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Morphological and structural investigations of Co–MgF2 granular thin films grown by thermionic vacuum arc
Thin Solid Films 518 (2010) 3945–3948
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Morphological and structural investigations of Co–MgF2 granular thin films grown by thermionic vacuum arc V. Ionescu a, M. Osiac b,⁎, C.P. Lungu c, O.G. Pompilian c, I. Jepu c, I. Mustata c, G.E. Iacobescu b a b c
Ovidius University, 900527, Constanta, Romania Faculty of Physics, University of Craiova, 200585, Craiova, Romania National Institute for Laser, Plasma and Radiation Physics, Bucharest-Magurele, 077125 Romania
a r t i c l e
i n f o
Article history: Received 3 June 2009 Received in revised form 1 February 2010 Accepted 12 February 2010 Available online 19 February 2010 Keywords: Thermionic vacuum arc Thin films Co–MgF2 plasma
a b s t r a c t Co–MgF2 granular films were deposited in thermionic vacuum arc plasma with simultaneous ignition of plasma in Co and MgF2 vapours. The samples were investigated by transmission electron microscopy which revealed Co grains of a few nm in diameter embedded in the MgF2 matrix. The crystalline phase for Co and MgF2 was studied by electron diffraction after selecting a typical area of the sample. Low-angle x-ray diffraction method was used to verify the bulk crystalline structure of the samples. The surface morphologies of the films were investigated by atomic force microscopy. The magneto-optical longitudinal Kerr rotation spectra of the films were also measured and compared. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Combinations of metal–nonmetal granular soft magnetic films such as Co–Al2O3, Co–MgO, Fe–SiO2, and Fe–SrF2 have attracted much interest in the last years, due to their electrical and magnetic properties, such as high electrical resistivity, low coercivity, low effective magnetic anisotropy, and very weak inter-particle magnetic interaction. Good magnetic properties including susceptibilities of 10− 3 up to −1 10 , electrical resistivity over 500 µΩ cm, saturation flux densities of about 1 T, and coercivities as low as 10 A/m have been obtained [1–4]. For example, Co–MgF2 granular system is used as thin film inductor in power converters for microprocessors, with fast response, high efficiency and small size. Magnesium fluoride (MgF2) was chosen as an insulating material because of its low evaporation point. This material has a high resistivity due to the wider gap and smaller dielectric constant than e.g. MgO and Al2O3, and it has a tendency to get separated from the co-evaporated metal [5,6]. Networks of magnetic tunnel junctions are formed in ferromagnetic metal (FM)-insulator granular films composed of metallic nanodots embedded in the insulating matrix. The tunneling magnetoresistance effect due to spin-dependent tunneling between neighbouring FM dots has been intensively studied [7–9]. In the present study we propose the use of a thermionic vacuum arc (TVA) deposition technique for coating smooth, nanostructured Co–MgF2 granular thin films onto glass substrates. We investigated
⁎ Corresponding author. E-mail address: [email protected] (M. Osiac). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.02.035
two samples placed at two different positions relative to the anode of the discharge that produces the Co plasma. 2. Experimental details The method used here based on TVA for deposition of granular films was described in details elsewhere [10,11]. It uses two identical set-ups which produce two separate electron beams emitted by externally heated cathodes. Each cathode is made from a tungsten wire with a diameter of 1 mm. The electron beams are accelerated by high anodic voltages and bombard simultaneously the crucibles placed at the anodes, as shown in Fig. 1. The crucibles contain each a few grams of Co and MgF2, respectively. By applying a high voltage between 1 and 6 kV on each of the anode–cathode sets, two different plasmas are formed in pure Co and MgF2 vapours. The chamber is pumped down by mechanical and diffusion pumps until a pressure of 2.7 * 10− 3 Pa is obtained. Both plasmas expand in the high vacuum chamber and reach the surface of the substrates of the samples, which are grounded. The currents used to heat the cathode filaments are between 40 and 55 A. The current of the discharge ignited in Co vapours is Idisch = 0.7 to 0.8 A at a voltage Udisch = 400 V. In the case of the discharge ignited in MgF2 vapours, the current and the voltage were Idisch = 0.4 to 0.5 A and Udisch = 300–400 V, respectively. The deposition rate rd and film thickness d were measured and controlled in situ using a FTM7 quartz microbalance. The thickness of the Co–MgF2 coatings was 200 nm. The film substrates were rectangular pieces of industrial glass with an area of 15 × 15 mm2. The studied thin film samples were located at P1 above and just in front of the Co crucible and P2 at mid-distance
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V. Ionescu et al. / Thin Solid Films 518 (2010) 3945–3948
Fig. 1. Experimental set-up for Co–MgF2 thin film deposition.
