X-ray induced change of initial permeability in polycrystalline YIG(Me4+)

M. WURLITZER and U. REINHOLD: X-Ray Induced Change of Initial Permeability 531

phys. stat. sol. (a) 44, 531 (1977)

Subject classification: 11; 18.3; 22.8.2

Sektion Yhysik der Karl-Marx- Universitdt Leipzig

X-Ray Induced Change of Initial Permeability in Polycrystalline YIG(Me4+)

BY M. WURLITZER and u. REINHOLD

Investigations of the influence of various irradiations on polycrystalline YIG(Me), Me = Si4+, Ti4+, Zr4+, a t the temperature of liquid nitrogen show that hard X-rays have completely equivalent effects on the initial permeability t o those of incandescent light. The relative decrease of the initial permeability depends merely on the total number of irradiated photons, irrespective of the time of irradiation. Energetic photons in the X-ray range are much more effective than infrared photons. The reduction of the effect in the range of soft X-rays can be explained merely by the strong reduction of the penetrating depth of the X-rays as a result of the absorption by the Y and Fe atoms. Very similar results are obtained for the various tetravalent ions, Me4+, which are arranged in the garnet lattice in different ways.

Untersuchungen des Einflusses verschiedener Bcst’rahlnngen auf polykristallines YIG(Me), Me = Si4+, Ti4+, Zr4+, bei dor Temperatur des fliissigcn Stickstoffs ergeben, dalj harte Rontgen- strahlen vollig aqiiivalente Wirkung auf dic Anfangspcrmeabilitat ausiiben wie Gluhlampenlicht. Die relativo Abnahme der Anfangspermeabilitat hangt nur von der geaamten Anzahl der einge- strahltcn Photonon ab, gleich, in welcher Zeit die Eiristrahlung erfolgt. Energiereiche Photonen im Bereich der Rontgenstrahlen sind wesentlich wirksamer als die Infrarotphotonen. Die Abnahme des Effektes im Bereich weicher Rontgenstrahlen 11113t sich allein durch die starke Reduktion der Eindringtiefe der Rontgenstrahlen als Folge der Absorption der Y- und Fe-Atome erklaren. Fur die verschiedenen vierwertigen Ionen, Me4+, die in verschiedener Weise im Granatgitter angeordnet sind, ergeben sich sehr ahnliche Resultate.

1. Introduction

The “irreversible” photomagnetic effect (PME) can be produced by X-rays as was already reported by Enz and van der Heide [9]. While the influence of light was investigated in detail in the wavelength range of 1 pm mainly in yttrium iron garnet (YIG), few experimental results on X-ray induced changes of permeability have been published until now [14 to 16, 181. The dependence of the effect on several parameters must be studied in order to test the possibility of applying the irreversible PME for example in an X-ray detector. The kind and degree of doping, the layer thickness of the material, the intensity and wavelength of the X-rays are essential.

I n this paper results of experiments on polycrystalline YIG with X-rays are reported and compared with those of analogous experiments obtained after irradiation with infrared light.

2. Experimental

Polycrystalline samples were prepared with the usual ceramic sintering technique. Part of the Fe3+ was substituted by Me4+ = Si4+, Ti4+, Zr4+ according to the scheme

532 111. WURLITZER and U. REINHOLD

The unixtual inductance was measured by an automatic recorder, and the relative permeability was calculated from these values. Besides a mutual inductance bridge was available for the determination of the loss factor (tan 6) or the complex permeability. All measurements were carried out a t 77 K after imrrrersing the sample into liquid nitrogen. Thc irradiation with light was done directly using a projector lamp, which was set a few centimetres over the sample, the latter being covered by a layer of liquid nitrogen a few millimetres thick. But the irradiation with X-rays was per- forriled laterally through a window consisting of a 0.25 mm thick copper foil, to which the sample was cemented within the bath. Further details aIe siirniriarizcd in Table 1.

T a b l e 1 sample

diameter 10 mm composition Y,Fej-,Me,O,J

thickncss number of turns 40 sintering temperature 1400 “C wire thickness (Cu) 0.05 mm measuring temperature 77 K

p-measurement radiation sources

time constant of the recorder p ( t ) N - 0.1 s

time interval of observation 2 to 104 s

frequency 10 kHz 80 t o 200 kV amplitude of magnetic field

2.5 to 0.25 mm Me = Si, Ti, Zr; x = 0 t o 0.04

a) projector lamp 250 JV b) TR 250, VEB Rohrenwerk Rudolstadt

(Cu target, continuous spectrum) voltage range of the X-ray tube ascd

corresponding half-value thickness d0.5 of YIG 0.6 t o 5 mm

29 kV, 25 mA

max. 0.5 Am-l c) TFX 60 (mecidal equipment)

3. The Influence of X-Ray Absorption

Soft X-rays are attenuated more strongly than infrared radiation in the range of the optical “window” when going through YlG. For example the intensity of a soft X-ray (A = 70 ptn) has dropped to 18:h of the incident radiation after penetrating a 0.2 mm thick single crystal layer; however, the intensity of infrared radiation ( I . = 1.06 p i , a =- 1.3 x 103 m-1 [ 7 ] ) is still 90% of the incident light.

