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Impedance and magnetization of CoFeZr nanoclusters embedded into alumina matrix
Journal of Alloys and Compounds 423 (2006) 176–180
Impedance and magnetization of CoFeZr nanoclusters embedded into alumina matrix A.M. Saad a , A.K. Fedotov b,∗ , I.A. Svito b , J.A. Fedotova b , B.V. Andrievsky c , Yu.E. Kalinin d , A.A. Patryn c , V.V. Fedotova e , V. Malyutina-Bronskaya b , A.V. Mazanik b , A.V. Sitnikov d a
Al-Balqa Applied University, P.O. Box 2041, Amman 11953, Jordan b Belarusian State University, Minsk 220050, Belarus c Technical University of Koszalin, Koszalin 119890, Poland d Voronezh State Technical University, Voronez 250770, Russia e Institute of Physics of Solids and Semiconductors of NAS, Minsk 220078, Belarus Received 7 September 2005; received in revised form 18 November 2005; accepted 5 December 2005 Available online 31 March 2006
Abstract Influence of metal-to-dielectric ratio in (CoFeZr)x (Al2 O3 )100–x composites and influence of gas ambient in sputtering chamber on the impedance and magnetization of the films containing soft ferromagnetic Co0.45 Fe0.45 Zr0.10 nanoclusters in amorphous alumina matrix have been investigated. The films of 3–5 m in thicknesses and with variable composition of 30 at.% < x < 60 at.% were sputtered on a single substrate from the compound target in the chamber containing argon or argon–oxygen gas mixture. The threshold character of impedance and magnetization behavior for the studied films was found to be in agreement with the earlier results obtained from dc carrier transport, magnetotransport and M¨ossbauer measurements. The change of electronic transport mechanisms from hopping (exponential) to metallic (power-like) behavior and the change of magnetic state from superparamagnetic to ferromagnetic beyond the percolation threshold concentration of xc ∼ 45–47 at.% for the films deposited in pure argon atmosphere were observed. The films sputtered in argon–oxygen gas mixture revealed that their conservation of hopping mechanism of ac electronic transport and superparamagnetic or non-magnetic state was due to the formation of semiconducting FeCo-based oxide shells separating metallic nanoclusters even beyond the percolation threshold of xc ∼ 55 at.%. © 2006 Elsevier B.V. All rights reserved. Keywords: Magnetic films and multilayers; Amorphization; Electronic transport; Magnetic measurements
1. Introduction Miniaturization of electronic magnetic devices and their aspiration to function at very high frequencies demanded that the magnetic materials to be of low coercivity, high permeability and high saturation magnetization as well as high electrical resistivity for a wide range of frequencies [1,2]. In accordance with the random anisotropy model of Herzer [3], magnetic materials with such alternative properties can be obtained by sintering the soft ferromagnetic films with grains smaller than the exchange correlation length. Composite films, containing amorphous FeCo-
∗
Corresponding author. Tel.: +375 29 6277495; fax: +375 17 2095445. E-mail address: [email protected] (A.K. Fedotov).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.12.115
based nanoclusters embedded into amorphous dielectric matrix, are potential candidates to achieve the mentioned goal [4–6]. However, FeCo-based nanoclusters are surrounded very often by oxide shells due to the influence of residual oxygen in the chamber during film preparation. These oxides showed the whole collection of different magnetic properties (ferromagnetic for ␥-Fe2 O3 or Fe3 O4 ; anti-ferromagnetic for CoO; ferromagnetic, super-paramagnetic or ferri-magnetic for CoFe2 O4 , depending on temperature and type of matrix, etc.) altering magnetization, coercivity, remanence, resistivity, etc. of FeCo-based composites [4,5]. The aim of the work was to investigate the effect of controllable oxygen content in the sputtering gas ambient on magnetic behavior and impedance of (Co0.45 Fe0.45 Zr0.10 )x (Al2 O3 )100–x nanocomposites.
