Structure, magnetic and magnetostrictive properties of as-deposited Fe–Ga thin films

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Abstract

Polycrystalline Fe100−xGax (19⩽x⩽23) films were grown on Si(1 0 0) substrates at different partial pressures of sputtering gas ranging from 3 to 7 μbar. Microstructural, magnetic and magnetostrictive properties were studied using X-ray diffraction (XRD), atomic force microscopy (AFM), energy dispersive spectroscopy (EDS) and magneto-optic Kerr effect (MOKE) magnetometry respectively. X-ray diffraction showed that all films have the body-centered cubic (bcc) Fe–Ga phase with the 〈1 1 0〉 direction out of the film plane. Magnetic characterization of the films showed that the films prepared at 3 μbar had weak uniaxial anisotropy whereas films grown at Ar pressures in the range 4–7 μbar were magnetically isotropic. The effective saturation magnetostriction constants (λeff) of the films were measured using the Villari effect. It was found that effective saturation magnetostriction constants were almost constant over the Ga composition range achieved by varying the sputtering pressure. The measured effective magnetostriction constants fit closely to the calculated saturation magnetostriction constants of 〈1 1 0〉 textured polycrystalline films with the 〈1 1 0〉 directions slightly canted with respect to the normal to the sample surface. It was found that a high pressure of the sputtering gas effected the magnetic softness of the films. The saturation field increased and remanence ratio decreased with increase in pressure.

Introduction

It is well known that magnetostrictive materials have many applications in sensors, actuators and energy harvesting devices [1], [2]. The desired characteristics for magnetostrictive materials are high Curie temperature, low coercivity, low saturation field, wide operating temperature range and high saturation magnetostriction constant. Among the available magnetostrictive materials, Fe–Ga has gained significant interest over other magnetostrictive materials such as Terfenol-D, due to a good combination of the required characteristics. Terfenol-D (Tb1−xDyxFe2) is brittle and cannot be used in a tensile or shear stress environment. It has a single-crystal magnetostriction constant up to 1200 ppm (1200×10−6), but has limited use due to a low Curie temperature, large coercivity (Hc), high saturation fields and a narrow operating temperature range. Terfenol-D is also expensive due to the high cost of Tb and Dy. Fe–Ga alloys have a more modest single-crystal magnetostriction constant (∼400 ppm) and are ductile, and meet all the required properties and can withstand tensile or shear stresses. Magnetostriction in bulk Fe–Ga was first discussed by Clark et al. [3] and has been studied by several groups [4], [5], [6], [7], [8], [9], [10], [11], [12]. All of these studies were made with bulk Fe–Ga alloys prepared either in single-crystal or polycrystalline form. With relatively low Hc these alloys can exhibit large eddy current losses in bulk form. Eddy current losses can be negligible by fabricating Fe–Ga alloys into nanoscale thin films [1], [13].

