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SiO2 multilayers
Materials Science and Engineering B 112 (2004) 120–122 www.elsevier.com/locate/mseb
Structural and magnetotransport properties of discontinuous Co/SiO2 multilayers J.C. Denardina,*, M. Knobela, L.S. Dornelesb,1, L.F. Schelpb a
Instituto de Fı´sica Gleb Wataghin (IFGW), Universidade Estadual de Campinas (UNICAMP), C.P. 6165, Campinas S.P., Brazil b Department de Fı´sica, CCNE, UFSM, 97105-900, Santa Maria, R.S., Brazil
Abstract Results of structural, magnetic and transport properties of magnetic Co/SiO2 granular multilayers, consisting of successive planes of nanosized cobalt clusters embedded in SiO2 and produced by sequential deposition, are presented. Microscopy shows that samples can be composed of a periodical array of Co nanoparticles. Relationships between the nanostructure and magnetotransport properties are discussed. ß 2004 Elsevier B.V. All rights reserved. Keywords: Granular systems; Magnetic nanoparticles; Spin-dependent electronic transport
1. Introduction Transport, magnetoresistance, magnetic, dielectric and optical properties of both magnetic and non-magnetic granular metals or cermets have been studied since late sixties [1]. Co-sputtering or co-evaporation are common methods to fabricate these granular solids. It usually results in a random distribution of clusters in the matrix, with a large dispersion of grain sizes and distances [1]. In metal/insulating cosputtered composites, one can observe different physical properties by changing the metallic concentrations, which makes these nanostructured materials an ideal environment to investigate interactions among magnetic particles [2–5], tunnel magnetoresistance [6], superparamagnetism [7] and several new phenomena that emerge with the reduced dimensions of the crystallites. In order to study the influence of the microstructure on the magnetic and magnetotransport properties of these granular materials, we have used an alternative method to fabricate granular multilayers consisting of successive planes of nanosized cobalt clusters embedded in SiO2 and produced by sequential deposition. In contrast to co-deposition, with this method the size and distance among clusters * Corresponding author. Tel.: +55197885504; fax: +55192893137. E-mail address: [email protected] (J.C. Denardin). 1 Bolsista do CNPq, Brasil. 0921-5107/$ – see front matter ß 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.05.016
can be controlled by varying the thickness of the layers. This allows one to obtain a periodic array of almost monodisperse nanoparticles in a solid matrix. Varying the nominal thickness of Co and SiO2 layers we obtained several different samples, ranging from insulating to metallic. TEM images show that the most insulating samples are composed of periodically arranged Co nanoparticles with approximately 3 nm in diameter. Samples that are close to metal-insulator transition are composed by a connected network of metallic paths. Measurements of magnetization, resistivity and magnetoresistance were made in the temperature range of 5– 300 K, and fields up to 4 kOe. Relationships between the nanostructure and magnetotransport properties are discussed and compared with previous results obtained in Co sputtered films.
2. Experiment Co/SiO2 multilayers were grown by DC (Co) and RF (SiO2) sputtering on Si substrates held at room temperature. A 10 nm buffer layer of SiO2 was deposited on the Si substract before the sequential deposition. Cross-section TEM characterization was performed using a Jeol JEM3010 ARP microscope. Magnetization and transport properties were measured in a Quantum Design MPMS XL7
J.C. Denardin et al. / Materials Science and Engineering B 112 (2004) 120–122
system in the temperature range 5–300 K and fields up to 4 kOe. Magnetoresistance was measured using an electrometer and the method of force voltage/measure current, due to the high resistance of the samples.