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2. Experimental procedures The studied film samples with thicknesses of 3–5 m in a form of Co0.45 Fe0.45 Zr0.10 nano-clusters, embedded into Al2 O3 matrix, were sputtered either in pure argon (Ar) at the pressure of 8.0 × 10−4 Pa (films of series 1) or in argon–oxygen (Ar–O) gas mixture at the common pressure of 9.6 × 10−2 Pa and partial pressure of oxygen of 4.4 × 10−2 Pa (films of series 2). Details of the sputtering procedure was described elsewhere [7–9]. The studied composites with metallic phase content of x between 30 and 65 at.% were investigated using SEM, TEM, XRD, M¨ossbauer, impedance and magnetization measurements. TEM microscopy of as-deposited films revealed their granular structure with randomly distributed amorphous metallic Co0.45 Fe0.45 Zr0.10 alloy clusters (clusters dimensions between 6 and 10 nm) in amorphous alumina [7]. Comparing SEM, XRD and M¨ossbauer data [7,9] for the films (samples) of series 1 and 2 concluded that the bcc–FeCo was a dominating phase formed in both series of nanocomposite films. Addition of O into Ar sputtering atmosphere resulted in the partial (before percolation threshold xc ) or nearly full (beyond xc ) oxidation of CoFeZr nanoclusters. Moreover, M¨ossbauer spectroscopy strongly revealed a different magnetic behavior for the two series of films [9]. On one hand, it showed superparamagnetic state exhibited for the films of series 1 below xc and magnetically-split spectra beyond xc . On the other hand, M¨ossbauer spectra for the films sputtered at Ar–O mixture (series 2) revealed their superparamagnetic or non-magnetic state for the whole range of CoFeZr clusters concentrations. For low temperature measurements, a set-up for the impedance determination consisting of a flowing cryostat, a precision ac bridge HP-4284 and a PC-based control system was used. In order to calculate the real and imaginary parts of impedance, the samples were measured for the changes of current amplitude and phase using frequencies of 102 to 106 Hz and applied voltage of 2 V at the temperature of 77–350 K [7,9]. The magnetization curves were obtained by alternation grade magnetometer at 300 K with applied magnetic induction of B ≤ 600 mT with B normal to the films surface [9]. The pick-up coil signal I, recorded at different B and related to the film volume, was proportional to the magnetization M.
3. Results and discussion The behavior of dc and ac carrier transport and also magnetic properties in binary metal–dielectric composites are strongly dependent on the composition of the material, in particular on its position relative to metal–insulator transition (MIT), or percolation threshold xc [10–13]. The latter is determined by the atomic fraction of metallic phase x in the composite, the ratio of the resistances of the metallic and the dielectric particles, and the geometrical parameters of the metallic and dielectric constituents such as their sizes, shape, distribution topology, etc. In accordance with the percolation theory [10], at x < xc , i.e. below percolation threshold, continuous currentconducting (percolating) cluster cannot be formed, so that carrier transport is mainly obtained through dielectric matrix between metallic particles. However, beyond the percolation threshold x > xc , metallic particles can contact each other electrically forming a continuous current-conducting clusters that shunt the dielectric layers shifting the composite to the metallic side of MIT. For many of the composite materials an additional semi-insulating phase (in shape of oxide “shells” or precipitates) can arise at the interface between metallic and dielectric phases due to the influence of the residual oxygen in the gas chamber during the composite preparation or due to the diffusion processes at the interface of metallic clusters with matrix. All that made the structure of composites more complicated
Fig. 1. Real part of impedance Rreal vs. atomic fraction x of metal components in nanocomposites of series 1 and 2 at room temperature and for frequencies of f = 0.1–10 kHz.
(at least three phases but not binary) so that a character of impedance, magnetization and other properties should be varying with composition differently than that for the binary composites. Fig. 1 shows the dependences of real part of impedance Rreal on atomic fraction x of CoFeZr alloy nanoclusters measured at the temperature of 295 K. Note that the shape of Rreal (x) curves is identical for the different frequencies in the range of 0.1–10 kHz and agree with dc resistivity behavior (see [7,9]). The dependences of impedance are strongly different for the films of series 1 than that of series 2. Firstly, the mentioned oxidation of nanoclusters for the films of series 2 resulted in an increase of their Rreal values between 2 and 3 orders of magnitude in comparison with that of series 1 samples for the same x values. Secondly, Rreal (f) dependences in the frequency range of 10–1000 kHz have mainly different character for the films of series 1 than that for the series 2. Thirdly, room temperature Rreal (x) dependences confirmed the shift of percolation threshold xc from 43 to 47 at.% for the films of series 1 to approximately 55 at.% for the films of series 2 as was revealed from the measurements of temperature dependences of dc resistance and magnetoresistance in [7,9]. Comparison of Rreal (T, f) dependences, presented in Figs. 2 and 3, shows that their behavior for the films of series 1 and 2 have one common feature. There is a strong change of Rreal (T, f) character at the percolation threshold xc and activational character of Rreal (T) curves for f ≤ 10 kHz on the dielectric side of MIT. However, it is seen from Figs. 2 and 3, the Rreal (T, f) dependences of the films of series 1 and 2 have many distinctions and sometimes rather strong. A first distinction is to relate to their different frequency responses as shown in Fig. 2. The figure shows that for the films of series 1 lying on dielectric side of MIT Rreal generally does not depend on frequency for the complete studied temperature range but for all other samples (of series 1 on metallic side and series 2 for the complete range of concentrations
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Fig. 2. (a and b) Real part of impedance Rreal vs. temperature for the films of series 1 lying on dielectric side (a) and metallic side (b) of MIT for f = 0.1 kHz (curve 1), f = 10 kHz (curve 2) and f = 1000 kHz (curve 3).