For technological applications, magnetostrictive thin films are of current interest for use in micro- and nano-electromechanical systems (MEMS and NEMS) and in particular for integrated magnetostrictive devices (MagMEMS) [14], [15]. Integrated magnetostrictive devices require a thin film of magnetoelastic material with high magnetostriction constant and low saturation field. Growth mechanisms of Fe–Ga alloys into thin films with optimised parameters that show high magnetostriction with low saturation field and low coercivity are not yet well established. Butera et al. [16], [17] prepared epitaxial Fe81Ga19 thin films on MgO(1 0 0) substrates by DC sputtering from a target with a fixed composition of Fe81Ga19. They used this target because at 19 at% Ga the maximum magnetostriction (up to 400 ppm) was reported in a quenched bulk Fe–Ga single crystal by Clark et al. [18]. They prepared thin films of thickness 90 nm under an Ar pressure of 3 mTorr (≈4 μbar) and studied angular dependence of the remanence and in-plane angular variation of the ferromagnetic resonance (FMR) field on epitaxial films for different sputtered powers. They observed cubic symmetry for low powers. They suggested that low power must be used to avoid the growth of a non-cubic magnetic phase. Basantkumar et al. [19] prepared Fe–Ga thin films on both glass and tungsten substrates using the RF sputtering technique. They used an Fe–Ga target with fixed composition (18.4 at% Ga) and sputtered at different powers and pressures to achieve films with different Ga composition and thickness. They studied the magnetostrictive properties, and reported a maximum magnetostriction of 147 ppm for a 146.5 nm Fe76.2Ga23.8 film. Finally, they integrated the films with an MEMS cantilever based on Si. Wang et al. [20] studied the structural and magnetostrictive properties of Fe81Ga19 thin films prepared by sputtering a target of composition Fe81Ga19. They fabricated films of 660 nm thickness on Si(1 0 0) substrates under Ar pressure of 1.2 Pa (12 μbar) and reported a magnetostriction constant approaching 50 ppm in as-deposited films. Dunlap et al. [21] prepared Fe100−xGax thin films over a wide composition range (0<x<36) of thickness around 800 nm using a combinatorial method by co-sputtering targets of both Fe and Fe50Ga50 on a Si(1 0 0) substrate. They characterized the films using X-ray diffraction (XRD) and Mössbauer spectroscopy but did not study the magnetostrictive properties over the composition range of interest.

The reported work so far on Fe–Ga thin films shows that changes in deposition parameters or film thickness may affect the net properties. Systematic investigation of both microstructural and magnetic properties and their correlation with deposition parameters is still needed. In general, films deposited at different Ar pressure, pAr, can have either compressive or tensile stress depending on the sputtering pressure pAr and substrate target distance d [22]. There may also be compositional variation with pAr for alloy films. For control of stress in sputtered deposited films, several approaches can be adopted; for example, choice of sputtering power, substrate–target distance, chamber geometry, substrate temperature and control of sputtering pressure. In our previous work, we reported [23] the effect of forming field on structure, magnetic and magnetostrictive properties of Fe–Ga films. In this work, microstructural, magnetic and magnetostrictive properties of Fe100−xGax films fabricated by varying the pressure of the sputtering gas were studied.

Section snippets

Experimental detail

Fe100−xGax (19⩽x⩽23) films of thickness 50±0.15 nm were grown on Si(1 0 0) substrates using a co-sputtering and evaporation technique described by us elsewhere [23]. A 5 cm diameter Fe (99.99%) target was DC sputtered with sputter power (PFe) 20 W, while Ga was simultaneously evaporated from a separate source at a fixed rate, RGa. The Ar pressure (pAr) was varied from 3 to 7 μbar. A 4×1 cm2 strip was cut from a Si(1 0 0) wafer, and before film deposition, the substrate was cleaned with acetone and IMS

Results and discussion

Fig. 1 shows the change in the at% Ga as a function of pAr. The at% Ga in the film decreases as pAr increases, and is effectively constant above pAr=5 μbar. As the mean free path (l) is inversely proportional to pressure (l∝1/pAr) of the sputtering gas, l decreases with an increase in pAr. The Ga atoms will be increasingly thermalized due to in-elastic collisions with the sputtering gas (Ar) during transportation towards the substrate as pAr is increased. We have only achieved a narrow at% Ga

Conclusions

The main conclusions of this study can be summarized as follows:

X-ray diffraction analysis shows that the as-deposited polycrystalline Fe–Ga films had 〈1 1 0〉 crystallographic texture normal to the film plane. Only a single-phase bcc (1 1 0) reflection peak was observed in the XRD patterns. Superlattice peaks were not observed in the XRD patterns for the DO3 structure. Lattice parameter was found to be increased with increase in at% Ga. MOKE measurements showed that low pAr films (3 μbar) had weak

Acknowledgements

This work was supported by EPSRC through Grant no. EP/D022509/1. One of the authors (A. Javed) is grateful to Higher Education Commission (HEC) Government of Pakistan and University of the Punjab, Lahore, Pakistan for awarding doctoral fellowship through Faculty Development Programme (FDP) and granting study leave.

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