PHOTO-INDUCED EFFECT ON PERMEABILITY IN COBALT DOPED Li- AND Mg-FERRITES

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PHOTO-INDUCED EFFECT ON PERMEABILITY IN

COBALT DOPED Li- AND Mg-FERRITES

K. Hisatake, K. Ohta

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C I , supplkment au no 4 , Tome 38, Avril 1977, page C1-219

PHOTO-INDUCED EFFECT ON PERMEABILITY

IN

COBALT DOPED Li-

AND

Mg-FERRITES

*

K. HISATAKE and K. OHTA

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

RCsume. - Des Cchantillons polycristallins de Li0,5Fe2,504 et de MgFezOc dopks au cobalt (0.001 a 0.006 atomes par formule unite) presentent une diminution photo-induite de la permeabi- lit6 initiale A 77 K. Pour des teneurs en cobalt plus elevees, une augmentation ou bien un maximum de l'accroissement de la permtabilite sont observes pendant les illuminations successives. Ces observations peuvent &tre qualitativement intcrpretks partir des modifications photo-induites de la concentration en Co2+.

Abstract. - Polycrystalline samples of both Lio.5Fe2.504 and MgFez04 : Co (0.001-0.006 atoms per formula unit) are found to exhibit a photo-induced decrease in initial permeability at 77 K.

With higher Co (0.008-0.08) contents, an increase or a maximum of increased permeability during successive flashes is observed. The observations can be qualitatively interpreted in terms of photo- induced change in [ C O ~ ~ ] concentration.

1 . Introduction. - A photo-induced decrease in initial permeability p was observed for the first time by Enz et al. [I] in YIG(Si) and attributed to higher energy centers due to photo-excitation of low energy centers. The higher energy centers and low energy centers are respectively far and near Si4+ ions.

Although the effect has been actively studied in garnets with several dopants [2, 3, 4, 51, not so many

studies on spinel ferrites [6, 71 have been reported. The reason for this is probably due to the fact that optical transparency of spinel is rather poorer than that of garnet. The authors have tried to observe this effect in several kinds of spinel ferrites, i. e., Mg-, Li- and NiZn-ferrites with Si dopant and MnZn-ferrite with no intentional dopant, but the results were not convincing. It is apparent that the part played by F e z + in spinel ferrites is still controversial and the effect is not so clear as it is in garnet. The aim of this report is, there- fore, the presentation of new results and some discussions on Mg- and Li-ferrites with small amounts of Co2+ substituted to Fe2+.

2. Experimental. - Very pure starting materials were used. Especially, the Co impurity content was less than 10 ppm. The samples of

LiI/2(L - ~ ) ~ ~ l / 2 ( 5 -x)C0x04

and

MgFe2-xCox04 (0

<

x

<

0.1)

(*) This work was partially supported by the Grant-in Aid

for Recording Research from Akai Electric Co. in Japan.

were prepared by the following technique : Stoichio- metric proportions of Li2C0, and or MgO, F e 2 0 3 and CoC204 were mixed and fired at 800 OC-1 200 OC for 5 hours, reground and then heated at 1 150 OC- 1400OC for another 5 hours. In order to obtain a better homogeneity of the samples, this procedure was sometimes repeated. These samples were cooled slowly in a furnace, but the Li-ferrite samples, were occa- s i o n a l ~ quenched rapidly in water from 850 OC in order to additionally investigate the order-disorder characteristics influence on this effect concerned. X-ray powder pattern confirmed that the final material was single phase spinel. Since the chemical analysis of very small Co contents is inaccurate in polycrystalline samples, the temperature of the second peak in p-T curve was used to give an estimate of the effective Co-contents.

After cooling the sample in the dark by immersion into liquid nitrogen, the 0.1 mm thick sample is a. c. demagnetized by a demagnetizer designed in the authors' laboratory [8]. Then the sample was illumi- nated with the xenon flash lamp.

3. Experimental Results. - A typical recorder trac- ing of photo-induced decrease in p is shown in figure 1. A drastic change in p occurs at the first flash and decreases at each successive flash. When a saturation value has been reached, demagnetization is again repeated. In this method, p is recovered to a certain level, depending on the samples. The ratio recovered by demagnetization is also listed on table 1. After the flash is removed, more than 10 hours are required to restore the original p of the sample at 77 K, due to thermal back reaction.

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CI-120 K. HISATAKE AND K. OHTA

MgFe,O,; 0.00 8 Co

77 k

f l a s h on

FIG. 2. - Photo-induced ,u increase, where the experimental condition being is similar to that shown in Fig. 1, for polycrys-

talline sample MgFe1.992Co0.00804.

FIG. I.

