Nano particles of iron oxides in SiO 2 glass prepared by ion implantation

J Radioanal Nucl Chem (2011) 287:341 346 DOI 10.1007/s10967-010-0858-0 Nano particles of iron oxides in SiO 2 glass prepared by ion implantation K. Nomura H. Reuther Received: 29 June 2010 / Published online: 10 October 2010 Ó Akadémiai Kiadó, Budapest, Hungary 2010 Abstract Quartz (SiO 2 ) glass was implanted with 5 9 10 16 57 Fe ions/cm 2 at a substrate temperature of 500 C, and annealed at temperatures between 700 and 950 C. The implanted and annealed plates were characterized by conversion electron Mössbauer spectroscopy (CEMS), and measured by a Kerr effect magnetometer or a vibration sample magnetometer. Kerr effect measurement of as-implanted SiO 2 glass showed ferromagnetism at room temperature. CEM spectrum of the as-implanted glass consisted of magnetic relaxation peaks of finely dispersed metallic Fe species, and paramagnetic doublets of Fe 3? and Fe 2? species. The sample heated at 700 C contained large grains of metallic Fe and a lot of oxidation products of Fe 2? species. After oxidation at temperatures higher than 800 C, the samples showed also ferromagnetism, which was attributed mainly to ferromagnetic e-fe 2 O 3 precipitated in SiO 2 matrix. Small amounts of a-fe 2 O 3 were produced at 950 C. The results suggest that ion implantation and oxidation make a transparent ferromagnetic glass possible. Keywords Nano particles of iron in SiO 2 matrix Ion implantation e-fe 2 O 3 Transparent magnetic glass Room temperature magnetism K. Nomura (&) School of Engineering, the University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan e-mail: k-nomura@t-adm.t.u-tokyo.ac.jp H. Reuther Forschungszentrum Dresden-Rossendorf e. V, Postfach 510119, 01324 Dresden, Germany Introduction Dilute magnetic ion doped oxide semiconductors are candidates for spintronics materials because they show room temperature ferromagnetism [1, 2]. However, it is still not so clear why the dilution of magnetic ions enhances ferromagnetic properties of oxide semiconductors. It is necessary to clarify the chemical and magnetic states of Fe ion doped oxides. We have investigated Fe doping effect of sol gel synthesized SnO 2 powders [3, 4]. Phonon densities of state of dilute Fe doped rutile, SnO 2 and TiO 2, have been investigated by nuclear inelastic scattering (NIS) using synchrotron radiation [5]. NIS is found to be able to determine if the doped 57 Fe ions formed clusters or are diluted. On the other hand, as one of the approach, we have prepared films of Sn 1-x 57 Fe x O 2-d by implantation with 1 9 10 17 and 5 9 10 16 Fe ions/cm 2 [6]. The as-implanted samples at room temperature and the post-annealed samples did not show any ferromagnetic property, but the sample implanted with Fe ions at 300 C showed Kerr effect, that is, bulk ferromagnetism. The Kerr effect disappeared after annealing. We have implanted 57 Fe ions into SnO 2 film on quartz heated at a temperature of 500 C [7]. As the result, larger rotation angles of Kerr effect were obtained than at the substrate temperature of 300 C. Kerr effect did not disappear even after annealing. SnO 2 (0.1 and 3% Sb) films implanted with 5 9 10 16 57 Fe ions/cm 2 at 500 C were post-annealed step by step from 400 to 800 C, and the magnetic properties and microstructures of these films have been investigated in detail [8, 9]. Kerr rotation angles increased a little with the increase of annealing temperatures up to 700 C. In the as implanted SnO 2 film, formation of fine magnetite is recognized. It is further found that the room temperature ferromagnetism is