As Enz and co-workers found out the irreversible PME depends on the current density of photons within the sample [9, 101. Only for sufficiently thin samples the radiation intensity is nearly homogeneous, and the effect of radiation (for example the drop of the permeability) can be regarded as a quantity which is independent of the dimensions of the sample (property of the material).

For X-rays i t is possible to increase the penetration depth by an increase of photon energy besides reducing the thickness of the saniple. Therefore i t is necessary to investigatc abovc all the influence of layer thickness. It has been established in several papers [12, 13, 171 that the decrease in permeability induced by infrared irradiation is especially large if the degree of the substitution is suitably chosen. Already relatively sniall deviations from the optimum doping, where a maximum effect takes place, result in a strong reduction of the effect. Because the doping of divalent and tetravalent cations essentially influences the concentration of Fez+ [ 171, optical absorption changes with doping for infrared radiation [7, 8, 111. The thickness of the

X-Ray Induced Change of Initial Permeability in Polycrystalline YIG(Me4+) 533

layer which diminishes the radiation intensity to a definite fraction of the radiation incident on the surface of the sample varies for samples of different compositions. Thus one cannot be sure whether the dependence of the PME on doping, measured on polycrystalline saniples, is simulated by infrared absorption. As we shall show, in the wavelength range under investigat,ion the absorption of X-rays does not depend essentially on the doping. A comparison of the dependence of the PME on doping after irradiation with infrared light with analogous experiments after irradiation with hard X-rays therefore offers the possibility of deciding whether a dependence of the P M E on the doping degree does exist.

3.1 Vuriation of layew thickness

At first we investigated the drop of the permeability of a sensitive Si-doped YICX sample (x = 0.013, cf. Section 3.2) during irradiation with X-rays. The thickness of the sample and the accelerating voltage were varied, the former by grinding down the sample. The p(t ) curves were taken and the relative decrease of the permeability was calculated from these values (Fig. 1). We found a strong dcpendence on the thickness of the sample for relatively soft X-rays (29 kV). After 2 h irradiation we obtained a drop of the permeability of only 107; for a solid sample of thickness h = 2.5 mm, but 65lYh for a sample ten times thinner. Qnulitatively, these results agrcc well with Hisatake’s results [14] found on YIG(Pb).

With hard X-rays (200 kV) one gets results similar to those obtained with incandescent light. The reduction of the permeability exceeds 50[y0 already after a few minutes and attains maximum values above SO(:/, after extended irradiation. The differences between thick and thin samples have diminished essentially.

Obviously there is a “saturation value” of the permeability drop which is practically reached in a short time a t sufficiently high intensity of radiation and low sample thickness, no matter which kind of radiation is used. We simply define this “saturation value” as PME.

3.2 Variation of doping

The PME thus defined was measured after irradiation with infrared light as well as after irradiation with hard X-rays. A comparison of the results from samples with different compositions is possible if the intensity is exactly the same. In Fig. 2 the results of infrared irradiation and X-ray irradiat,ion are compared. There is no doubt that there exists a similar dependence of the P M E on doping: a small effect in the undoped material (x = O), a maximum change of the permeability by 80 to 907; at x, x 0.013, reduction of the effect for higher dopings, x > 2,. The coincidence of the results holds for the different kinds of irradiation as well as for various dopants, Xi, Ti, Zr.

