Giant magnetoresistive coatings using thermionic vacuum arc ...

Giant magnetoresistive coatings using thermionic vacuum arc 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 by simultaneous thermionic vacuum arc (TVA) discharges in high vacuum conditions. The magnetic metal or alloy (Fe, Co, Ni, Permalloy) together with the noble non magnetic metal were deposited on substrates having different positions relative to the discharges. Local magnetic interactions and Fe–phase composition were obtained by M¨ ossbauer spectroscopy whereas the magnetoresistance effects were measured by a dc method using a four–point configuration with perpendicular to plane magnetic fields up to 0.8 T. The surface 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, M¨ ossbauer spectroscopy, atomic force microscopy

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Introduction

The granular metallic thin films consisting of single domain ferromagnetic clusters embedded in a non–magnetic metallic matrix and presenting giant magnetoresistance (GMR), acquired in the last years a great interest, in challenge to the typical 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 comparable or even larger than of multilayered in the usual current in plane (CIP) geometry [2]. The GMR effect is very sensitive to preparation parameters and theoretical estimations suggest that it is intimately related to the density and size distribution of the magnetic clusters. Indeed, when the cluster density is low, their magnetic behavior is super paramagnetic like [3], and the GMR effect increases when the B16

Czechoslovak Journal of Physics, Vol. 56 (2006), Suppl. B

Giant magnetoresistive coatings using thermionic vacuum arc technology

cluster number increases up to the so named percolation threshold when the reciprocal interaction of magnetic domains leads to GMR decrease. Also the cluster dimensions are of great importance because the responsibility for the appearance of the GMR effect is supposed to be the spin dependent scattering of the conduction electrons at the cluster–nonmagnetic metal interfaces and – to a less extend – within the ferromagnetic granules. Therefore when the cluster surface to volume rate is optimum the GMR effect is expected to be maximum. For a certain concentration of the magnetic metal the cluster dimensions can be changed by special thermal treatment after the deposition or, when possible, during the film formation.Using simple arguments about a random distribution of noninteracting magnetic moments of the same magnitude, it can be proved that the magnetoresistance must display a quadratic dependence on the relative magnetization. However, there are numerous reports that for many practical systems such a law is not fulfilled in the range of medium and high applied magnetic fields. In fact the experimental data point to the complexity of this phenomenon in the real systems and show that the origin of the spin–dependent scattering mechanism remains still uncertain in 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 discharge, known as thermionic vacuum arc (TVA) technology was used [5–8]. The method uses an electron beam emitted by an externally heated cathode (a tungsten grounded filament) accelerated by a high anodic voltage. The electron beam can evaporate the anode materials as neutral pure particles and facilitate their deposition on the substrate when the electron energy and current intensity are not too high. When the anode potential is increased up to a certain value, the evaporation rate increases as much as to allow an electrical discharge to be ignited in the evaporated pure material and the discharge is maintained even when the discharge current is as low as a few hundreds mA. By using the TVA technology, the metal deposition takes place in high or ultrahigh vacuum conditions, without the presence of any gas. This method allows, as can be seen in Fig. 1, the simultaneous deposition of granular films with different Fe–Cu concentrations and different size of the magnetic clusters, giving rise to the possibility of a detailed examination of microscopic–macroscopic correlations concerning GMR phenomena. The samples were 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 as in sputtering cases) bombardment. – The evaporated atoms are partially ionized in the plasma generated near the anode. Czech. J. Phys. 56 (2006)

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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 positioned. – The substrate arrangement allows us to obtain samples with a continuous concentration variation of the magnetic metal within the nonmagnetic matrix. As result of these circumstances the deposited films are expected to have special properties 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 sample depending 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 two anodes and two TVA guns. The metal vapor “pressure” during the deposition process was about 50 Pa (near the anode) decreasing towards 10−4 Pa (near the sample B18

