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Structural and magnetic properties of TM–SiO2 (TM = Fe, Co, Ni) films
Journal of Magnetism and Magnetic Materials 262 (2003) 102–106
Structural and magnetic properties of TM–SiO2 (TM = Fe, Co, Ni) films L.M. Socolovskya,*, J.C. Denardina, A.L. Brandla, M. Knobela, X.X. Zhangb a
! de Materiais e Baixas Temperatures, Departamento de F!ısica da Materia Condensada, Instituto de F!ısica ‘‘Gleb Wataghin’’, Laboratorio Universidade Estadual de Campinas, CP 6165, Campinas (CP) 13083-970, Brazil b Physics Department and Institute of Nanoscience and Technology (INST), Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China
Abstract TMx–(SiO2)1x (TM=Fe, Co, Ni) thin films were prepared in a wide concentration range (0.35 pxp1). Structure was studied with transmission electron microscopy (TEM), X-ray diffraction (XRD) and small angle X-ray scattering (SAXS). Magnetic and magnetotransport properties were investigated by means of magnetization and Hall effect measurements. TEM images display nanometric spherical structures embedded in a SiO2 amorphous matrix, with typical sizes increasing from 3 to 5 nm when TM volume concentration x is increased. SAXS measurements indicate a complex structure formed by nanosized objects. XRD measurements show that the structure is composed by amorphous SiO2 and TM crystallites. Slightly above the percolation threshold all samples display giant Hall effect. The observed magnetic properties are dependent on x, and display an evolution resulting from the progressive increase of the mean particle size. r 2003 Elsevier Science B.V. All rights reserved. PACS: 73.63.Bd; 75.50.Tt; 75.75.+a Keywords: Magnetic thin films; Magnetic nanoparticles; SAXS
1. Introduction Giant Hall effect (GHE) is a new property found in granular metals [1], usually in transition metal–dielectric alloys. It was found that near the critical concentration of metal in the composite where a metal–insulator transition occurs, both the ordinary and extraordinary parts of the Hall effect display a huge increase when compared to *Corresponding author. Tel.: +55-19-37885504. E-mail address: leandros@ifi.unicamp.br (L.M. Socolovsky).
the value measured in the metallic regime. Although the GHE in non-magnetic granular systems has been successfully accounted for within the local quantum interference model [2,3], the understanding of the GHE in magnetic granular materials is still far from complete [1,4]. To gain a deeper understanding of GHE in magnetic materials, it is believed that a systematic investigation is essential. Therefore, in order to investigate this effect thoroughly we have prepared a set of Fe– SiO2, Co–SiO2, Ni–SiO2 samples by magnetron co-sputtering, and studied their structural, magnetotransport and magnetic properties.
0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0304-8853(03)00028-3
L.M. Socolovsky et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 102–106
(a)
Intensity [counts]
0.87
(b)
(c)
0.85
0.84
0.72
0.61
103
0.73 0.56 0.54
0.58
0.41 0.32 0.49
40
43
46
49
40
43
46 2Θ
49
40
43
46
49
Fig. 1. (a) XRD diffractograms of the Fex–(SiO2)1x. (b) Cox–(SiO2)1x. (c) Nix–(SiO2)1x.
2. Experimental Thin films of Fe–SiO2, Co–SiO2, Ni–SiO2 were deposited onto Kapton and glass sheets by magnetron co-sputtering. Substrate was kept at room temperature during the process. Pressure before sputtering was 107 Torr and during the process it was kept at 5 mTorr, the chamber filled with Ar. The glass substrates were rotated during sputtering, to ensure composition uniformity. The Fe, Co, or Ni, volume fraction was controlled by the relative sputtering rates, and were then determined by energy-dispersive X-ray spectroscopy using a Philips EDAX XL30 device. We will use x to indicate the transition metal (TM) volume fraction. X-ray diffraction (XRD) was obtained in a Philips PW 1820 diffractometer, equipped with a single-crystal monochromator. Scans were done in the samples evaporated on glass using CuKa radiation, in the usual y–2y configuration. No filters were used. Transmission electron microscopy (TEM) was done using a Jeol JEM-3010 ARP microscope at the Microscopy Facility.1 Small angle X-ray scattering (SAXS) was measured in the SAS station of the LNLS, using a wavelength of l=0.1758 nm. Data were collected 1 ! Laboratorio Nacional de Luz S!ıncrotron (LNLS), Campinas,SP, Brazil.
in transmission geometry with a linear position sensitive detector placed at 773.0 mm from the sample. The detector was mounted vertically. The camera length corresponds to a scattering vector q ( 1. Measureranging from 0.01322 to 0.36425 A ments were done in a vacuum of 101 Torr. No mathematical desmearing was required, because of the small dimensions of the X-ray spot (1 mm vertical and 5 mm horizontal, approximately). Obtained data were corrected through the appropriate procedures. Zero-field cooling magnetization (ZFC) was measured with a SQUID magnetometer. Hall resistance measurements were performed using the Van der Pauw method.