between the anodes as illustrated in Fig. 1. The distance between the Co crucible and P1 sample was 250 mm, while the distance between the Co crucible and P2 sample was 275 mm. The substrates were maintained at T = 300 K during deposition. The transmission electron microscopy (TEM) analyses were carried out using a Philips CM120ST microscope operating at 100 kV with Cs = 1.2 mm and ≈2 Å resolution. The x-ray diffraction (XRD) patterns were obtained using a Shimadzu model 6000 diffractometer operating with Cu Kα radiation (40 kV, 30 mA). The atomic force microscopy (AFM) data were recorded in non-contact mode using a Park XE-100 equipment (silicon tip with conical shape). The AFM scans were taken over the area 5 × 5 μm2. We used the horizontal line by line flattening as planarization method. Finally, the magnetic properties of the films were investigated via longitudinal Magneto-Optic Kerr Effect (MOKE), using p-polarized, 633 nm He–Ne laser light (10 mW power) as an incident light. The incident beam was modulated with an electro-optic-modulator. The magnetic field was applied parallel to the films and lays in the incidence plane of He–Ne laser. 3. Results and discussion The TEM image of P1 sample, presented in Fig. 2(a) shows a network-like structure of nanoscale grains of Co (dark areas) and thin intergrain boundaries of MgF2 (light areas). The grain size is about 3–4 nm in diameter with intergrain regions of 3–6 nm width. The TEM image of P2 sample presented in Fig. 2(b) shows the presence of some very fine dark spots (Co) fixed in a lighter area surface (MgF2
Fig. 2. Cross-sectional TEM images for: a) sample P1; b) sample P2; and c) SAED pattern for sample P1.
Fig. 3. XRD patterns of: a) sample P1; and b) sample P2.
V. Ionescu et al. / Thin Solid Films 518 (2010) 3945–3948
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For sample P1 the well defined diffraction rings shown in SAED pattern (Fig. 2(c)) correspond to the interplanar distances of 0.212 nm and 0.194 nm. These values fit well with the distances of the (100) and (101) diffraction planes for the hexagonal-close-packed (hcp) phase of Co. This measurement proves the presence of crystalline structure of Co grains in the MgF2 matrix of sample P1. Low-angle XRD analyses were performed to establish the presence of crystalline phases in the coatings. In the XRD patterns of both samples P1 and P2 the 2θ diffraction peaks can be assigned to (110), (111), (210), (211) and (301) crystalline planes of MgF2 tetragonal phase (Fig. 3). In Fig. 3(a), we can see for sample P1 the smooth slope of the XRD curve in the 44°–47° range. The Co lines inserted from the database JCPDS—International Centre for Diffraction Data, which overlap this angle range, allowed us to assign this slope to the presence of Co in the MgF2 matrix. Fig. 3(b) shows that by XRD measurements it is not possible to detect the presence of Co in sample P2. The topographical