M (emu/cm3 Oe)
6
3. Results and discussion When varying the nominal thickness of Co and SiO2 layers it is possible to obtain different samples, ranging from insulating to metallic. The two samples presented here are labeled in order of increasing resistance as: (A) [Co (0.7 nm)/SiO2 (2 nm)]x10 and (B) [Co (0.7 nm)/SiO2 (3 nm)]x10. As shown in the TEM image of Fig. 1, the most insulating sample (sample B) is composed of periodically arranged Co nanoparticles with 3 nm in diameter. This sample is a result of a sequential deposition of 10 bi-layers composed of 0.7 nm of Co and 3 nm of SiO2. As the first layer of Co clusters is obtained by growing an ultra thin layer of Co, the large difference between the surface energies of Co and SiO2 leads to tridimensional growth of Co and to the formation of clusters on the SiO2 buffer layer. The next amorphous SiO2 layer wets perfectly this granular Co layer and small surface undulations appear. In the next Co layer the clusters are formed preferentially on the hollows of the wavy surface. The periodic repetition of this phenomenon is at the origin of the observed self-organized growth [8]. The clusters of the sample shown in Fig. 1 are superparamagnetic at room temperature, and the blocking temperature occurs at 100 K in the zero-field cooled/field cooling (ZFC/FC) curve, as shown in Fig. 2. These features make this sample suited for the study of dipolar interactions [5] and tunnel magnetoresistance. In sample A the blocking temperature is observed at approximately 170 K (see Fig. 2). This increase in the blocking temperature is associated with the appearance of larger particles in this sample, which can remain blocked at higher temperatures.
121
5
sample A
4 3 FC
2
0
0
sample B
TB
1 ZFC
50 100 150 200 250 300
T (K) Fig. 2. Zero-field cooled (ZFC), field cooled (FC) magnetization curves for sample A and sample B. The field used for these measurements was 20 Oe.
From analysis of the ZFC/FC curves, shown in Fig. 2, one can directly associate the position of the maximum of the ZFC curve with the mean blocking temperature TB [5]. Such value allows one to estimate the diameter of particles Dm for sample B, using the the Ne´ el [5] expression for the relaxation time of a superparamagnetic particle t = t0 exp(Ea/kBT), where Ea = KV is the particle’s energy barrier separating the energy minima, K is the uniaxial anisotropy constant of bulk hcp cobalt (K = 6 106 erg/cm3), V is the particle’s volume, kB is the Boltzmann’s constant, and T is the absolute temperature. The pre-exponential factor t0 is a constant related to gyromagnetic precession, and may be considered to be of the order of 10 9 to 10 10 s [5]. The blocking temperature TB, for a system of particles with mean volume V, is defined as the temperature at which t = tm, the measurement time (typically on the order of 100 s for dc measurements). For the value of TB = 100 K, we can calculate the corresponding mean diameter of the particles Dm = 5 nm, that is larger than the value observed in Fig. 1. The discrepancy between the experimental and expected values of Dm could be explained by a larger anisotropy constant of the particles, for instance, to interface effects with the SiO2 matrix, and/or the effect of a collective magnetic state arising from the dipolar interactions between T(K) 10
4
10
3
10
2
10
1
10
0
100
25.0
11.11
ρ (Ω cm)
(b)
6.25
4.00
sample B
sample A
-1
10
0.1
0.2
T Fig. 1. Cross-section TEM image of a [Co 0.7 nm/SiO2 3 nm]10 (sample B) granular multilayer.
0.3
-1/2
0.4
0.5
-1
(K )
Fig. 3. Resistivity (in logarithmic scale) vs. T 1/2 for samples A and B and respective fitting to the law r(T) = r0 exp{(T0/T)1/2}.
122
J.C. Denardin et al. / Materials Science and Engineering B 112 (2004) 120–122
At 100 K and above there is no observable coercivity in the curves, indicating that the nanoparticles are superparamagnetic.
