Conversion electron M6ssbauer spectroscopic study of YIG substituted with Bi, Ti, Ga and La

Transcription

1 Hyperfine Interactions 84(1994) Conversion electron M6ssbauer spectroscopic study of YIG substituted with Bi, Ti, Ga and La K. Nomura a, T. Hanai a, R. Sadamoto a, Y. Ujihira b, T. Ryuo c and M. Tanno ~ "Faculty of Engineering, University of Tokyo, Ilongo 7-3-1, Bunkyo-ku, Tokyo 113, Japan bresearch Center for Advanced Science and Technology, University of Tokyo, Komaba 4-6-1, Meguro-ku, Tokyo 153, Japan CAdvanced Functional Materials Research Center, Shin-Estu Chemical Co., Ltd., Isobe Annaka-city, Gunma. Japan YIG films, substituted with Bi, Ti, Ga and La, were grown on a (111) plane of Gd3Ga5012 or (Gd, Ca)3(Ga, Mg, Zr)5Ol2 by a liquid phase epitaxial method. With the increase of the concentration of Bi atoms, which are substituted at 24c sites of YIG, the magnetic hyperfine fields at 16a and 24d Fe sites increased and the direction of magnetic moments changed from parallel to 45 ~ to the (111) plane. With the increase of Ga concentration, the hyperfine fields at 16a and 24d Fe sites decreased. The hyperfine interaction and the substitution effect of YIG films are discussed. 1. Introduction Yttrium-iron-garnet (YIG) has been used as a magnetic thin film (-1 I.tm) with magnetization perpendicular to the plane for a magnetic bubble memory. Recently, it was proved to be useful as a magneto-optical isolator because it shows transparency in the near infrared region and a prominent Faraday effect [1]. YIG has ferrimagnetism and a cubic structure, which is formed as [C]3[A]2[D]3OI2. The unit cell is composed of eight sublattices. [C] ions are located at dodecahedral (24c) sites. [A] and [D] ions occupy octahedral (16a) sites and tetrahedral (24d) sites, respectively [2]. Especially YIG crystals substituted with diamagnetic Bi 3 ions show a large Faraday rotation of -104 deg/cm. The Curie point increases with the increase of the concentration of Bi substitutes [Bi], i.e. dtc/d[bi] = 38 K. These data were explained by the molecular field theory of three-sublattice garnets [3]. YIG single crystals can be used as a magneto-static wave device for the transmission of information using a microwave. The magnetization can be controlled by the addition of Ga ions. The magnetic hyperfine structure and the substitution effect of YIG films substituted with Bi, Ti, Ga and La, which were developed for a magneto-optical isolator and a magneto-static wave device, were investigated by conversion electron Mdssbauer spectroscopy (CEMS), X-ray diffractometry and magnetic observations. 9 J.C. Baltzer AG, Science Publishers

