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Magnetic properties of iron-and cobalt-implanted silicone polymers
Vacuum 58 (2000) 551}560
Magnetic properties of iron-and cobalt-implanted silicone polymers夽 B.Z. Rameev *, B. Aktas , R.I. Khaibullin, V.A. Zhikharev, Yu.N. Osin, I.B. Khaibullin Gebze Institute of Technology, P.K. 141 41400, Gebze/Kocaeli, Turkey Kazan Physical-Technical Institute, Sibirsky Trakt 10/7, 420029 Kazan, Russia
Abstract Composite metal}polymer thin "lms were formed by implantation of 40 KeV Fe> and Co> ions into viscous and solid silicone substrates in the dose range of (0.3}1.5);10 ions/cm. Morphology and magnetic properties of the synthesized "lms were investigated. TEM investigations showed that the microstructure of metal granular layers synthesized in silicone polymers strongly depends on an initial relaxation state of the target. Magnetic resonance studies revealed essential di!erences in the magnetic properties of thin metal "lms synthesized at di!erent initial states of silicone substrate and with respect to the type of implanted ions (Fe or Co). A shift and broadening of FMR signal with the samples cooling are observed both for perpendicular and parallel orientation of the "lms with respect to the magnetic "eld. These e!ects are associated with the increase of the inter-particle interactions and superparamagnetism blocking in the particles. It was found that the viscous initial state of the polymer is a favourable factor for the high anisotropy of FMR response in Fe-implanted samples in contrast to the Co-based ones. The reasons for this have been discussed with respect to the morphology of Fe and Co synthesized "lms. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Ion implantation; Ferromagnetic resonance; Granular magnetic "lms; Polymers; Superparamagnetism
1. Introduction The investigation of granular metal}polymer "lms has great practical signi"cance for the development of magnetic recording and data storage devices, magnetoresistivity sensors, etc. [1]. 夽
Paper presented at the 11th International School on Vacuum, Electron and Ion Technologies, 20}25 September 1999, Varna, Bulgaria. * Corresponding author. Fax: #90-262-6538490. E-mail address: [email protected] (B.Z. Rameev). 0042-207X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 2 1 9 - 0
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Among the di!erent techniques available for preparation of such "lms, the method of high dose implantation has a few advantages. They are easy control of metal content in the polymer layer, homogeneous dispersion of metal particles inside the composite "lm and the possibility of controlling the size of metal granules. Recently, a new approach of ion implantation has been developed [2], where at the moment of implantation organic matrix is held at the viscous-#ow relaxation state and then polymerized. Such a technique, which has been used in our study, permits to control not only the average size, but also the size spread of nanoparticles and morphology of synthesized "lms as a function of target viscosity. This work presents the results of investigations on iron and cobalt implantation into viscous and solid silicone polymers by electron microscopy and magnetic resonance methods.
2. Experimental Commercial silicone polymers were used as substrates for implantation. A mixture of viscous silicone resin (low-molecular methyl-phenyl-siloxane, 97% of weight) with a curing agent (diethyldicaprilat of tin, 3%) was prepared, and thin (150 lm) silicone "lms were deposited on glass plates by centrifugation of a viscous mixture. (40 keV) Fe> and Co> ions were then implanted into both viscous-#ow and fully cured solid silicone "lms with doses of (0.3}1.5);10 ions/cm and an ion current density of 4 lA/cm. The dynamic viscosity of viscous substrates measured by a capillary viscometer was equal to 20 Pa s at an initial moment of the implantation. The viscosity of solid silicone polymers has a value of the order of 10}10 Pa s [3]. The microstructure and crystalline phases of the synthesized iron and cobalt granular "lms were studied by transmission electron microscopy (TEM) on the EM-125 electron microscope. Magnetic resonance spectra of the irradiated samples were recorded at temperatures ranging from 4.2 to 300 K with the Bruker EMX spectrometer operating in X-band (9.5 GHz). As usual, the "eld derivative of microwave power absorption (dP/dH) was registered as a function of the applied magnetic "eld H. For comparison, magnetic resonance spectra of the commercial 3.5 magnetic diskette (`Verbatima, HD, 1.44 MB) were also recorded at room temperature.