0
(R - R0)/R0 (%)
-1 -2
300 K
-3
4. Conclusion
-4
100 K
-5 -6 -20 -15 -10
25 K -5
0
5
10
15
20
H (kOe) Fig. 4. DR/R0 = [R(H) R (0)]/R (0) for sample B as function of magnetic field, measured at different temperatures.
individual particles, in agreement with previous conclusions obtained from the analysis of ZFC/FC curves [5]. The dominant type of conductance in the samples was initially determined from the analysis of the temperature dependence of resistivity. Shown in Fig. 3 are the resistivity versus temperature curves measured for samples A and B, plotted as r (in logarithmic scale) versus T 1/2. This linear dependence of r and a negative TCR are typical of insulating granular systems and they are attributed to thermally activated and/or bias assisted tunneling processes, and a broad distribution of particle sizes and separations [1]. These curves can be well fitted by a r(T) = r0 exp{(T0/T)1/2} law [1], where the value of T0 is proportional to s/d, where s is the grain separation and d is the grain diameter. Fig. 4 show the results of magnetoresistance for the sample B, where TMR ratios of about 3% were obtained at room temperature and 5% at 25 K. MR was measured with both the electric current and magnetic field parallel to the film plane. At 25 K the resistance of sample is about 109 Ohms, and the measurement was only possible with the use of an electrometer. Magnetization curves for these samples are well saturated for fields above 2 kOe (see Fig. 5). The coercive field at 25 K is ~500 Oe, measured by both MR and magnetization.
200
25 K 100 K 300 K
200
25 K 100 K
Acknowledgements This work was financially supported by FAPESP, FAPERGS and CNPq. The TEM images were performed at the Laborato´ rio de Microscopia Eletroˆ nica (LME, LNLS), Campinas, Brazil.
References [1] [2] [3] [4] [5]
0
0
3
M (emu/cm )
3
M (emu/cm )
400
Results of structural, magnetic and transport properties obtained in granular Co/SiO2 multilayers were presented. Maximum GMR values were observed in samples that display T n behavior, which is a signature of hopping or tunneling. An enhancement of MR values with decreasing temperature well below the blocking temperature of the system was observed. As at lower temperatures (5 K or below) these systems can present enhanced GMR values, due to coulomb blockade effects [9], a more detailed study at lower temperatures is being performed in these systems. The increase of density requirements in the magnetic data storage has motivated the research of nanostructured magnetic devices. Granular films consisting of magnetic nanoparticles in an insulating matrix could present promising features for ultrahigh density magnetic recording media [10], respecting the superparamagnetic limit and the effect of dipolar interactions. In contrast with the co-deposition, with this method the size and distance between clusters can be controlled by varying the thickness of the layers. This allows one to obtain periodic arrays of monodisperse nanoparticles in a solid matrix.
-200
-400 -20 -15 -10 -5
0
[6]
-200 -3
300 K -2
-1
0
1
2
3
H (kOe) 5
10
15 20
[7] [8] [9]
H (kOe) [10] Fig. 5. Magnetization curves measured at different temperatures for samples B. The curves are well saturated at 20 kOe.
B. Abeles, P. Sheng, M.D. Coutts, Y. Arie, Adv. Phys. 24 (1975) 40. M. Va´ zquez, A. Hernando, Adv. Mater. 7 (1995) 1021. G. Herzer, Phys. Scripta T49 (1993) 307. A. Hernando, T. Kulik, Phys. Rev. B 49 (1994) 7064. J.C. Denardin, A.L. Brandl, M. Knobel, P. Panissod, A.B. Pakhomov, H. Liu, X.X. Zhang, Phys. Rev. B 65 (2002) 064422. A. Milner, A. Gerber, B. Groisman, M. Karpovsky, A. Gladkikh, Phys. Rev. Lett. 76 (1996) 475. A. Slawska-Waniewska, M. Gutowski, H.K. Lachowicz, T. Kulik, H. Matyja, Phys. Rev. B 46 (1992) 14594. J.L. Maurice, J. Briatico, J. Carrey, F. Petroff, L.F. Schelp, A. Vaures, Philos. Mag. A 79 (1999) 2921. L.F. Schelp, A. Fert, F. Fettar, P. Holody, S.F. Lee, J.L. Mourice, F. Petroff, A. Vaures, Phys. Rev. B 56 (1997) R5747. V. Skumryev, S. Stoyanov, Y. Zhang, G. Hadjipanayis, D. Givord, J. Nogue’s, Nature 423 (2003) 850.
