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Magnetic and transport properties of annealed Fe(SiO) alloys
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 284 (2004) 165–171 www.elsevier.com/locate/jmmm
Magnetic and transport properties of annealed Fe(SiO) alloys M. Anasa, C. Bellouardb,, L. Moreaub, M. Vergnatb b
a Haluoleo University, Kampus Bumi Tridharma Kendari 93231, Indonesia Laboratoire de Physique des Mate´riaux, (U.M.R. C.N.R.S. No 7556), Universite´ Henri Poincare´ Nancy 1, B.P. 239, 54506 Vandæuvre-le`s-Nancy, Cedex, France
Received 21 January 2004; received in revised form 22 June 2004 Available online 21 July 2004
Abstract Previous experiments performed with co-evaporated Fec(SiO)1c alloys have shown that this composite is superparamagnetic for an atomic concentration c between 0.3 and 0.5 and that it exhibits a tunnel magnetoresistance (TMR) effect for 0:4pcp0:5. We present here a further study of annealed alloys in the superparamagnetic phase performed by transmission electron microscopy, infrared absorption spectroscopy, magnetic and transport measurements. Infrared absorption spectroscopy shows that the SiO matrix decomposes in SiOx and Si, even for asdeposited alloys. The TMR observed in as-deposited alloys is thus attributed to tunneling between amorphous FeSi granules. The decomposition of the insulating matrix is activated by annealing and enhanced compared to pure SiO by the presence of Fe, as if Fe attracts Si to form an FeSi amorphous alloy. The TMR is almost unchanged until an annealing temperature of 300 1C. Above 300 1C and before reaching the crystallization temperature, the TMR steeply decreases. These results are readily attributed to the presence of amorphous FeSi. r 2004 Elsevier B.V. All rights reserved. PACS: 61.46.+w; 73.50.Jt; 75.20.g; 75.47.m; 75.75.+a; 73.63.b; 81.15.Ef Keywords: Superparamagnetism; Tunneling magnetoresistance; Granular alloy; Amorphous alloy; Co-evaporation
1. Introduction Magnetoresistance (MR) resulting from spinpolarized tunneling was first experimentally observed by Julliere [1] in Fe/Ge/Co and Fe/Ge/Pb Corresponding author. Tel: +33-3-83-68-48-23; Fax: +33-
3-83-68-48-01. E-mail address: [email protected] (C. Bellouard).
junctions. It consists of a change in the tunnel conductance as a function of the relative orientation of the magnetization in the two ferromagnetic layers. The large MR response in low magnetic fields has stimulated considerable interest. However, fabrication of pinhole-free insulating barrier remains a major problem. MR was also observed in granular metal/insulator films [2] which consist of randomly distributed ferromagnetic particles in
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.06.034
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an insulating matrix. For applications, these films are easier to fabricate, are very robust due to the protective insulating oxide, and are not as susceptible to electrical breakdown as tunnel junctions. They can be elaborated by codeposition or by alternate deposition of the magnetic metal and the insulating matrix (to obtain the so-called discontinuous multilayers). When there is no chemical mixing between the granules and the matrix, annealing at a moderate temperature (200–300 1C) only modifies the density and size of magnetic grains, it can then improve or lower the MR. For instance in the case of the Fe(SiO2) alloy, the MR effect is increased or decreased depending on the composition of the alloy [3]. In the same way, the MR effect is enhanced with annealing in Co/SiO2 multilayers [4] whereas annealing weakly decreases the MR observed for discontinuous Co80Fe20/ Al2O3 [5]. Otherwise, in the case of good wetting of the metal on the insulating layer during the alternate deposition, an annealing process is necessary to get isolated grains and a MR effect [6]. When there is a chemical interaction between the insulator and the metal, annealing has a more drastic effect on TMR. For instance, an annealing process is essential to induce a MR effect in Fe–PbO alloy as it creates a magnetic Fe2O3 compound between the metallic grains and the insulating matrix [7]. On the contrary, for the Fe/ ZrO2 system, annealing produces an iron oxidation likely responsible for the low MR observed in this system [8]. All these experiments show that annealing is an useful tool to check the interaction between the granules and the matrix and can then provide some indications about the absence or amplitude of a MR effect. We have recently performed a study of the structural and transport properties in co-evaporated Fex(SiO)1x [9] granular films as a function of the Fe volume fraction x. It has been shown that a superparamagnetic behaviour is obtained for 0:13pxp0:27 together with a tunnel magnetoresistance (TMR) for 0:2pxp0:27. Nevertheless, the maximum amplitude of the tunnel magnetoresistance is of the order of 2% at 100 K for 70 kOe which is rather low compared to other systems. To further investigate
the interplay between the structure and morphology of the grains and the MR effect, an annealing study with alloys in the superparamagnetic phase (0:13pxp0:27) is presented. Their structural and magnetotransport properties have been followed at each annealing step in the range 200–9501C by infrared absorption spectroscopy, transmission electron microscopy (TEM), magnetic and magnetoresistance measurements. It will be shown that these experiments offer a new insight into the behaviour of as-deposited samples.