AFM images and subsequent statistical data analysis, including the calculation of the root mean square roughness (Rq), gave detailed information about the surface morphology. The morphological AFM images of samples P1 and P2 are presented in Fig. 4(a) and (b), respectively. The Rq values are 3.8 nm for P1 and 4.7 nm for P2. The in-plane size of the surface asperities was found to be almost uniform for both samples, only a small number of these asperities being higher than 10 nm for P1 and higher than 20 nm for P2. Similar results were reported by Hosoya et al. [1] in the case of Fe–SrF2 co-evaporated granular films deposited onto MgO substrates. In Fig. 5 we present the MOKE hysteresis loops (longitudinal mode) at room temperature for samples P1 and P2. It can be observed that the saturation magnetization for sample P1 is about two times higher than that of the P2 sample; the coercivities resulting from MOKE measurements are: 0.038 T for P1 and 0.054 T for P2. 4. Conclusions In order to obtain Co–MgF2 composite thin films in plasmas generated in the vapours of Co and MgF2 materials employing the TVA technique the substrates are usually placed at 250–275 mm above the evaporating anodes. From the SAED pattern obtained by TEM analysis we could reveal the presence of hcp crystalline phase of Co and of nanometrical Co grains embedded in the MgF2 insulating matrix, only in the film placed just above the anode which produces the Co plasma. The XRD patterns in the bulk of the same film revealed a random orientation of Co in the MgF2 crystalline matrix. This sample has a low surface roughness with crack-free and densely packed microstructure. The best magneto-optical properties are obtained for the film with the
Fig. 4. AFM top view for a scan area of 5 × 5 µm2 in a) sample surface P1; and b) sample surface P2.
structure). We could not notice the existence of Co nanograins embedded in MgF2 matrix in this case. Coonley et al. [6] reported a similar structure for a granular soft magnetic Co–MgF2 film prepared by evaporative co-deposition onto glass substrate, with 41% volume of Co. In their study, the Co grain size was between 2.7 nm and 3.7 nm in diameter, and the intergrain region widths in the range of 1 to 2 nm.
Fig. 5. MOKE measurements on samples P1 and P2 at room temperature.
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Co nanograins fixed in the MgF2 structure. The Co–MgF2 granular thin film with the thickness of about 200 nm was deposited onto glass substrates placed within the Co plasma. References [1] H. Hosoya, H. Arita, K. Hamada, Y. Takahashi, K. Higashi, K. Oda, M. Ueda, J. Phys. D Appl. Phys. 39 (2006) 5103. [2] Y. Xu, X. Yan, J. Mater. Res. 11 (1996) 2506. [3] S. Honda, T. Okada, M. Nawate, M. Tokumoto, Phys. Rev. B 56 (1997) 14566. [4] H. Fujimori, S. Mitani, S. Ohnuma, Mater. Sci. Eng. B 3 (1995) 219. [5] C.R. Sullivan, S.R. Sanders, IEEE Trans. Power Electron. 11 (1996) 228.