-

,u time dependence for polycrystalline sample

MgFel. 998C00.00204 ring under successive illumination with present results. At each flash, ,u increases in steps and a xenon flash lamp (1.4 x 1 0 - 3 s.) Driving frequency and field saturation is reached. A considerable increase in by

are 20 kHz and 5 rnOe, respectively. illumination is often observed among samples whose Co value is higher than in those which exhibit the photo-induced decrease in ,u, irrespective of Li-or-Mg- On the other hand, photo-induced increase in p ferrite.

has been observed, as shown in figure 2. This repre- This feature is reported only on Mg-ferrite (x=0.008) sents the most pronounced effect observed among the in this text as shown in figure 2 in while the similar

Photo-induced p decrease and its associated values at 77 X. Dejinitions are introduced here ; pd ;

,u 30 minutes after demagnetization ; pill ; the saturated ,u value after illumination. ; ,u just after demagnetization before illumination. ; ,u at the moment of demagnetization after illumination. Apil, ;

1 2 2

P d

-

P i l l ; AKj ; p d e m

-

P d ; A ~ d e m ; P d e m

-

P i l l ; 1) A ~ i l l l ~ d

(%)

; 2) ~ ~ d / l * d l e m ; 3) ~f-';em/~ill

(%I.

Photo-induced ,u

decrease Disaccommodation Ratio recovered

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PHOTO-INDUCED EFFECT ON PERMEABILITY IN COBALT DOPED Li- AND Mg-FERRITES C1-221

behaviour on Li-ferrite (x = 0.02) is obtained. It should be noted furthermore that the anomalous behavior has been observed with higher Co contents, as shown in figure 3. In this figure, the increased p becomes maximum during the successive flashes. This is also confirmed during the continuous white light illumination. The second peak in the p-T curve is located at 98 K before illumination.

flash on

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Mg Fe20, ; 0 . 0 3 Co

FIG. 3.

-

Photo-induced p increase which has a maximum under successive flashes.

4. Discussions. - In order to explain the three

different types of response to illumination, a tentative picture will be presented : This can be seen in figure 4a, b and c, where two curves are shown in each case. Photo-induced change in magnetocrystalline aniso- tropy

K,

is represented by a vertical wavy line, while

FIG. 4.

-

K1 as a function of temperature for three different Co contents before and after illumination.

thermal change in K, is along the line itself. In each

figure, the concentration of [Co2+], is assumed to be higher in the upper line before illumination, than in the lower line after illumination. The main part of Co is assumed to be in the divalent state before illumination [9].

If the incident light oxidizes Co according to

and the back reaction is hindered due to the small concentration of Co, photoinduced change of K , would increase or decrease, depending on the initial sign of K , before illumination and, accordingly, a p-decrease or a p-increase results. This assumption is based on data showing that the Co2' to K1 contri-

bution may be stronger than that of Fez+ [lo]. This picture contrasts with the situation in most previous studies of garnets, where the higher energy center could be induced by illumination. The possible thermal effect, accompanied by illumination should be safely neglected. When the light is removed, the relaxation time to the original state is too long to be attributed to it. An important indication of photo-induced increase of p is the observed shift in the second peak in the p-T curve forwards lower temperatures (by successive illumination).

So far, only the photo-induced change of total K,

has been considered. It should be noted, however, that besides Kl, the photo-acceleration of disaccommodation is to be taken into account. Also, in the present samples, the often observed latter process is to be separated by demagnetization. In the following report [Ill, this situation will be discussed in more detail. Photo-induced increase of p is often masked by the relaxation process, since its change is inverse and competing. The relaxation time is, however, very sensitive to the sample preparation [9]. This process is sometimes to be eliminated, as it is the case in figure 2 or figure 3. There are rather scatters in data of an increased p or the maximum of p experiment. This may probably due to inhomogeneous distri- bution of cobalt.

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C1-222 K. HISATAKE AND K. OHTA

References

[I] ENZ, U. and VAN DER HEIDE, H., Sotid State Commun. 6 [7] BERNSTEIN, P. and MERCERON, T., J. Physique Cotlog. 38

(1968) 347.

_.

, (1977) C 1-211.

MEISELAAR, R. and HYUBERTS, M. A. H., Philips Res. Rep.

29 (1974) 453.

LOTGERING, F. K., J. Phys. Chem. Solidr 36 (1975) 1183. ENZ, U., METSELAAR, R. and R ~ J N ~ E R ~ E , J. Physique Colloq.

32 (1971) C1-703.

KUTS, P. S., KOVALENKO, V. F., SOV. Phys. Sol. Stare 17

, ,

[8] WSATAKE, K. and OHTA, K., Proceedings of I. C. M. (Moscow) VI (1973) p. 188.

[9] DE LAU, J. G. M., Philips Res. Rep. Suppl. 6 (1975) 1. [lo] SELEZNEV, V. N., PKHOV, I. K., DRONIN, A. I. and SHAPO-

VALOV, V. A., SOV. Phys. Solid State 12 (1 970) 683.

(1975) 960. [Ill ICHINOSE, N, YOKOYAMA, H., HISATAKE, K. and OHTA, K.,