Gordon E. Fish

Soft magnetic materials

The state of art is reviewed with an emphasis on recent research results and a view to identifying areas in which further developments in materials and processing might lead to even better properties and greater application of novel technology. Basis magnetic considerations are discussed, namely, B-H loop shape, core loss, magnetic anisotropy and annealing, and magnetostriction and stress effects. Materials and applications for power frequency devices are examined, covering core loss considerations, silicon steel development, metallic glasses, and high silicon materials. High-frequency and pulse applications, magnetic recording heads, and sensor and transducer applications are also discussed. Basic research questions and future directions with respect to core loss, magnetization, and stability are examined. >

Effects of crystalline precipitates on the soft magnetic properties of metallic glasses

Effects of crystalline precipitates, nucleating at temperatures slightly below the crystallization temperatures, on the soft magnetic properties are reported for some selected metallic glasses. Emphasis is placed on the effects due to the difference of the crystalline symmetry of the precipitates. The glassy alloys selected for the present study are Fe85B15, Fe77B23, and Fe60Ni25B15, which tend to precipitate bcc‐Fe, bct‐Fe3B, and fcc Fe‐Ni crystalline particles, respectively. It is concluded that the differences in structure of the precipitates are more important than their morphology in determining the ac properties of the materials at frequencies greater than 50 kHz.

Frequency dependence of core loss in rapidly quenched Fe‐6.5 wt. %Si

The frequency, field, and temperature dependencies of core loss in rapidly quenched Fe‐6.5 wt. %Si ribbons are reported. The annealed material exhibits (001)〈uv0〉 fiber texture, resulting in isotropic properties in plane; equivalent losses and B‐H loops are seen in tape‐wound toroids and in annuli punched from ribbon. Hysteresis losses predominate in the present Fe‐6.5%Si up to about 1 kHz, as the thin gauge and high resistivity reduce eddy current losses from those of conventional nonoriented steels. The 2‐kHz/1.0‐T core losses are a factor of 6–8 higher in conventional M‐15 than in Fe‐6.5%Si. The core losses of Fe‐6.5%Si are about 40% higher at 77 than at 373 K. The loss data are not in agreement with the predictions of Bertotti’s theory.

On the nature of the remaining amorphous matrix after nanocrystallization of Fe77Si14B9 with Cu and Nb addition

Abstract FeSiBNbCu amorphous alloys usually crystallize in two stages. The first crystallization stage for near-eutectic and hypoeutectic compositions corresponds to the nucleation and growth of α-Fe(Si) nanocrystals. The surrounding amorphous matrix in turn crystallizes in the second stage which occurs at high temperatures. We have measured the variations in the Curie temperature Tc and crystallization temperature Tx with composition of the remaining amorphous matrix after α-Fe(Si) nanocrystallization and found that the remaining amorphous matrix contains only 3–4 at.% Nb. Mass balance considerations then impose the conclusion that, contrary to some previous reports, the α-Fe(Si) nanocrystals still contain at least 1.5 at.% Nb. This conclusion is shown to be consistent with the lattice parameter and magnetic properties of the α-Fe(Si) nanocrystals.

Spin waves in amorphous Fe1−xBx alloys

The temperature dependence of spin excitations has been studied in amorphous Fe1−xBx (x = 0.18 and 0.14) by inelastic neutron scattering. The spin‐wave stiffness constant D was determined directly from the magnon dispersion curves (E = Dq2) over the temperature range from below room temperature (T/TC = 0.48 and 0.36, respectively, for x = 0.18 and 0.14 alloys) up to 548 K (T/TC = 0.75 and 0.99), which is just below the crystallization temperature. For both alloys the temperature dependence of D was found to be proportional to (T/TC)5/2 up to (T/TC) greater than 0.8. The extrapolated values of D at T = 0 were D = 167 and 138 meV A2, respectively, and were nearly twice as large as those determined from the T3/2 coefficient found from magnetization studies. These anomalies may be related to the Invar characteristics of the thermal expansion in these alloys. Limited data were also obtained for an alloy Fe76B24 which exhibits a higher D (≳174 meV A2).

Polarization analysis of the magnetic excitations in Invar and non‐Invar amorphous alloys

Conventional spin wave theory works remarkably well in describing the spin dynamics of both Invar and non‐Invar isotropic ferromagnets, with the important exception that for Invar systems the magnetization decreases much more rapidly with temperature than can be explained based on the measured spin wave dispersion relations. We have been carrying out triple‐axis polarized inelastic neutron scattering experiments on the amorphous ferromagnets Fe86B14 (Invar system) and Fe40Ni40P14B6 (METGLAS■ 2826) in order to separate the longitudinal magnetic fluctuations from the transverse (spin wave) excitations, and thereby determine if the presence of longitudinal excitations might resolve this discrepancy. The present measurements exhibit longitudinal excitations below Tc, but in both materials. Possible interpretations of the results are discussed.