[6] K.D. Coonley, G.J. Mehas, C.R. Sullivan, U.J. Gibson, IEEE Trans. Magn. 36 (2000) 3463. [7] H. Fujimori, S. Mitani, S. Ohnuma, Mater. Sci. Eng. B 31 (1995) 219. [8] A. Milner, A. Gerber, B. Groisman, M. Karporsky, A. Gladkikh, Phys. Rev. Lett. 76 (1996) 475. [9] Y. Hayakawa, N. Hasegawa, A. Makino, S. Mitani, H. Fujimori, J. Magn. Magn. Mater. 154 (1996) 175. [10] I. Mustata, C.P. Lungu, A.M. Lungu, V. Zaroski, M. Blideran, V. Ciupina, Vacuum 76 (2004) 131. [11] C.P. Lungu, I. Mustata, G. Musa, A.M. Lungu, V. Zaroschi, K. Iwasaki, R. Tanaka, Y. Matsumura, I. Iwanaga, H. Tanaka, T. Oi, K. Fujita, Surf. Coat. Technol. 200 (2005) 399.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Morphological and structural investigations of Co–MgF2 granular thin films grown by thermionic vacuum arc V. Ionescu a, M. Osiac b,⁎, C.P. Lungu c, O.G. Pompilian c, I. Jepu c, I. Mustata c, G.E. Iacobescu b a b c
Ovidius University, 900527, Constanta, Romania Faculty of Physics, University of Craiova, 200585, Craiova, Romania National Institute for Laser, Plasma and Radiation Physics, Bucharest-Magurele, 077125 Romania
a r t i c l e
i n f o
Article history: Received 3 June 2009 Received in revised form 1 February 2010 Accepted 12 February 2010 Available online 19 February 2010 Keywords: Thermionic vacuum arc Thin films Co–MgF2 plasma
a b s t r a c t Co–MgF2 granular films were deposited in thermionic vacuum arc plasma with simultaneous ignition of plasma in Co and MgF2 vapours. The samples were investigated by transmission electron microscopy which revealed Co grains of a few nm in diameter embedded in the MgF2 matrix. The crystalline phase for Co and MgF2 was studied by electron diffraction after selecting a typical area of the sample. Low-angle x-ray diffraction method was used to verify the bulk crystalline structure of the samples. The surface morphologies of the films were investigated by atomic force microscopy. The magneto-optical longitudinal Kerr rotation spectra of the films were also measured and compared. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Combinations of metal–nonmetal granular soft magnetic films such as Co–Al2O3, Co–MgO, Fe–SiO2, and Fe–SrF2 have attracted much interest in the last years, due to their electrical and magnetic properties, such as high electrical resistivity, low coercivity, low effective magnetic anisotropy, and very weak inter-particle magnetic interaction. Good magnetic properties including susceptibilities of 10− 3 up to −1 10 , electrical resistivity over 500 µΩ cm, saturation flux densities of about 1 T, and coercivities as low as 10 A/m have been obtained [1–4]. For example, Co–MgF2 granular system is used as thin film inductor in power converters for microprocessors, with fast response, high efficiency and small size. Magnesium fluoride (MgF2) was chosen as an insulating material because of its low evaporation point. This material has a high resistivity due to the wider gap and smaller dielectric constant than e.g. MgO and Al2O3, and it has a tendency to get separated from the co-evaporated metal [5,6]. Networks of magnetic tunnel junctions are formed in ferromagnetic metal (FM)-insulator granular films composed of metallic nanodots embedded in the insulating matrix. The tunneling magnetoresistance effect due to spin-dependent tunneling between neighbouring FM dots has been intensively studied [7–9]. In the present study we propose the use of a thermionic vacuum arc (TVA) deposition technique for coating smooth, nanostructured Co–MgF2 granular thin films onto glass substrates. We investigated
⁎ Corresponding author. E-mail address: [email protected] (M. Osiac). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.02.035
two samples placed at two different positions relative to the anode of the discharge that produces the Co plasma. 2. Experimental details The method used here based on TVA for deposition of granular films was described in details elsewhere [10,11]. It uses two identical set-ups which produce two separate electron beams emitted by externally heated cathodes. Each cathode is made from a tungsten wire with a diameter of 1 mm. The electron beams are accelerated by high anodic voltages and bombard simultaneously the crucibles placed at the anodes, as shown in Fig. 1. The crucibles contain each a few grams of Co and MgF2, respectively. By applying a high voltage between 1 and 6 kV on each of the anode–cathode sets, two different plasmas are formed in pure Co and MgF2 vapours. The chamber is pumped down by mechanical and diffusion pumps until a pressure of 2.7 * 10− 3 Pa is obtained. Both plasmas expand in the high vacuum chamber and reach the surface of the substrates of the samples, which are grounded. The currents used to heat the cathode filaments are between 40 and 55 A. The current of the discharge ignited in Co vapours is Idisch = 0.7 to 0.8 A at a voltage Udisch = 400 V. In the case of the discharge ignited in MgF2 vapours, the current and the voltage were Idisch = 0.4 to 0.5 A and Udisch = 300–400 V, respectively. The deposition rate rd and film thickness d were measured and controlled in situ using a FTM7 quartz microbalance. The thickness of the Co–MgF2 coatings was 200 nm. The film substrates were rectangular pieces of industrial glass with an area of 15 × 15 mm2. The studied thin film samples were located at P1 above and just in front of the Co crucible and P2 at mid-distance
3946
V. Ionescu et al. / Thin Solid Films 518 (2010) 3945–3948
Fig. 1. Experimental set-up for Co–MgF2 thin film deposition.
