Micro-structure and magnetic properties of Fe–Cu nanocomposites for anisotropic permanent magnets

https://doi.org/10.1016/j.jallcom.2004.11.059Get rights and content

Abstract

Melt spun ribbons of FexCu100−x were obtained by different procedures, in order to maximize the dispersion of the Fe atoms in the Cu matrix. Subsequent thermal and mechanical treatments were used for a controlled crystallization process of the soft magnetic phase. A detailed analysis of structural aspects and crystallization dynamics and their relation with the magnetic behavior of the Fe–Cu ribbons has been obtained via Mössbauer spectroscopy, X-ray diffraction (XRD) and magnetic measurements. In order to obtain composite permanent magnets with shape anisotropy, the optimal conditions and the suitable processing were established.

Introduction

The Rare-Earth (RE) based intermetallics, rich in 3d transition elements, represent to date highest performance magnets. There are complex systems, involving exchange spring phenomena, which also exhibit as a main constituent a RE containing phase. Intense efforts are presently done in the search for cheaper RE-free permanent magnets with similar or even improved magnetic properties. Enhanced energy products can be reached by increasing both the coercivity and the remanent magnetization, the RE ions being responsible for the strong intrinsic anisotropy. Therefore, in RE-free permanent magnets, alternative effects should induce a preferred orientation of the magnetic moments along an easy axis. In particular, the shape anisotropy effects of the ferromagnetic aggregates in Fe–Cu systems might be exploited.

Fe-wires with reduced diameters (down to 30 nm) in plastically deformed copper matrix were obtained by Levi [1] in 1960. He reported that after inserting Fe-wires in a Cu-matrix and a subsequent drawing, this system has shown an intrinsic coercivity of about 400 Oe for composites containing iron wires with diameters smaller than 50 nm. In 1971, by lamination of a cast Fe–Cu compound, Kawabuchi and Ogawa [2] reported similar magnetic results. However, such magnets were not attractive mainly due to high costs compared with performances.

The last two decades brought various alternative procedures to obtain a larger range of new materials by promoting a higher miscibility of different metals. The FexCu100−x phase diagram obtained by classical metallurgy methods shows for 4 < x < 87 a solid state immiscibility. Therefore, to overstep the mentioned limits, unconventional methods such as rapidly quenching techniques [3] or mechanical attrition [4], [5] were used. By these procedures, mostly FexCu100−x nanocrystallised systems can be obtained. Interesting properties are related to interface mechanisms, induced by temperature or external mechanical effects, at the large surface area offered by the nanograins. For example, Hernando et al. reported on peculiar magnetic properties of granular Fe–Cu systems obtained by melt spinning (see for example [6]). A precipitated α-Fe (body-centered-cube, bcc) phase, with different grain shapes and sizes, was obtained in a Cu (face-centered-cube, fcc) matrix for various FexCu100−x compositions (x = 20, 50, 80). These heterogeneous nanosystems indicated magnetization different from the values provided by the usual spherical α-Fe grains in Cu. It is reported that the anisotropy field, HK, is higher than that of Fe (HK  700 Oe), reaching for the Fe precipitates with a length/diameter ratio of 4.4, values of about 8600 Oe at room temperature. Thus, other anisotropy contributions (e.g., shape anisotropy) must be taken into account in order to explain the increasing of the overall anisotropy.

We re-iterate the idea that permanent magnets based on shape anisotropy could be obtained in a controlled way by the segregation of acicular α-Fe aggregates in a Cu matrix, following suitable thermal annealing in the presence of an external magnetic field. Therefore, initial Fe–Cu solid solutions or nanocomposites with highly dispersed Fe atoms in the Cu matrix are required. The full structural characterization of the “as prepared” alloys and the detailed study of the subsequent crystallization process are very important aspects, by providing valuable information concerning the processing of the granular permanent magnets implying shape anisotropy. This work reports on the Fe phase distribution in Fe–Cu melt spun ribbons, as precursors for shape anisotropy magnets. The data are acquired before and after different thermal or mechanical treatments. The main results, derived from Mössbauer spectroscopy, are corroborated with X-ray diffraction (XRD) and magnetic measurements. The effect of the applied magnetic field on the crystallization process will be reported in a subsequent paper.

Section snippets

Experimental details

The FexCu100−x alloys with x = 10, 20, 50 and 80 (samples S0, S1, S2 and S3) were prepared by the melt spinning techniques starting from technical pure iron and electrolytic Cu. A wheel velocity of 30 m/s was used for all preparations. For the composition with x = 20, different Ar overpressures ranging between 40 and 60 kPa were used in order to maximize the Fe dispersion inside the Cu matrix. The pressure of 40 kPa was selected as optimal. Ribbons from the corresponding sample (S1′) were subsequently

Results and discussions

Some relevant XRD patterns of the analyzed samples are presented in Fig. 1. The sample S3, representative for the Fe rich compositional range of the Fe–Cu phase diagram, shows only diffraction peaks typical to the body-centered-cube (bcc) structure of the α-Fe phase. There are two aspects concerning the equi-atomic composition (sample S2). Firstly, new peaks assigned to the Cu face-centered-cube (fcc) structure have appeared. Then, it has to be mentioned that the peaks belonging to the bcc

Conclusions

The corroboration of various data (Mössbauer, XRD, magnetic measurements) emphasizes several aspects related to the melt spun Fe–Cu composites. The XRD evidence only two definite structures: a bcc one assigned to α-Fe and a fcc one assigned to the impure Cu matrix, their relative amounts depending on the Fe content. The Mössbauer investigations point to three different iron configurations assigned to: bcc α-Fe phase, small fcc-Fe clusters of 4–5 atoms in the Cu matrix and larger (tenths of

Acknowledgement

The financial support through the Romanian national program “Matnantech”, Project no. 134/2003 is highly acknowledged.

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