, remanence/saturation ratio, coercivity factor) and rheological properties (tap density, apparent density and wettability) for a number of γ-Fe2O3samples are analyzed. The samples are the following: I. γ-Fe2O3produced from densified α-FeOOH at various densification times; II. Densified γ-Fe2O3withdrawn from the densifier at different densification times; III. Densified and non-densified materials available on the market. The magnetic data as a function of the densification degree (i.e. vs. apparent density) confirm the increase of interactions and the re-assembling of the loose particles in a distribution of single particles, sheaf and ring shaped clusters. The wettability of the I samples is largely different from that of II samples; an hypothesis is made: densification produces a rough cleaning of the particles surface thus bringing out the reactive sites of the surface that can therefore be easily wetted out. Samples III confirm this hypothesis.     

Correlation between magnetic and electroacoustic parameters of audio magnetic tapes

Coercivity, squareness, remanent coercivity, coercivity factor, rotational hysteresis loss and its integral were measured on two sets of tapes (γ-Fe2O3and crO2) and related to their electroacoustic performances. The static magnetic measurements explain some of the properties of the particulate media: it was found that while CrO2rotates either coherently or incoherently depending on the direction of the applied field, γ-Fe2O3rotates incoherently, and the departure from the model of "fanning chain of spheres" can be related to the imperfections of the particles which are sources of demagnetizing fields. Good agreement with audio performance is found not only for parameters like coercivity and squareness, whose role is well known, but also with the other static parameters which were previously disregarded as ruling analogue recording.     

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 fractionp. The range of variability ofpis from 0.0003 to 0.20. The squareness ratio and the rotational and alternating hysteresis integrals change withp, while the coercive field, the initial anhysteretic susceptibility, and the areas between the remanence curves are not dependent onp. 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.     

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 CrO2. It is found that in all these materials but pure γ-Fe2O3a conflict of anisotropy is active. As a result, the values of remanence ratio jrcan 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 whilej_{r} < 1/2for acicular magnetite and CrO2, the introduction of cobalt always produces an increase of jr: 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, Hcvs. 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 Hc, has a beneficial effect on the morphology of the particles.     

Thermal stability of Co-modified iron oxides. III. Effect onoverwrite, erasability and dynamic coercivity

For pt.II see ibid., vol.MAG-23, p.48 (1987). The instability properties of Co-modified iron oxides are closely related to the volume scaling factor (i.e. the Co-containing/Co-free volume ratio). It is shown that this factor influences the audio erasability (in the video mode) and the ratio between printing and storage coercivity. Under such circumstances, bulk doped materials are found to be very unstable. However, the nature of the Co-containing phase can also have a large influence on the stability properties so that bulk-doped materials can be stabilized to some extent. Such improvement can be accounted for by the single ion contribution to the magnetocrystalline anisotropy constant and to its temperature dependence. The influence of the volume scaling factor on overwrite is also discussed     

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

Most properties of CrO2in 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 CrO2is 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 CrO2vs. 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.     

Magnetization reversal in sputtered Co-Cr films

The magnetic characteristics of a set of r.f. sputtered Co-Cr films of various composition and thickness are examined in order to evaluate their performance for perpendicular recording. The mechanism of the magnetization reversal is discussed on the bases of: i) the shape of the rotational hysteresis loss as a function of the internal rotating field, ii) the value of the rotational hysteresis integral, iii) the dependence of the coercivity Hc, and the remanence coercivity Hr, on the angle between the easy direction and the field direction. In particular the measurements of rotational hysteresis were performed by monitoring the rate of change of the angular velocity of a top containing the specimen spinning in a magnetic field perpendicular to its axis. The deviation of the experimental behaviour from the ideal models are related to the amount of anomalies in the columnar structure of the film.     

Magnetic properties of new (NP) hydrothermal particles

A new type of uniaxial particles for magnetic recording whose precursors are produced via an original hydrothermal process was recently introduced in the market. The characterizing feature of these materials is their extremely uniform size and ellipsoidal shape. Most of their properties can be ascribed to the lack of external (pores, dendrites, sharp edges) as well as internal (because of ellipsoidal shape) - self-demagnetizing sources, which suggests the name of "Non-Polar (NP)" particles. As a result of this morphology, they show unique magnetic properties. The magnetization reversal mechanism is not accounted for by any known mechanism as fanning or curling and the rotational hysteresis gives very sharp Wrvs. H curves with the lowest values for the integral ever measured for particulate media. The behaviour of coercivity versus packing fraction for Cobalt-surface-modified NP particles is described by a new "constricted magnetization" model. The reasons for these unique characteristics and the practical impact that these materials may have in the field of magnetic recording are discussed.