x) Rreal is strongly dependent on frequency, especially at low temperatures. A second critical distinction is to relate to the main change of carrier transport mechanism where activational process transformed into the power-like (metallic) character beyond the percolation threshold for the films of series 1, sputtered in pure Ar. As follows from Fig. 2a, for the samples lying on the dielectric side of MIT, the linearization of the real part of impedance in coordinates ln (Rreal ) versus (1/T)0.25 shows that the Rreal (T) dependences are described by Mott law [14] for the temperature range of 77–350 K. This confirms the conclusion in [7,9] concerning the predominance of a variable range of hopping mechanism of dc carrier transport by the localized states in alumina matrix. However, beyond xc ≈ 43–47 at.% Rreal increases with temperature (Fig. 2b) testifying transition to the metallic behavior of the samples. For the films of series 2, the transition over percolation threshold xc did not change activational character of low frequency Rreal (T) dependences, but resulted only in lowering of hopping activation energies as shown in Fig. 3b, as x increased. This confirmed M¨ossbauer data [9] for creating semiconducting oxide shells around CoFeZr nanoclusters during sputter-
Fig. 3. (a and b) Temperature dependences of real part of impedance for the films of series 2 lying on dielectric side (a) and metallic side (b) of MIT for f = 0.1 kHz (curve 1), f = 10 kHz (curve 2) and f = 1000 kHz (curve 3).
ing of the films in the Ar–O gas mixture in the sputtering chamber. Behavior of the normalized magnetization curves J(B) (in arbitrary units) is shown in Figs. 4 and 6. Fig. 5 shows the dependences of the derivative of magnetization (dJ/dB)0 , which is proportional to the magnetic susceptibility of the samples as B → 0, on the metallic elements content x for the films of both series. For the films of series 1 lying on dielectric side of MIT and far from percolation threshold (x ≤ 35 at.%), the J(B) reveal almost a linear increase of J with B increasing and that looks like paramagnetic state (Fig. 4a). At higher concentrations of metallic phase up to percolation threshold of xc ≈ 43–47 at.% [7,9], the room temperature magnetization curves show neither evidence of saturation nor hysteresis effects, confirming a M¨ossbauer data [9] about superparamagnetic state of metallic alloy nanoclusters at x ≤ xc . In this case, both the remanence and coercivity are almost zero at 300 K. Moreover, the J values measured at constant B increase as x increases although (dJ/dB)0 is almost constant up to xc , where it increases sharply as seen in Fig. 5, curve a to a maximal value for this series of samples.
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Fig. 4. (a and b) Normalized magnetization curves J(B) at room temperature for the films of series 1 with x < xc (a) and x ≥ xc (b)—curve 1: x = 35.0 at.%, curve 2: x = 41.7 at.%, curve 3: x = 44.8 at.%, curve 4: x = 47.0 at.%, curve 5: x = 50.0 at.%, curve 6: x = 56.2 at.%, curve 7: x = 63.3 at.%.
Fig. 5. Variations of the derivative (dJ/dB)0 with concentration of the metallic components x for the films of series 1 (curve a) and series 2 (curve b), estimated from J(B) curves in Figs. 4 and 6.