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˚/s was stabilized and controlled with position). The evaporation rate of 10 ± 1 A an 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 special heating during the process. Before deposition, the substrates were ultrasonically cleaned in acetone and alcohol and dried in hot air. A series of four Fe–Cu thin films (250 ± 1 nm thick) with different Fe relative content were obtained on both Kapton and Si substrates. The samples were positioned at the following distances from the Fe/Cu anodes, respectively: 28 cm/34 cm, 30 cm/32 cm, 32 cm/30 cm and 34 cm/28 cm. Samples labeled by Pr1, Pr2, Pr3 and Pr4 are ordered in a decreasing Fe relative content. Local magnetic interactions and Fe–phase composition were obtained by Mössbauer spectroscopy whereas the magnetoresistance effects were measured by a dc method using a four–point configuration with perpendicular to plane magnetic fields up to 0.8 T. Typical dependence of the magnetoresistance ratio M R(B) = R(B)−R(0) vs. the applied magnetic field, B, was evidenced, as can R(0) be seen for sample Pr3 in Fig. 2. Effects of order of percents and saturating fields of about 0.2 T were obtained at room temperature, strongly related to sample composition. The room temperature Mössbauer 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 analyzed films. The spectra evidenced mainly two iron phases, namely the bcc α−Fe and iron dispersed in the f cc Cu matrix. The relative content of the α − Fe phase decreases in samples with higher Cu content. It is worth mentioning that the ferromagnetic α − 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 Cu content (being represented by a singlet with small positive isomer shifts, in samples Pr3, Pr4). To continue with the Fe–Cu composition we present in Fig. 4 the CIP like measurements for GMR effect for as deposited sample and for the same sample after a post deposition thermal treatment of 2 hours and 30 minutes at 410 ◦ C. In Czech. J. Phys. 56 (2006)

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Fig. 3. Room temperature M¨ ossbauer spectra of the Fe–Cu analyzed samples.

this and following cases the thicknesses of the films were 120 nm, therefore smaller than in the case of the sample presented in Fig. 2. One can see the beneficial influence of the thermal treatment. Given on the figure are the distances of the sample to the corresponding anode. For comparison we present in Fig. 5 the GMR effect measured for a Co–Cu granular sample. There the same GMR improving effect by post thermal sample treatment is observed but the magnitude of the effect is greater than in case of Fe–Cu composition. An even smaller GMR effect appears in the case of Ni–Cu combination as can be seen in Fig. 6. But in this case (as in the Fe–Cu case presented in Fig. 3) we can see a very special kind of quadratic dependence of the GMR effect. While for Fe–Cu the GMR curve is higher and narrower, for Ni–Cu case it is larger and higher. This kind of dependence can suggest a narrow dimensional distribution of the magnetic cluster, because until a certain magnetic field value they remain as in the field absence and when this value is over–passed they are all oriented and the resistance decreases very rapidly. Further on the resistance value remains constant up to the maximum magnetic induction value used by us (0.8 T). This kind of dependence can be useful because it furnish a great signal only for a certain magnetic inductance value. The future studies by electronic B20



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 to appear. In Fig. 7 we present an AFM image of the Co–Cu film which indicates that the Czech. J. Phys. 56 (2006)

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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 the case of Co–Cu compound the GMR dependence is more parabolic [7] suggesting the existence of clusters presenting different dimensions which are oriented along the applied magnetic field for different induction values. The resistance decrease is less abrupt as it is seen from Fig. 5.

Fig. 7. AFM image of the Co–Cu film.

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Conclusion

The results provide evidence for a composite thin films consisting of α − Fe nanoparticles embedded in the Cu matrix. The size of the Fe nanoparticles and their dispersion, related also to the purity of the Cu matrix, are analyzed starting from the Mössbauer spectra and finally correlated with the magnetoresistance effects and AFM measurements. The greatest GMR effect appear in the case of Co–Cu structure and for Fe–Cu and Ni–Cu cases, very special kind (quadratic) of GMR dependence appear. That could suggest a very narrow distribution of the cluster dimensions within the copper structure. This can be explained by the special conditions of deposition in TVA technology, already mentioned in the introductory chapter. References [1] T. Lucinski, A. Hutten, H. Bruckl, S. Heitmann, T. Hempel, G. Reiss: J. Magn. Magn. Mater 269 (2004) 78. [2] F. Spizzoa et al: J. Magn. Magn. Mater 262 (2003) 88. [3] Changzeng Wang et al: J. Magn. Magn. Mater 277 (2004) 273. [4] S. Honda, M. Nawate, M. Tanaka and T. Okada: J. Appl. Phys. 82 (1997) 764. [5] G. Musa, H. Ehrich, M. Mausbach: Journal of Vacuum Science and Technology A12 (1994) 2887. [6] C. P. Lungu, I. Mustata, G. Musa, V. Zaroschi, A. M. Lungu and K. Iwasaki: Vacuum 76 (2004)127. [7] I. Mustata, C. P. Lungu, A. M. Lungu, V. Zaroski, M. Blideran and V. Ciupina: Vacuum 76 (2004) 131. [8] V. Kuncser, I. Mustata, C. P. Lungu, A. M. Lungu, V. Zaroschi, W. Keune, B. Sahoo, F. Stromberg, M. Walterfang, L. Ion and G. Filoti: Surf. and Coat. Techn 200 (2005) 980.


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