3. Results and discussion 3.1. Structural investigations The microstructure of the synthesized granular metal "lms is strongly dependent on the viscosity of the implanted silicone polymer and the type of embedded ions (Fe or Co). Fe-implanted samples. Fig. 1A and B represents the TEM plane visual images of iron granular "lms synthesized in viscous (A) and solid (B) polymer substrates implanted with a dose of 1.25;10 ions/cm. Both isolated iron nanoparticles and large planar agglomerates (with sizes of 100}300 nm) of the needle-like particles are observed in the samples implanted into the viscous state (Fig. 1A). The increase of implantation dose results in the enlargement of the mean size of these agglomerates, and the particles form a connected fractal-like network. In irradiated solid silicone samples, the synthesis of isolated drop-like iron particles with a mean size of about 30 nm is
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Fig. 1. TEM images of iron (A, B) and cobalt (C, D) granular "lms formed by ion implantation in viscous (A, C) and solid (B, D) silicone polymers at a dose of 1.25;10 ions/cm.
observed (Fig. 1B). Electron di!raction studies point out that most of the implanted iron ions enter into the b.c.c. lattice of metallic a-Fe. Only weak traces of iron oxides and silicides have also been observed. Co-implanted samples. Fig. 1C and D shows the microstructure of cobalt granular "lms produced in viscous (C) and solid (D) polymers implanted with a dose of 1.25;10 ions/cm. It is seen that large (100}200 nm) Co groups of closely packed particles are formed in viscous substrates. In implanted solid polymers the cobalt granular "lm consists of widely separated "ne particles and
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their size being spread in a rather broad range until 100 nm. Electron di!raction measurements obtained on cobalt particles indicate the hexagonal a-Co metallic phases in the implanted polymers only. The data on electron microscopic cross-section studies showed that synthesized iron and cobalt "lms are about 30}50 nm thick and they are buried at a depth of 20}25 nm from the polymer surface. 3.2. Magnetic resonance studies Fig. 2 shows the dependencies of magnetic resonance spectra on orientation (h) of the static magnetic "eld (H) relative to the normal of "lm (n) for viscous and solid samples implanted with Fe (Fig. 2a) and Co (Fig. 2b) ions with a dose of 1.25;10 ions/cm. All spectra contain the components H with the orientation dependencies typical for ferromagnetic resonance (FMR) in $+0 thin granular magnetic "lms: when H#n (h"03), then absorption lines are shifted to the high-"eld side, when HNn (h"903) they are shifted to the low-"eld side. The only component of FMR line is observed for all samples under orientation h"903, and a sort of `structurea of FMR absorption is observed for solid samples for h"03. Figs. 3 and 4 show the dependencies of spectra on temperature for the same samples (3 * implanted with Fe, 4 * implanted with Co, (a) viscous samples and (b) solid ones) for h"03 and h"903. There are extra resonance signals in the spectra. Low-"eld absorption signal (H ) is * observed in the range 1000}2500 Oe (Figs. 3a and 4a) for the magnetic "eld orientation h"03. One can also see two isotropic signals where the "rst one is isotropic signal H with g"2. The signal is . observable for Fe-implanted sample (Fig. 3a, both orientations) and disappears below 200 K. The second one is present in all experimental spectra at H"1600 Oe. The paramagnetic behaviour of that one, till the lowest temperatures, and its small linewidth (see Figs. 3 and 4) permits us to identify it as a signal of glass plate and this signal can be excluded from our further consideration. The observed signals reveal distinct temperature dependencies. FMR signal: In the viscous samples (Fe and Co both), the FMR signal H shifts to the high $+0 "eld side for the orientation h"03 and to lower "elds for the orientation h"903 with the temperature decrease. For all samples the signal gradually broadens, excepting the Co-implanted viscous one, where at lower temperatures a sort of `structurea of FMR absorption evolves under orientation h"03 and the narrowing arises under orientation h "903. ¸ow-,eld absorption: In the Fe-implanted sample the low-"eld absorption H gradually * broadens with temperature decrease (Fig. 3a). In contrast to this, the signal of low-"eld absorption of the Co-implanted sample may be clearly distinguished only at temperatures far below 200 K (Fig. 4a). Isotropic signal with g"2: In the Fe-implanted viscous sample isotropic signal H gradually . disappears at temperatures about 100 K (Fig. 3a). As can be seen from Figs. 2}4, the values of magnetic resonance "eld, FMR line shapes and their orientation and temperature dependencies strongly depend on the relaxation state of the substrate It should be noted, that the signal of low-"eld absorption and isotropic signal at g"2 were observed in some other samples Fe- and Co-implanted with smaller doses and exhibit a similar behaviour.