2. Experimental details Fex(SiO)1x films have been deposited by coevaporation of Fe and SiO on a substrate kept at 100 1C. More details about the elaboration are given in Ref. [9]. In this paper, the volume fraction x has been evaluated using the bulk density of Fe and SiO. For an easier comparison with other systems, we will mention below the co-evaporated alloys by their atomic concentration c. The superparamagnetic behaviour observed for 0:13pxp0:27 corresponds to 0:3pcp0:5. The metallic behaviour appears for x ¼ 0:35 or c ¼ 0:6. Annealing has been performed in a quartz tube with a base pressure of 108 Torr. The heating is performed with a constant rate of 10 K/min. It is stopped and the quartz tube is removed from the furnace as soon as the annealing temperature T a is reached. Silicon substrates were used for infrared absorption spectrometry performed with a PerkinElmer 2000 spectrometer with a resolution of 4 cm1. Transmission electron microscopy has been performed with 500 A˚ thick films deposited on carbon grids. The films were also heated in the quartz tube. The magnetic properties were determined with a superconducting quantum interference device (SQUID) magnetometer. Field cooled (FC) and zero field cooled (ZFC) magnetization measurements have been performed as a function of increasing temperature for an applied field of 20 Oe.
ARTICLE IN PRESS M. Anas et al. / Journal of Magnetism and Magnetic Materials 284 (2004) 165–171
Transport measurements have been performed only in the current in plane (CIP) geometry with a superconducting coil equipped cryostat. The magnetoresistance is defined by: MRðH Þ ¼
RðH ¼ 0Þ RðH Þ : R ð H ¼ 0Þ
3. Results and discussion
167
increase and a shift of three absorption bands around 450, 750 and 1000 cm1 are observed. These bands are attributed to the infrared active vibrations of the Si–O–Si bonds. In particular, the main band at 1000 cm1 is due to the asymmetric stretching vibration of the oxygen atom in the Si–O–Si group. Fig. 3 shows the position of the main band as a function of the annealing temperature for pure SiO and for c ¼ 0:3. The
3.1. TEM and infrared spectroscopy 0.8 c = 0.30
Absorbance
The results obtained from electron diffraction are presented in the phase diagram (Fig. 1). For cp0:3, the alloy keeps its amorphous structure until 600 1C. It then crystallizes in a non-identified phase. The crystallization temperature is determined by the appearence of several fine rings. For an intermediate concentration 0:4pcp0:6, the crystallization temperature of BCC iron decreases with increasing Fe content. This result is similar to the annealing of Fec(SiO2)1c, where the crystallization temperature decreases from 700 to 480 1C when the iron fraction is increased from 0.3 to 0.79 [10]. Fig. 2. shows the infrared absorption spectra of Fe0.3(SiO)0.7 alloys for several annealing temperatures between 200 and 950 1C. The spectra evolve in the same way as those of a SiO compound [11]. For an annealing temperature above 200 1C, an
950°C
0.4
800°C 650°C
0 600
900
1200
500°C 360°C 200°C NR 1500
Wavenumber (cm-1) Fig. 2. Infrared absorption spectra as a function of the annealing temperature T a for the Fe0.3(SiO)0.7 alloy.
1100
Ta (°C)
600
400
SiO2
1080
α - Fe crystals
Wave number (cm-1)
800
unknown phase
amorphous
1060 1040 1020 c = 0.3
1000
200
pure SiO 980 0 0.0
0.2
0.4
0.6
0.8
0
200
400
600
800
1000
Ta (°C)
c (atomic fraction) Fig. 1. Phase diagram of the Fec(SiO)1c alloys as a function of the atomic fraction c and of the annealing temperature T a .
Fig. 3. Position of the maximum of the main infrared absorption band as a function of the annealing temperature T a for the Fe0.3(SiO)0.7 alloy (atomic percent) and pure SiO.
ARTICLE IN PRESS M. Anas et al. / Journal of Magnetism and Magnetic Materials 284 (2004) 165–171
followed during annealing. As the qualitative behaviour is identical for both alloys, only the results for c ¼ 0:5 are described. We recall that, according to Fig. 1, this alloy is expected to crystallize in the pure Fe BCC structure around T a ¼ 500 1C. First of all, we note that the moment per Fe atom measured at 5 K remains almost constant until T a ¼ 500 1C, then it slowly decreases to get, at 660 1C, 85% of the value of the as-deposited alloy. For 315 1CpT a p660 1C, the temperature dependence of the ZFC magnetization exhibits a well-defined maximum at a so-called blocking temperature T ¼ T B . The evolution of T B with annealing temperature is plotted in Fig. 4. It is almost constant until T a ¼ 500 1C, close to the crystallization temperature, then it steeply increases with T a . To provide a quantitative evolution of the size distribution of particles, the magnetization curve MðHÞ at 300 K , which is always above the blocking temperature, has been fitted by the sum of two Langevin Lðmi Þ, according functions to: MðHÞ ¼ M s ðT Þ xL m1 þ ð1 xÞL m2 : The mean particle moment is then given by:
1 m ¼ x 1x : m þ m 1
2
This treatment does not mean that there are two typical sizes in the particle distribution, it just 50 1500
TB <µ>
40
1000
30
20
<µ> (µB)
band corresponding to the Fe alloy is always at a higher frequency than the band of pure SiO, even for the as-deposited alloy. Both SiO and Fe(SiO) bands shift with annealing temperature until the frequency attributed to SiO2 is reached at 800 1C for the Fe alloy and at 950 1C for pure SiO. From previous experiments, it is generally assumed that there is a linear relation between the oxygen content n of the SiOn compound and the frequency of the maximum band [11,12]. The shift of the band is then the signature of the decomposition of SiO into Si and SiOn, with n41. The global shift of the frequencies for the Fe(SiO) compound compared to annealed pure SiO indicates that the decomposition of SiO is favoured by the presence of Fe. The remaining Si can form Si grains as in pure SiO [11] or can mix with Fe. We note that the formation of FeSi alloy was suspected in a Fe(SiO2) compound where the extra Si content in the magnetic particles was due to the sputtering process [10]. This assumption was asserted by the presence of Fe5Si3 in Fe(SiO2) films prepared at 500 1C. In our case, the assumption of the presence of an FeSi alloy even in the as-deposited alloys is in agreement with both the electron diffraction and the magnetization results as only an amorphous phase was observed for co0.6 with a reduced magnetic moment per Fe atom [9]. Following the linear relation between the frequency n of the main infrared band and the oxygen content n of the silicon oxide phase SiOn proposed by Rinnert et al. [11] (n ¼ ðn 918Þ=81), a stoichiometry n ¼ 1:32 was found for the asdeposited Fe0.4(SiO)0.6 alloy (atomic percent). This alloy would then correspond to (SiO1.32)0.45 (Fe0.73Si0.27)0.55. This is in agreement with the magnetic moment measured for this alloy, 1.4 mB/ Fe atom [9], as the magnetic moment per Fe atom of a Fe0.7Si0.3 is roughly 1.5 mB/Fe atom [13]. Nevertheless, the composition deduced from infrared spectroscopy should correspond to a mean value as a gradient of Fe composition may exist between grains or inside a grain.