342 K. Nomura, H. Reuther attributed mainly to formation of fine maghemite produced by post-annealing. This is not intrinsic, but extrinsic for dilute magnetism. In these transparent films, finely dispersed maghemite in matrixes is relatively stable up to higher temperatures rather than maghemite itself, which is normally converted to hematite above 400 C. Recently we have also reported that yttrium aluminum garnet (YAG) shows ferromagnetic behavior by diluting Fe ions and found that magnetic sextets appeared in Mössbauer spectra with decreasing Fe concentration [10]. Dilute magnetic insulator also is attractive for a basic research as well as the application. Implantation techniques may be used as fabrication of nano magnetic materials formed in the transparent materials. Thus, we have tried to implant Fe ions into quartz directly in this study, and investigated the matrix effects on formation of iron oxides. Kerr rotation angle [rad.] 0.006 0.004 0.002 0.000-0.002-0.004 As implanted with 5x10 16 Fe ions/cm 2 (a) SiO 2, λ=300nm (b) 0.1%Sb doped SnO 2, λ= 300nm -0.006-20 -10 0 10 20 Applied field H [koe] Fig. 1 Polar Kerr rotation curves of (a) SiO 2 glass plate and (b) SnO 2 (0.1%Sb) films implanted with 57 Fe at substrate temperature of 500 C, measured by MCD mode at k = 300 nm Experimental A quartz glass plate was implanted with 5 9 10 16 57 Fe ions/cm 2 at the substrate temperature of 500 C in vacuum, using energy of 100 kev [7]. The iron profile peak is expected to be located at about 40 nm depth with a maximum Fe concentration of 5 at.%. Polar Kerr effect of the as-implanted sample was measured with magnetic circular dichroism (MCD) mode. The sample was postannealed at 700, 800 and 950 C in air. Magnetic properties were measured by a vibration sample magnetometer (VSM; Riken Denshi Co.). CEM spectra were observed using a homemade back scattering type of He? 5% CH 4 gas flow counter [11] and a c source of 57 Co/Cr matrix. Doppler velocity was calibrated with a-fe foil at room temperature. Results and discussion Polar Kerr rotation curves at light wave length of k = 300 nm by MCD mode were measured for SiO 2 plate implanted with 57 Fe at a substrate temperature of 500 C. The result is shown in Fig. 1 as compared with that of SnO 2 (0.1%Sb) film. It is clear that the Kerr rotation direction of SiO 2 implanted with Fe is reversed and the angle is larger, by a factor of three, than that of SnO 2 film implanted with the same amount of Fe. CEM spectrum of as-implanted SiO 2 glass is shown in Fig. 2. Mössbauer parameters analyzed are listed in Table 1. CEM spectrum was decomposed into two doublets and a broad magnetic relaxation peak. Two doublets are assigned to Fe 3? and Fe 2? paramagnetic species, respectively, from the values of isomer shift (d). The broad peak was fitted as an approximation by Blume Tjon two stage relaxation model [12], 1.30 1.25 1.20 1.15 0.95 Fig. 2 CEM spectrum of Fe implanted SiO 2 at a substrate temperature of 500 C offered by Mosswinn program [13], assuming the magnetic field of a-fe and asymmetry parameter (g) of electric field gradient (EFG) = 0. However, the resultant parameters are not conclusive. In the case of Fe implanted SnO 2 film, the doublets of Fe 3? (d = 0.39 mm/s, D = 0.73 mm/s) and Fe 2? (d = mm/s, D = 1.99 mm/s) species and two broad sextets of fine magnetite (d = 0.33 mm/s, B hf = 47.2 T and d = 0.64 mm/s, B hf = 43.1 T) are observed [8]. The iron products in the SnO 2 films are clearly different from iron species produced in the SiO 2 matrix by implantation with 57 Fe. Thus it is found that finely dispersed metallic Fe species are produced together with Fe 3? and Fe 2? paramagnetic species in SiO 2 matrix. The metallic Fe particles aggregated in SiO 2 matrix are considered to induce the large Kerr effect.