Beyond the percolation threshold xc the shape of magnetization curves is strongly changed as shown in Fig. 4b revealing saturation without any hysteresis that is the characteristic of soft ferromagnetic materials in agreement with the magnetically-
split M¨ossbauer spectra [9]. At the same time both of J at constant B (Fig. 4b) and (dJ/dB)0 (Fig. 5, curve a) values decrease as x increases. In general, the behavior of magnetization curves J(B) for the films of series 2 with oxidized nanoclusters (Fig. 6) looks like that of the previous films of series 1. In particular, at x xc (i.e. far below the percolation threshold of xc ≈ 55 at.% [9]) the magnetization curves of the films of series 2 also do not show saturation and exhibit no hysteresis, indicating that the oxidized alloy nanoclusters are also in superparamagnetic state. However, in this case the J(B) curves indicate lower values of magnetization J as measured at constant B (Fig. 6a) and (dJ/dB)0 values (Fig. 5, curve b) than that for the films of series 1 (Fig. 4a). Beyond the percolation threshold xc the shape of magnetization curves for the films of series 2 is mainly changed from non-saturation to saturation and also without any features of hysteresis (Fig. 6b). Contrary to the case of x < xc , M¨ossbauer spectra of the samples deposited in Ar–O gas mixture showed non-magnetic behavior independently to the metal component content, i.e. below and beyond percolation threshold of xc ≈ 55 at.% [9]. Moreover, in this case local Fe states observed in [9] demonstrated the threshold character as for the shape of J(B) curves. Beyond xc the values of (dJ/dB)0 increase, as seen in Fig. 5(curve b), as x increases
Fig. 6. (a and b) Normalized magnetization curves J(B) at room temperature for the films of series 2 for x < xc (a) and x ≥ xc (b)—curve 1: x = 34.9 at.%, curve 2: x = 43.4 at.%, curve 3: x = 54.0 at.%, curve 4: x = 59.6 at.%, curve 5: x = 61.5 at.%.
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and J(B = 0.5 T) values are twice higher than for the films of series 1 from comparison of Figs. 4b and 6b. 4. Conclusion The impedance and magnetization measurements have confirmed that the ac carrier transport properties and magnetic state of the studied composite films, containing soft ferromagnetic CoFeZr nanoclusters embedded into amorphous alumina matrix, mainly depend on the oxygen additions into the sputtering chamber. Moreover, the threshold character of the impedance and magnetization behavior with metallic phase content for the studied films was found to be in clear correlation with the results obtained earlier from dc carrier transport and magnetotransport measurements [7,9] and M¨ossbauer spectroscopy [9]. In particular, the formation of continuous current-conducting percolation net of CoFeZr nanoclusters is in agreement with the change of electronic transport mechanisms from hopping (exponential) to metallic (power-like) one beyond the percolation threshold concentration of xc ∼ 45–47 at.% for the films of series 1 deposited in pure Ar atmosphere. At the same time, in the films of series 2 even beyond the percolation threshold of xc ∼ 55 at.% may explain the conservation of the hopping mechanism of ac electronic transport due to the influence of semiconducting FeCo-based oxide shells separating CoFeZr nanoclusters.
Acknowledgement The authors would like to acknowledge the financial support from VISBY Program of the Swedish Institute. References [1] Y. Imry, in: W.P. Kirk, M.A. Reed (Eds.), Nanostructures and Mesoscopic Systems, Academic, New York, 1992, p. 11. [2] M.A. Willard, J.H. Claassen, V.G. Harris, IEEE-NANO, S2.1 Nanomagn. II 51 (2001). [3] G. Herzer, IEEE Trans. Magn. 26 (1990) 1397; G. Herzer, IEEE Trans. Magn. 25 (1989) 3327. [4] R. Skomski, J. Phys. Condens. Matter 15 (2003) R841. [5] D.S. Jiles, Acta Mater. 51 (2003) 5907. [6] S.D. Bader, Surf. Sci. 500 (2002) 172. [7] A.M. Saad, A.V. Mazanik, Yu.E. Kalinin, J.A. Fedotova, A.K. Fedotov, S. Wrotek, A.V. Sitnikov, I.A. Svito, Rev. Adv. Mater. Sci. 8 (2004) 152. [8] Yu.E. Kalinin, A.N. Remizov, A.V. Sitnikov, Bull. Voronezh State Techn. Univ. Mater. Sci. N113 (2003) 43. [9] A.M. Saad, B. Andrievsky, A. Fedotov, J. Fedotova, T. Figielski, Yu.E. Kalinin, V. Malyutina-Bronskaya, A. Mazanik, A. Patryn, A. Sitnikov, I. Svito, Phys. Stat. Solidi C (2006) in press. [10] G. Grimmet, Percolation, second ed., Springer-Verlag, Berlin, 1999. [11] R. Wood, IEEE Trans. Magn. 36 (2000) 36. [12] C. Baker, S.K. Hasanain, S. Ismat Shaha, J. Appl. Phys. 96 (2004) 11. [13] X. Huang, Z. Chen, J. Cryst. Growth 271 (2004) 287. [14] N.F. Mott, E.A. Devis, Electron Processes in Noncrystalline Materials, Clarendon Press, Oxford, 1979.