TB (K)
168
500
c = 0.5
10 100
200
300
400
500
600
0 700
Ta (°C)
3.2. Magnetic properties The magnetic properties of Fec(SiO)1c alloys with c ¼ 0:4 and 0:5 (atomic fraction) have been
Fig. 4. Position of the maximum of the ZFC curve (the so- called blocking temperature T B ) and mean particle moment m as a function of the annealing temperature T a for the Fe0.5(SiO)0.5 alloy (atomic percent).
ARTICLE IN PRESS M. Anas et al. / Journal of Magnetism and Magnetic Materials 284 (2004) 165–171
provides a good fitting of the data, and allows us to evaluate easily the mean moment of the particle distribution om4. It is plotted in Fig. 4 as a function of T a . It increases from T a ¼ 315 1C although the blocking temperature clearly changes only above 500 1C. This feature indicates that despite the blocking temperature being unchanged when approaching the crystallization temperature, there is an evolution of the size or moment particle distribution. This is also clearly shown in Fig. 5 where the FC and ZFC magnetizations have been plotted as a function of temperature for the asdeposited alloy and for T a ¼ 500 and 570 1C. When the annealing temperature is increased, the low field magnetization measured at 100 K above the blocking temperature T B is increased and its shape above T B is modified. In the case of a single particle moment m, the magnetization far above T B can be expressed by the Curie law: MðHÞ ¼ M s ðT Þ
mðT ÞH : 3kB T
The temperature dependence of the saturation magnetization in the Curie law allows to take into account the temperature dependence of the magnetization of the amorphous FeSi grains before crystallization. This temperature dependence has to be introduced as the increase of magnetization with decreasing temperature is steeper than a 400 c = 0.5
as-d. ZFC as-d. FC 500°C ZFC 500°C FC 570°C ZFC 570°C FC
3
M (emu/cm )
300
200
100
0
0
50
100
T (K) Fig. 5. Magnetization of the ZFC and FC curves as a function of temperature for the Fe0.5(SiO)0.5 as-deposited alloy (atomic percent), and for annealing temperatures: T a ¼ 500 and 570 1C.
169
simple 1/T dependence. For instance, the increase of magnetization of the as deposited sample between 50 and 25 K is larger than a factor 2. Within the assumption of the Curie law, an increase of low field magnetization MðHÞ can be directly related to an increase of m. In the present case of a size distribution, it seems reasonable to correlate the increase of low field magnetization with annealing temperature T a with the increase of mean particle moment, as the saturation magnetization M s remains constant or decreases with T a Moreover, one can notice that the bending of the FC–ZFC curves changes around 30 K for the asdeposited alloy whereas this change occurs around 60 K for T a ¼ 500 1C. Below this temperature, the magnetization no longer follows a Curie law and some particles become blocked. Despite there being no tail in the difference between the FC and ZFC curves, the evolution of the FC-ZFC curves suggests a broadening of the particule moment or size distribution when reaching the crystallization temperature. Finally, on one hand, annealing gives rise to an increase of the mean magnetic particle moment of the distribution, which can be associated to an increase of the size or of the Fe concentration of the FeSi particles. On the other hand, no increase of the magnetization is observed. On the contrary, the magnetization slightly decreases with annealing above 500 1C, it must then correspond to an average of 2.2 mB/Fe atom with a much lower value due to poorly or non magnetic iron. During annealing, as a part of the FeSi alloy gets a higher Fe concentration to form the Fe BCC structure when crossing the crystallization temperature, another part, whose fraction increases, is lost in the matrix. As a matter of fact, annealing results in a demixion-like process. From infrared spectroscopy, we have deduced that, during annealing, Si leaves the matrix to either form Si grains or a FeSi alloy. The magnetization measurements only agree with the second assumption : BCC iron coexists with amorphous FeSi alloy which can be non or poorly magnetic. 3.3. Transport properties The room-temperature resistivity is plotted as a function of T a for c ¼ 0:4 and 0:5 in Fig. 6a. For
ARTICLE IN PRESS M. Anas et al. / Journal of Magnetism and Magnetic Materials 284 (2004) 165–171
170
both compositions, annealing has no appreciable effect until T a ¼ 400 1C. Above this temperature, very different behaviours are observed for these close compositions. A sharp increase of the resistivity is observed for c ¼ 0:4 whereas it decreases with annealing for c ¼ 0:5. First of all, neglecting any chemical interaction between the granules and the matrix, annealing has generally two effects upon the evolution of the grains size distribution: it increases the mean size of the grains and, as a matter of fact, decreases their density or increases the separation between grains. Moreover, in our case, things are more complicated because of the decomposition of the SiO matrix which becomes more oxidized. The gap energy of the dielectric is then increased. Annealing has then conflicting effects on the tunneling mechanism between grains. On one hand, the increase of mean 2.5
