PHOTO-INDUCED EFFECT ON THE PERMEABILITY OF YIG SINGLE CRYSTALS WITH Pb IMPURITY

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PHOTO-INDUCED EFFECT ON THE

PERMEABILITY OF YIG SINGLE CRYSTALS WITH

Pb IMPURITY

N. Ichinose, H. Yokoyama, K. Hisatake, K. Ohta

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C1, supplkment au no 4, Tome 38, Avril 1977, page (21-215

PHOTO-INDUCED EFFECT ON THE PERMEABILITY OF YIG

SINGLE CRYSTALS WITH Pb IMPURITY

N. ICHINOSE, H. YOKOYAMA

Toshiba Research and Development Center, Tokyo Shibaura Electric Company, Ltd., Kawasaki, Japan and

K. HISATAKE, K. OHTA

Faculty of Science and Engineering, Aoyama Gakuin University, Setagaya-ku, Tokyo, Japan

RbumB. - On etudie, sur un monocristal de grenat d'yttrium Fer de type-pcomportant des impuretks de plomb, l'influence sur la permhbilitt de I'irradiation en lumitre blanche. On a mis en evidence dans cet Cchantillon de YIG type-p I'existence d'un effet photomagnttique irreversible et d'une dCsaccommodation photo acctlerke. Ces effets sont expliques par un modtle A deux centres modifiC, dans lequel I'ion Fe4+ joue un rBle important.

Abstract.

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The influence of irradiation with white light on the permeability at 77 K is investigated on p-type yttrium iron garnet single crystals with Pb impurity. Theexistence of an irreversible photo-magnetic effect and photo-accelerated disaccommodation is demonstrated in p-type YIG.

These effects are explained by a modified two center model in which Fe4+ plays an important role.

1. Introduction. - In the first paper on photo- magnetic effects, Teale et al. [I] have reported that, at low temperatures, the field for ferromagnetic reso- nance of single crystal samples of Si-doped YIG (Y3Fe4.,Si0.,O,,) is appreciably influenced by irradiation with i. r. light. This effect has been interpreted as the result of a photo-induced change in the ferrous ions distribution in the sample. The same mechanism is responsible for photo-induced change in optical dichroism [2] and strain [3]. These phenomena are designated as class I effects.

The so-called class I1 effects have been discovered by Enz et al. [4], who observed photo-induced changes in the permeability and coercive force of single Si-doped YIG (Y3Fe4.975Si,,oz50,,) crystal. Concerned with these phenomena, a two center model has been propos- ed by Lems et al. [5]. In this model, two types of centers, labelled I (Fez+ nearby Si4+) and 11 (Fez+ far away from Si4+), are assumed to be present. Since the pioneering work by Enz et al., the photo-induced decrease in permeability (abbreviated PDP hereafter) has been extensively investigated in both single- crystalline and polycrystalline n-type YIG [6, 71.

In class I effects (high Si doping level), photomagnetic effect is of a reversible nature, but it is irreversible in class I1 effects (low Si doping level). Whether or not the Fe4+-Fe3+ system shows photomagnetic effects in p-type YIG similar to those due to Fez+-Fe3+ transi- tions in n-type YIG, is a n interesting research subject. This paper reports observations and possible explana-

tions on the YIG single crystals with Pb impurity and Ca-or Sr-substituted YIG.

2. Experimental procedure.

-

YIG single crystals

were prepared from a molten solution of PbO-PbF,-Bz03 or PbO-B,03 by slow cooling in air from 1 350 OC to 1 050 OC. A 200 ml capacity platinum crucible was charged with about 1 kg of hydrostatically pressed raw materials, placed in a ceramic case and covered with bubble alumina. Crystals embedded in solidified flux were separated from the matrix phase by a hot mixed solution of nitric and acetic acids. The P b concentration was determined from spectroscopic and chemical analysis, respectively.

For measuring PDP, the following samples and procedure were used. Toroidal-like, irregular form samples were employed for the measurement on YIG (Pb). Their dimensions are approximately 4 mm in diameter, 0.3 mm thick, with a 1.2 mm hole in the center. On YIG (Sr), toroidal ring was formed preci- sely in a dimension of 6.1 mm in diameter, 1.1 mm thick and 3.4 mm hole. The primary winding (10 turns) carries a 20 kHz, 0.3 mmA sinusoidal current on YIG (Pb). The permeability value is obtained 2.5 seconds after demagnetization. The sample is kept in the dark for a sufficiently long time to enable the observation of possible disaccommodation effects. Thereafter, the sample is illuminated with a tungsten lamp which

applies as light intensity of approximately 0.05 W C ~ - ~ on the surface of the sample until a saturation value

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(21-216 N. ICHINOSE, H. YOKOYAMA, K. ,HISATAKE AND K. OHTA

is reached. Finally, demagnetization leads to a p,,, permeability value. The signal from the secondary winding was recorded by a pen-recorder whose time constant was 0.1 second. Permeability calibration was accomplished with a large toroidal ring cut from each ingot.

Electric conduction-typing for these crystals was checked by the conventional method.

