We are thankful to Tokyo Ohka Foundation for financial support of ... 211â212. (1994) 455â459. 2) H. Uchida, M. Wada, K. Koike, H. H. Uchida, V. Koeninger, Y.
Materials Transactions, Vol. 45, No. 2 (2004) pp. 225 to 228 Special Issue on Materials and Devices for Intelligent/Smart Systems #2004 The Japan Institute of Metals
Ar Ion Beam Irradiation Effects on Magnetostrictive Characteristics of Tb-Fe Thin Film Mitsuaki Takeuchi1; * , Yoshihito Matsumura1 , Hirohisa Uchida1 and Toshiro Kuji2 1
Department of Applied Science, School of Engineering, Tokai University, Hiratsuka 259-1292, Japan Department of Information and Communication Technology, School of High-technology of Human Welfare, Tokai University, Hiratsuka 259-1292, Japan 2
TbFe2 films prepared by a flash evaporation system onto Si(100) or polyimide substrate have been irradiated with different Ar ion doses at zero, 1:3 1017 and 2:7 1017 cm2 and at 10 kV. Magnetostrictive properties, i.e., saturated magnetostriction and magnetostrictive susceptibility, of TbFe2 film with disordered structure were improved by Ar ion beam irradiation. This result was probably caused increasing of in-plane compressive stress corresponding to change of volume magnetostriction. (Received July 29, 2003; Accepted September 9, 2003) Keywords: terbium iron alloy, magnetostriction, magnetostrictive susceptibility, thin film, ion beam irradiation, ion implantation, flush evaporation, residual stress, compressive stress, volume magnetostriction
1.
Introduction
The giant magnetostrictive films exhibit promising high applicability to devices for micro-machines, sensor systems and surface acoustic wave filters1–3) due to high response velocity and huge stress created by the magnetostriction. For these applications, high magnetostrictive susceptibilities with appropriate small hysteresis at low magnetic field are required. Since 1990, we have systematically investigated the magnetostrictive properties of the TbFe2 , DyFe2 , and (Tb,Dy)Fe2 compounds using different techniques of thin film formation systems,4) i.e., flash evaporation,5–9) ion beam sputtering,10) ion plating,11) electron beam evaporation,11) and magnetron sputtering.12) We previously reported that RFe2 (R=rare earth elements) films include high concentration of gases such as O2 , H2 O, CO, or CO2 during a film formation process. In particular, oxygen markedly affects magnetic and magnetostrictive properties (Uchida et al., 2002).4) The giant magnetostrictive films are affected by vacuum condition, chemical composition, microstructure, and internal stress. Among them, internal stress could dominate the magnetostrictive properties (Schatz et al., 1994, Xu et al., 2001).13,14) Wada et al. (1996) have reported that the magnetostrictive properties of Tb-Dy-Fe thin films prepared by ion beam sputtering system using Ar may be attributed to the internal stress induced by Ar inclusion during the deposition.10) Nevertheless, due to high costs and technical difficulties, the ion beam sputtering system is only used in laboratory. Magnetron sputtering system is alternative ways to form TbFe2 films. Post-treatment, i.e., Ar ion beam irradiation, is necessary to improve magnetostrictive properties. However, quantitative effects of Ar ion beam irradiation on magnetostrictive characteristics and residual stress have not been clear yet. In this study, the effect of Ar ion beam irradiation on the magnetic and magnetostrictive characteristics of TbFe2 films was quantitatively discussed.
*Graduate
Student, Tokai University.
2.