MR (% )
2 1.5 1 c = 0.4 c = 0.5
0.5 (b)
0
80 c = 0.4 c = 0.5
ρ (Ω cm)
60 40 20 0 (a)
0
100
200
300
400
grain size may increase the tunnel probalility, on the other hand, the increase of the intergrain distance and of the potential barrier tends to lower the tunneling. The first case is prevailing for x ¼ 0:5 whereas the second case corresponds to x ¼ 0:4. 3.4. Magnetoresistance Fig. 6b shows the magnetoresistance MR measured at 70 kOe and 50 K (for which the signal is saturated) for c ¼ 0:4 and 0:5 as a function of the annealing temperature. The results are very similar for both alloys. A reasonable annealing to about 200–300 1C, as was performed for some discontinuous multilayers [4,5], has a poor effect on the MR effect. Above 3001C and before crystallization, annealing drastically decreases the MR effect which becomes lower than 0.5% for T a ¼ 500 1C. We note that both alloys exhibit the same TMR behaviour during annealing whereas opposite variation has been observed for their resistivity. On the contrary, for Fe(SiO2) alloys, annealing can increase or decrease the TMR effect, depending on the composition [3]. This discrepancy between both Fe(SiO) and Fe(SiO2) systems can be attributed to the different interactions between the granules and the matrix. The stoichiometric Fe(SiO2) system is rather stable, as no FeSi alloy has been detected until an annealing at 900 1C [14]. In our case, as was discussed above, we can assume that the Si content of the metallic grains increases with annealing temperature. This assumption is rather conflicting with the increase of the mean particle moment. To conciliate both results, we propose the following picture : the Fe-rich core of the magnetic particles which grows during annealing is surrounded by an FeSi alloy which becomes richer in Si. This weak or non magnetic interface between magnetic granules and insulating SiOx can explain the decreasing MR effect with annealing.
500
Ta (°C) Fig. 6. Resistivity r at 300 K (a) and magnetoresistance MR (b) measured at 50 K and 70 kOe as a function of the annealing temperature T a for the Fec(SiO)1c alloys with c ¼ 0:4 and 0:5.
4. Conclusion Structural, magnetic and transport properties of co-evaporated granular Fex(SiO)1x alloys have
ARTICLE IN PRESS M. Anas et al. / Journal of Magnetism and Magnetic Materials 284 (2004) 165–171
been followed during annealing. Infrared absorption spectroscopy shows that the SiO matrix decomposes in SiOx and Si even for as-deposited alloys. This decomposition is activated by annealing and enhanced with respect to pure SiO by the presence of Fe, as if Fe attracts Si to form an FeSi amorphous alloy. The magnetization remains largely reduced with respect to bulk iron even above the crystallization temperature. This reveals that an FeSi alloy is always present in the composite. This interaction between the metal and the insulating matrix explains the rather low TMR effect in Fe(SiO) with respect to the Fe(SiO2) system as the polarization of conduction electrons in the FeSi alloy is damaged compared to bulk iron. The TMR is almost constant until an annealing temperature of 300 1C, and then steeply decreases to a value lower than 0.5% (measured at 50 K) before the crystallization temperature. References [1] X. Julliere, Phys. Lett. 54A (1975) 225. [2] Y. Goldstein, J.I. Gittleman, Solid State Commun 9 (1971) 1197.
171
[3] S. Honda, T. Okada, M. Nawate, J. Magn. Magn. Mater. 165 (1997) 153. [4] B. Dieny, S. Sankar, M.R. McCartney, D.J. Smith, P. Bayle-Guillemaud, A.E. Berkowitz, J. Magn. Magn. Mater. 185 (1998) 283. [5] G.N. Kakazei, Y.G. Pogorelov, A.M.L. Lopes, J.B. Sousa, S. Cardoso, P.P. Freitas, M.M. Pereira de Azevedo, E. Snoeck, J. Appl. Phys. 90 (2001) 4044. [6] S. Sankar, B. Dieny, A.E. Berkowitz, J. Appl. Phys. 81 (1997) 5512. [7] Y.-H. Huang, J.-H. Hsu, J.W. Chen, C.-R. Chang, Appl. Phys. Lett. 72 (1998) 2171; J.-H. Hsu, Y.-H. Huang, J. Magn. Magn. Mater. 203 (1999) 94. [8] P. Auric, J.S. Micha, O. Proux, L. Giacomoni, J.R. Regnard, J. Magn. Magn. Mater. 217 (2000) 175. [9] M. Anas, C. Bellouard, M. Vergnat, J. Appl. Phys. 88 (2000) 6075. [10] R.L. Holtz, A.S. Edelstein, P. Lubitz, C.R. Gossett, J. Appl. Phys. 64 (1988) 4251. [11] H. Rinnert, M. Vergnat, A. Burneau, J. Appl. Phys. 89 (2001) 237. [12] P.G. Pai, S.S. Chao, Y. Takagi, G. Lucovsky, J. Vac. Sci. Technol. B 4 (1986) 689. [13] G. Marchal, Ph. Mangin, M. Piecuch, C. Janot, J. Hubsch, J. Phys. F: Metal Phys. 7 (1977) L165. [14] D.P. Yang, Y.D. Zhang, S. Hui, J. Appl. Phys. 91 (2002) 8198.
