F. Masoli

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.

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.

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.

Influence of the packing density on the coercivity of iron particles for magnetic recording

The magnetic properties of iron particles aggregates of different packing density p are analysed. In some cases, when p increases, the coercive field has an initial increase followed, after a distinct maximum, by a decrease. This behavior is ascribed to the formation of chains of particles whose effect is opposite to the effect due to the formation of clusters. The chain formation increases the maximum anisotropy field because of the increase of the shape anisotropy. From the rotational loss measurements it is shown that fanning is the magnetization reversal model.

Evolution of the interactions effects with Co-doping of iron oxide particles

Abstract The evolution of the magnetic properties, and particularly of the coercive field, of aggregates of particles of pure and Co-modified iron oxides on packing is analyzed. When the interactions, evaluated with the Δ plot area, increase, the coercivity of the Co-modified particles shows an anomalous behavior, which is imputed to a reduction of the canting of the magnetic moments in the surface layer.

Magnetization processes in iron particles for magnetic recording

Abstract The magnetic properties and the magnetization reversal processes are analysed on commercial α-Fe particles for magnetic recording. The experimental results are discussed on the basis of two recent models where the particles are assumed to be composed of chain-of-spheres contacting each other at a point (Kz. Ohshima) or at an area (Y. Ishii and M. Sato).

Magnetic anisotropy distribution in Ba ferrite particles

The distribution of anisotropy fields in Ba ferrite particles and its evolution with the addition of Co/Ti ions is studied. The width of the anisotropy distribution increases with increase in doping content, and further is larger at low temperature. The result is ascribed to the role of the shape anisotropy, the effect of which is enhanced by doping and decreasing temperature.

Magnetization processes in CoNiW films

The torque curves of hard magnetic CoNiW films with perpendicular magnetic anisotropy are analysed. Using the dependence of the rotational hysteresis energy losses on the field, both in the film plane and perpendicular to it, a conclusion is drawn about the magnetic structure of the films and the mechanism of magnetization reversal consisting of an inhomogeneous rotation of the magnetization vector.

Magnetization processes in double-layer films for perpendicular recording

Abstract We examine the influence of the NiFe underlayer on the magnetization reversal processes of CoCr films by rotational hysteresis measurements. We analyse both the case of the NiFe film deposited over the CoCr layer and the case of the CoCr film deposited over the NiFe layer. In the first case no appreciable variations occur for the switching process of the CoCr film with respect to the case of a single layer of CoCr. In the second case the rotational losses of the CoCr layer increase with respect to the single CoCr layer ones for both in plane and perpendicular rotations. The rotational hysteresis integral increases in case of perpendicular rotation. These results indicate that, when the CoCr is deposited over the NiFe layer, the reversal mechanism of CoCr becomes “more incoherent”. This effect is ascribed to the enhancement of the influence of the CoCr “initial layer” (which is in contact with the NiFe substratum) on the top layer of CoCr.

Rotational and alternating loss in amorphous ferromagnet

Abstract The ratio of the rotational hysteresis loss to alternating loss is examined as a function of the relative magnetization in amorphous Metglas 2826. The behaviour of this ratio does not coincide with that of most polycrystalline materials. Such an anomalous behaviour is ascribed to the strong interaction effects among domains.