1 08 1

Fig. 1. Relative change of initial permeability of poly- 2 0 6 crystalline Y,l?e5,Si,012 (5 = 0.013) induced by different 5 kinds of irradiation. x X-rays 29 kV (25mA), 0 X-rays 204 200 kV (I5 mA), o infrared (incandescent lamp 250 W); a variation of sample thickness h: (1) 0.25, (2) 2.55, (3) 0.25, o2 (4) 2.22, (5) 0.25, (6) 1.85, (7) 2.22, (8) 2.55 mm (pa permeability after cooling to 77 K in the dark, p(t ) per-

meability after an irradiation time t at 77 K) o zo 40 60 80 ion IZO tlmin) -

534

0.03

0.9

2.7 4.6 6.3 8.3

0.1

1.3

iV1. WURLITZER and U. REINHOLD

26 111

93 P I

144 P I 186 233 300

45 ~ 2 ~ 4 1

107 121

Fig. 2. Photomagnetic effect as a function of doping of Y,Fes -zMe,O,,. a) after intense irradiation with incandescent light (projector lamp 250 W), b) after irradiation with X-rays (200 kV, 20 mA, Cu target). I undoped material (r = O ) , 0 Me = Si (solid curve), > Me - Ti (for comparison w i t h Si), 0 Me 7- Zr (for comparison with Si) (p,i permeability after cooling to 77 K in the dark, plrr permeability after a) or b) at

77 K)

3.3 Discussion

To begin with, we can limit our analysis to regarding only the absorption of the

If No photons of a monochromatic radiation arc incident on a sample and N ( z ) is X-rays in YIG, in order to explain tlhe results.

the amount of photons arriving after penetrating the layer thickness x, N ( x ) = N0e--aS .

We denote a as the linear attenuation coefficient. The mass attenuation coefficient a lp (p density) of YIG can be calculated from the mass attenuation coefficients of yttrium, iron, and oxygen [5]

E) = CY k)y + CFe t)Fe + co k) * (1)

YIG 0

c, are the fractions of the elements constituting the total mass of >.'I(:. Table 2 suniniarizes the data for YIG and monochromatic radiation which are

important for our experiments. The numerical values show that the main portion of the absorption originates from the heavy metal atoms Y and Fe, and only a small portion is due to the 0 atoms. The low Si doping contributes to the absorption only neglibly. The fraction of the absorbed photons due to the PME (presence of Fez+) can therefore be vcry small only. The half-value thickness varies by more than one order of magnitude in the wavelength range under consideration.

Table 2

1 0

(10-l2 m)

71 42 20 18 13 10 8 6

5.0 1.1 0.14 0.10 0.05 0.029 0.021 0.016

a ( lo3 m-l)

26 5.8 0.74 0.52 0.26 0.15 0.11 0.083

X-Ray Induced Change of Initial Permeability in Polycrystalline YIG(Me4t) 535

Only qualitative statements are possible with the continuous radiation spectrum used. The upper energy limit of the light quanta is simply related to the accelerating voltage U by hum,, = eU. A relation 1, = 213 Amin is valid for the maximum intensity wavelength [5 ] . If U = 30 kV, the most intense wavelength of the continuous radiation is 60 pm, and the corresponding half-value thickness is smaller than 0.1 mm. This explains the dependence on thickness according to Fig. 1 for soft radiation. But for U = 200 kV (A, x 9 pm) the half-value thickness reaches 5 mm, and the sample of 2.2 mm thickness attenuates less than 30% of the most intense part of the radiation, so that nearly the same effect arises as for the sample with a thickness of 0.25 mm, which is approximately homogeneously penetrated.

The same effect of X-rays and infrared irradiation is demonstrated especially well, if equivalent radiation intensities are selected (which give rise to the same effect). This was shown in the experiment giving Fig. 3. The relative decrease of permc- ability was plotted versus a logarithmic time scale. The curves for two 1R radiation intensities can be brought to coincidence by parallel shifting (distance variation of the illuminant). The curves for X-ray irradiation conform well to the two curves to be compared (IR) over a wide range of time.

The samples are practically penetrable for the X-rays used in these experiments (200 kV, Fig. 2 and 3). The radiation can be regarded as sufficiently homogeneous within the sample as the following data show:

thickness of the sample 0.2 mm, average wavelength m, half-value thickness of the layer 4.6 mm, fraction of the total absorption due to Si

The same behaviour of the material towards both kinds of irradiation (infrared and X-ray radiation) indicates that it is not important for the production of the YME how large the energy of the photons is in the wavelength range under consideration (ratio of photon energy for infrared and for X-rays, 1 : lo5). The effect of 11% radiation and of X-rays is completely equivalent if the samples are penetrated homogeneously and if the intensities of the radiation are selected suitably. Of course, equivalent photon current densities (IR and X-rays) differ much in their amounts. An X-ray photon current produces a much stronger effect as compared with a corresponding infrared photon current: an energetic (primary) photon can induce secondary processes in the course of which several secondary photons arise. Thus fewer photons with respect to the arriving photon stream are sufficient to give rise to the same effect as in the infrared spectrum. The sensitivity to radiation which produces the irreversible PME: can vary with wavelength and show an anomaly in a certain wavelength range (IR or X-rays). However, we do not believe that this is possible for the maximum value of the PME ( t -+ a), which was measured for example in Fig. 2. According to previous results [18] the same relaxation (with an average time constant of s a t 77 K) is

doping (z x 0.04) 0.2 yo.