Journal of Magnetism and Magnetic Materials 284 (2004) 165–171 www.elsevier.com/locate/jmmm
Magnetic and transport properties of annealed Fe(SiO) alloys M. Anasa, C. Bellouardb,, L. Moreaub, M. Vergnatb b
a Haluoleo University, Kampus Bumi Tridharma Kendari 93231, Indonesia Laboratoire de Physique des Mate´riaux, (U.M.R. C.N.R.S. No 7556), Universite´ Henri Poincare´ Nancy 1, B.P. 239, 54506 Vandæuvre-le`s-Nancy, Cedex, France
Received 21 January 2004; received in revised form 22 June 2004 Available online 21 July 2004
Abstract Previous experiments performed with co-evaporated Fec(SiO)1c alloys have shown that this composite is superparamagnetic for an atomic concentration c between 0.3 and 0.5 and that it exhibits a tunnel magnetoresistance (TMR) effect for 0:4pcp0:5. We present here a further study of annealed alloys in the superparamagnetic phase performed by transmission electron microscopy, infrared absorption spectroscopy, magnetic and transport measurements. Infrared absorption spectroscopy shows that the SiO matrix decomposes in SiOx and Si, even for asdeposited alloys. The TMR observed in as-deposited alloys is thus attributed to tunneling between amorphous FeSi granules. The decomposition of the insulating matrix is activated by annealing and enhanced compared to pure SiO by the presence of Fe, as if Fe attracts Si to form an FeSi amorphous alloy. The TMR is almost unchanged until an annealing temperature of 300 1C. Above 300 1C and before reaching the crystallization temperature, the TMR steeply decreases. These results are readily attributed to the presence of amorphous FeSi. r 2004 Elsevier B.V. All rights reserved. PACS: 61.46.+w; 73.50.Jt; 75.20.g; 75.47.m; 75.75.+a; 73.63.b; 81.15.Ef Keywords: Superparamagnetism; Tunneling magnetoresistance; Granular alloy; Amorphous alloy; Co-evaporation
1. Introduction Magnetoresistance (MR) resulting from spinpolarized tunneling was first experimentally observed by Julliere [1] in Fe/Ge/Co and Fe/Ge/Pb Corresponding author. Tel: +33-3-83-68-48-23; Fax: +33-
3-83-68-48-01. E-mail address: [email protected] (C. Bellouard).
junctions. It consists of a change in the tunnel conductance as a function of the relative orientation of the magnetization in the two ferromagnetic layers. The large MR response in low magnetic fields has stimulated considerable interest. However, fabrication of pinhole-free insulating barrier remains a major problem. MR was also observed in granular metal/insulator films [2] which consist of randomly distributed ferromagnetic particles in
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.06.034
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M. Anas et al. / Journal of Magnetism and Magnetic Materials 284 (2004) 165–171
an insulating matrix. For applications, these films are easier to fabricate, are very robust due to the protective insulating oxide, and are not as susceptible to electrical breakdown as tunnel junctions. They can be elaborated by codeposition or by alternate deposition of the magnetic metal and the insulating matrix (to obtain the so-called discontinuous multilayers). When there is no chemical mixing between the granules and the matrix, annealing at a moderate temperature (200–300 1C) only modifies the density and size of magnetic grains, it can then improve or lower the MR. For instance in the case of the Fe(SiO2) alloy, the MR effect is increased or decreased depending on the composition of the alloy [3]. In the same way, the MR effect is enhanced with annealing in Co/SiO2 multilayers [4] whereas annealing weakly decreases the MR observed for discontinuous Co80Fe20/ Al2O3 [5]. Otherwise, in the case of good wetting of the metal on the insulating layer during the alternate deposition, an annealing process is necessary to get isolated grains and a MR effect [6]. When there is a chemical interaction between the insulator and the metal, annealing has a more drastic effect on TMR. For instance, an annealing process is essential to induce a MR effect in Fe–PbO alloy as it creates a magnetic Fe2O3 compound between the metallic grains and the insulating matrix [7]. On the contrary, for the Fe/ ZrO2 system, annealing produces an iron oxidation likely responsible for the low MR observed in this system [8]. All these experiments show that annealing is an useful tool to check the interaction between the granules and the matrix and can then provide some indications about the absence or amplitude of a MR effect. We have recently performed a study of the structural and transport properties in co-evaporated Fex(SiO)1x [9] granular films as a function of the Fe volume fraction x. It has been shown that a superparamagnetic behaviour is obtained for 0:13pxp0:27 together with a tunnel magnetoresistance (TMR) for 0:2pxp0:27. Nevertheless, the maximum amplitude of the tunnel magnetoresistance is of the order of 2% at 100 K for 70 kOe which is rather low compared to other systems. To further investigate
the interplay between the structure and morphology of the grains and the MR effect, an annealing study with alloys in the superparamagnetic phase (0:13pxp0:27) is presented. Their structural and magnetotransport properties have been followed at each annealing step in the range 200–9501C by infrared absorption spectroscopy, transmission electron microscopy (TEM), magnetic and magnetoresistance measurements. It will be shown that these experiments offer a new insight into the behaviour of as-deposited samples.
