Influence of the order-disorder transition on the magnetic properties of Fe₇₅Al_25-xSi_x alloys

Highlights

• Fe₇₅Al_25-xSi_x alloys were completely disordered by ball milling technique.

• Si addition makes more difficult to disorder Fe₇₅Al₂₅ alloy.

• There is an opposite competition between Si and Al in the magnetism of Fe.

• There is no influence of structural parameters in the magnetism of the alloys.

Abstract

The influence of the different crystal structures and the variation of the lattice parameter on the evolution of the magnetism in the order-disorder transition produced by crushing and mechanical milling in the intermetallic Fe₇₅Al_25-xSi_x alloys (x = 7.5, 12.5, 17.5, 25) has been systematically studied by means of XRD measurements, Mössbauer spectroscopy and magnetic measurements. The results indicate that with the addition of Si to binary Fe₇₅Al₂₅ alloy the mechanical deformation needed to disorder the alloys increases. At the same time the variation of the lattice parameter due to the disorder is reduced as Si is added. The magnetic measurements indicate that there is a complex behaviour in ternary alloys with an opposite influence of Si and Al during the order-disorder transition. However, when the transition is fulfilled there is a linear relationship between structural and magnetic parameters.

Introduction

The interest in iron-rich intermetallic compounds is based on a wide range of potential applications including high-temperature structural materials, functional materials for magnetic applications, diffusion barriers, contacts, and interconnections in microelectronics [1]. Fe aluminides and Fe silicides are considered for structural applications because they unite high strength with excellent corrosion resistance in oxidizing and sulfiding environment [2]. The Fe-rich compounds Fe₃Al and Fe₃Si possess a very high magnetic susceptibility which makes them useful as soft magnetic materials [3]. The relationship between structural and magnetic properties in Fe–Al and Fe–Si alloys has been extensively studied during the last years [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14].

The mechanical and magnetic properties of these binary alloys can be improved by mixing Al and Si. For example, an almost 2:3 mixture of these two alloys, called Sendust, has been used as magnetic head material in recorders [15]. Nowadays, FeSiAl alloys have attracted much interest due to their high permeability [16]. Some researchers prepared flaky FeSiAl metallic powders and investigated the relationship between the crystal structure and the microwave properties [17], [18], [19]. The novel powders have a good anti-EMI (Electromagnetic Interference) characteristic in communication and computer systems [20], [21].

The effect of Si introduction in the magnetic properties of stoichiometric Fe₃Al alloy is twofold; on the one hand it contributes to the decrease of the lattice parameter which makes the magnetism weaker and, on the other hand, the introduction of an additional electron makes the hybridisation to increase and therefore the magnetic moment decreases [22]. However, as far as the structure is concerned the Fe-rich side of the phase diagram of the Fe–Al, Fe–Si and Fe–Al–Si systems are quite similar [23], [24]. Indeed, for a 75 at% Fe, the stable structure at room temperature is the D0₃ structure, which is stoichiometric for this Fe content.

Mechanical deformation is known to induce A2 disordered structure in the three systems [25], [26]. Moreover, in Fe–Al alloys there is a relationship between the enhancement of the magnetization and the structural order-disorder transition. This relationship has been explained in terms of the change of the local environment of Fe atoms, where the Fe magnetic moment depends on the number of Al atoms as nearest neighbours decreases [27], [28]. Furthermore, mechanical deformation causes an increase in volume [29], [30] and the influence of this increase on the magnetic moment has been proved both theoretically and experimentally in a few studies [31], [32], [33].

The relationship between the crystal structure and magnetic properties of Fe₇₅Al_25.xSi_x alloys prepared by different non-equilibrium techniques, as mechanical alloying [34], [35], and melt spinning [17], [36], has been reported. However, we are not aware of a systematic investigation of the influence of the order-disorder transition on the magnetic properties of Fe₇₅Al_25-xSi_x, and the relationship between the variation of the lattice parameter and the magnetic properties in the D0₃ range of the phase diagram. The present investigation, therefore, fills in this gap.