Impedance and magnetization of CoFeZr nanoclusters embedded into alumina matrix A.M. Saad a , A.K. Fedotov b,∗ , I.A. Svito b , J.A. Fedotova b , B.V. Andrievsky c , Yu.E. Kalinin d , A.A. Patryn c , V.V. Fedotova e , V. Malyutina-Bronskaya b , A.V. Mazanik b , A.V. Sitnikov d a
Al-Balqa Applied University, P.O. Box 2041, Amman 11953, Jordan b Belarusian State University, Minsk 220050, Belarus c Technical University of Koszalin, Koszalin 119890, Poland d Voronezh State Technical University, Voronez 250770, Russia e Institute of Physics of Solids and Semiconductors of NAS, Minsk 220078, Belarus Received 7 September 2005; received in revised form 18 November 2005; accepted 5 December 2005 Available online 31 March 2006
Abstract Influence of metal-to-dielectric ratio in (CoFeZr)x (Al2 O3 )100–x composites and influence of gas ambient in sputtering chamber on the impedance and magnetization of the films containing soft ferromagnetic Co0.45 Fe0.45 Zr0.10 nanoclusters in amorphous alumina matrix have been investigated. The films of 3–5 m in thicknesses and with variable composition of 30 at.% < x < 60 at.% were sputtered on a single substrate from the compound target in the chamber containing argon or argon–oxygen gas mixture. The threshold character of impedance and magnetization behavior for the studied films was found to be in agreement with the earlier results obtained from dc carrier transport, magnetotransport and M¨ossbauer measurements. The change of electronic transport mechanisms from hopping (exponential) to metallic (power-like) behavior and the change of magnetic state from superparamagnetic to ferromagnetic beyond the percolation threshold concentration of xc ∼ 45–47 at.% for the films deposited in pure argon atmosphere were observed. The films sputtered in argon–oxygen gas mixture revealed that their conservation of hopping mechanism of ac electronic transport and superparamagnetic or non-magnetic state was due to the formation of semiconducting FeCo-based oxide shells separating metallic nanoclusters even beyond the percolation threshold of xc ∼ 55 at.%. © 2006 Elsevier B.V. All rights reserved. Keywords: Magnetic films and multilayers; Amorphization; Electronic transport; Magnetic measurements
1. Introduction Miniaturization of electronic magnetic devices and their aspiration to function at very high frequencies demanded that the magnetic materials to be of low coercivity, high permeability and high saturation magnetization as well as high electrical resistivity for a wide range of frequencies [1,2]. In accordance with the random anisotropy model of Herzer [3], magnetic materials with such alternative properties can be obtained by sintering the soft ferromagnetic films with grains smaller than the exchange correlation length. Composite films, containing amorphous FeCo-
∗
Corresponding author. Tel.: +375 29 6277495; fax: +375 17 2095445. E-mail address: [email protected] (A.K. Fedotov).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.12.115
based nanoclusters embedded into amorphous dielectric matrix, are potential candidates to achieve the mentioned goal [4–6]. However, FeCo-based nanoclusters are surrounded very often by oxide shells due to the influence of residual oxygen in the chamber during film preparation. These oxides showed the whole collection of different magnetic properties (ferromagnetic for ␥-Fe2 O3 or Fe3 O4 ; anti-ferromagnetic for CoO; ferromagnetic, super-paramagnetic or ferri-magnetic for CoFe2 O4 , depending on temperature and type of matrix, etc.) altering magnetization, coercivity, remanence, resistivity, etc. of FeCo-based composites [4,5]. The aim of the work was to investigate the effect of controllable oxygen content in the sputtering gas ambient on magnetic behavior and impedance of (Co0.45 Fe0.45 Zr0.10 )x (Al2 O3 )100–x nanocomposites.
