R. Valenzuela

Novel Applications of Ferrites

The applications of ferrimagnetic oxides, or ferrites, in the last 10 years are reviewed, including thin films and nanoparticles. The general features of the three basic crystal systems and their magnetic structures are briefly discussed, followed by the most interesting applications in electronic circuits as inductors, in high-frequency systems, in power delivering devices, in electromagnetic interference suppression, and in biotechnology. As the field is considerably large, an effort has been made to include the original references discussing each particular application on a more detailed manner.

Correlations between low-field microwave absorption and magnetoimpedance in Co-based amorphous ribbons

Microwave power absorption measurements at 9.4 GHz were carried out on as-cast amorphous ribbons of nominal composition Co66Fe4B12Si13Nb4Cu. Two absorptions were observed: a small signal at a low dc field (<0.01T) and another one at a high dc field (∼0.1682T). The high-field signal shows all the features corresponding to ferromagnetic resonance. The low-field absorption (LFA) signal exhibits different characteristics such as hysteresis and a minimum in power absorption at zero magnetic field. A correlation between this LFA signal and magnetoimpedance measurements showed that both electromagnetic processes are associated with the same phenomenon.

Equation of motion of domain walls and equivalent circuits in soft ferromagnetic materials

The equation of motion for ferromagnetic domain walls is compared with initial permeability measurements of Co‐based soft ferromagnetic amorphous ribbons at frequencies in the range 100 Hz–13 MHz. The analysis of these results by complex permeability methods leads to an equivalent circuit formed by a parallel RL arrangement in series with a small resistance RW due to resistance of the measurement coil. This equivalent circuit, however, cannot model the irreversible magnetization behavior for fields higher than the pinning (or propagation) field. It is shown that the equation of motion is also consistent only with the reversible magnetization mechanism, if the displacement term, x, represents instead the bulging of the pinned wall. The correlations between the terms in the equation of motion and the elements of the equivalent circuit are established.

Effects of nanocrystallization upon the soft magnetic properties of Co‐based amorphous alloys

Amorphous samples of Vitrovac(R) 6025 metallic alloys were nanocrystallized by thermal annealing at temperatures in the range 576–716 K, well below the crystallization temperature (831 K, as obtained by differential scanning calorimetry at 20 K/min) and times in the range 10–90 min. Initial magnetic permeability exhibited a maximum of ∼109 000 (from ∼20 000 for the as‐quenched state) for annealing times of ∼15 min, and then a decrease toward a plateau value (30 000–40 000) for longer times. Domain wall relaxation frequency showed the opposite behavior, a minimum of ∼9 kHz (from 40 kHz for the as‐quenched value) for the permeability maximum, and then a stabilization at ∼15 kHz. Transmission electron microscopy observations confirmed that for annealing times up to 60 min., crystallite size is smaller than 30 nm. Permeability and relaxation frequency results are interpreted in terms of the dependence of wall bulging on anisotropy and free‐wall surface area between pinning sites.

Domain wall pinning, bulging, and displacement in circumferential domains in CoFeBSi amorphous wires

A detailed study of the complex inductance response of as-cast CoFeBSi wires as a function of frequency (100 Hz–13 MHz range) and circular field amplitude [0.1–17 A/m room mean square (rms) on the surface of the wire] in the giant magnetoimpedance arrangement is presented. The analysis of experimental results show evidence of magnetization processes associated with circumferential domain walls, such as domain wall pinning, bulging, and displacement. The unpinning field was measured as 0.2 A/m (rms) at 5 kHz. It is shown that the analysis of spectroscopic plots of real and imaginary inductance leads to characterization of the magnetization process involved.

Domain wall relaxation in amorphous ribbons

Monodomain, polycrystalline hexagonal ferrites (BaFe12O19) and amorphous ribbons (Vitrovac 6025) were measured as a function of frequency with rf magnetic fields from 5 Hz to 13 MHz of various amplitudes. Monodomain ferrites showed the features of the spin rotation magnetization mechanism, while amorphous ribbons showed the low‐frequency dispersion generally attributed to domain wall relaxation. However, at relatively high applied fields, a third dispersion is resolved at lower frequencies from domain wall relaxation, which can be attributed to magnetic hysteresis. By using some of the complex impedance formalisms, the relaxation character of domain wall bulging is confirmed, while hysteresis dispersion exhibited a complex behavior.

Effect of the grain size distribution on the magnetization curve

Using theoretical expressions for the magnetization curve of polycrystalline ferrimagnets given previously, we have considered the effect of a grain size distribution on the magnetization versus applied magnetic field curve. We have observed the effects of various theoretical probability distribution functions and compared them with an experimental curve of yttrium iron garnet, Y3Fe5O12. The best fitting was obtained with a normal logarithmic distribution function for the grain size distribution of the sample.

Analytical prediction of the magnetization curve and the ferromagnetic hysteresis loop

We have obtained theoretical expressions for the ferromagnetic magnetization curve and hysteresis loop using an extension of the general ideas of the Globus model for polycrystalline ferrimagnets. In this work we take into account the force which results from the variation of the total energy (magnetic energy plus surface energy) in order to find the value of the critical field. Our theoretical magnetization curve agrees well with the experimental curve and our hysteresis loop has the general qualitative features of the corresponding experimental loops.

Circumferential magnetization curves of Co-rich amorphous wires under tensile stress

Circumferential magnetization curves were obtained for low, negative magnetostriction amorphous wires of nominal composition (Co0.94Fe0.06)72.5B15Si12.5, under tensile stress up to 250 MPa, by using the magnetoimpedance effect. The various magnetization processes, i.e., spin rotation, domain wall bulging, and domain wall displacement were resolved by frequency measurements in the 5 Hz–13 MHz range, and by varying the applied field amplitude between 0.28 and 12 A/m (root mean square). The results show that reversible magnetization processes (domain wall bulging and spin rotation) are damped by tensile stress, while the irreversible process (domain wall displacement) is enhanced by stress. These results are interpreted in terms of an increase in anisotropy and a reorienting of magnetization in circumferential domains, both as a result of the stress-induced anisotropy.

Magnetic properties of substituted W and X hexaferrites

Magnetic measurements were performed on Sr‐hexaferrites of type Me2+2–Sr(W) with Me2+2=Fe2+, Ni2+, Co2+, Zn2+, and Me2+‐Sr(X) with Me2+=Fe2+, Zn2+ in the temperature range 5K 750 K. The Curie temperatures are found to be 781 and 721 K for Fe2‐Sr(W) and Fe‐Sr(X) systems, respectively. The saturation magnetization of a Zn2‐Sr(W) hexaferrite has been observed to be maximum at 5 and 300 K among all the samples investigated, namely, 115 and 82 emu/g. The saturation magnetization of Fe2‐Sr(W) and Fe‐Sr(X) hexaferrites are almost the same, whereas at 300 K this value is higher for Fe2‐Sr(W). The coercivity of the Fe‐Sr(X) sample has been found to be higher than the Fe2‐Sr(W) hexaferrite after 8 h of milling.