Fig. 3. Aplp as a function of lg t at 77 K. (0 X-ray irradiation and o infrared irradiation with selected

(intensities under the conditions of Pig. 2)

536 11. WUELIYLER m t l U. REINHOLD

induced by the different radiations, which results in a breakdown of the pernieability. Once the radiation has produced the maximum possible amount of relaxing centres, no further permeability drop takcs place as irradiation is continued. This final state is only a function of doping and does not depend either on the kind of radiation or on the time in which it was reached (if recombination can be neglected).

We can now also state that the measured dependence on doping after Fig. 2 cannot essentially be niodificd hy the infrared absorption coefficient a.

4. Dependence on Radiation Intensity and Wavelength

The variation of permeability with time was measured in the following experiments on a selected sample (x = 0.013, niaximum effect) :

a) dependence on radiation intensity I, at a constant accelerating voltage U = 200 kV by variation of sample-target distance and by variation of filter thickness,

b) dependence on accelerating voltage a t a constant anode current (15 mA) KO accelerating voltage smaller than 80 kV was used in order to ensure that the

sample was irradiated homogeneously. Fig. 4 shows the relative change of permeability Ap/p as a function of irradiation time (logarithmic scale) for the example of varying filter thickness. But the curves are typical for all experiments, especially for infrared irradiation, too :

1. There is a widc range where the relative decrease of permeability is nearly pro- portional to the logarithm of time. For very short and very long times the curve deflects so that both limits ( t --f 0 and t -+ 00) will be approached asymptotically with vanish- ing slope.

2. The graphs are symmetric with respect to a point of inflexion t = t,. 3 . The graphs of different intensities can be brought to coincidence by parallel shift.

Thesc statements are also valid if one considers the reciprocal susceptibility 1/x instead of the permeability p. If the distance between sample and target is varied instead of the filter thickness, the same results are obtained. Varying the accelerating voltage gives quite analogous curves which can likewise be brought to coincidence by parallel shift. Increasing the accelerating voltage results in an acceleration of the permeability drop and therefore in a shift of the curves opposite to the lg t axis. But the drop of the permeability occurs at least 40 times more slowly than with an intense irradiation with infrared light (250 W projector lamp).

4.1 Discussion

The experimental results can be described by a function f ( x ) = - f ( 1/x) independent of the type of irradiation which depends only on the property of the sample (x = t i tw) .

Fig. 4. Relative decrease of initial pcrnieability as a fuiiction of irradiation time t (logarithmic scale) a t 77 I< for five different filter thickncsses. (1) 0.25 mm Cu, (2) 0.5 mm Cu + 0.5 mm Al, (3) 1.0 mm C n + + 0.5 mm Al, (4) 2.0 mm Cu $- 0.5 mm dl, (5) 4.0 mm Cu + 0.5 mm Al; U = 200 kV, Isnode = 20 m 9

X-Kay Induced Change of Initial Permeability in Polycrystalline Y1G(I~te4+) 537

Fig. 5. t i 1 as a function of intensity I, (corrcsponding to the measured exposurc rate dq/dt). (a) for various filter thick- nesses a t a constant accelerating voltage U :- 200 kV, (b) for dlffcrmt accelerating voltages as a constant anode current of

15 mA m

0 7 2 4 r, ( ~ o - ~ w i ~ ~ j - -

t , is determined mainly by the irradiation (intensity, wavelength).

In Fig. 5, t i 1 was plotted as a function of the exposure rate, which was measured by a VA-J-18 ionmeter of VEB Vakutronik, Dresden. The exposure rate dqldt (SI in A kg-l) is a measure of the total radiation intensity a t the surface of the sample, inte- grated over all wavelengths of the continuous spectrum. For example, for a wavelength of 10 prn (most intense wavelength of the continuous spectrum caused by U = 200 kV) one can convert the exposure rate dqldt into radiation intensity (81 in W r ~ i - ~ ) (1 A kg-l x 1.4 x lo4 Wm-2). A linear connection between tG1 and I, exists only for the case of a constant accelerating voltage

t i 1 - I. or 5 - tI, . Therefore the relative decrease of permeability depends merely on the total radiation energy density (Jnir2), no matter in what time a given amount of energy is radiated. For infrared radiation this has been known for long [9]. Obviously this statement does not hold if the spectrum is shifted (case b). The ratio Io/hvo (yo = c/A,) is plotted instead of I . (Fig. 6) to linearize curve b. We can regard this quantity as a measure of the photon current density. For both experiments there results the same slope, the parallel shift being a result of differences in the distance sample to target for the different experiments and radiation measurements and of errors in the determination of t,.