2 422 K. Nomura et al. / Conversion electron M6ssbauer spectroscopy of YIG 2. Experimental details YIG films, doped with Bi, Ga, Ti and La, were grown on a (111) plane of Gd3GasOl2 (GGG) or (Gd, Ca)3(Ga, Mg, Zr)5012 by a liquid phase epitaxial method (LPE) in a bath of PbO and B203 [2]. Ti-doped BiYIG was prepared as an optical isolator. Ga-doped YIG films were prepared for a magneto-static wave device. The thicknesses of the prepared films were about several tens p.m. The YIG films were covered with a thin carbon film (10-20 nm thick) in order to avoid the electrification effect of insulator samples during the measurement of conversion electron MOssbauer (CEM) spectra using a backscattering type gas flow counter [4]. The CEM spectra were resolved into two sextets of Lorentzian, corresponding to 16a and 24d Fe sites, by computer fitting. 3. Results and discussion 3.1. BiYIG FOR A MAGNETO-OFFICAL ISOLATOR CEM spectra of Bi~Y3_xlG (x = 0, 0.2, 1.2 and 1.4) were measured. The M0ssbauer parameters obtained from these spectra and lattice constants are shown in table 1. With the increase of the concentration of Bi substitutes at 24c sites of the YIG crystal, the lattice constant increased and the magnetic hyperfine fields both at 16a and 24d sites, Hin(a) and Hin(d), increased a little. Hin(a) and Hin(d) for Bil.4Y1.6IG were 50.3 T and 41.7 T, respectively. The Hin(a) reached saturation faster than Hin(d). Hin(a) and Hin(d) for a sputtered epitaxial film of Bi3FesO12 were reported to be 49.8 T and 42.7 T, respectively [3]. The intensity ratio of magnetic peaks at 2 and 5 to peaks 1 and 6 decreased from 1.1 to 0.56 at 16a sites and from 1.3 to 0.55 at 24d sites, respectively. This indicates that the number of magnetic moments perpendicular to the (111) surface increased in Bi-doped YIG films, whereas the undoped YIG contained almost all magnetic moments parallel to the surface. By doping with a large amount of Bi, the spins were oriented in the direction of 45 ~, on average, with respect to the (I11) plane. The isomer shift for iron at 16a sites, IS(a), increased with the doping of Bi although IS(d) for 24d sites did not. This supports localized molecular orbital theory [5]. Doping with a small amount of Bi provided the narrow linewidths at 16a sites. Therefore, the MOssbauer parameters of Fe 3+ ions at 16a sites were affected more than those at 24d sites. The quadrupole splitting (QS) was at highest 0.09 mm/s at 16a sites and 0.12 mm/s at 24d sites for highly Bi-doped specimens, respectively. The QS was very small because the overall structure of YIG films was kept cubic. The peak area ratio of 16a to 24d Fe 3+ ions was less than 0.57, which was lower than the occupation ratio of 0.66 for an ideal structure of YIG. This may be caused by the incorporation of y3+ or Pb 2+ from the melt of PbO and B203 [6]. The absorption coefficient of Ti-doped Bil.4Yz6IG was 73 cm -1 at 785 nm wavelength, which was much smaller compared with 160 cm -1 of BiYIG. On the

3 Hyperfine fields (Hi~, T), isomer shifts (IS, mm/s), quadrupole splitting (QS, mm/s), linewidths (W, ram/s), intensity ratio of magnetic peaks (PR = P2.s/PI,6), area ratio (16a/24d) of the octahedral (a) and tetrahedral (d) sublattices, and lattice constant (/~) of YIG films. Films H~(a) Hi~(d) IS(a) IS(d) QS(a) QS(d) W(a) W(d) PR(a) PR(d) 16a/24d Lattice C 1. YIG Bio.2Y2.sFe5O Bil.2Y1.sFesO12 49, Bil4Y~.6FesO~ Bit.4Y1.6TioAFe4.9OI , Y3Gao.6Fe Lao.o3Y2.97G ao.9 Fe4A O 12 41, Lao.~Yz96Gao.97Fe4.o3O12 40, Bil.4Yl.6Gao.l~Fea g~,r t~ C~ 4x Table 1

4 424 K. Nomura et al. / Conversion electron MOssbauer spectroscopy of YIG (a) i l l l l ] l i l l l l i l b I I I I i i i,' I I I I I I,,,, (b) I la I I I I I 0 0 I,, l "~ ~. I. :: St # t~ rl.,,~ \..~l ~. ~'~.74 I,~ J \ ~:... _z..r -.4.L~::".." "~=2-..; :';'~'.2--..;"-7.~::.'-~:~ ',''" ~ i,'" i ' " i ":.~ I ' I(~ "'" ](oi),' ll,l:lllli::l', :l l i I i ) la,' iw: "~"~ 9 ~...,.~:q,~,~, ~.; -.,, ":q,, "=:-~", -, d,-~?. ], ~ ) I I i i, q I )I i I i, i i I] Velocity (mm/s) Fig. 1. CEM spectra of (a) YIG, (b) Bil.4Y1.6FesO]2, and (c) Bil.4Y1.6Tio.lFe4.9Otz. other hand, the Faraday rotation constants for Ti-doped Bi].4Y2.6IG and BiYIG were x 103 deg/cm and deg/cm, respectively. The doping of Ti provided the excellent improvement of transparency. It is considered that doped Ti 3+ prevents Fe 3+ from changing into Fe 4+ due to the incorporation of Pb 2+. CEM spectra of YIG, Bi].4Y].6IG and Ti-doped Bi].4Y1.6IG films are shown in fig. 1. The Hin of Tidoped BiYIG decreased slightly and the number of spins oriented perpendicular to the (111) plane increased with respect to Bi].4Y].6IG. The peak ratio of 16a to 24d iron sites became close to the ideal occupation ratio. Ti 3 ions might prevent y3+ or Pb 2+ from incorporating into 16a sites, in addition to a charge compensation effect.