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555
Fig. 2. The orientation dependencies of FMR spectra of iron (a) and cobalt (b) granular "lms synthesized in viscous (on the left) and solid (on the right) silicone polymers at a dose of 1.25;10 ions/cm. The h is the angle between the static magnetic "eld and the normal to the "lms plane.
during the implantation and the type of implanted ions (Fe or Co). The FMR signal of the granular magnetic "lm results from the single particles entering the "lm. (Only at highest doses of implantation, when the continuous metal layer starts to form, the anisotropy of the FMR signal due to the shape of the sample as a whole would be essential [4].) An isolated spherical magnetic particle should provide the Lorentzian-like line shape of the resonance absorption at H"3400 Oe. According to the electron microscopy studies the shape of particles essentially deviate from spherical (typical size of particles is about 100 nm in plane of the thin "lm compared with the
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Fig. 3. The temperature dependencies of FMR spectra of iron granular "lms synthesized in viscous (Fig. 3a) and solid (Fig. 3b) silicone polymers at a dose of 1.25;10 ions/cm. The FMR absorption H , low-"eld absorption H and $+0 * isotropic paramagnetic signal at g"2 H are labelled. .
thickness of granular layer about 30 nm) and one can expect the contribution of particle shape anisotropy in FMR. Besides, magnetocrystalline anisotropy (together with the orientational distribution of the easy axes of anisotropy) and the interparticle magnetic dipolar interaction may contribute to the shifts mentioned above and the FMR lines broadening. In this way the analysis of the FMR spectra peculiarities observed in the implanted silicone samples should be performed with respect to the structural features of the synthesized granular metal "lms.
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557
Fig. 4. The temperature dependencies of FMR spectra of cobalt granular "lms synthesized in viscous (a) and solid (b) silicone polymers at a dose of 1.25;10 ions/cm. The FMR absorption H , low-"eld absorption H are labelled. $+0 *
3.3. Fe- and Co-implanted viscous samples Structural investigations showed that in the viscous Fe-implanted samples, the planar agglomerates of iron needle-like particles are formed. The orientation dependence of FMR spectra is determined mainly by the shape anisotropy of these agglomerates [5]. The temperature dependency of FMR line in the Fe-implanted sample is related to the increase of e!ective value of magnetization of the agglomerates with the temperature lowering. In viscous Co-implanted samples the closely packed cobalt agglomerates consist of particles of oblate ellipsoidal or spherical form, which do not touch each other as do some of the Fe particles at the same doses. Therefore, the more pronounced temperature dependence of Co-implanted sample is to be related mainly with the interparticle magnetic dipolar interaction, which is very signi"cant in the thin "lm of high local particle concentration. It is also suggested that the fraction of "ne iron
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and cobalt particles, which is isolated well from the particle agglomerates, exhibits the superparamagnetic (SPM) properties [6,7] * the magnetic moment direction of these particles #uctuates quickly in the "lm plane at higher temperatures. The strong dipolar interaction leads to: (1) a shift of the basic FMR signal H for Co-implanted sample and the disappearing of the signal of $+0 the isolated particles H under decreasing temperature as the averaging e!ect of thermal #uctu. ations of magnetization is reduced [8]; (2) an appearance of the additional resonance absorption H in low-"eld range of FMR spectra, when the value of static magnetic "eld H is lower than the * value of magnetic dipolar "elds from neighbouring particles. 