G. Bottoni

Interparticle interactions and magnetic parameters of iron powders for magnetic recording

Abstract The evolution of the coercivity Hc and the switching field distribution (SFD) of some commercial iron particles for magnetic recording with the packing density p is analyzed. For some samples the curves Hc vs. p show a maximum and the corresponding SFD vs. p plots have a minimum. This behavior is ascribed to the different role of the axial interactions and of the lateral interactions among the particles. The cases of positive and negative interactions are also discussed.

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 H c , and the remanence coercivity H r , 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.

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.

Distribution of anisotropy field in recording media deduced from the hysteresis curve

The distribution of anisotropy fields in magnetic recording media is carried out by performing the second derivative of the magnetization curve detected in perpendicular direction with respect to the easy axis of the system, following a procedure similar to that used for the soft magnetic materials. The method is applied to the CoCr films with the anisotropy direction perpendicular to the film plane. It was found that the field at the peak of the distribution function agrees with the anisotropy field measured by the torque method. For particulate media, the shape of the anisotropy distribution function depends both on the particle orientation ratio and on the interparticle interactions; as a consequence, the anisotropy distribution function determined on these materials does not represent the intrinsic anisotropy field distribution of the particles but the anisotropy distribution as it appears in that particular specimen. In any case, this function is useful for understanding the magnetic behavior of the ...

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.

Analysis of the magnetization switching using the rotational hysteresis integral

The relation of the rotational hysteresis integral with the magnetization switching mode is experimentally analysed in materials which are subjected to different treatments which can modify the magnetization switching mode. As expected, the value of the integral depends on the basic processes of magnetization. They evolve with the evolution of the switching in the expected way, both when the switching changes from wall motion to rotation of the magnetization, and when, in single-domain particles, the rotation occurs with different reversal modes.

Effect of the coupling between Co and CoO in metal evaporated tapes

Abstract In metal evaporated (ME) tapes the interaction between the ferromagnetic Co grains and the antiferromagnetic CoO layer may affect the magnetic behaviour at low temperatures. In fact, when the tapes are cooled from room temperature in the remanence state or in the presence of a strong magnetic field, a shift in the hysteresis cycle is observed. This is an effect of the presence of unidirectional exchange anisotropy, which originates in the exchange coupling between the ferromagnetic and antiferromagnetic phases, and also affects the magnetic behaviour in the presence of a rotating magnetic field. The torque curves have been measured at 80 K. For tapes cooled in the remanence state or in the presence of a strong magnetic field, they show a superposition of sin 2 θ and sin θ torque functions, which indicates the simultaneous presence of the uniaxial anisotropy of the ferromagnetic phase, and of the unidirectional exchange anisotropy. The rotational hysteresis energy loss at 80 K, deduced from the area enclosed in the torque curves, shows anomalous behaviour, with no decrease with increasing field, and also finite values at applied fields larger than the anisotropy field of the ME tapes. This is due to the drag of the ferromagnetic Co moments by the antiferromagnetic CoO exchange-coupled spins during the rotation of the field direction.

The effect of multiple anisotropies in fine particles

Abstract To investigate the effects of multiple anisotropies, morphology and size on magnetic properties of fine particles, cobalt-modified materials with different shapes were tested at temperatures from liquid nitrogen to 400 K. Some interesting and original conclusions were drawn: (a) When multiple easy axes are available, thermal fluctuations can induce the magnetization to switch from one axis to the other; the overall effect will be an increase of the fraction of particles with superparamagnetic behaviour. (b) The phenomenon will be greater for materials where the conflicting anisotropy constants are similar (isotropic particles); thus, for a given composition, the lower the shape anisotropy and the larger the superparamagnetic fraction. (c) Porosity and particle defects will contribute to increase the super-paramagnetic fraction. (d) In practical media (tapes) the effect of the superparamagnetic fraction is much lower than expected: a “constricted magnetization” phenomenon could account for such behaviour. (e) The lack of interactions predicted for truly isotropic media is experimentally verified only at extremely low temperatures. (f) Partial orientation in the plane of the strongest anisotropy axis must be taken into account for explaining the behaviour of SFD; under such assumption, “quasi-spherical” particles will behave quite differently from elongated ones. (g) Rotational hysteresis, CF and (1 − S∗) for isotropic particles seems to indicate that the rotational mechanism might not be accounted for by known models.