Structural and magnetotransport properties of discontinuous Co/SiO2 multilayers J.C. Denardina,*, M. Knobela, L.S. Dornelesb,1, L.F. Schelpb a
Instituto de Fı´sica Gleb Wataghin (IFGW), Universidade Estadual de Campinas (UNICAMP), C.P. 6165, Campinas S.P., Brazil b Department de Fı´sica, CCNE, UFSM, 97105-900, Santa Maria, R.S., Brazil
Abstract Results of structural, magnetic and transport properties of magnetic Co/SiO2 granular multilayers, consisting of successive planes of nanosized cobalt clusters embedded in SiO2 and produced by sequential deposition, are presented. Microscopy shows that samples can be composed of a periodical array of Co nanoparticles. Relationships between the nanostructure and magnetotransport properties are discussed. ß 2004 Elsevier B.V. All rights reserved. Keywords: Granular systems; Magnetic nanoparticles; Spin-dependent electronic transport
1. Introduction Transport, magnetoresistance, magnetic, dielectric and optical properties of both magnetic and non-magnetic granular metals or cermets have been studied since late sixties [1]. Co-sputtering or co-evaporation are common methods to fabricate these granular solids. It usually results in a random distribution of clusters in the matrix, with a large dispersion of grain sizes and distances [1]. In metal/insulating cosputtered composites, one can observe different physical properties by changing the metallic concentrations, which makes these nanostructured materials an ideal environment to investigate interactions among magnetic particles [2–5], tunnel magnetoresistance [6], superparamagnetism [7] and several new phenomena that emerge with the reduced dimensions of the crystallites. In order to study the influence of the microstructure on the magnetic and magnetotransport properties of these granular materials, we have used an alternative method to fabricate granular multilayers consisting of successive planes of nanosized cobalt clusters embedded in SiO2 and produced by sequential deposition. In contrast to co-deposition, with this method the size and distance among clusters * Corresponding author. Tel.: +55197885504; fax: +55192893137. E-mail address: [email protected] (J.C. Denardin). 1 Bolsista do CNPq, Brasil. 0921-5107/$ – see front matter ß 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.05.016
can be controlled by varying the thickness of the layers. This allows one to obtain a periodic array of almost monodisperse nanoparticles in a solid matrix. Varying the nominal thickness of Co and SiO2 layers we obtained several different samples, ranging from insulating to metallic. TEM images show that the most insulating samples are composed of periodically arranged Co nanoparticles with approximately 3 nm in diameter. Samples that are close to metal-insulator transition are composed by a connected network of metallic paths. Measurements of magnetization, resistivity and magnetoresistance were made in the temperature range of 5– 300 K, and fields up to 4 kOe. Relationships between the nanostructure and magnetotransport properties are discussed and compared with previous results obtained in Co sputtered films.
2. Experiment Co/SiO2 multilayers were grown by DC (Co) and RF (SiO2) sputtering on Si substrates held at room temperature. A 10 nm buffer layer of SiO2 was deposited on the Si substract before the sequential deposition. Cross-section TEM characterization was performed using a Jeol JEM3010 ARP microscope. Magnetization and transport properties were measured in a Quantum Design MPMS XL7
J.C. Denardin et al. / Materials Science and Engineering B 112 (2004) 120–122
system in the temperature range 5–300 K and fields up to 4 kOe. Magnetoresistance was measured using an electrometer and the method of force voltage/measure current, due to the high resistance of the samples.