2. Experimental details Fex(SiO)1x films have been deposited by coevaporation of Fe and SiO on a substrate kept at 100 1C. More details about the elaboration are given in Ref. [9]. In this paper, the volume fraction x has been evaluated using the bulk density of Fe and SiO. For an easier comparison with other systems, we will mention below the co-evaporated alloys by their atomic concentration c. The superparamagnetic behaviour observed for 0:13pxp0:27 corresponds to 0:3pcp0:5. The metallic behaviour appears for x ¼ 0:35 or c ¼ 0:6. Annealing has been performed in a quartz tube with a base pressure of 108 Torr. The heating is performed with a constant rate of 10 K/min. It is stopped and the quartz tube is removed from the furnace as soon as the annealing temperature T a is reached. Silicon substrates were used for infrared absorption spectrometry performed with a PerkinElmer 2000 spectrometer with a resolution of 4 cm1. Transmission electron microscopy has been performed with 500 A˚ thick films deposited on carbon grids. The films were also heated in the quartz tube. The magnetic properties were determined with a superconducting quantum interference device (SQUID) magnetometer. Field cooled (FC) and zero field cooled (ZFC) magnetization measurements have been performed as a function of increasing temperature for an applied field of 20 Oe.
ARTICLE IN PRESS M. Anas et al. / Journal of Magnetism and Magnetic Materials 284 (2004) 165–171
Transport measurements have been performed only in the current in plane (CIP) geometry with a superconducting coil equipped cryostat. The magnetoresistance is defined by: MRðH Þ ¼
RðH ¼ 0Þ RðH Þ : R ð H ¼ 0Þ
3. Results and discussion
167
increase and a shift of three absorption bands around 450, 750 and 1000 cm1 are observed. These bands are attributed to the infrared active vibrations of the Si–O–Si bonds. In particular, the main band at 1000 cm1 is due to the asymmetric stretching vibration of the oxygen atom in the Si–O–Si group. Fig. 3 shows the position of the main band as a function of the annealing temperature for pure SiO and for c ¼ 0:3. The
3.1. TEM and infrared spectroscopy 0.8 c = 0.30
Absorbance
The results obtained from electron diffraction are presented in the phase diagram (Fig. 1). For cp0:3, the alloy keeps its amorphous structure until 600 1C. It then crystallizes in a non-identified phase. The crystallization temperature is determined by the appearence of several fine rings. For an intermediate concentration 0:4pcp0:6, the crystallization temperature of BCC iron decreases with increasing Fe content. This result is similar to the annealing of Fec(SiO2)1c, where the crystallization temperature decreases from 700 to 480 1C when the iron fraction is increased from 0.3 to 0.79 [10]. Fig. 2. shows the infrared absorption spectra of Fe0.3(SiO)0.7 alloys for several annealing temperatures between 200 and 950 1C. The spectra evolve in the same way as those of a SiO compound [11]. For an annealing temperature above 200 1C, an
950°C
0.4
800°C 650°C
0 600
900
1200
500°C 360°C 200°C NR 1500
Wavenumber (cm-1) Fig. 2. Infrared absorption spectra as a function of the annealing temperature T a for the Fe0.3(SiO)0.7 alloy.
1100
Ta (°C)
600
400
SiO2
1080
α - Fe crystals
Wave number (cm-1)
800
unknown phase
amorphous
1060 1040 1020 c = 0.3
1000
200
pure SiO 980 0 0.0
0.2
0.4
0.6
0.8
0
200
400
600
800
1000
Ta (°C)
c (atomic fraction) Fig. 1. Phase diagram of the Fec(SiO)1c alloys as a function of the atomic fraction c and of the annealing temperature T a .
Fig. 3. Position of the maximum of the main infrared absorption band as a function of the annealing temperature T a for the Fe0.3(SiO)0.7 alloy (atomic percent) and pure SiO.
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followed during annealing. As the qualitative behaviour is identical for both alloys, only the results for c ¼ 0:5 are described. We recall that, according to Fig. 1, this alloy is expected to crystallize in the pure Fe BCC structure around T a ¼ 500 1C. First of all, we note that the moment per Fe atom measured at 5 K remains almost constant until T a ¼ 500 1C, then it slowly decreases to get, at 660 1C, 85% of the value of the as-deposited alloy. For 315 1CpT a p660 1C, the temperature dependence of the ZFC magnetization exhibits a well-defined maximum at a so-called blocking temperature T ¼ T B . The evolution of T B with annealing temperature is plotted in Fig. 4. It is almost constant until T a ¼ 500 1C, close to the crystallization temperature, then it steeply increases with T a . To provide a quantitative evolution of the size distribution of particles, the magnetization curve MðHÞ at 300 K , which is always above the blocking temperature, has been fitted by the sum of two Langevin Lðmi Þ, according functions to: MðHÞ ¼ M s ðT Þ xL m1 þ ð1 xÞL m2 : The mean particle moment is then given by:
1 m ¼ x 1x : m þ m 1
2
This treatment does not mean that there are two typical sizes in the particle distribution, it just 50 1500
TB <µ>
40
1000
30
20
<µ> (µB)
band corresponding to the Fe alloy is always at a higher frequency than the band of pure SiO, even for the as-deposited alloy. Both SiO and Fe(SiO) bands shift with annealing temperature until the frequency attributed to SiO2 is reached at 800 1C for the Fe alloy and at 950 1C for pure SiO. From previous experiments, it is generally assumed that there is a linear relation between the oxygen content n of the SiOn compound and the frequency of the maximum band [11,12]. The shift of the band is then the signature of the decomposition of SiO into Si and SiOn, with n41. The global shift of the frequencies for the Fe(SiO) compound compared to annealed pure SiO indicates that the decomposition of SiO is favoured by the presence of Fe. The remaining Si can form Si grains as in pure SiO [11] or can mix with Fe. We note that the formation of FeSi alloy was suspected in a Fe(SiO2) compound where the extra Si content in the magnetic particles was due to the sputtering process [10]. This assumption was asserted by the presence of Fe5Si3 in Fe(SiO2) films prepared at 500 1C. In our case, the assumption of the presence of an FeSi alloy even in the as-deposited alloys is in agreement with both the electron diffraction and the magnetization results as only an amorphous phase was observed for co0.6 with a reduced magnetic moment per Fe atom [9]. Following the linear relation between the frequency n of the main infrared band and the oxygen content n of the silicon oxide phase SiOn proposed by Rinnert et al. [11] (n ¼ ðn 918Þ=81), a stoichiometry n ¼ 1:32 was found for the asdeposited Fe0.4(SiO)0.6 alloy (atomic percent). This alloy would then correspond to (SiO1.32)0.45 (Fe0.73Si0.27)0.55. This is in agreement with the magnetic moment measured for this alloy, 1.4 mB/ Fe atom [9], as the magnetic moment per Fe atom of a Fe0.7Si0.3 is roughly 1.5 mB/Fe atom [13]. Nevertheless, the composition deduced from infrared spectroscopy should correspond to a mean value as a gradient of Fe composition may exist between grains or inside a grain.