5 K. Nomura et al. / Conversion electron MOssbauer spectroscopy of YIG Ga-DOPED YIG FOR MAGNETO-STATIC WAVE DEVICE In highly Ga-doped YIG films, La 3+ ions were also doped in 24c Y sites in order to avoid the serious decrease of the lattice constant of the epitaxial films. The change of the lattice constant of these Ga-doped YIG films was within 0.01 /~. With the increase of doped diamagnetic Ga 3+, both Hin(a ) and Hin(d ) decreased and the linewidths became broad. This is due to the weakening of super-exchange interaction at 16a and/or 24d sites. The spin orientation of all Ga-doped YIG was parallel to the (111) plane. The spectrum of highly Ga-doped YIG could not be fitted well enough to discuss the site population of 16a and 24d precisely. The hyperfine fields are plotted against magnetization of these YIG crystals in fig. 2. On Ga-doping, the 50 16a b-.c_ 7- i 20 i,,, i I i r, t, i i, i, i I, TENs (G) Fig. 2. Hyperfine fields and saturation magnetization of Ga-doped YIG. saturation magnetization of YIG decreased more drastically, from 1800 to 320 G compared with Hin, changing from Hi.(a) = 47.9 T and Hi.(d) T to Hi.(a) = 40 T and Hin(d ) --34 T, respectively. The saturation magnetization of LaGaYIG, 4riMs = 365 G, was approximately independent of temperature. The temperature dependence was about 50 ppm/~ at 5.5 GHz, which corresponds to 1/40 of that of YIG. It is known that the perpendicular resonance field depends on the saturation magnetization and the anisotropic field. The Ga-doped BiYIG, Bil.4Y1.6GaoAsFe4.85O12, prepared in order to adjust the saturation magnetization (4~Ms = 1550 G) to the anisotropic field (H k = 1550 Oe), showed a stable perpendicular resonance field in the temperature range of -60 to 80 ~ The Hi,(a) and Hin(d ) decreased slightly, and the number of spins oriented perpendicular to the (11 l) plane increased by the doping of Ga.

6 426 K. Nomura et al. / Conversion electron MOssbauer spectroscopy of YIG 4. Conclusions It was observed that with the increase of the concentration of Bi atoms doped into 24c sites of YIG, the magnetic hyperfine fields of Fe 3 ions at 16a and 24d sites increased and the IS of Fe 3 ions at 16a sites increased. These results suggested that electrons are transferred from 02- to Fe 3+ at 16a sites rather than at 24d sites. The peak area ratio of 16a and 24d sites was small compared with that of the ideal structure. By the doping of Bi into Y sites, the magnetic spins were oriented from parallel to the (111) plane to 45 ~ whereas the spins were kept parallel to the (111) plane by the doping of Ga ions into Fe sites. The change in hyperfine fields was small compared with the drastic decrease of saturation magnetization by the doping of Ga. Ti-doped BiYIG and Ga-doped BiYIG showed excellent characteristics of an optical isolator and a magneto-static wave, respectively. CEMS is useful for the analysis of YIG films grown epitaxially on a thick substrate. References [1] P.J. Picone and A.H. Morrish, J. Appl. Phys. 53(1982)2471. [2] T. Hibiya, Oyo Buturi 55(1986)109. [3] M. Pardavi-Horv~ith, L. Botty~, I.S. Szilcs, P.E. Wigen and M. Gomi, Hyp. Int. 54(1990)639. [4] K. Nomura and Y. Ujihira, Bunseki Kagaku 33(1984)T81. [5] F.J. Kahn, P.S. Pershan and J.P. Reeika, Phys. Rev. 186(1969)891. [6] J. hoh, K. Saneyoshi, T. Toriyama and K. Hisatake, J. Appl. Phys. 60(1986)1461.