3. Results and discussion XRD show the existence of two phases (Fig. 1): one amorphous with a maximum located at E 25 (not seen in the graph), which corresponds to SiO2, and the other one crystalline, identified by a peak in the samples with x above 0.50. This peak is located, in the case of samples with Fe and Ni, at the positions corresponding to the (1 1 0) and (1 1 1) reflections of the BCC Fe and FCC Ni, respectively. At higher x the intensity of the crystalline peak grows, and its width decreases, indicating an increase in the crystallite size. For
L.M. Socolovsky et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 102–106
104
Co–SiO2 samples, at the low concentration region, a wide peak located between 40 and 50 appears. With further increase of Co concentration three peaks are visible in the same region. These broadened peaks correspond to the (1 0 0), (0 0 2) and (1 0 1) reflections of the HCP-cobalt. At the highest concentrations small rounded peaks appear in the high-angle region for all systems. After experimental corrections one can calculate the mean crystallite size hDi from the highest peak in each system using the Scherrer formula: hDi ¼ 60 Cox-(SiO2)1-x (TEM) Co x-(SiO2)1-x (XRD) Nix-(SiO2)1-x (TE M) Ni x-(SiO2)1-x (XRD) Fex-(SiO2)1-x (XRD)
[nm]
50 40 30 20 10 0 0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
x [vol.] Fig. 2. Mean particle size estimated from XRD and TEM.
0:94l=b cosy; where l is the wavelength of the Xray, b the width of the peak (measured in radians in the scale 2y) and y is the position of the maximum of the peak. Results are shown in Fig. 2, together with mean size estimations obtained from TEM images. TEM photographs show a system of spherical particles with sizes varying from about 3 to 5 nm when x increases (not shown here). Above x E 0.5 the system shows arrangements of touching spheres. At higher x complex structures are viewed. Average size of the spherical particles was estimated from several TEM photographs. SAXS spectra (Fig. 3) display similar features for the three systems: a fall in the intensity with q, a flat and rather broad peak located between 0.02 ( 1, and a rounded peak that shifts to and 0.09 A lower q when x increases. The objects that produce these peaks have rather broad size dispersion (evidenced by the width of the peaks in the SAXS spectrum), in agreement with TEM measurements. We discard interference effects between particles, because if it were the case, when x grows, interparticle distances would diminish, leading to a shift of the peak to higher q values [5]. So, this peak indicates that the average size of these particles increases with concentration. This peak has characteristics of a tridimensional object. These scatterers have typical sizes of few nm [6].
Fig. 3. SAXS spectra of the three studied systems.
L.M. Socolovsky et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 102–106
More experiments are being carried out in order to accurately determine the origin of each peak. Giant Hall effect measurements in the three systems are shown in Fig. 4. Effects of the order of 100 mO cm are found for samples with x E 0.50, the effect strongly decreasing with the increase in TM concentration. The minimum effect (E 0.1 mO cm) was recorded at x=1. We were not able to measure Hall effect for Fe–SiO2 near x=0.50 due to the high resistivity of the sample.
Fe Co Ni
ρ (µΩ cm) xy
100
10
1
0.1
0.5
0.6
0.7
0.8
0.9
1.0
x [vol.]
Fig. 4. Measured GHE in the three systems (at T=5 K).
ZFC measurements in the three systems (Fig. 5) display a peak at low x. At higher x the ZFC curves show a more intrincated profile, similar to the one found in percolated systems composed by small magnetic particles [7]. In this figure one can see that the blocking temperature in each system becomes higher when x increases, which is caused by a progressive increase of the mean particle size, in full agreement with the structural measurements. It is worth noting that the width of the distribution function also increases with x, fact that is infered from both structural and magnetic measurements. The co-sputtering process allows one to obtain systems of metallic (Co, Fe, Ni) nanocrystalline particles embedded in an amorphous matrix. We have obtained (Fig. 2) estimations of the particle size obtained by XRD and TEM. These measurements agree in this point: the size of the small particles grow smoothly when x increases, and the percolation threshold just indicates the establishment of conduction paths, but the system as a whole keeps their granular characteristics. ZFC measurements confirm this fact, showing an evolution from a superparamagnetic system for low x, to a percolated one for high x. This kind of measurement confirms the point of the increasing
(b)
(a)
(c)
x = 0.73
x = 0.50 x = 0.61
x = 0.58
M [a.u]
x = 0.44 x = 0.59
x = 0.49 x = 0.41
100
200
300 0
x = 0.45
x = 0.35
x = 0.34 0
100
105
200
300 0
x = 0.34 100
200
300
T [K] Fig. 5. ZFC measurements of: (a) Fex–(SiO2)1x; (b) Cox–(SiO2)1x; and (c) Nix–(SiO2)1x. Applied field was 20 Oe.