3.4. Fe- and Co-implanted solid samples. The granular metal "lms formed in the solid samples consist of widely separated nanoparticles. In this case the orientation dependence of FMR spectra is generally caused by particle shape anisotropy. It is known that implantation of metal ions in solid substrates leads to the formation of oblate ellipsoidal metal particles with short axes being perpendicular to the implanted polymer surface [5]. The orientation dependence of FMR spectra of Fe-implanted solid sample, observed in Fig. 3a is smaller than that for the viscous-implanted sample. The magnitude of the orientation shift of the FMR line results from the following factors: the e!ective magnetization and the particle shape anisotropy. The e!ective value of magnetization, M of the granular thin "lms may be calculated from magnetic resonance data by using a modi"ed Kittel's equation [9]. In a rough approximation, M "Q;M, where Q is the volume percentage of magnetic metal particles in a composite metal}polymer "lm and M is the magnetization of the particles. In the case of isolated drop-like iron particles of the solid samples, the value M should be smaller than for large planar agglomerates of viscous silicone samples (Fig. 1A and B) as well as the value of the particle shape anisotropy. This is not the case of the Co-implanted viscous sample, an orientational shift of the FMR line for that is smaller than for the solid one. This may be due to the shape of individual Conanoparticles which is much more close to sphere in the viscous polymer than in the solid one. On the other hand, the granular metal "lms formed in the solid samples characterized by considerable dispersion of granule sizes, that results in the formation of `structurea of FMR absorption in the Co-implanted solid sample and in the appreciable FMR response from granules with largest sizes in the high-"eld range. A shift of resonance signals with the decreasing temperature under orientation h"03 for both solid samples is much smaller than for the viscous ones. The possible reasons for that are the following: (1) for the Fe-implanted solid sample the value of temperature-dependent e!ective magnetization is much smaller than for the viscous one; (2) for both solid samples, the shift is determined by the average value of dipole}dipole interactions between individual granules more diluted than the particle agglomerates of viscous sample. A shift of resonance signals at
The detailed analysis of the temperature behaviour of FMR lines observed in both viscous and solid samples will be published.
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559
Fig. 5. The FMR spectra of commercial 3.5 diskette at a room temperature for two di!erent orientations (h) of the applied DC magnetic "eld relative to the normal of diskette plane.
temperatures below 70 K for orientation h"903 obviously should be related to the increase of remanent magnetization at these temperatures. A reduction of thermal #uctuations of magnetic moments and SPM blocking in the particles produce random dipolar "elds [8]. Thereby, the considerable inhomogeneity of microstructure of the solid samples mentioned above result in the much pronounced broadening of the resonance lines at lower temperatures. FMR spectra of a commercial magnetic diskette are presented in Fig. 5. The calculated value of M &200 Oe is lower than the one in iron granular "lms synthesized in viscous silicone polymers at higher doses (&400 Oe) [2]. However, the observed zero-"eld FMR absorption and the asymmetric line shape at h"903 indicate a high remanence of a diskette at room temperature. It is not observed in most of our experimental samples at temperatures higher than 100 K, where the interparticle magnetic dipolar interactions and SPM e!ects in the plane of the magnetic "lm lead to fast disorientation of magnetic particle moments and the vanishing of a "lm magnetization at room temperature.
4. Summary The study of structural and magnetic properties of ion beam implanted thin iron and cobalt granular "lms in silicone polymers has been performed. Magnetic resonance studies have revealed the essential di!erences in the thin metal "lms synthesized at di!erent initial states of silicone substrate. It was shown that the viscosity of an initial state of silicone target and the type of implanted ions strongly in#uence the structural and magnetic properties of the synthesized "lms.
Acknowledgements This work is supported by Grant No. 99-03-32 548 of the Russian Basic Research Foundation and by Grant No. 99-A-01-02-14 of Research Fund of Gebze Institute of Technology.