A.M. Saad et al. / Journal of Alloys and Compounds 423 (2006) 176–180
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2. Experimental procedures The studied film samples with thicknesses of 3–5 m in a form of Co0.45 Fe0.45 Zr0.10 nano-clusters, embedded into Al2 O3 matrix, were sputtered either in pure argon (Ar) at the pressure of 8.0 × 10−4 Pa (films of series 1) or in argon–oxygen (Ar–O) gas mixture at the common pressure of 9.6 × 10−2 Pa and partial pressure of oxygen of 4.4 × 10−2 Pa (films of series 2). Details of the sputtering procedure was described elsewhere [7–9]. The studied composites with metallic phase content of x between 30 and 65 at.% were investigated using SEM, TEM, XRD, M¨ossbauer, impedance and magnetization measurements. TEM microscopy of as-deposited films revealed their granular structure with randomly distributed amorphous metallic Co0.45 Fe0.45 Zr0.10 alloy clusters (clusters dimensions between 6 and 10 nm) in amorphous alumina [7]. Comparing SEM, XRD and M¨ossbauer data [7,9] for the films (samples) of series 1 and 2 concluded that the bcc–FeCo was a dominating phase formed in both series of nanocomposite films. Addition of O into Ar sputtering atmosphere resulted in the partial (before percolation threshold xc ) or nearly full (beyond xc ) oxidation of CoFeZr nanoclusters. Moreover, M¨ossbauer spectroscopy strongly revealed a different magnetic behavior for the two series of films [9]. On one hand, it showed superparamagnetic state exhibited for the films of series 1 below xc and magnetically-split spectra beyond xc . On the other hand, M¨ossbauer spectra for the films sputtered at Ar–O mixture (series 2) revealed their superparamagnetic or non-magnetic state for the whole range of CoFeZr clusters concentrations. For low temperature measurements, a set-up for the impedance determination consisting of a flowing cryostat, a precision ac bridge HP-4284 and a PC-based control system was used. In order to calculate the real and imaginary parts of impedance, the samples were measured for the changes of current amplitude and phase using frequencies of 102 to 106 Hz and applied voltage of 2 V at the temperature of 77–350 K [7,9]. The magnetization curves were obtained by alternation grade magnetometer at 300 K with applied magnetic induction of B ≤ 600 mT with B normal to the films surface [9]. The pick-up coil signal I, recorded at different B and related to the film volume, was proportional to the magnetization M.
3. Results and discussion The behavior of dc and ac carrier transport and also magnetic properties in binary metal–dielectric composites are strongly dependent on the composition of the material, in particular on its position relative to metal–insulator transition (MIT), or percolation threshold xc [10–13]. The latter is determined by the atomic fraction of metallic phase x in the composite, the ratio of the resistances of the metallic and the dielectric particles, and the geometrical parameters of the metallic and dielectric constituents such as their sizes, shape, distribution topology, etc. In accordance with the percolation theory [10], at x < xc , i.e. below percolation threshold, continuous currentconducting (percolating) cluster cannot be formed, so that carrier transport is mainly obtained through dielectric matrix between metallic particles. However, beyond the percolation threshold x > xc , metallic particles can contact each other electrically forming a continuous current-conducting clusters that shunt the dielectric layers shifting the composite to the metallic side of MIT. For many of the composite materials an additional semi-insulating phase (in shape of oxide “shells” or precipitates) can arise at the interface between metallic and dielectric phases due to the influence of the residual oxygen in the gas chamber during the composite preparation or due to the diffusion processes at the interface of metallic clusters with matrix. All that made the structure of composites more complicated
Fig. 1. Real part of impedance Rreal vs. atomic fraction x of metal components in nanocomposites of series 1 and 2 at room temperature and for frequencies of f = 0.1–10 kHz.
(at least three phases but not binary) so that a character of impedance, magnetization and other properties should be varying with composition differently than that for the binary composites. Fig. 1 shows the dependences of real part of impedance Rreal on atomic fraction x of CoFeZr alloy nanoclusters measured at the temperature of 295 K. Note that the shape of Rreal (x) curves is identical for the different frequencies in the range of 0.1–10 kHz and agree with dc resistivity behavior (see [7,9]). The dependences of impedance are strongly different for the films of series 1 than that of series 2. Firstly, the mentioned oxidation of nanoclusters for the films of series 2 resulted in an increase of their Rreal values between 2 and 3 orders of magnitude in comparison with that of series 1 samples for the same x values. Secondly, Rreal (f) dependences in the frequency range of 10–1000 kHz have mainly different character for the films of series 1 than that for the series 2. Thirdly, room temperature Rreal (x) dependences confirmed the shift of percolation threshold xc from 43 to 47 at.% for the films of series 1 to approximately 55 at.% for the films of series 2 as was revealed from the measurements of temperature dependences of dc resistance and magnetoresistance in [7,9]. Comparison of Rreal (T, f) dependences, presented in Figs. 2 and 3, shows that their behavior for the films of series 1 and 2 have one common feature. There is a strong change of Rreal (T, f) character at the percolation threshold xc and activational character of Rreal (T) curves for f ≤ 10 kHz on the dielectric side of MIT. However, it is seen from Figs. 2 and 3, the Rreal (T, f) dependences of the films of series 1 and 2 have many distinctions and sometimes rather strong. A first distinction is to relate to their different frequency responses as shown in Fig. 2. The figure shows that for the films of series 1 lying on dielectric side of MIT Rreal generally does not depend on frequency for the complete studied temperature range but for all other samples (of series 1 on metallic side and series 2 for the complete range of concentrations
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Fig. 2. (a and b) Real part of impedance Rreal vs. temperature for the films of series 1 lying on dielectric side (a) and metallic side (b) of MIT for f = 0.1 kHz (curve 1), f = 10 kHz (curve 2) and f = 1000 kHz (curve 3).