According to this result the P M E seems to depend merely on the total number of radiated photons in the wavelength range under consideration. To make this statement more precise i t would be necessary to use monochromatic X-rays with continuously variable wavelength, However, this is difficult to carry out because of the high X-ray intensities required.

After the postulates of the two-centre model [lo] the initial susceptibility must be represented by the equation

1 - = A, + A,(1 - e-pl) x

Fig. 6. t;' as a function of lo/hv,. (a) for different filt,er thickncsses. (b) for different accelerating voltages

538 M. WURLITZER and U. REINHOLD: X-Ray Induced Change of Initial Pernieability

if the recombination can be neglected. This relation can he derived from the theory of aftereffect, if one takes into consideration that a relaxation is induced with an average relaxation freqnency f , = 1 kHz a t 77 K. If the p(t) curves are measured with f 2 2 10 kHz, then the pcriod of the alternating field is much shorter than the average &ne constant of relaxation and NBel’s theory applied to an individual wall gives for t,his case [3]

(notation after [GI) . Here M , is the saturation magnetization, a factor depending on the direction of the applied field with respect to the direction of the wall, S’ the wall area/domain volunie, a the elastic wall constant, y the factor considering the wall type, W,, the aftereffect constant, and d the wall thickness.

If one assumes proportionality between the aftereffect constant W,, and the number ?z of relaxing electrons and that dn N [n(m) - n( t ) ] dt during irradiation, and hence n(t) = n(m) (1 - e-pt), then (4) takes the form of (3). The graphs x = f(lg/3t) after (3) with the fitted values ~ ( m ) = (A, + AJ1 and ~ ( 0 ) = A i l are nearly symmetric, hut they are much too steep. That means that the results cannot be explained by the movement of an individual wall. But good analytic reproduction of the experimental curves could be reached by taking into consideration a distribution of a-values of the different walls.

Aclcnowledgsments

The authors wish to thank Prof. Dr. sc. W. Holzmiiller and Ur. F, Yliimer for facilities to carry out the experiments on the X-ray equipment and Mrs. M. Stein- hardt for the careful analysis of the results.

References [I] K. A. WINQARDH, Z. Phys. 8, 363 (1922). [2] LANDOLT-B~RNSTEIN, Vol. I, Part 1, Springer-Verlag, 1950. [3] L. KEEL, J. Phys. Radium 13, 249 (1952). [4] M. A. BLOCHIN, Physik der Rontgenstrahlen, Verlag Technik, Berlin 1957. [5] H. NEPF, Grundlagen und Anwendung der Rontgen-Feinstruktur-Analyse, Verlag R. Olden-

[a] 31. WURLITZER, phys. stat. sol. 2, 1750 (1962). [7] D. L. WOOD a n d J . P. REMEIKA, J . appl. l’hys. 37, 1232 (1966). [8] K. NASSAU, J. Crystal Growth 2 , 218 (1968). [9] U. ENZ and H. VAN DER HEIDE, Solid State Cornmun. 6, 347 (1968).

bourg, Miinchen 1962.

-1OJ W. I,F:MR, P. J. RIJNIERSE, P. I?. BONGERS, and U. ENZ, l’hys. Rev. Letters 21, 1643 (1968). -111 J. F. R. HAWICES and R. W. TEALE, J. Phys. C 6, 481 (1972). -121 fir. WURL~TZKR and W. CIBURA, phys. stat. sol. (a) Z1, K95 (1974). 131 R. MEWELAAH. and M. A. H. HUYBERTS, Philips Res. Rep. 29, 453 (1974). 141 K. HISATAKE, K. OHTA, N. ICHTNOSE, and H. YOKOYAMA, phys. stat. sol. (a) 26, K79 (1974). 151 K . HISATAKE and T. MATSUYAMA, Japan J. appl. Phys. 13,2063 (1974). 161 K. HISATAKE, K. OFITA, and Y. Nono, phys. stat. sol. (a) 30, K83 (1975). 171 31. WURLITZER and W. SCRALLER, phys. stat. sol. (a) 28, K41 (1975). 181 &I. WKRLITZER and F. PLUNER, phys. stat. sol. (a) 33, K55 (1976).

(Received September 8, 1WY)

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X-ray induced change of initial permeability in polycrystalline YIG(Me4+)