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References [1] Kryder MH. Thin Solid Films 1992;216:174. [2] Khaibullin RI, Abdullin SN, Stepanov AL, Osin YuN, Khaibullin IB. Pis'ma Zh Tekh Fiz (Russia) 1996;22:48. [transl.: Tech Phys Lett (USA) 1996;22:112]. [3] Schatz M. In: AL Klebanckij, editor. Silikonovyij Kauchuk. Leningrad: Khimiya Press, 1975 (in Russian). [4] Netzelmann U. J Appl Phys 1990;68:1800. [5] Petukhov V, Zhikharev V, Ibragimova M, Zheglov E, Bazarov V, Khaibullin I. Solid State Commun 1996;97:361. [6] Neel L. C R Acad Sci 1949;228:664. [7] Bean CP. Livingston JD. J Appl Phys 1959;30:120S. [8] Sharma VK, Baiker A. J Chem Phys 1981;75:5596. [9] Rubinstein M, Das BN, Koon NC, Chrisey DB. Horwitz, J. Phys Rev B 1994;50:184.
Magnetic properties of iron-and cobalt-implanted silicone polymers夽 B.Z. Rameev *, B. Aktas , R.I. Khaibullin, V.A. Zhikharev, Yu.N. Osin, I.B. Khaibullin Gebze Institute of Technology, P.K. 141 41400, Gebze/Kocaeli, Turkey Kazan Physical-Technical Institute, Sibirsky Trakt 10/7, 420029 Kazan, Russia
Abstract Composite metal}polymer thin "lms were formed by implantation of 40 KeV Fe> and Co> ions into viscous and solid silicone substrates in the dose range of (0.3}1.5);10 ions/cm. Morphology and magnetic properties of the synthesized "lms were investigated. TEM investigations showed that the microstructure of metal granular layers synthesized in silicone polymers strongly depends on an initial relaxation state of the target. Magnetic resonance studies revealed essential di!erences in the magnetic properties of thin metal "lms synthesized at di!erent initial states of silicone substrate and with respect to the type of implanted ions (Fe or Co). A shift and broadening of FMR signal with the samples cooling are observed both for perpendicular and parallel orientation of the "lms with respect to the magnetic "eld. These e!ects are associated with the increase of the inter-particle interactions and superparamagnetism blocking in the particles. It was found that the viscous initial state of the polymer is a favourable factor for the high anisotropy of FMR response in Fe-implanted samples in contrast to the Co-based ones. The reasons for this have been discussed with respect to the morphology of Fe and Co synthesized "lms. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Ion implantation; Ferromagnetic resonance; Granular magnetic "lms; Polymers; Superparamagnetism
1. Introduction The investigation of granular metal}polymer "lms has great practical signi"cance for the development of magnetic recording and data storage devices, magnetoresistivity sensors, etc. [1]. 夽
Paper presented at the 11th International School on Vacuum, Electron and Ion Technologies, 20}25 September 1999, Varna, Bulgaria. * Corresponding author. Fax: #90-262-6538490. E-mail address: [email protected] (B.Z. Rameev). 0042-207X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 2 1 9 - 0
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B.Z. Rameev et al. / Vacuum 58 (2000) 551}560
Among the di!erent techniques available for preparation of such "lms, the method of high dose implantation has a few advantages. They are easy control of metal content in the polymer layer, homogeneous dispersion of metal particles inside the composite "lm and the possibility of controlling the size of metal granules. Recently, a new approach of ion implantation has been developed [2], where at the moment of implantation organic matrix is held at the viscous-#ow relaxation state and then polymerized. Such a technique, which has been used in our study, permits to control not only the average size, but also the size spread of nanoparticles and morphology of synthesized "lms as a function of target viscosity. This work presents the results of investigations on iron and cobalt implantation into viscous and solid silicone polymers by electron microscopy and magnetic resonance methods.
2. Experimental Commercial silicone polymers were used as substrates for implantation. A mixture of viscous silicone resin (low-molecular methyl-phenyl-siloxane, 97% of weight) with a curing agent (diethyldicaprilat of tin, 3%) was prepared, and thin (150 lm) silicone "lms were deposited on glass plates by centrifugation of a viscous mixture. (40 keV) Fe> and Co> ions were then implanted into both viscous-#ow and fully cured solid silicone "lms with doses of (0.3}1.5);10 ions/cm and an ion current density of 4 lA/cm. The dynamic viscosity of viscous substrates measured by a capillary viscometer was equal to 20 Pa s at an initial moment of the implantation. The viscosity of solid silicone polymers has a value of the order of 10}10 Pa s [3]. The microstructure and crystalline phases of the synthesized iron and cobalt granular "lms were studied by transmission electron microscopy (TEM) on the EM-125 electron microscope. Magnetic resonance spectra of the irradiated samples were recorded at temperatures ranging from 4.2 to 300 K with the Bruker EMX spectrometer operating in X-band (9.5 GHz). As usual, the "eld derivative of microwave power absorption (dP/dH) was registered as a function of the applied magnetic "eld H. For comparison, magnetic resonance spectra of the commercial 3.5 magnetic diskette (`Verbatima, HD, 1.44 MB) were also recorded at room temperature.
