E. Amano

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.

ac impedance analysis on crystalline layered and polycrystalline bismuth titanate

A comparison is made on the dielectric properties of the ferroelectric perovskite bismuth titanate for both crystalline (along different directions) and polycrystalline (with and without flux) samples, using the complex ac impedance technique. Results for the impedance, conductivity activation energies, bulk dielectric constant, and temperature behavior of the low‐frequency dielectric constant are presented. We discuss these results in terms of the conduction mechanisms depending on the nature of the sample, and compare results for crystalline and polycrystalline samples. Also, we show the important influence of the flux on the dielectric properties of the different samples, which must be taken into account in the use of these materials as substrates for the growth of superconducting thin‐film ceramic oxides.

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.

Characterisation of magnetic materials by impedance spectroscopy

Abstract Impedance spectroscopy has been used to investigate the magnetic properties of (Ni,Zn)Fe 2 O 4 . Two magnetic relaxations were observed corresponding to domain wall bulging and domain rotation. At higher applied fields a second magnetic mechanism, involving irreversible domain movement, was also observed. An equivalent circuit consisting of two RL parallel elements and a Warburg-type magnetic impedance has been used to model these phenomena.

Effects of tensile stress on the domain wall dynamics of Co-based amorphous ferromagnetic wires

Abstract Complex permeability measurements were performed on Co 72.5 B 15 Si 12.5 amorphous wires, as a function of tensile stress, at room temperature and in the frequency range 5 Hz – 13 MHz. Rotational and initial permeabilities showed a decrease as tensile stress increased. The domain wall relaxation frequency exhibited a linear behavior as stress increased. These results are interpreted in terms of the magnetoelastic contribution to anisotropy.

Reversible and irreversible domain wall movement in Metglas® amorphous ribbons

Abstract Magnetization mechanisms in Metglas® 2605 ferromagnetic amorphous ribbons have been investigated by Impedance Spectroscopy. At applied fields below 14 mOe, permeability was independent of frequency up to ∼ 0.5 MHz, where a relaxation occurs. For higher applied fields, the permeability at low frequencies was a function of applied field. A complex dispersion towards the low field value occurs at ∼ 5 kHz. The different character of these two magnetization mechanisms have been demonstrated by means of complex permeability plots. The low field permeability is associated with reversible domain wall movement and the high field mechanism with irreversible domain wall movement.

Characterization of Joule-heated Co-rich amorphous alloys under applied tensile stress by the inductance spectroscopy method

Abstract The inductance response of stress-annealed (1.55 A for 5 min, with tensile stress in the 300–615 MPa range) amorphous ribbons of Vitrovac ® 6025 was measured by applying an AC magnetic field (5 Hz—13 MHz) in the longitudinal and transverse axis. Results can be interpreted in terms of a combination of axial domains isolated and surrounded by a fine system of transverse domains.

Domain wall relaxation in Co-rich amorphous wires

Abstract The complex permeability in low-magnetostriction, as-cast CoFeBSi amorphous wires has been measured in the frequency range 5 Hz-13 MHz, and the applied field range 0.5–5.7 A/m. Cole-Cole plots showed characteristics associated with domain wall relaxation processes; wall relaxation frequencies exhibited an inverse behavior compared with the permeability. These results are interpreted in terms of a complex, multidomain structure in the core of the wire.

Curie-Weiss behavior in polycrystalline barium titanate from ac measurements

Abstract The dielectric constant of undoped, polycrystalline barium titanate in the paraelectric phase was obtained from the complex impedance plane representation in the 180–400°C temperature range, between 5 Hz and 13 MHz. A Curie-Weiss behavior is observed, with a Curie constant C o = 10.48 × 104 (K). These results are discussed and compared with results obtained with fixed frequency measurements.