106
L.M. Socolovsky et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 102–106
particle size through the increase in the blocking temperature. SAXS measurements agree in the existence of a distribution of nanometric-sized objects and exhibit similar features and changes with x whatever be the measured system. GHE appears at concentrations slightly above the effective percolation threshold for conductivity, and strongly decreases at higher x. Taking into account that we have a size distribution we can think that this effect is related not to a certain particle size, but to an interplay between sizes of individual grains and distances among them.
Acknowledgements This work was supported by CNPq and Funda@*ao de Amparo a" Pesquisa do Estado de
S*ao Paulo and partially supported by National Synchrotron Light Laboratory, Brazil. L.M.S. thanks Dr. I. Torriani and Dr. A. Craievich for discussions. X.X.Z. thanks financial support of Hong Kong RGC grant (HKUST6159/99P).
References [1] A.B. Pakhomov, X. Yan, B. Zhao, Appl. Phys. Lett. 67 (1995) 3497. [2] X.X. Zhang, C. Wan, H. Liu, Z.Q. Li, P. Sheng, J.J. Lin, Phys. Rev. Lett. 86 (2001) 5562. [3] C.C. Wan, P. Sheng, Phys. Rev. B 66 (2002) 075309. [4] J.C. Denardin, A.B. Pakhomov, M. Knobel, X.X. Zhang, J. Phys.: Condens. Matter 12 (2000) 3397. [5] A. Craievich, Mater. Res. 5 (1) (2002) 1. [6] L.M. Socolovsky, et al., in preparation. [7] B.V.B. Sarkissian, J. Phys. F. 208 (1981) 2191.
Structural and magnetic properties of TM–SiO2 (TM = Fe, Co, Ni) films L.M. Socolovskya,*, J.C. Denardina, A.L. Brandla, M. Knobela, X.X. Zhangb a
! de Materiais e Baixas Temperatures, Departamento de F!ısica da Materia Condensada, Instituto de F!ısica ‘‘Gleb Wataghin’’, Laboratorio Universidade Estadual de Campinas, CP 6165, Campinas (CP) 13083-970, Brazil b Physics Department and Institute of Nanoscience and Technology (INST), Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China
Abstract TMx–(SiO2)1x (TM=Fe, Co, Ni) thin films were prepared in a wide concentration range (0.35 pxp1). Structure was studied with transmission electron microscopy (TEM), X-ray diffraction (XRD) and small angle X-ray scattering (SAXS). Magnetic and magnetotransport properties were investigated by means of magnetization and Hall effect measurements. TEM images display nanometric spherical structures embedded in a SiO2 amorphous matrix, with typical sizes increasing from 3 to 5 nm when TM volume concentration x is increased. SAXS measurements indicate a complex structure formed by nanosized objects. XRD measurements show that the structure is composed by amorphous SiO2 and TM crystallites. Slightly above the percolation threshold all samples display giant Hall effect. The observed magnetic properties are dependent on x, and display an evolution resulting from the progressive increase of the mean particle size. r 2003 Elsevier Science B.V. All rights reserved. PACS: 73.63.Bd; 75.50.Tt; 75.75.+a Keywords: Magnetic thin films; Magnetic nanoparticles; SAXS
1. Introduction Giant Hall effect (GHE) is a new property found in granular metals [1], usually in transition metal–dielectric alloys. It was found that near the critical concentration of metal in the composite where a metal–insulator transition occurs, both the ordinary and extraordinary parts of the Hall effect display a huge increase when compared to *Corresponding author. Tel.: +55-19-37885504. E-mail address: leandros@ifi.unicamp.br (L.M. Socolovsky).
the value measured in the metallic regime. Although the GHE in non-magnetic granular systems has been successfully accounted for within the local quantum interference model [2,3], the understanding of the GHE in magnetic granular materials is still far from complete [1,4]. To gain a deeper understanding of GHE in magnetic materials, it is believed that a systematic investigation is essential. Therefore, in order to investigate this effect thoroughly we have prepared a set of Fe– SiO2, Co–SiO2, Ni–SiO2 samples by magnetron co-sputtering, and studied their structural, magnetotransport and magnetic properties.