TB (K)
168
500
c = 0.5
10 100
200
300
400
500
600
0 700
Ta (°C)
3.2. Magnetic properties The magnetic properties of Fec(SiO)1c alloys with c ¼ 0:4 and 0:5 (atomic fraction) have been
Fig. 4. Position of the maximum of the ZFC curve (the so- called blocking temperature T B ) and mean particle moment m as a function of the annealing temperature T a for the Fe0.5(SiO)0.5 alloy (atomic percent).
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provides a good fitting of the data, and allows us to evaluate easily the mean moment of the particle distribution om4. It is plotted in Fig. 4 as a function of T a . It increases from T a ¼ 315 1C although the blocking temperature clearly changes only above 500 1C. This feature indicates that despite the blocking temperature being unchanged when approaching the crystallization temperature, there is an evolution of the size or moment particle distribution. This is also clearly shown in Fig. 5 where the FC and ZFC magnetizations have been plotted as a function of temperature for the asdeposited alloy and for T a ¼ 500 and 570 1C. When the annealing temperature is increased, the low field magnetization measured at 100 K above the blocking temperature T B is increased and its shape above T B is modified. In the case of a single particle moment m, the magnetization far above T B can be expressed by the Curie law: MðHÞ ¼ M s ðT Þ
mðT ÞH : 3kB T
The temperature dependence of the saturation magnetization in the Curie law allows to take into account the temperature dependence of the magnetization of the amorphous FeSi grains before crystallization. This temperature dependence has to be introduced as the increase of magnetization with decreasing temperature is steeper than a 400 c = 0.5
as-d. ZFC as-d. FC 500°C ZFC 500°C FC 570°C ZFC 570°C FC
3
M (emu/cm )
300
200
100
0
0
50
100
T (K) Fig. 5. Magnetization of the ZFC and FC curves as a function of temperature for the Fe0.5(SiO)0.5 as-deposited alloy (atomic percent), and for annealing temperatures: T a ¼ 500 and 570 1C.
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simple 1/T dependence. For instance, the increase of magnetization of the as deposited sample between 50 and 25 K is larger than a factor 2. Within the assumption of the Curie law, an increase of low field magnetization MðHÞ can be directly related to an increase of m. In the present case of a size distribution, it seems reasonable to correlate the increase of low field magnetization with annealing temperature T a with the increase of mean particle moment, as the saturation magnetization M s remains constant or decreases with T a Moreover, one can notice that the bending of the FC–ZFC curves changes around 30 K for the asdeposited alloy whereas this change occurs around 60 K for T a ¼ 500 1C. Below this temperature, the magnetization no longer follows a Curie law and some particles become blocked. Despite there being no tail in the difference between the FC and ZFC curves, the evolution of the FC-ZFC curves suggests a broadening of the particule moment or size distribution when reaching the crystallization temperature. Finally, on one hand, annealing gives rise to an increase of the mean magnetic particle moment of the distribution, which can be associated to an increase of the size or of the Fe concentration of the FeSi particles. On the other hand, no increase of the magnetization is observed. On the contrary, the magnetization slightly decreases with annealing above 500 1C, it must then correspond to an average of 2.2 mB/Fe atom with a much lower value due to poorly or non magnetic iron. During annealing, as a part of the FeSi alloy gets a higher Fe concentration to form the Fe BCC structure when crossing the crystallization temperature, another part, whose fraction increases, is lost in the matrix. As a matter of fact, annealing results in a demixion-like process. From infrared spectroscopy, we have deduced that, during annealing, Si leaves the matrix to either form Si grains or a FeSi alloy. The magnetization measurements only agree with the second assumption : BCC iron coexists with amorphous FeSi alloy which can be non or poorly magnetic. 3.3. Transport properties The room-temperature resistivity is plotted as a function of T a for c ¼ 0:4 and 0:5 in Fig. 6a. For
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both compositions, annealing has no appreciable effect until T a ¼ 400 1C. Above this temperature, very different behaviours are observed for these close compositions. A sharp increase of the resistivity is observed for c ¼ 0:4 whereas it decreases with annealing for c ¼ 0:5. First of all, neglecting any chemical interaction between the granules and the matrix, annealing has generally two effects upon the evolution of the grains size distribution: it increases the mean size of the grains and, as a matter of fact, decreases their density or increases the separation between grains. Moreover, in our case, things are more complicated because of the decomposition of the SiO matrix which becomes more oxidized. The gap energy of the dielectric is then increased. Annealing has then conflicting effects on the tunneling mechanism between grains. On one hand, the increase of mean 2.5