x) Rreal is strongly dependent on frequency, especially at low temperatures. A second critical distinction is to relate to the main change of carrier transport mechanism where activational process transformed into the power-like (metallic) character beyond the percolation threshold for the films of series 1, sputtered in pure Ar. As follows from Fig. 2a, for the samples lying on the dielectric side of MIT, the linearization of the real part of impedance in coordinates ln (Rreal ) versus (1/T)0.25 shows that the Rreal (T) dependences are described by Mott law [14] for the temperature range of 77–350 K. This confirms the conclusion in [7,9] concerning the predominance of a variable range of hopping mechanism of dc carrier transport by the localized states in alumina matrix. However, beyond xc ≈ 43–47 at.% Rreal increases with temperature (Fig. 2b) testifying transition to the metallic behavior of the samples. For the films of series 2, the transition over percolation threshold xc did not change activational character of low frequency Rreal (T) dependences, but resulted only in lowering of hopping activation energies as shown in Fig. 3b, as x increased. This confirmed M¨ossbauer data [9] for creating semiconducting oxide shells around CoFeZr nanoclusters during sputter-
Fig. 3. (a and b) Temperature dependences of real part of impedance for the films of series 2 lying on dielectric side (a) and metallic side (b) of MIT for f = 0.1 kHz (curve 1), f = 10 kHz (curve 2) and f = 1000 kHz (curve 3).
ing of the films in the Ar–O gas mixture in the sputtering chamber. Behavior of the normalized magnetization curves J(B) (in arbitrary units) is shown in Figs. 4 and 6. Fig. 5 shows the dependences of the derivative of magnetization (dJ/dB)0 , which is proportional to the magnetic susceptibility of the samples as B → 0, on the metallic elements content x for the films of both series. For the films of series 1 lying on dielectric side of MIT and far from percolation threshold (x ≤ 35 at.%), the J(B) reveal almost a linear increase of J with B increasing and that looks like paramagnetic state (Fig. 4a). At higher concentrations of metallic phase up to percolation threshold of xc ≈ 43–47 at.% [7,9], the room temperature magnetization curves show neither evidence of saturation nor hysteresis effects, confirming a M¨ossbauer data [9] about superparamagnetic state of metallic alloy nanoclusters at x ≤ xc . In this case, both the remanence and coercivity are almost zero at 300 K. Moreover, the J values measured at constant B increase as x increases although (dJ/dB)0 is almost constant up to xc , where it increases sharply as seen in Fig. 5, curve a to a maximal value for this series of samples.
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Fig. 4. (a and b) Normalized magnetization curves J(B) at room temperature for the films of series 1 with x < xc (a) and x ≥ xc (b)—curve 1: x = 35.0 at.%, curve 2: x = 41.7 at.%, curve 3: x = 44.8 at.%, curve 4: x = 47.0 at.%, curve 5: x = 50.0 at.%, curve 6: x = 56.2 at.%, curve 7: x = 63.3 at.%.
Fig. 5. Variations of the derivative (dJ/dB)0 with concentration of the metallic components x for the films of series 1 (curve a) and series 2 (curve b), estimated from J(B) curves in Figs. 4 and 6.