between the anodes as illustrated in Fig. 1. The distance between the Co crucible and P1 sample was 250 mm, while the distance between the Co crucible and P2 sample was 275 mm. The substrates were maintained at T = 300 K during deposition. The transmission electron microscopy (TEM) analyses were carried out using a Philips CM120ST microscope operating at 100 kV with Cs = 1.2 mm and ≈2 Å resolution. The x-ray diffraction (XRD) patterns were obtained using a Shimadzu model 6000 diffractometer operating with Cu Kα radiation (40 kV, 30 mA). The atomic force microscopy (AFM) data were recorded in non-contact mode using a Park XE-100 equipment (silicon tip with conical shape). The AFM scans were taken over the area 5 × 5 μm2. We used the horizontal line by line flattening as planarization method. Finally, the magnetic properties of the films were investigated via longitudinal Magneto-Optic Kerr Effect (MOKE), using p-polarized, 633 nm He–Ne laser light (10 mW power) as an incident light. The incident beam was modulated with an electro-optic-modulator. The magnetic field was applied parallel to the films and lays in the incidence plane of He–Ne laser. 3. Results and discussion The TEM image of P1 sample, presented in Fig. 2(a) shows a network-like structure of nanoscale grains of Co (dark areas) and thin intergrain boundaries of MgF2 (light areas). The grain size is about 3–4 nm in diameter with intergrain regions of 3–6 nm width. The TEM image of P2 sample presented in Fig. 2(b) shows the presence of some very fine dark spots (Co) fixed in a lighter area surface (MgF2
Fig. 2. Cross-sectional TEM images for: a) sample P1; b) sample P2; and c) SAED pattern for sample P1.
Fig. 3. XRD patterns of: a) sample P1; and b) sample P2.
V. Ionescu et al. / Thin Solid Films 518 (2010) 3945–3948
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For sample P1 the well defined diffraction rings shown in SAED pattern (Fig. 2(c)) correspond to the interplanar distances of 0.212 nm and 0.194 nm. These values fit well with the distances of the (100) and (101) diffraction planes for the hexagonal-close-packed (hcp) phase of Co. This measurement proves the presence of crystalline structure of Co grains in the MgF2 matrix of sample P1. Low-angle XRD analyses were performed to establish the presence of crystalline phases in the coatings. In the XRD patterns of both samples P1 and P2 the 2θ diffraction peaks can be assigned to (110), (111), (210), (211) and (301) crystalline planes of MgF2 tetragonal phase (Fig. 3). In Fig. 3(a), we can see for sample P1 the smooth slope of the XRD curve in the 44°–47° range. The Co lines inserted from the database JCPDS—International Centre for Diffraction Data, which overlap this angle range, allowed us to assign this slope to the presence of Co in the MgF2 matrix. Fig. 3(b) shows that by XRD measurements it is not possible to detect the presence of Co in sample P2. The topographical AFM images and subsequent statistical data analysis, including the calculation of the root mean square roughness (Rq), gave detailed information about the surface morphology. The morphological AFM images of samples P1 and P2 are presented in Fig. 4(a) and (b), respectively. The Rq values are 3.8 nm for P1 and 4.7 nm for P2. The in-plane size of the