0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0304-8853(03)00028-3
L.M. Socolovsky et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 102–106
(a)
Intensity [counts]
0.87
(b)
(c)
0.85
0.84
0.72
0.61
103
0.73 0.56 0.54
0.58
0.41 0.32 0.49
40
43
46
49
40
43
46 2Θ
49
40
43
46
49
Fig. 1. (a) XRD diffractograms of the Fex–(SiO2)1x. (b) Cox–(SiO2)1x. (c) Nix–(SiO2)1x.
2. Experimental Thin films of Fe–SiO2, Co–SiO2, Ni–SiO2 were deposited onto Kapton and glass sheets by magnetron co-sputtering. Substrate was kept at room temperature during the process. Pressure before sputtering was 107 Torr and during the process it was kept at 5 mTorr, the chamber filled with Ar. The glass substrates were rotated during sputtering, to ensure composition uniformity. The Fe, Co, or Ni, volume fraction was controlled by the relative sputtering rates, and were then determined by energy-dispersive X-ray spectroscopy using a Philips EDAX XL30 device. We will use x to indicate the transition metal (TM) volume fraction. X-ray diffraction (XRD) was obtained in a Philips PW 1820 diffractometer, equipped with a single-crystal monochromator. Scans were done in the samples evaporated on glass using CuKa radiation, in the usual y–2y configuration. No filters were used. Transmission electron microscopy (TEM) was done using a Jeol JEM-3010 ARP microscope at the Microscopy Facility.1 Small angle X-ray scattering (SAXS) was measured in the SAS station of the LNLS, using a wavelength of l=0.1758 nm. Data were collected 1 ! Laboratorio Nacional de Luz S!ıncrotron (LNLS), Campinas,SP, Brazil.
in transmission geometry with a linear position sensitive detector placed at 773.0 mm from the sample. The detector was mounted vertically. The camera length corresponds to a scattering vector q ( 1. Measureranging from 0.01322 to 0.36425 A ments were done in a vacuum of 101 Torr. No mathematical desmearing was required, because of the small dimensions of the X-ray spot (1 mm vertical and 5 mm horizontal, approximately). Obtained data were corrected through the appropriate procedures. Zero-field cooling magnetization (ZFC) was measured with a SQUID magnetometer. Hall resistance measurements were performed using the Van der Pauw method.
3. Results and discussion XRD show the existence of two phases (Fig. 1): one amorphous with a maximum located at E 25 (not seen in the graph), which corresponds to SiO2, and the other one crystalline, identified by a peak in the samples with x above 0.50. This peak is located, in the case of samples with Fe and Ni, at the positions corresponding to the (1 1 0) and (1 1 1) reflections of the BCC Fe and FCC Ni, respectively. At higher x the intensity of the crystalline peak grows, and its width decreases, indicating an increase in the crystallite size. For
L.M. Socolovsky et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 102–106
104
Co–SiO2 samples, at the low concentration region, a wide peak located between 40 and 50 appears. With further increase of Co concentration three peaks are visible in the same region. These broadened peaks correspond to the (1 0 0), (0 0 2) and (1 0 1) reflections of the HCP-cobalt. At the highest concentrations small rounded peaks appear in the high-angle region for all systems. After experimental corrections one can calculate the mean crystallite size hDi from the highest peak in each system using the Scherrer formula: hDi ¼ 60 Cox-(SiO2)1-x (TEM) Co x-(SiO2)1-x (XRD) Nix-(SiO2)1-x (TE M) Ni x-(SiO2)1-x (XRD) Fex-(SiO2)1-x (XRD)
50 40 30 20 10 0 0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
x [vol.] Fig. 2. Mean particle size estimated from XRD and TEM.