Nano particles of iron oxides in SiO 2 glass 343 Table 1 Mössbauer parameters of Fe implanted SiO 2 at a substrate temperature of 500 C Doublet (1); Fe 3? 21.8 0.19(1) 0.63 (2) 0.70 (3) Doublet (2); Fe 2? 14.7 0.96 (1) 2.15(2) 0.70 (3) Magn. relax. peaks; Fe 0 63.5 0.01(3) D* 0.52 (2) 33.3(fixed) Tentative D*, V zz (EFG) =-0.3 (3) [10 21 V/m 2 ], g (EFG) = 0.00 (fixed), Relax. time s (s) = 10-8.63 The Fe implanted SiO 2 plate was annealed at 700 C for 2 h in air. The CEM spectrum and the analyzed parameters are shown in Fig. 3, and Table 2, respectively. It is clear from the Mössbauer parameters that small parts of fine metallic Fe are aggregated to form the large grains of metallic Fe (12%) in the SiO 2 matrix, and that large parts of fine metallic Fe are oxidized into two kinds of paramagnetic Fe 2? species (Doublet (2); area intensity = 28%, Doublet (3); area int. = 34%). The oxidation enlarged the volume of the iron oxides. The Mossbauer parameters of Fe 2? oxides produced in SiO 2 matrix are different from those of fayalite (Fe 2 SiO 4 ) (A; d = 1.11 mm/s, D = 2.82 mm/s, B; d = 1.22 mm/s, D = 2.88 mm/s, orthoferrosilite (FeSiO 3 )(A; d = 1.21 mm/s, D = 2.44 mm/s, B; d = 1.11 mm/s, D = 1.64 mm/s), and wustite (FeO x ) (d = 1.15 mm/s, D = 0.73 mm/s), which can be produced by the oxidation of silicon iron alloys [14]. The isomer shift of Fe 2? species produced in SiO 2 matrix is very close to that of wustite although the quadruple splitting is larger than that of conventional wustite. Iron-silica compounds such as fayalite and orthoferrosilite would not be produced at 700 C because iron products only were observed by heating at temperatures higher than 800 C. Thus, the main product of Fe 2? species may be fine wustite because the fine particle has generally a large quadrupole splitting due to the large surface effect. The magnetic components of iron oxides are considered to be precursors of epsilon iron oxides, which are produced at 800 C. Fe implanted SiO 2 glass annealed at 700 C was further annealed at 800 C for 2 h. The CEM spectrum and parameters are shown in Fig. 4 and Table 3, respectively. It is found that Fe species produced by annealing at 800 C became almost all Fe 3? ions, and that the major constituents produced reflected magnetic sextets (about 75%). The 1.30 1.25 1.20 1.15 Fig. 3 CEM spectrum of Fe implanted SiO 2, annealed at 700 C for 2h Fig. 4 CEM spectrum of Fe implanted SiO 2, annealed at 800 C for 2h Table 2 Mössbauer parameters of Fe doped SiO 2 annealed at 700 C for 2 h Doublet (1); Fe 3? 18.5 0.42 (1) 0.72 (2) 0.77 (3) Doublet (2); Fe 2? 27.8 1.08 (1) 1.95 (2) 0.59 (3) Doublet (3); Fe 2? 33.5 1.15 (1) 2.77 (2) 0.37 (3) Magn. sextet (1); Fe 0 11.2 0.00 (3) 0 0.71 (2) 32.4 (1) Magn. sextet (2); Fe 3? 8.8 0.38 (5) -0.38 1.5 (1) 42.9 (4) Magn. sextet (3); Fe 3? 2.3 0.40 (3) 0.03 0.54 (2) 25.7 (2)

344 K. Nomura, H. Reuther Table 3 CEM parameters of Fe implanted SiO 2, annealed at 800 C for 2 h Doublet (1); Fe 3? 17.7 0.37 (1) 0.76 (2) 0.84 (3) Doublet (2); Fe 3? 6.9 0.46 (1) 2.22 (2) 0.63 (3) Magn. sextet (1); Fe 3? 19.3 0.24 (2) 0.19 (3) 1.17 (3) 25.6 (1) Magn. sextet (2); Fe 3? 19.6 0.37 (5) 0.10 (5) 1.37 (6) 40.0 (2) Magn. sextet (3); Fe 3? 