3. Results and Discussion. - Table 1 shows an emission spectroscopic analysis for the present sample. A slight deviation from the stoichiometry is also found. Iron ions are deficient, as compared with yttrium ions in the stoichiometric composition. According to reference [8], PDP is most marked in Y,Fe,-,Si,OI2 polycrystalline samples where Si concentration x = 0.007 5. PDP almost vanishes at a 0.0025 Si concentration. Assuming that data could be applicable to single crystals, the present samples are found to be sufficiently free from Si-contamination. Also, fluorine ions substituted for oxygen ions coming form the PbO-PbF2-B,O, flux in the sample (No. 2) might play a charge compensating role. However, no appreciable PDP difference between samples No. 1 and No. 2 could be observed, as shown in figure I (a) and (6). Pb2+ ions come from the flux used in growing the sample and enter the dodecahedra1 sites. It is generally assumed that they induce Fe4+ ions in the tetrahedral or octahedral sites [9]. This is confirmed by the fact that these crystals show p-type conduction. Samples cited in table 1 exhibit a large PDP, but it is difficult to compare with results reported by other researchers, because of the lack of detailed information on sample dimensions, surface state and so on. The effectiveness in table 1 is defined as (pd

-

pi)/pd x 100

(%),

where p, and pi represent permeability before illumination (30 minutes after demagnetization) and satufated value after illumination, respectively. After the light is removed, a demagnetization is repeated. Most part of PDP is, in this case, of Enz's type as shown in figure 1 (a). At 77 K, an additional disaccommodation measurement was carried out before illumination. DA of sample No. 1 is pronounced but it is not observed in sample No. 2. This difference is pro- bably due to the amount of Pb impurity.

The type I center would not be limited to Si4+-Fez+ pair. In the present case, let us assume that a Pb2+-Fe4+ pair acts as a type I center. The Fe4+ ion (3d4) has a degenerate ground state on a site distant from Pb2+ and is able to contribute to the magnetocrystalline

Sample thickness ( 4 No. 1 (0.030) No. 2 (0.034)

Y I G (No.2)

FIG. 1. - Time dependence of the initial permeability of YIG with Pb impurity at 77 K. (a) Sample No. 1 (b) Sample No. 2

(Refer to table I for compositions)

anisotropy, leading to the decrease in permeability as well as Fe2+(3d6). Several authors have already reported on related phenomena due to Fe4+ in

Characteristics of the samples studied Results of analysis

Y Fe Pb F Effectiveness ₍

_%)

-

68

+

15

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PHOTO-INDUCED EFFECT ON THE PERMEABILITY OF YIG SINGLE CRYSTALS WITH Pb IMPURITY C1-217 YIG [lo, 11, 121. In order to confirm this fact, the

PDP in p-type substituted garnets Y,-,M,Fe,O,, (M = Ca, Sr) was also investigated. One of the authors has already observed PDP in polycrystalline Ca-doped YIG [13].

Y I G ( S r ) 7 7 K

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Dem. Dem.

FIG. 2. - Photo-induced permeability changes at 77 K for single crystal sample of Y Q . ~ ~ S ~ O . O ~ F ~ S O I Z .

Figure 2 shows the PDP at 77 K for Y0.97Sr0.03 Fe,Ol, single crystal sample, which contains a detectable amount of Pb impurity in X-ray fluorescent analysis. (The content of Sr is nominal but estimated from the X-ray fluorescent peak to be of the same order.) From this figure, it is seen that the permeability behavior for these crystals is more complicated than in the case of YIG with Pb impurity : A mixture of PDP in the class I1 effects, DA and photo-accelerated 110, 141 or light-enhanced DA [7] is found in this figure. They are to be separated respectively by repeating irradiation and demagnetization, since Enz's type PDP is insensitive to demagnetization and DA or photo-accelerated DA is reversible with demagnetization.

Conclusion.

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PDP observed in p-type YIG single crystals with Pb impurity may be explained by a modified two center model, in which Fe4+ plays an important role. Furthermore, it is confirmed that small amounts of ~ eions are necessary to obtain PDP in ~ + substituted garnets Y1 -,M,Fe,O,, (M = Ca, Sr). In the single crystal samples of Yo~g,Sro,,3Fe50,,, a combination of Enz's type PDP and photo-accelerated DA has been found.

References

[l] TEAL, R. W. and TEMPLE, D. W., Phys. Rev. 19 (1969) 904. [8] WURLITZER, M. and CXBURA, W., Phys. Stat. Sol. (a) 21

[2] DILLON J. F., Jr., GYORGY, E. M. and REMEIKA, J. P., (1974) K 95.

Phys. Rev. Lett. 22 (1969) 643. [9] MERCERON, T., Phys. Stzt. Sol. (a) 15 (1973) 539.

13] J . F. Jr., GYORGy, E- M, and REMEIKAy J+ P., Appl _[lo]_HISATARE,_{K. and OHTA,}_K.,_{Proc. Int. Con$ on Ferrites}₁ Phys. Lett. 15 (1969) 221. (1970) 14.

[4] ENZ, U. and VAN DER HEIDE, H., Solid State Commun. 10

(1972) 1021. [ l l ] HJSATAKE, K., OHTA, K., ICHINOSE, N. and YOKOYAMA, H., [5] LEMS, W., METSELAAR, R. RIJNIERSE P. J., and ENZ, U., J. Phys. Stat. Sol. (a) 26 (1974) K 75.

Appl. Phys. 41 (1970) 1248. 1121 METSELAAR, R., HUYBERTS, M. A. H., and LOGMANS, H.,

[6] LEMS, W., METSELAAR, R., RIJNIERSF, P. J., and ENZ, U., J. J. Appl. Phys. 46 (1975) 3171.

Phvsiaue 32 (1971) 704. [I 31 HISATAKE, K., Japan. J. Appl. Phys. 13 (1974) 2067.

[7] METSELAAR, R. ahd H ~ B E R T S , M. A. H., Philips Res. Rept. [14] MARAIS, A. and MERCERON, T., Phys. Stat. Sol. (a) 22