Spin dynamics of amorphous Fe90−xNixZr10

Neutron inelastic scattering experiments have been performed in order to study the long wavelength spin dynamics of the amorphous Invar system Fe90−xNixZr10 (for x=5,10). Spin waves were observed over the entire range of wave vectors (0.05–0.15 A−1) and temperatures (0.3–0.9 TC) under study. The spin‐wave energies are well described by the quadratic dispersion relation Eq=Dq2+Δ, where Δ is a small gap due primarily to dipolar interactions. The stiffness parameter D renormalizes with temperature as D(T)=D(0) ×[1−A(T/TC)5/2] throughout the temperature range under study. The values of D(0) corresponding to x=5, 10, are 42.1 and 78.0 meV A2, respectively, with A=0.83. In the temperature range 0.55–0.90 TC the intrinsic spin‐wave linewidths Γq can be fitted equally well to the q4 dependence predicted by hydrodynamic spin‐wave theory, or to the q5 dependence predicted from the topological disorder in amorphous systems, but they did not show the T2 behavior expected from magnon interactions. These findings sugge...

Spin dynamics of the amorphous Invar alloy Fe0.86B0.14

High‐resolution neutron scattering studies have been made of the long wavelength spin excitations in a ribbon sample of amorphous Fe0.86B0.14, which exhibits Invar properties. Spin waves were observed in the wave vector range 0.05 A−1≤q≤0.12 A−1 at temperatures between 300 K (0.54 Tc) and 500 K (0.90 Tc). The spin wave energies are well described by a dispersion relation E=Dq2+Δ. The small energy gap Δ of ≊0.04 meV is attributed primarily to the dipole‐dipole interaction. The stiffness constant renormalizes with temperature as D=D(0)[1.0−0.86 (T/Tc) 5/2] in the range of temperatures under study, with D(0)=132 meV A2. This value of D(0) is approximately twice as large as that calculated from the T3/2 coefficient of the magnetization, a discrepancy common to many Invar materials. Plots of the intrinsic linewidths against q4 and T2 reveal that the data are consistent with the Γ∝q4[Tln(kT/E)]2 dependence predicted for a Heisenberg ferromagnet. There are no anomalies in the spin‐wave lifetimes at long waveleng...

Spin dynamics of amorphous Fe90−xNixZr10 (invited)

Amorphous Fe90−x Ni x Zr10 is a system that exhibits a relatively high degree of magnetic exchange frustration, which becomes stronger as the system approaches the composition of amorphous pure iron. Thus, while samples with moderate amounts of iron (x≥5) are ferromagnetic, the samples with the highest concentration of iron (x≤1) behave like reentrant spin glasses. We have performed a detailed neutron scattering study of the spin‐wave excitations in this system for x=1, 5, 10, 20. In all cases, well‐defined spin‐wave excitations were observed below a transition temperature T c that decreased from 455 K (for x=20) to 250 K (for x=1). For x=5, 10, 20 the spin‐wave stiffness coefficient follows the temperature dependence expected for a conventional ferromagnet but the spin‐wave excitations broaden considerably at low temperatures. For x=1 the spin‐wave stiffness coefficient softens at low temperatures and an elastic component of the scattering, associated with the development of a spin‐glass order parameter, appears below T≊0.28T c . A coexistence of propagating spin‐wave excitations and spin freezing phenomena is observed below this temperature down to T=0.09T c . These results are discussed in terms of the relevant current theories.

Research and development opportunities for rapidly solidified soft magnetic materials

Abstract The state-of-the-art in rapidly solidified soft magnetic materials is reviewed with a view to identifying areas in which further research and development in materials and processing might lead to greater application of novel technology. The prime examples of these materials are metallic glasses. The substitution of them for conventional materials has been demonstrated to reduce core loss in a wide variety of devices operating from d.c. to several MHz. Fundamental research is especially needed in glass formation and stability and in mechanisms of core loss. For line frequency applications, improved production and manufacturing engineering offer the most immediate prospects. Better alloys, high quality thin ribbons, and surface coatings would benefit high frequency applications such as switch-mode power supplies and pulse compression systems. Sensor applications are presently limited by available techniques for interfacing the sensing element to its surroundings. Thin films with properties comparable with those of bulk metallic glasses would be useful for sensors and would extend the frequency range for inductive devices by reducing eddy current losses.