Experimental
To reveal effects on magnetostriction by Ar ion beam irradiation, the samples were prepared by the flash evaporation system because the system is low energy process.5–9) Base pressure was 8:8 105 Pa, and substrate temperature was maintained at 400 K. The TbFe2 powder pulverized into 40–100 mm was evaporated onto substrates by tungsten heater at 2300 K. The TbFe2 films were deposited on either single crystal Si (100) wafer (0.28 mm thick) or polyimide substrate (0.13 mm thick), with deposition rate of 16 nm/s. The TbFe2 films prepared by flash evaporation system were irradiated with Ar ion beam. The Ar ion beam irradiation was done with a plasma cathode ion source.15) Vacuum conditions were in the range of 105 Pa as background and 1:5 102 Pa as irradiation Ar gas pressure. The Ar ion beam by the potential of 10 kV applied between the ion source and the sample target. The flux was 4:2 1014 ions/cm2 s. The TbFe2 film was treated with the Ar ion beam at 1.3 or 2:7 1017 doses. The structure of the film was examined by X-ray diffraction (XRD) using Cu K radiation. The chemical composition of the film was determined using energy dispersive X-ray spectroscopy (EDX), and the amount of Ar atom implanted to film was determined by thermal desorption spectroscopy (TDS). The magnetization of the film samples were measured using vibrating sample magnetometer (VSM) in the range of plus and minus of 1200 kAm1 . The magnetostriction of film sample was measured by a bending cantilever beam method in parallel direction to the film plane.16) For the calculation of the magnetostriction an effective Young’s modulus of the film was assumed to be 76 GPa.17) The residual stress inside the film was determined from measurement of bending cantilever method using micrometer (Riethmuller and Benecke,1988).18)
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Fig. 2 TDS spectra of TbFe2 film exposed to different irradiation doses.
Fig. 1
3.
EDX spectra of TbFe2 film exposed to different irradiation doses.
Results and Discussions
3.1 Film composition and quantity of implanted Ar The composition of the film(TbFe2 ) did not vary before and after the Ar ion beam irradiation. EDX spectra of TbFe2 films treated with Ar ion beam irradiation at 1:3 1017 and 2:7 1017 doses shows Ar peak as shown in Fig. 1. Figure 2 shows TDS spectra of TbFe2 films exposed to Ar ion beam. The measurement was carried out with heating the sample up to 1000 K at 0.4 K/s and keeping at 1000 K to 9000 s for below 1:0 104 Pa of base pressure. Desorption Ar peaks from TbFe2 films treated with Ar ion beam irradiation were detected at about 2000 s of measurement time. Films with 1:3 1017 and 2:7 1017 doses desorbed Ar atoms of about 1012 and 1013 assuming that TDS spectra of as-deposited film is back ground. These results indicate that Ar atoms were implanted into the film during ion beam irradiation. 3.2 Film structure XRD diffraction patterns of TbFe2 films treated with Ar ion beam irradiation are shown in Fig. 3. No distinct diffraction peaks from TbFe2 phase were observed in asdeposited sample, suggesting that the as-deposited film was expected to be amorphous structure. XRD patterns shown in Fig. 3 indicate that films seem to be composed of extremely fine grains or be in amorphous state. In previous studies, we reported that Tb-Dy-Fe films are in nano-crystalline structure (grain size < 10 nm) when ratio R of vacuum-to-deposition rate in a film formation process is less than 5:0 104 Pa s nm1 (Wada et al., 1996).7) The ratio R is defined as p=r, where p is pressure during film formation and r is
Fig. 3 XRD diffraction patterns of TbFe2 films at different Ar ion irradiation doses.
deposition rate. The ratio R was 5:0 105 Pa s nm1 in this work so that the TbFe2 film could be nano-structured. The result may lead to small magnetostrictive hysteresis because of extremely low magneto-crystalline anisotropy. 3.3 Magnetic and magnetostrictive characteristics Figure 4 shows the magnetization hysteresis loops of TbFe2 films for the Ar ion irradiation doses of zero, 1:3 1017 and 2:7 1017 ions/cm2 . Magnetization loops of perpendicular to film plane were corrected as demagnetizing coefficient n ¼ 1. Crystalline TbFe2 film has a perpendicular magnetic anisotropy.4) The as-deposited film shows similar perpendicular magneto anisotropy corresponding to larger residual magnetization parallel to film plane Mr== than that of perpendicular to film plane Mr? . In contrast, Mr== increased and Mr? decreased with increasing Ar ion dose. This indicates that magnetic anisotropy changed from perpendicular to in-plane. The change caused by Ar ion beam irradiation seems to originate from the magnetoelastic coupling energy which will be discussed later in 3.4. These samples seem to be affected strongly by magneto-shape anisotropy and residual stress due to the low magnetocrystalline anisotropy of the disordered structure shown in XRD diffraction patterns.