MR (% )
2 1.5 1 c = 0.4 c = 0.5
0.5 (b)
0
80 c = 0.4 c = 0.5
ρ (Ω cm)
60 40 20 0 (a)
0
100
200
300
400
grain size may increase the tunnel probalility, on the other hand, the increase of the intergrain distance and of the potential barrier tends to lower the tunneling. The first case is prevailing for x ¼ 0:5 whereas the second case corresponds to x ¼ 0:4. 3.4. Magnetoresistance Fig. 6b shows the magnetoresistance MR measured at 70 kOe and 50 K (for which the signal is saturated) for c ¼ 0:4 and 0:5 as a function of the annealing temperature. The results are very similar for both alloys. A reasonable annealing to about 200–300 1C, as was performed for some discontinuous multilayers [4,5], has a poor effect on the MR effect. Above 3001C and before crystallization, annealing drastically decreases the MR effect which becomes lower than 0.5% for T a ¼ 500 1C. We note that both alloys exhibit the same TMR behaviour during annealing whereas opposite variation has been observed for their resistivity. On the contrary, for Fe(SiO2) alloys, annealing can increase or decrease the TMR effect, depending on the composition [3]. This discrepancy between both Fe(SiO) and Fe(SiO2) systems can be attributed to the different interactions between the granules and the matrix. The stoichiometric Fe(SiO2) system is rather stable, as no FeSi alloy has been detected until an annealing at 900 1C [14]. In our case, as was discussed above, we can assume that the Si content of the metallic grains increases with annealing temperature. This assumption is rather conflicting with the increase of the mean particle moment. To conciliate both results, we propose the following picture : the Fe-rich core of the magnetic particles which grows during annealing is surrounded by an FeSi alloy which becomes richer in Si. This weak or non magnetic interface between magnetic granules and insulating SiOx can explain the decreasing MR effect with annealing.
500
Ta (°C) Fig. 6. Resistivity r at 300 K (a) and magnetoresistance MR (b) measured at 50 K and 70 kOe as a function of the annealing temperature T a for the Fec(SiO)1c alloys with c ¼ 0:4 and 0:5.
4. Conclusion Structural, magnetic and transport properties of co-evaporated granular Fex(SiO)1x alloys have
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been followed during annealing. Infrared absorption spectroscopy shows that the SiO matrix decomposes in SiOx and Si even for as-deposited alloys. This decomposition is activated by annealing and enhanced with respect to pure SiO by the presence of Fe, as if Fe attracts Si to form an FeSi amorphous alloy. The magnetization remains largely reduced with respect to bulk iron even above the crystallization temperature. This reveals that an FeSi alloy is always present in the composite. This interaction between the metal and the insulating matrix explains the rather low TMR effect in Fe(SiO) with respect to the Fe(SiO2) system as the polarization of conduction electrons in the FeSi alloy is damaged compared to bulk iron. The TMR is almost constant until an annealing temperature of 300 1C, and then steeply decreases to a value lower than 0.5% (measured at 50 K) before the crystallization temperature. References [1] X. Julliere, Phys. Lett. 54A (1975) 225. [2] Y. Goldstein, J.I. Gittleman, Solid State Commun 9 (1971) 1197.
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[3] S. Honda, T. Okada, M. Nawate, J. Magn. Magn. Mater. 165 (1997) 153. [4] B. Dieny, S. Sankar, M.R. McCartney, D.J. Smith, P. Bayle-Guillemaud, A.E. Berkowitz, J. Magn. Magn. Mater. 185 (1998) 283. [5] G.N. Kakazei, Y.G. Pogorelov, A.M.L. Lopes, J.B. Sousa, S. Cardoso, P.P. Freitas, M.M. Pereira de Azevedo, E. Snoeck, J. Appl. Phys. 90 (2001) 4044. [6] S. Sankar, B. Dieny, A.E. Berkowitz, J. Appl. Phys. 81 (1997) 5512. [7] Y.-H. Huang, J.-H. Hsu, J.W. Chen, C.-R. Chang, Appl. Phys. Lett. 72 (1998) 2171; J.-H. Hsu, Y.-H. Huang, J. Magn. Magn. Mater. 203 (1999) 94. [8] P. Auric, J.S. Micha, O. Proux, L. Giacomoni, J.R. Regnard, J. Magn. Magn. Mater. 217 (2000) 175. [9] M. Anas, C. Bellouard, M. Vergnat, J. Appl. Phys. 88 (2000) 6075. [10] R.L. Holtz, A.S. Edelstein, P. Lubitz, C.R. Gossett, J. Appl. Phys. 64 (1988) 4251. [11] H. Rinnert, M. Vergnat, A. Burneau, J. Appl. Phys. 89 (2001) 237. [12] P.G. Pai, S.S. Chao, Y. Takagi, G. Lucovsky, J. Vac. Sci. Technol. B 4 (1986) 689. [13] G. Marchal, Ph. Mangin, M. Piecuch, C. Janot, J. Hubsch, J. Phys. F: Metal Phys. 7 (1977) L165. [14] D.P. Yang, Y.D. Zhang, S. Hui, J. Appl. Phys. 91 (2002) 8198.