Beyond the percolation threshold xc the shape of magnetization curves is strongly changed as shown in Fig. 4b revealing saturation without any hysteresis that is the characteristic of soft ferromagnetic materials in agreement with the magnetically-
split M¨ossbauer spectra [9]. At the same time both of J at constant B (Fig. 4b) and (dJ/dB)0 (Fig. 5, curve a) values decrease as x increases. In general, the behavior of magnetization curves J(B) for the films of series 2 with oxidized nanoclusters (Fig. 6) looks like that of the previous films of series 1. In particular, at x xc (i.e. far below the percolation threshold of xc ≈ 55 at.% [9]) the magnetization curves of the films of series 2 also do not show saturation and exhibit no hysteresis, indicating that the oxidized alloy nanoclusters are also in superparamagnetic state. However, in this case the J(B) curves indicate lower values of magnetization J as measured at constant B (Fig. 6a) and (dJ/dB)0 values (Fig. 5, curve b) than that for the films of series 1 (Fig. 4a). Beyond the percolation threshold xc the shape of magnetization curves for the films of series 2 is mainly changed from non-saturation to saturation and also without any features of hysteresis (Fig. 6b). Contrary to the case of x < xc , M¨ossbauer spectra of the samples deposited in Ar–O gas mixture showed non-magnetic behavior independently to the metal component content, i.e. below and beyond percolation threshold of xc ≈ 55 at.% [9]. Moreover, in this case local Fe states observed in [9] demonstrated the threshold character as for the shape of J(B) curves. Beyond xc the values of (dJ/dB)0 increase, as seen in Fig. 5(curve b), as x increases
Fig. 6. (a and b) Normalized magnetization curves J(B) at room temperature for the films of series 2 for x < xc (a) and x ≥ xc (b)—curve 1: x = 34.9 at.%, curve 2: x = 43.4 at.%, curve 3: x = 54.0 at.%, curve 4: x = 59.6 at.%, curve 5: x = 61.5 at.%.
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and J(B = 0.5 T) values are twice higher than for the films of series 1 from comparison of Figs. 4b and 6b. 4. Conclusion The impedance and magnetization measurements have confirmed that the ac carrier transport properties and magnetic state of the studied composite films, containing soft ferromagnetic CoFeZr nanoclusters embedded into amorphous alumina matrix, mainly depend on the oxygen additions into the sputtering chamber. Moreover, the threshold character of the impedance and magnetization behavior with metallic phase content for the studied films was found to be in clear correlation with the results obtained earlier from dc carrier transport and magnetotransport measurements [7,9] and M¨ossbauer spectroscopy [9]. In particular, the formation of continuous current-conducting percolation net of CoFeZr nanoclusters is in agreement with the change of electronic transport mechanisms from hopping (exponential) to metallic (power-like) one beyond the percolation threshold concentration of xc ∼ 45–47 at.% for the films of series 1 deposited in pure Ar atmosphere. At the same time, in the films of series 2 even beyond the percolation threshold of xc ∼ 55 at.% may explain the conservation of the hopping mechanism of ac electronic transport due to the influence of semiconducting FeCo-based oxide shells separating CoFeZr nanoclusters.
Acknowledgement The authors would like to acknowledge the financial support from VISBY Program of the Swedish Institute. References [1] Y. Imry, in: W.P. Kirk, M.A. Reed (Eds.), Nanostructures and Mesoscopic Systems, Academic, New York, 1992, p. 11. [2] M.A. Willard, J.H. Claassen, V.G. Harris, IEEE-NANO, S2.1 Nanomagn. II 51 (2001). [3] G. Herzer, IEEE Trans. Magn. 26 (1990) 1397; G. Herzer, IEEE Trans. Magn. 25 (1989) 3327. [4] R. Skomski, J. Phys. Condens. Matter 15 (2003) R841. [5] D.S. Jiles, Acta Mater. 51 (2003) 5907. [6] S.D. Bader, Surf. Sci. 500 (2002) 172. [7] A.M. Saad, A.V. Mazanik, Yu.E. Kalinin, J.A. Fedotova, A.K. Fedotov, S. Wrotek, A.V. Sitnikov, I.A. Svito, Rev. Adv. Mater. Sci. 8 (2004) 152. [8] Yu.E. Kalinin, A.N. Remizov, A.V. Sitnikov, Bull. Voronezh State Techn. Univ. Mater. Sci. N113 (2003) 43. [9] A.M. Saad, B. Andrievsky, A. Fedotov, J. Fedotova, T. Figielski, Yu.E. Kalinin, V. Malyutina-Bronskaya, A. Mazanik, A. Patryn, A. Sitnikov, I. Svito, Phys. Stat. Solidi C (2006) in press. [10] G. Grimmet, Percolation, second ed., Springer-Verlag, Berlin, 1999. [11] R. Wood, IEEE Trans. Magn. 36 (2000) 36. [12] C. Baker, S.K. Hasanain, S. Ismat Shaha, J. Appl. Phys. 96 (2004) 11. [13] X. Huang, Z. Chen, J. Cryst. Growth 271 (2004) 287. [14] N.F. Mott, E.A. Devis, Electron Processes in Noncrystalline Materials, Clarendon Press, Oxford, 1979.