Giant magnetoresistive coatings using thermionic vacuumarc technology

A. Anghel, C. P. Lungu, I. Mustata, V. Zaroschi, A.M. Lungu,I. Barbu, M. Badulescu, O. Pompilian

National Institute for Lasers, Plasma and Radiation Physics,P.O.Box, MG–36, 77125, Magurele–Bucharest, Romania

G. Schinteie, D. Predoi, V. Kunkser, G. Filoti

National Institute for Materials Physics,P.O. Box MG 7,77125, Magurele–Bucharest, Romania

N. Apetroaei

Al. I. Cuza University, Iasi,Blvd. Carol I, Nr. 11, 700506 Iasi, Romania

Received 25 May 2006

Giant magnetoresistive (GMR) coatings on silicon, glass and Kapton were prepared bysimultaneous thermionic vacuum arc (TVA) discharges in high vacuum conditions. Themagnetic metal or alloy (Fe, Co, Ni, Permalloy) together with the noble non magneticmetal were deposited on substrates having different positions relative to the discharges.Local magnetic interactions and Fe–phase composition were obtained by Mossbauer spec-troscopy whereas the magnetoresistance effects were measured by a dc method usinga four–point configuration with perpendicular to plane magnetic fields up to 0.8 T. Thesurface morphology of the coatings was characterized by atomic force microscopy (AFM)in contact mode.

PACS : 52.77.-J

Key words: thermionic vacuum arc, giant magnetoresistance, Mossbauer spectroscopy,atomic force microscopy

1 Introduction

The granular metallic thin films consisting of single domain ferromagnetic clus-ters embedded in a non–magnetic metallic matrix and presenting giant magnetore-sistance (GMR), acquired in the last years a great interest, in challenge to thetypical multilayered systems [1]. From the point of view of practical applications,granular systems are very convenient because they are relatively easy to produce,present a good thermal stability and exhibit magnetoresistance effects comparableor even larger than of multilayered in the usual current in plane (CIP) geometry[2]. The GMR effect is very sensitive to preparation parameters and theoreticalestimations suggest that it is intimately related to the density and size distributionof the magnetic clusters. Indeed, when the cluster density is low, their magneticbehavior is super paramagnetic like [3], and the GMR effect increases when the

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cluster number increases up to the so named percolation threshold when the re-ciprocal interaction of magnetic domains leads to GMR decrease. Also the clusterdimensions are of great importance because the responsibility for the appearance ofthe GMR effect is supposed to be the spin dependent scattering of the conductionelectrons at the cluster–nonmagnetic metal interfaces and – to a less extend – withinthe ferromagnetic granules. Therefore when the cluster surface to volume rate isoptimum the GMR effect is expected to be maximum. For a certain concentrationof the magnetic metal the cluster dimensions can be changed by special thermaltreatment after the deposition or, when possible, during the film formation.Usingsimple arguments about a random distribution of noninteracting magnetic mo-ments of the same magnitude, it can be proved that the magnetoresistance mustdisplay a quadratic dependence on the relative magnetization. However, there arenumerous reports that for many practical systems such a law is not fulfilled inthe range of medium and high applied magnetic fields. In fact the experimentaldata point to the complexity of this phenomenon in the real systems and showthat the origin of the spin–dependent scattering mechanism remains still uncertainin many aspects [4].

2 TVA method

For preparation of granular nanostructured GMR films, an original technology,which is placed between electron beam evaporation and electrical vacuum arc dis-charge, known as thermionic vacuum arc (TVA) technology was used [5–8]. Themethod uses an electron beam emitted by an externally heated cathode (a tung-sten grounded filament) accelerated by a high anodic voltage. The electron beamcan evaporate the anode materials as neutral pure particles and facilitate their de-position on the substrate when the electron energy and current intensity are nottoo high. When the anode potential is increased up to a certain value, the evap-oration rate increases as much as to allow an electrical discharge to be ignited inthe evaporated pure material and the discharge is maintained even when the dis-charge current is as low as a few hundreds mA. By using the TVA technology, themetal deposition takes place in high or ultrahigh vacuum conditions, without thepresence of any gas. This method allows, as can be seen in Fig. 1, the simultaneousdeposition of granular films with different Fe–Cu concentrations and different sizeof the magnetic clusters, giving rise to the possibility of a detailed examination ofmicroscopic–macroscopic correlations concerning GMR phenomena. The sampleswere settled at different distances from the Fe anode (dFe) and Cu anode (dCu).The principal characteristics of this technology are the followings:

– The deposition is made in high or ultrahigh vacuum conditions.

– The metal particles are evaporated from the anode by electronic (not ionic asin sputtering cases) bombardment.

– The evaporated atoms are partially ionized in the plasma generated near theanode.

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Fig. 1. Simultaneous deposition by TVA of Fe–Cu films.

– The plasma is expanded in the vacuum space where the substrates are posi-tioned.

– The substrate arrangement allows us to obtain samples with a continuousconcentration variation of the magnetic metal within the nonmagnetic matrix.

As result of these circumstances the deposited films are expected to have specialproperties as:

1. High purity with no gas inclusions.

2. High uniformity of granule dimensional distribution.

3. A fine, dense, well structured matrix.

4. High GMR effect even for as deposited films.

5. A continuous variation of magnetic metal density from one to another sampledepending of their relative distance to the corresponding plasma source.

The following presented data will support the above made suppositions.

