https://www.jstage.jst.go.jp/article/matertrans1989/31/8/31_8_743/_pdf

High Saturation Magnetization and Soft Magnetic Properties of bcc Fe–Zr–B Alloys with Ultrafine Grain Structure

High Saturation Magnetization and Soft Magnetic Properties of bcc Fe–Zr–B and Fe–Zr–B–M (M=Transition Metal) Alloys with Nanoscale Grain Size

This paper reviews our results on the development of a new type of soft magnetic material with high saturation magnetic flux density (B s ) combined with excellent soft magnetic properties. A mostly single bcc structure composed of bcc grains with about 10-20 nm in size surrounded by a small amount of intergranular amorphous layers was obtained by crystallization of amorphous phases prepared by melt-spinning and sputtering technique in Fe-rich regions of Fe-M-B (M=Zr, Nb, Hf) ternary systems. The typical nanocrystalline bcc Fe 90 Zr 7 B 3 , Fe 89 Hf 7 B 4 and Fe 84 Nb 7 B 9 alloys subjected to the optimum annealing exhibit high B s above 1.5 T as well as high effective permeability (μ e ) at 1 kHz above 20000. The high B s for the Fe-M-B alloys is resulting from the high Fe concentrations owing to high glass-forming ability of M(Zr, Hf, Nb) and B. The origin of the good soft magnetic properties for the alloys are listed as follows. (1) The apparent anisotropy is decreased by the combined effects of the formation of the nanoscale bcc structure and the achievement of rather strong magnetic coupling between the bcc grains through the intergranular ferromagnetic amorphous phase. (2) The small saturation magnetostriction (λ s ) results from the nonequilibrium bcc phase. The solute-rich inter-glanular amorphous phase with high Curie temperature (T c ) and high thermal stability has an important role in the achievement of the good soft magnetic properties through the formation of the nanoscale bcc structure and the attainment of the rather strong magnetic coupling between the bcc grains. The soft magnetic properties of the nanocrystalline Fe-M-B alloys were improved through the decrease in the bcc grain size and the increase in T c of the intergranular amorphous phase by optimizing heating rate in the crystallization process and adding small amounts of elements. For example, the improved Fe 84 Zr 3.5 Nb 3.5 N 8 Cu 1 alloy shows the high μ e of 100000 combined with the high B s of 1.53 T. This excellent μ e is comparable to those of nanocrystalline Fe 73.5 Si 13.5 B 9 Nb 3 Cu 1 and the zero-magnetostrictive Co based amorphous alloys, and the high B s is comparable to those of the Fe based amorphous alloys with good soft magnetic properties. The Fe-M-B based alloys also have very low core losses, the sufficient thermal stability and the low stress-sensibility of the soft magnetic properties. Therefore, the nanocrystalline Fe-M-B alloys are expected as practical magnetic materials for magnetic transformers, inductors, and other devices and parts.

Nanocrystalline Soft Magnetic Fe–M–B (M=Zr, Hf, Nb) Alloys Produced by Crystallization of Amorphous Phase (Overview)

It is by now well established that in antiferromagnetic \ensuremath{\gamma}-Fe, stabilized in the form of precipitates in a Cu matrix or by epitaxial growth on an appropriate substrate, magnetic and/or crystalline symmetries are broken. Little is known, however, on the physical effects driving the symmetry reduction, and on the interplay of crystalline and magnetic symmetry breaking. We have used a recently developed unconstrained vector-field description of noncollinear magnetism, implemented in an ab initio spin-density-functional code, to search for the magnetic and crystalline structure of \ensuremath{\gamma}-Fe, stabilized by different types of constraints. We show that in near face-centered-cubic \ensuremath{\gamma}-Fe, stabilized by three-dimensional constraints, the magnetic ground state is a spin-spiral with propagation vector $\stackrel{\ensuremath{\rightarrow}}{q}=2\ensuremath{\pi}/a\ifmmode\times\else\texttimes\fi{}(0.2,0,1)$ at an equilibrium atomic volume of $\ensuremath{\Omega}=10.63{\mathrm{\AA{}}}^{3},$ very close to the propagation vector ${q}_{\mathrm{exp}}=2\ensuremath{\pi}/a\ifmmode\times\else\texttimes\fi{}(0.1,0,1),$ determined experimentally, but at considerably lower volume than the atomic volume of the \ensuremath{\gamma}-Fe precipitates in Cu on which the experiments were performed $(\ensuremath{\Omega}=11.44{\mathrm{\AA{}}}^{3}).$ At these larger volumes our calculations predict an helical spin solution at $\stackrel{\ensuremath{\rightarrow}}{q}=2\ensuremath{\pi}/a\ifmmode\times\else\texttimes\fi{}(0,0,0.6)$ to be the ground state. Epitaxially stabilized \ensuremath{\gamma}-Fe is found to be unstable against both tetragonal distortion as well as monoclinic shear deformation, and the structural distortions suppress the formation of spin-spiral states, in agreement with experimental observations on Fe/Cu(100) films.

Broken symmetries in the crystalline and magnetic structures of γ-iron

The authors present self-consistent spin-polarized electronic structure calculations for realistic models of amorphous transition metal alloys. The atomic structure is prepared by a simulated molecular-dynamic quench, based on interatomic forces calculated using hybridized nearly-free-electron tight-binding-bond theory. The electronic structure is calculated in the local spin-density approximation, using a linear muffin-tin orbital (LMTO) supercell approach. Detailed results for crystalline and amorphous alloys of Ni, Co and Fe with Zr are presented. NixZr1-x alloys are predicted to be paramagnetic for x or=0.75 the competition between ferromagnetic and antiferromagnetic exchange interactions leads to the formation of negative moments on isolated Fe sites. The number of negative Fe moments increases strongly for x>or=0.90, leading to a decrease of the average moment in the Fe-rich limit. The authors show that the formation of negative local Fe moments is related to a large number of contracted Fe-Fe pairs. The predictions of local spin-density theory are found to be in excellent agreement with experiment. For all crystalline and amorphous alloys the local exchange splitting of the 3d states is shown to be correlated linearly with the local magnetic moment, with a slope of approximately=1 eV mu B-1. This relation holds for all types of magnetic order and also for the crystalline and amorphous pure metals.

Local spin-density theory of itinerant magnetism in crystalline and amorphous transition metal alloys

The temperature dependence of the AC susceptibility Fe-rich Fe-Zr amorphous alloys having mictomagnetic or spin-glasslike behaviour has been measured in a superposed DC field. The Fe-Zr amorphous alloys containing more than 8.6 at.% Zr clearly show sharp peaks at the Curie temperature TC and the spin cluster freezing temperature Tsf. These two temperatures seem to coincide at a critical concentration for the Fe-7.6 at.% Zr alloy, which is considered to be the percolation limit for the long-range ferromagnetic interactions.

http://iopscience.iop.org/article/10.1088/0305-4608/16/7/018/pdf

Y Nakagawa

Papers

Mictomagnetism of Fe-rich Fe-Zr amorphous alloys studied by AC susceptibility in a superposed DC field