M (emu/cm3 Oe)
6
3. Results and discussion When varying the nominal thickness of Co and SiO2 layers it is possible to obtain different samples, ranging from insulating to metallic. The two samples presented here are labeled in order of increasing resistance as: (A) [Co (0.7 nm)/SiO2 (2 nm)]x10 and (B) [Co (0.7 nm)/SiO2 (3 nm)]x10. As shown in the TEM image of Fig. 1, the most insulating sample (sample B) is composed of periodically arranged Co nanoparticles with 3 nm in diameter. This sample is a result of a sequential deposition of 10 bi-layers composed of 0.7 nm of Co and 3 nm of SiO2. As the first layer of Co clusters is obtained by growing an ultra thin layer of Co, the large difference between the surface energies of Co and SiO2 leads to tridimensional growth of Co and to the formation of clusters on the SiO2 buffer layer. The next amorphous SiO2 layer wets perfectly this granular Co layer and small surface undulations appear. In the next Co layer the clusters are formed preferentially on the hollows of the wavy surface. The periodic repetition of this phenomenon is at the origin of the observed self-organized growth [8]. The clusters of the sample shown in Fig. 1 are superparamagnetic at room temperature, and the blocking temperature occurs at 100 K in the zero-field cooled/field cooling (ZFC/FC) curve, as shown in Fig. 2. These features make this sample suited for the study of dipolar interactions [5] and tunnel magnetoresistance. In sample A the blocking temperature is observed at approximately 170 K (see Fig. 2). This increase in the blocking temperature is associated with the appearance of larger particles in this sample, which can remain blocked at higher temperatures.
121
5
sample A
4 3 FC
2
0
0
sample B
TB
1 ZFC
50 100 150 200 250 300
T (K) Fig. 2. Zero-field cooled (ZFC), field cooled (FC) magnetization curves for sample A and sample B. The field used for these measurements was 20 Oe.
From analysis of the ZFC/FC curves, shown in Fig. 2, one can directly associate the position of the maximum of the ZFC curve with the mean blocking temperature TB [5]. Such value allows one to estimate the diameter of particles Dm for sample B, using the the Ne´ el [5] expression for the relaxation time of a superparamagnetic particle t = t0 exp(Ea/kBT), where Ea = KV is the particle’s energy barrier separating the energy minima, K is the uniaxial anisotropy constant of bulk hcp cobalt (K = 6 106 erg/cm3), V is the particle’s volume, kB is the Boltzmann’s constant, and T is the absolute temperature. The pre-exponential factor t0 is a constant related to gyromagnetic precession, and may be considered to be of the order of 10 9 to 10 10 s [5]. The blocking temperature TB, for a system of particles with mean volume V, is defined as the temperature at which t = tm, the measurement time (typically on the order of 100 s for dc measurements). For the value of TB = 100 K, we can calculate the corresponding mean diameter of the particles Dm = 5 nm, that is larger than the value observed in Fig. 1. The discrepancy between the experimental and expected values of Dm could be explained by a larger anisotropy constant of the particles, for instance, to interface effects with the SiO2 matrix, and/or the effect of a collective magnetic state arising from the dipolar interactions between T(K) 10
4
10
3
10
2
10
1
10
0
100
25.0
11.11
ρ (Ω cm)
(b)
6.25
4.00
sample B
sample A
-1
10
0.1
0.2
T Fig. 1. Cross-section TEM image of a [Co 0.7 nm/SiO2 3 nm]10 (sample B) granular multilayer.
0.3
-1/2
0.4
0.5
-1
(K )
Fig. 3. Resistivity (in logarithmic scale) vs. T 1/2 for samples A and B and respective fitting to the law r(T) = r0 exp{(T0/T)1/2}.
122
J.C. Denardin et al. / Materials Science and Engineering B 112 (2004) 120–122
At 100 K and above there is no observable coercivity in the curves, indicating that the nanoparticles are superparamagnetic.