surface asperities was found to be almost uniform for both samples, only a small number of these asperities being higher than 10 nm for P1 and higher than 20 nm for P2. Similar results were reported by Hosoya et al. [1] in the case of Fe–SrF2 co-evaporated granular films deposited onto MgO substrates. In Fig. 5 we present the MOKE hysteresis loops (longitudinal mode) at room temperature for samples P1 and P2. It can be observed that the saturation magnetization for sample P1 is about two times higher than that of the P2 sample; the coercivities resulting from MOKE measurements are: 0.038 T for P1 and 0.054 T for P2. 4. Conclusions In order to obtain Co–MgF2 composite thin films in plasmas generated in the vapours of Co and MgF2 materials employing the TVA technique the substrates are usually placed at 250–275 mm above the evaporating anodes. From the SAED pattern obtained by TEM analysis we could reveal the presence of hcp crystalline phase of Co and of nanometrical Co grains embedded in the MgF2 insulating matrix, only in the film placed just above the anode which produces the Co plasma. The XRD patterns in the bulk of the same film revealed a random orientation of Co in the MgF2 crystalline matrix. This sample has a low surface roughness with crack-free and densely packed microstructure. The best magneto-optical properties are obtained for the film with the
Fig. 4. AFM top view for a scan area of 5 × 5 µm2 in a) sample surface P1; and b) sample surface P2.
structure). We could not notice the existence of Co nanograins embedded in MgF2 matrix in this case. Coonley et al. [6] reported a similar structure for a granular soft magnetic Co–MgF2 film prepared by evaporative co-deposition onto glass substrate, with 41% volume of Co. In their study, the Co grain size was between 2.7 nm and 3.7 nm in diameter, and the intergrain region widths in the range of 1 to 2 nm.
Fig. 5. MOKE measurements on samples P1 and P2 at room temperature.
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V. Ionescu et al. / Thin Solid Films 518 (2010) 3945–3948
Co nanograins fixed in the MgF2 structure. The Co–MgF2 granular thin film with the thickness of about 200 nm was deposited onto glass substrates placed within the Co plasma. References [1] H. Hosoya, H. Arita, K. Hamada, Y. Takahashi, K. Higashi, K. Oda, M. Ueda, J. Phys. D Appl. Phys. 39 (2006) 5103. [2] Y. Xu, X. Yan, J. Mater. Res. 11 (1996) 2506. [3] S. Honda, T. Okada, M. Nawate, M. Tokumoto, Phys. Rev. B 56 (1997) 14566. [4] H. Fujimori, S. Mitani, S. Ohnuma, Mater. Sci. Eng. B 3 (1995) 219. [5] C.R. Sullivan, S.R. Sanders, IEEE Trans. Power Electron. 11 (1996) 228.
[6] K.D. Coonley, G.J. Mehas, C.R. Sullivan, U.J. Gibson, IEEE Trans. Magn. 36 (2000) 3463. [7] H. Fujimori, S. Mitani, S. Ohnuma, Mater. Sci. Eng. B 31 (1995) 219. [8] A. Milner, A. Gerber, B. Groisman, M. Karporsky, A. Gladkikh, Phys. Rev. Lett. 76 (1996) 475. [9] Y. Hayakawa, N. Hasegawa, A. Makino, S. Mitani, H. Fujimori, J. Magn. Magn. Mater. 154 (1996) 175. [10] I. Mustata, C.P. Lungu, A.M. Lungu, V. Zaroski, M. Blideran, V. Ciupina, Vacuum 76 (2004) 131. [11] C.P. Lungu, I. Mustata, G. Musa, A.M. Lungu, V. Zaroschi, K. Iwasaki, R. Tanaka, Y. Matsumura, I. Iwanaga, H. Tanaka, T. Oi, K. Fujita, Surf. Coat. Technol. 200 (2005) 399.