3 Results and discussion

The Fe–Cu, Ni–Cu, and Co–Cu films were prepared in a TVA system(2 × 10−4 Pa base pressure) by simultaneous deposition of Fe and Cu from twoanodes and two TVA guns. The metal vapor “pressure” during the deposition pro-cess was about 50 Pa (near the anode) decreasing towards 10−4 Pa (near the sample

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position). The evaporation rate of 10 ± 1 A/s was stabilized and controlled withan accuracy of 10 percent. In situ thickness measurements were performed using a“Cressington” thickness–meter.

All films were deposited onto Kapton, Si and glass substrates with no specialheating during the process. Before deposition, the substrates were ultrasonicallycleaned in acetone and alcohol and dried in hot air. A series of four Fe–Cu thinfilms (250 ± 1 nm thick) with different Fe relative content were obtained on bothKapton and Si substrates. The samples were positioned at the following distancesfrom the Fe/Cu anodes, respectively: 28 cm/34 cm, 30 cm/32 cm, 32 cm/30 cm and34 cm/28 cm. Samples labeled by Pr1, Pr2, Pr3 and Pr4 are ordered in a decreasingFe relative content. Local magnetic interactions and Fe–phase composition wereobtained by Mossbauer spectroscopy whereas the magnetoresistance effects weremeasured by a dc method using a four–point configuration with perpendicular toplane magnetic fields up to 0.8 T. Typical dependence of the magnetoresistanceratio MR(B) = R(B)−R(0)

R(0) vs. the applied magnetic field, B, was evidenced, as canbe seen for sample Pr3 in Fig. 2. Effects of order of percents and saturating fields ofabout 0.2 T were obtained at room temperature, strongly related to sample com-position. The room temperature Mossbauer spectra shown in Fig. 3 prove directly

Fig. 2. The dependence of the GMR effect for the Fe–Cu sample Pr3.

differences in phase composition and local magnetic interactions for the analyzedfilms. The spectra evidenced mainly two iron phases, namely the bcc α−Fe and irondispersed in the fcc Cu matrix. The relative content of the α−Fe phase decreasesin samples with higher Cu content. It is worth mentioning that the ferromagneticα − Fe phase, represented by a magnetic sextet in samples with low Cu content(e.g. samples Pr1 and Pr2) becomes superparamagnetic in samples with higher Cucontent (being represented by a singlet with small positive isomer shifts, in samplesPr3, Pr4). To continue with the Fe–Cu composition we present in Fig. 4 the CIPlike measurements for GMR effect for as deposited sample and for the same sampleafter a post deposition thermal treatment of 2 hours and 30 minutes at 410 ◦C. In

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Fig. 3. Room temperature Mossbauer spectra of the Fe–Cu analyzed samples.

this and following cases the thicknesses of the films were 120 nm, therefore smallerthan in the case of the sample presented in Fig. 2. One can see the beneficial influ-ence of the thermal treatment. Given on the figure are the distances of the sampleto the corresponding anode. For comparison we present in Fig. 5 the GMR effectmeasured for a Co–Cu granular sample. There the same GMR improving effect bypost thermal sample treatment is observed but the magnitude of the effect is greaterthan in case of Fe–Cu composition. An even smaller GMR effect appears in the caseof Ni–Cu combination as can be seen in Fig. 6. But in this case (as in the Fe–Cucase presented in Fig. 3) we can see a very special kind of quadratic dependenceof the GMR effect. While for Fe–Cu the GMR curve is higher and narrower, forNi–Cu case it is larger and higher. This kind of dependence can suggest a narrowdimensional distribution of the magnetic cluster, because until a certain magneticfield value they remain as in the field absence and when this value is over–passedthey are all oriented and the resistance decreases very rapidly. Further on the re-sistance value remains constant up to the maximum magnetic induction value usedby us (0.8 T). This kind of dependence can be useful because it furnish a great sig-nal only for a certain magnetic inductance value. The future studies by electronic

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Fig. 4. The GMR effect of the Fe–Cu Sample No. 5 (dFe = 32.5 cm, dCu = 38.5 cm).

Fig. 5. CoCu GMR effect (dCo = 41.5 cm, dCu = 34 cm).

microscopy will allow us to evaluate the precise condition of such phenomena toappear.

In Fig. 7 we present an AFM image of the Co–Cu film which indicates that the

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Fig. 6. The GMR effect of the NiCu film (dNi = 32.9 cm, dCu = 38.2 cm).

cluster dimensions of about 50 nm are very near to one another. In contrast, in thecase of Co–Cu compound the GMR dependence is more parabolic [7] suggestingthe existence of clusters presenting different dimensions which are oriented alongthe applied magnetic field for different induction values. The resistance decrease isless abrupt as it is seen from Fig. 5.

Fig. 7. AFM image of the Co–Cu film.

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4 Conclusion

The results provide evidence for a composite thin films consisting of α − Fenanoparticles embedded in the Cu matrix. The size of the Fe nanoparticles and theirdispersion, related also to the purity of the Cu matrix, are analyzed starting fromthe Mossbauer spectra and finally correlated with the magnetoresistance effects andAFM measurements.

The greatest GMR effect appear in the case of Co–Cu structure and for Fe–Cuand Ni–Cu cases, very special kind (quadratic) of GMR dependence appear. Thatcould suggest a very narrow distribution of the cluster dimensions within the copperstructure. This can be explained by the special conditions of deposition in TVAtechnology, already mentioned in the introductory chapter.

References

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