0:94l=b cosy; where l is the wavelength of the Xray, b the width of the peak (measured in radians in the scale 2y) and y is the position of the maximum of the peak. Results are shown in Fig. 2, together with mean size estimations obtained from TEM images. TEM photographs show a system of spherical particles with sizes varying from about 3 to 5 nm when x increases (not shown here). Above x E 0.5 the system shows arrangements of touching spheres. At higher x complex structures are viewed. Average size of the spherical particles was estimated from several TEM photographs. SAXS spectra (Fig. 3) display similar features for the three systems: a fall in the intensity with q, a flat and rather broad peak located between 0.02 ( 1, and a rounded peak that shifts to and 0.09 A lower q when x increases. The objects that produce these peaks have rather broad size dispersion (evidenced by the width of the peaks in the SAXS spectrum), in agreement with TEM measurements. We discard interference effects between particles, because if it were the case, when x grows, interparticle distances would diminish, leading to a shift of the peak to higher q values [5]. So, this peak indicates that the average size of these particles increases with concentration. This peak has characteristics of a tridimensional object. These scatterers have typical sizes of few nm [6].
Fig. 3. SAXS spectra of the three studied systems.
L.M. Socolovsky et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 102–106
More experiments are being carried out in order to accurately determine the origin of each peak. Giant Hall effect measurements in the three systems are shown in Fig. 4. Effects of the order of 100 mO cm are found for samples with x E 0.50, the effect strongly decreasing with the increase in TM concentration. The minimum effect (E 0.1 mO cm) was recorded at x=1. We were not able to measure Hall effect for Fe–SiO2 near x=0.50 due to the high resistivity of the sample.
Fe Co Ni
ρ (µΩ cm) xy
100
10
1
0.1
0.5
0.6
0.7
0.8
0.9
1.0
x [vol.]
Fig. 4. Measured GHE in the three systems (at T=5 K).
ZFC measurements in the three systems (Fig. 5) display a peak at low x. At higher x the ZFC curves show a more intrincated profile, similar to the one found in percolated systems composed by small magnetic particles [7]. In this figure one can see that the blocking temperature in each system becomes higher when x increases, which is caused by a progressive increase of the mean particle size, in full agreement with the structural measurements. It is worth noting that the width of the distribution function also increases with x, fact that is infered from both structural and magnetic measurements. The co-sputtering process allows one to obtain systems of metallic (Co, Fe, Ni) nanocrystalline particles embedded in an amorphous matrix. We have obtained (Fig. 2) estimations of the particle size obtained by XRD and TEM. These measurements agree in this point: the size of the small particles grow smoothly when x increases, and the percolation threshold just indicates the establishment of conduction paths, but the system as a whole keeps their granular characteristics. ZFC measurements confirm this fact, showing an evolution from a superparamagnetic system for low x, to a percolated one for high x. This kind of measurement confirms the point of the increasing
(b)
(a)
(c)
x = 0.73
x = 0.50 x = 0.61
x = 0.58
M [a.u]
x = 0.44 x = 0.59
x = 0.49 x = 0.41
100
200
300 0
x = 0.45
x = 0.35
x = 0.34 0
100
105
200
300 0
x = 0.34 100
200
300
T [K] Fig. 5. ZFC measurements of: (a) Fex–(SiO2)1x; (b) Cox–(SiO2)1x; and (c) Nix–(SiO2)1x. Applied field was 20 Oe.
106
L.M. Socolovsky et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 102–106
particle size through the increase in the blocking temperature. SAXS measurements agree in the existence of a distribution of nanometric-sized objects and exhibit similar features and changes with x whatever be the measured system. GHE appears at concentrations slightly above the effective percolation threshold for conductivity, and strongly decreases at higher x. Taking into account that we have a size distribution we can think that this effect is related not to a certain particle size, but to an interplay between sizes of individual grains and distances among them.
Acknowledgements This work was supported by CNPq and Funda@*ao de Amparo a" Pesquisa do Estado de
S*ao Paulo and partially supported by National Synchrotron Light Laboratory, Brazil. L.M.S. thanks Dr. I. Torriani and Dr. A. Craievich for discussions. X.X.Z. thanks financial support of Hong Kong RGC grant (HKUST6159/99P).
References [1] A.B. Pakhomov, X. Yan, B. Zhao, Appl. Phys. Lett. 67 (1995) 3497. [2] X.X. Zhang, C. Wan, H. Liu, Z.Q. Li, P. Sheng, J.J. Lin, Phys. Rev. Lett. 86 (2001) 5562. [3] C.C. Wan, P. Sheng, Phys. Rev. B 66 (2002) 075309. [4] J.C. Denardin, A.B. Pakhomov, M. Knobel, X.X. Zhang, J. Phys.: Condens. Matter 12 (2000) 3397. [5] A. Craievich, Mater. Res. 5 (1) (2002) 1. [6] L.M. Socolovsky, et al., in preparation. [7] B.V.B. Sarkissian, J. Phys. F. 208 (1981) 2191.