Ar Ion Beam Irradiation Effects on Magnetostrictive Characteristics of Tb-Fe Thin Film
Fig. 4
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The magnetic hysteresis loops of TbFe2 film at different Ar ion irradiation doses.
Fig. 5 The field dependence of magnetostriction for TbFe2 film exposed to different Ar ion beam irradiation doses.
Figure 5 shows magnetostriction == of TbFe2 films with different Ar ion beam irradiation doses. In all cases the magnetostrictive hysteresis was not found within our experimental precision, but the magnetostriction at 1200 kAm1 of magnetic field increased by Ar ion beam irradiation. These effects may be caused by increasing of the in-plane compression stress inside the film due to Ar ion implantation. As shown in Fig. 6, Ar ion implantation yielded increase in magnetostrictive susceptibility d== =dH in the low magnetic field. The magnetostrictive susceptibility reached the maximum value and decreased with magnetic field. The applied magnetic field giving the maximum value of magnetostrictive susceptibility decreased by Ar ion beam irradiation. Large magnetostrictive susceptibility of TbFe2 film implanted with Ar atoms may be induced by increasing magnetoelastic energy resulted in change of residual stress inside the film due to Ar ion beam irradiation. 3.4 Residual stress Figure 7 shows the residual stress of in-plane direction as a function of Ar ion dose. In film material, film receives stress as the result of bending substrate. The residual stress of asdeposited TbFe2 film was calculated from the difference in thermal expansion coefficients of Si single crystal and TbFe2 , 4 106 K1 , and 12 106 K1 from TbDyFe data of Ferromagnetic Materials,17) respectively. The as-deposited TbFe2 film must have tensile stress in the direction parallel to
Fig. 6 The magnetostrictive susceptibility of TbFe2 film under different Ar ion doses.
Fig. 7 The residual stress of in-plane direction inside the TbFe2 film as a function of Ar ion dose.
the film surface due to the fact that thermal contraction of the film is larger than that of Si (100) substrate. The residual stress inside the film varied from tensile stress to compression stress with increasing amount of Ar ion dose. Perry has estimated that large changes in residual stress and strain
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distributions are not only introduced in ion implanted zone but also are induced beyond ion implanted zone due to the increase of dense dislocation networks.19) In the TbFe2 film the dense dislocation networks at depths beyond the Ar implanted zone most likely caused change of volume magnetostriction corresponding to the increase of compressive stress parallel to film plane. Following Schatz et al., films with in-plane anisotropy show high magnetostriction at low magnetic fields due to the easy rotation of the spins even if the motion of the 180 degree domain walls does not contribute to magnetostriction. However films with perpendicular anisotropy need far higher applied fields to obtain the same values of magnetization and magnetostriction and the contribution of all domains to the magnetostriction gives rise to a higher maximum value.13) However, the TbFe2 film irradiated by Ar ion beam showed large magnetostriction and high magnetostrictive susceptibility. The high magnetostrictive susceptibility is probably caused by change of volume magnetostriction corresponding to the dense dislocation networks introduced by Ar ion beam irradiation. 4.
Conclusions
TbFe2 film, deposited onto Si(100) or polyimide substrate by flash evaporation, have been implanted with Ar ions. Magnetostrictive properties, i.e., saturated magnetostriction and magnetostrictive susceptibility, of TbFe2 film with disordered structure were improved by Ar ion beam irradiation due to increasing of in-plane compressive stress. The increase of compressive stress probably caused change of volume magnetostriction. Acknowledgement We are thankful to Tokyo Ohka Foundation for financial support of this work.
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