36 0.37 (1) -0.07 (1) (2) 46.0 (1) magnetic components can be deconvoluted into three sextets with large line widths, assuming that the peak intensity ratio of each sextet is 3:2:1:1:2:3 because the iron ions are distributed at random into silica matrix. Three hyperfine fields are close to those of e-fe 2 O 3 (B hf = 25.6, 38.9, and 44.5 T) [15] although the line widths are large. The area intensity ratio of three sextets is close to 1:1:2, which is corresponding to that of e-fe 2 O 3. The crystal structure of e-fe 2 O 3 contains four Fe sites, but two sites of e-fe 2 O 3 cannot be distinguished from each other because of very similar environments of Fe ions. Consequently magnetic sextet (3) with large hyperfine field has intensity around twice as much as magnetic sextet (1) or (2). These sextets are characteristic to each site of fine e-fe 2 O 3. It is known that e-fe 2 O 3 is formed as an intermediate polymorph between c-fe 2 O 3 and a-fe 2 O 3 due to the limited agglomeration in small cavities of silica matrix [16]. The structure contains three non-equivalent anion and four cation positions. One Fe 3? position has tetrahedral coordination and the other three positions have octahedral coordination. The IS values indicate three octahedral sites (d = 0.37 039 mm/s) and one tetrahedral site (d = 0.21 mm/s). Two of the octahedral sites are characterized by magnetic hyperfine field values close to each other (45 T) and are overlapped as the resulting. The third octahedral site exhibits a smaller hyperfine field (ca. 40 T). The magnetic field at tetrahedral site is low (about 25 T) [15]. In case of Fe implanted SnO 2 films, fine magnetite is produced with paramagnetic species by implantation, and easily converted to fine c-fe 2 O 3 by post annealing [5] because of the same spinel structure of magnetite and maghemite. In the case of Fe implanted SiO 2, the magnetite was not clearly observed, but the metallic Fe could be found. Even if the metal species are oxidized, magnetite or maghemite are not produced in the small cavities of SiO 2 matrix. On the other hand, it is reported that e-fe 2 O 3 is formed as an intermediate compound in sol gel silica matrix at temperature of 1000 C [17]. The paramagnetic doublets of Fe 3? species (25%) are considered to be superparamagnetic components due to small particles of e-fe 2 O 3 or Fe oxides dispersed into the SiO 2 matrix. Fig. 5 CEM spectrum of Fe implanted SiO 2, annealed at 950 C for 2h CEM spectrum and parameters of Fe implanted SiO 2 heated at 950 C for 2 h after heated at 700 C are shown in Fig. 5 and Table 4, respectively. Mössbauer spectrum consists of two doublets and four sextets. The two doublets are considered to be due to the same superparamagnetic components as those of SiO 2 heated at 800 C although the isomer shift and quadruple splitting were smaller. The area intensities of paramagnetic components were almost the same when heating at 800 and 950 C. They suggest that the heating at higher temperature induces only structure relaxation. The hyperfine fields of three sextets are corresponding to those of orthorhombic e-fe 2 O 3 although the intensity of the sextet with a small hyperfine field of 25 T became low by heating at 950 C, and a new magnetic sextet with a large hyperfine field of 50 T appeared. The new sextet with B hf = 50 T, d = 0.37 mm/s, D = -0.22 mm/s and area intensity = 8% is clearly assigned to fine hematite (a-fe 2 O 3 ) although the hyperfine field is small by 1 T as compared with normal a-fe 2 O 3 particle and the negative value of D =-0.22 mm/s is characteristic for hematite, not for maghemite. It is known that hematite shows very weak ferromagnetism at room temperature whereas maghemite shows strong ferromagnetism. VSM data of Fe implanted SiO 2 heated at 800 and 950 C for 2 h are shown in Fig. 6. It is clear that ferromagnetic components are included in both heated samples. The gradient of magnetization and the coercive force both