0
(R - R0)/R0 (%)
-1 -2
300 K
-3
4. Conclusion
-4
100 K
-5 -6 -20 -15 -10
25 K -5
0
5
10
15
20
H (kOe) Fig. 4. DR/R0 = [R(H) R (0)]/R (0) for sample B as function of magnetic field, measured at different temperatures.
individual particles, in agreement with previous conclusions obtained from the analysis of ZFC/FC curves [5]. The dominant type of conductance in the samples was initially determined from the analysis of the temperature dependence of resistivity. Shown in Fig. 3 are the resistivity versus temperature curves measured for samples A and B, plotted as r (in logarithmic scale) versus T 1/2. This linear dependence of r and a negative TCR are typical of insulating granular systems and they are attributed to thermally activated and/or bias assisted tunneling processes, and a broad distribution of particle sizes and separations [1]. These curves can be well fitted by a r(T) = r0 exp{(T0/T)1/2} law [1], where the value of T0 is proportional to s/d, where s is the grain separation and d is the grain diameter. Fig. 4 show the results of magnetoresistance for the sample B, where TMR ratios of about 3% were obtained at room temperature and 5% at 25 K. MR was measured with both the electric current and magnetic field parallel to the film plane. At 25 K the resistance of sample is about 109 Ohms, and the measurement was only possible with the use of an electrometer. Magnetization curves for these samples are well saturated for fields above 2 kOe (see Fig. 5). The coercive field at 25 K is ~500 Oe, measured by both MR and magnetization.
200
25 K 100 K 300 K
200
25 K 100 K
Acknowledgements This work was financially supported by FAPESP, FAPERGS and CNPq. The TEM images were performed at the Laborato´ rio de Microscopia Eletroˆ nica (LME, LNLS), Campinas, Brazil.
References [1] [2] [3] [4] [5]
0
0
3
M (emu/cm )
3
M (emu/cm )
400
Results of structural, magnetic and transport properties obtained in granular Co/SiO2 multilayers were presented. Maximum GMR values were observed in samples that display T n behavior, which is a signature of hopping or tunneling. An enhancement of MR values with decreasing temperature well below the blocking temperature of the system was observed. As at lower temperatures (5 K or below) these systems can present enhanced GMR values, due to coulomb blockade effects [9], a more detailed study at lower temperatures is being performed in these systems. The increase of density requirements in the magnetic data storage has motivated the research of nanostructured magnetic devices. Granular films consisting of magnetic nanoparticles in an insulating matrix could present promising features for ultrahigh density magnetic recording media [10], respecting the superparamagnetic limit and the effect of dipolar interactions. In contrast with the co-deposition, with this method the size and distance between clusters can be controlled by varying the thickness of the layers. This allows one to obtain periodic arrays of monodisperse nanoparticles in a solid matrix.
-200
-400 -20 -15 -10 -5
0
[6]
-200 -3
300 K -2
-1
0
1
2
3
H (kOe) 5
10
15 20
[7] [8] [9]
H (kOe) [10] Fig. 5. Magnetization curves measured at different temperatures for samples B. The curves are well saturated at 20 kOe.
B. Abeles, P. Sheng, M.D. Coutts, Y. Arie, Adv. Phys. 24 (1975) 40. M. Va´ zquez, A. Hernando, Adv. Mater. 7 (1995) 1021. G. Herzer, Phys. Scripta T49 (1993) 307. A. Hernando, T. Kulik, Phys. Rev. B 49 (1994) 7064. J.C. Denardin, A.L. Brandl, M. Knobel, P. Panissod, A.B. Pakhomov, H. Liu, X.X. Zhang, Phys. Rev. B 65 (2002) 064422. A. Milner, A. Gerber, B. Groisman, M. Karpovsky, A. Gladkikh, Phys. Rev. Lett. 76 (1996) 475. A. Slawska-Waniewska, M. Gutowski, H.K. Lachowicz, T. Kulik, H. Matyja, Phys. Rev. B 46 (1992) 14594. J.L. Maurice, J. Briatico, J. Carrey, F. Petroff, L.F. Schelp, A. Vaures, Philos. Mag. A 79 (1999) 2921. L.F. Schelp, A. Fert, F. Fettar, P. Holody, S.F. Lee, J.L. Mourice, F. Petroff, A. Vaures, Phys. Rev. B 56 (1997) R5747. V. Skumryev, S. Stoyanov, Y. Zhang, G. Hadjipanayis, D. Givord, J. Nogue’s, Nature 423 (2003) 850.