3. Results and discussion 3.1. Structural investigations The microstructure of the synthesized granular metal "lms is strongly dependent on the viscosity of the implanted silicone polymer and the type of embedded ions (Fe or Co). Fe-implanted samples. Fig. 1A and B represents the TEM plane visual images of iron granular "lms synthesized in viscous (A) and solid (B) polymer substrates implanted with a dose of 1.25;10 ions/cm. Both isolated iron nanoparticles and large planar agglomerates (with sizes of 100}300 nm) of the needle-like particles are observed in the samples implanted into the viscous state (Fig. 1A). The increase of implantation dose results in the enlargement of the mean size of these agglomerates, and the particles form a connected fractal-like network. In irradiated solid silicone samples, the synthesis of isolated drop-like iron particles with a mean size of about 30 nm is
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Fig. 1. TEM images of iron (A, B) and cobalt (C, D) granular "lms formed by ion implantation in viscous (A, C) and solid (B, D) silicone polymers at a dose of 1.25;10 ions/cm.
observed (Fig. 1B). Electron di!raction studies point out that most of the implanted iron ions enter into the b.c.c. lattice of metallic a-Fe. Only weak traces of iron oxides and silicides have also been observed. Co-implanted samples. Fig. 1C and D shows the microstructure of cobalt granular "lms produced in viscous (C) and solid (D) polymers implanted with a dose of 1.25;10 ions/cm. It is seen that large (100}200 nm) Co groups of closely packed particles are formed in viscous substrates. In implanted solid polymers the cobalt granular "lm consists of widely separated "ne particles and
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their size being spread in a rather broad range until 100 nm. Electron di!raction measurements obtained on cobalt particles indicate the hexagonal a-Co metallic phases in the implanted polymers only. The data on electron microscopic cross-section studies showed that synthesized iron and cobalt "lms are about 30}50 nm thick and they are buried at a depth of 20}25 nm from the polymer surface. 3.2. Magnetic resonance studies Fig. 2 shows the dependencies of magnetic resonance spectra on orientation (h) of the static magnetic "eld (H) relative to the normal of "lm (n) for viscous and solid samples implanted with Fe (Fig. 2a) and Co (Fig. 2b) ions with a dose of 1.25;10 ions/cm. All spectra contain the components H with the orientation dependencies typical for ferromagnetic resonance (FMR) in $+0 thin granular magnetic "lms: when H#n (h"03), then absorption lines are shifted to the high-"eld side, when HNn (h"903) they are shifted to the low-"eld side. The only component of FMR line is observed for all samples under orientation h"903, and a sort of `structurea of FMR absorption is observed for solid samples for h"03. Figs. 3 and 4 show the dependencies of spectra on temperature for the same samples (3 * implanted with Fe, 4 * implanted with Co, (a) viscous samples and (b) solid ones) for h"03 and h"903. There are extra resonance signals in the spectra. Low-"eld absorption signal (H ) is * observed in the range 1000}2500 Oe (Figs. 3a and 4a) for the magnetic "eld orientation h"03. One can also see two isotropic signals where the "rst one is isotropic signal H with g"2. The signal is . observable for Fe-implanted sample (Fig. 3a, both orientations) and disappears below 200 K. The second one is present in all experimental spectra at H"1600 Oe. The paramagnetic behaviour of that one, till the lowest temperatures, and its small linewidth (see Figs. 3 and 4) permits us to identify it as a signal of glass plate and this signal can be excluded from our further consideration. The observed signals reveal distinct temperature dependencies. FMR signal: In the viscous samples (Fe and Co both), the FMR signal H shifts to the high $+0 "eld side for the orientation h"03 and to lower "elds for the orientation h"903 with the temperature decrease. For all samples the signal gradually broadens, excepting the Co-implanted viscous one, where at lower temperatures a sort of `structurea of FMR absorption evolves under orientation h"03 and the narrowing arises under orientation h "903. ¸ow-,eld absorption: In the Fe-implanted sample the low-"eld absorption H gradually * broadens with temperature decrease (Fig. 3a). In contrast to this, the signal of low-"eld absorption of the Co-implanted sample may be clearly distinguished only at temperatures far below 200 K (Fig. 4a). Isotropic signal with g"2: In the Fe-implanted viscous sample isotropic signal H gradually . disappears at temperatures about 100 K (Fig. 3a). As can be seen from Figs. 2}4, the values of magnetic resonance "eld, FMR line shapes and their orientation and temperature dependencies strongly depend on the relaxation state of the substrate It should be noted, that the signal of low-"eld absorption and isotropic signal at g"2 were observed in some other samples Fe- and Co-implanted with smaller doses and exhibit a similar behaviour.