Nano particles of iron oxides in SiO 2 glass 345 Table 4 CEM parameters of Fe implanted SiO 2, annealed at 950 C for 2 h Doublet (1); Fe 3? 14.0 0.39 (1) 0.52 (3) 0.67 (8) Doublet (2); Fe 3? 10.8 0.32 (1) 1.93 (2) 0.83 (3) Magn. sextet (1); Fe 3? 10.8 0.24 (2) -0.03 (3) 1.4 (2) 24.6 (4) Magn. sextet (2); Fe 3? 18.7 0.37 (2) 0.03 (4) 1.68 (2) 40.1 (2) Magn. sextet (3); Fe 3? 37.3 0.37 (1) -0.03 (1) 0.99 (2) 46.4 (1) Magn. sextet (4); Fe 3? 8.2 0.37 (1) -0.22 (1) 0.46 (2) 50.0 (1) Magnetization (μ B /f.u.) 0.6 0.4 0.2 0.0-0.2-0.4-0.6 Quarz glass implanted with 57 Fe, annealed at -2000 0 2000 External field (Oe) 950 C, the area intensity of magnetic sextet (1) became small when heated at 950 C. A part of tetrahedral site in e-fe 2 O 3 may be considered to change easily into a-fe 2 O 3 by substitution of Si ions at Fe ions in tetrahedral site of e-fe 2 O 3 because Si ions are originally coordinated with four oxygen atoms. If Fe implanted and annealed SiO 2 glass could be applied for transparent magnets, e-fe 2 O 3 nano-crystals should be grown into rod-like shape by introducing Ba or Sr ions in quartz glass because e-fe 2 O 3 with a large coercive field (20 koe) was obtained in the presence of Ba and Sr ions [19]. Conclusions Fig. 6 VSM curves of Fe implanted SiO 2, annealed at 800 C and 950 C for 2 h decreased after heating at 950 C. They suggest that heating at 950 C let grow a small amount of weak ferromagnetic iron oxides. These results are reflected in CEM spectra as shown in Figs. 4 and 5. In the case of Fe implanted SnO 2 film heated above 600 C, a-fe 2 O 3 (d = 0.38 mm/s, D =-0.19 mm/s, B hf = 50.5 T, C = 0.35 mm/s) was produced together with maghemite (d = 0.33 mm/s, D =-0.04 mm/s, B hf = 47.9 T, C = 0.9 mm/s). Two kinds of paramagnetic Fe 3? species (d = 0.34 0.37 mm/s, D = 0.77 0.85 mm/s, C = 0.44 0.49 mm/s, and d = 0.26 0.33 mm/s, D = 1.35 1.51 mm/s, C = 0.65 0.90 mm/s) were clearly observed. One component with large isomer shifts may be superparamagnetic a-fe 2 O 3 and c-fe 2 O 3, and another with lower isomer shifts and large quadrupole splitting may be dilute Fe 3? in SnO 2 matrix. Almost all iron oxides became superparamagnetic a-fe 2 O 3 at 800 C [9]. However, in case of Fe implanted SiO 2 glass, even if heated at high temperature of 950 C, fine e-fe 2 O 3 remains although the part is converted to a small amount of hematite. The particle sizes of hematite supported on silica have been studied by Mössbauer spectra, in which the doublet and sextet have been observed [18]. Compared the area intensity ratio in three magnetic sextets (1), (2), and (3) for the samples heated at 800 and A SiO 2 glass plate was implanted with 5 9 10 16 57 Fe ions/ cm 2 at a substrate temperature of 500 C and post-annealed from 700 C to 800 C or to 950 C in air. The following results were obtained by measurements of magnetic properties and CEMS. (1) Fe implanted SiO 2 glass shows Kerr effect at light wavelength k = 300 nm, and after the annealing at high temperatures also shows ferromagnetic hysteresis. (2) Metallic iron clusters are formed with Fe 2? and Fe 3? paramagnetic species in as-implanted SiO 2 glass. (3) When the sample is heated at 700 C in air, metallic Fe clusters are aggregated to grow large grains of metallic Fe together with formation of paramagnetic ferrous oxides in SiO 2 matrix. (4) When further heated at 800 C in air, fine grains of e-fe 2 O 3 with orthorhombic structure are formed as main products in SiO 2 matrix. (5) The deformation of e-fe 2 O 3 occurs and a little amount of hematite is produced in SiO 2 by heating at 950 C. (6) Room temperature ferromagnetism comes from finely dispersed metallic Fe species implanted in SiO 2 glass, heat-treated below 700 C, and from formation of e-fe 2 O 3 for SiO 2 glass heated above 800 C. These results suggest the possibility that a transparent and magnetic glass will be realized by Fe ion implantation and oxidation.

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