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555
Fig. 2. The orientation dependencies of FMR spectra of iron (a) and cobalt (b) granular "lms synthesized in viscous (on the left) and solid (on the right) silicone polymers at a dose of 1.25;10 ions/cm. The h is the angle between the static magnetic "eld and the normal to the "lms plane.
during the implantation and the type of implanted ions (Fe or Co). The FMR signal of the granular magnetic "lm results from the single particles entering the "lm. (Only at highest doses of implantation, when the continuous metal layer starts to form, the anisotropy of the FMR signal due to the shape of the sample as a whole would be essential [4].) An isolated spherical magnetic particle should provide the Lorentzian-like line shape of the resonance absorption at H"3400 Oe. According to the electron microscopy studies the shape of particles essentially deviate from spherical (typical size of particles is about 100 nm in plane of the thin "lm compared with the
556
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Fig. 3. The temperature dependencies of FMR spectra of iron granular "lms synthesized in viscous (Fig. 3a) and solid (Fig. 3b) silicone polymers at a dose of 1.25;10 ions/cm. The FMR absorption H , low-"eld absorption H and $+0 * isotropic paramagnetic signal at g"2 H are labelled. .
thickness of granular layer about 30 nm) and one can expect the contribution of particle shape anisotropy in FMR. Besides, magnetocrystalline anisotropy (together with the orientational distribution of the easy axes of anisotropy) and the interparticle magnetic dipolar interaction may contribute to the shifts mentioned above and the FMR lines broadening. In this way the analysis of the FMR spectra peculiarities observed in the implanted silicone samples should be performed with respect to the structural features of the synthesized granular metal "lms.
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557
Fig. 4. The temperature dependencies of FMR spectra of cobalt granular "lms synthesized in viscous (a) and solid (b) silicone polymers at a dose of 1.25;10 ions/cm. The FMR absorption H , low-"eld absorption H are labelled. $+0 *
3.3. Fe- and Co-implanted viscous samples Structural investigations showed that in the viscous Fe-implanted samples, the planar agglomerates of iron needle-like particles are formed. The orientation dependence of FMR spectra is determined mainly by the shape anisotropy of these agglomerates [5]. The temperature dependency of FMR line in the Fe-implanted sample is related to the increase of e!ective value of magnetization of the agglomerates with the temperature lowering. In viscous Co-implanted samples the closely packed cobalt agglomerates consist of particles of oblate ellipsoidal or spherical form, which do not touch each other as do some of the Fe particles at the same doses. Therefore, the more pronounced temperature dependence of Co-implanted sample is to be related mainly with the interparticle magnetic dipolar interaction, which is very signi"cant in the thin "lm of high local particle concentration. It is also suggested that the fraction of "ne iron
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and cobalt particles, which is isolated well from the particle agglomerates, exhibits the superparamagnetic (SPM) properties [6,7] * the magnetic moment direction of these particles #uctuates quickly in the "lm plane at higher temperatures. The strong dipolar interaction leads to: (1) a shift of the basic FMR signal H for Co-implanted sample and the disappearing of the signal of $+0 the isolated particles H under decreasing temperature as the averaging e!ect of thermal #uctu. ations of magnetization is reduced [8]; (2) an appearance of the additional resonance absorption H in low-"eld range of FMR spectra, when the value of static magnetic "eld H is lower than the * value of magnetic dipolar "elds from neighbouring particles. 3.4. Fe- and Co-implanted solid samples. The granular metal "lms formed in the solid samples consist of widely separated nanoparticles. In this case the orientation dependence of FMR spectra is generally caused by particle shape anisotropy. It is known that implantation of metal ions in solid substrates leads to the formation of oblate ellipsoidal metal particles with short axes being perpendicular to the implanted polymer surface [5]. The orientation dependence of FMR spectra of Fe-implanted solid sample, observed in Fig. 3a is smaller than that for the viscous-implanted sample. The magnitude of the orientation shift of the FMR line results from the following factors: the e!ective magnetization and the particle shape anisotropy. The e!ective value of magnetization, M of the granular thin "lms may be calculated from magnetic resonance data by using a modi"ed Kittel's equation [9]. In a rough approximation, M "Q;M, where Q is the volume percentage of magnetic metal particles in a composite metal}polymer "lm and M is the magnetization of the particles. In the case of isolated drop-like iron particles of the solid samples, the value M should be smaller than for large planar agglomerates of viscous silicone samples (Fig. 1A and B) as well as the value of the particle shape anisotropy. This is not the case of the Co-implanted viscous sample, an orientational shift of the FMR line for that is smaller than for the solid one. This may be due to the shape of individual Conanoparticles which is much more close to sphere in the viscous polymer than in the solid one. On the other hand, the granular metal "lms formed in the solid samples characterized by considerable dispersion of granule sizes, that results in the formation of `structurea of FMR absorption in the Co-implanted solid sample and in the appreciable FMR response from granules with largest sizes in the high-"eld range. A shift of resonance signals with the decreasing temperature under orientation h"03 for both solid samples is much smaller than for the viscous ones. The possible reasons for that are the following: (1) for the Fe-implanted solid sample the value of temperature-dependent e!ective magnetization is much smaller than for the viscous one; (2) for both solid samples, the shift is determined by the average value of dipole}dipole interactions between individual granules more diluted than the particle agglomerates of viscous sample. A shift of resonance signals at
The detailed analysis of the temperature behaviour of FMR lines observed in both viscous and solid samples will be published.
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Fig. 5. The FMR spectra of commercial 3.5 diskette at a room temperature for two di!erent orientations (h) of the applied DC magnetic "eld relative to the normal of diskette plane.
temperatures below 70 K for orientation h"903 obviously should be related to the increase of remanent magnetization at these temperatures. A reduction of thermal #uctuations of magnetic moments and SPM blocking in the particles produce random dipolar "elds [8]. Thereby, the considerable inhomogeneity of microstructure of the solid samples mentioned above result in the much pronounced broadening of the resonance lines at lower temperatures. FMR spectra of a commercial magnetic diskette are presented in Fig. 5. The calculated value of M &200 Oe is lower than the one in iron granular "lms synthesized in viscous silicone polymers at higher doses (&400 Oe) [2]. However, the observed zero-"eld FMR absorption and the asymmetric line shape at h"903 indicate a high remanence of a diskette at room temperature. It is not observed in most of our experimental samples at temperatures higher than 100 K, where the interparticle magnetic dipolar interactions and SPM e!ects in the plane of the magnetic "lm lead to fast disorientation of magnetic particle moments and the vanishing of a "lm magnetization at room temperature.
4. Summary The study of structural and magnetic properties of ion beam implanted thin iron and cobalt granular "lms in silicone polymers has been performed. Magnetic resonance studies have revealed the essential di!erences in the thin metal "lms synthesized at di!erent initial states of silicone substrate. It was shown that the viscosity of an initial state of silicone target and the type of implanted ions strongly in#uence the structural and magnetic properties of the synthesized "lms.
Acknowledgements This work is supported by Grant No. 99-03-32 548 of the Russian Basic Research Foundation and by Grant No. 99-A-01-02-14 of Research Fund of Gebze Institute of Technology.
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