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Structural modifications induced by electronic energy deposition during the slowing down of heavy ions in matter
Nuclear Instruments and Methods North-Holland, Amsterdam
in Physics
Research
B39 (1989) l-6
STRUCTURAL MODIFICATIONS INDUCED BY ELECTRONIC ENERGY DEPOSITION DURING THE SLOWING DOWN OF HEAVY IONS IN MATTER M. TOULE~ONDE, CYRIL (CEA, CNRSj,
E. BALANZAT,
S. BOUFFARD
and J.C. JOUSSET
Rue Claude Bloch, BP 5133, 14040 Cam Cedex, France
The defects or the phase transformations induced by high density electronic excitation are studied in thick targets. Two kinds of materials have been studied: electrical conductors and magnetic insulators. These materials are insensitive to individual electronic excitations such as those created by photon or electron irradiations. In amorphous metallic alloys a two step process of damage creation has been observed: defect pr~uction followed by a change of dimensions of the samples. In high-ir, superconductors large electronic excitations enhance the rate of decrease of the critical temperature except in La,CuO, where Y& increases upon ion irradiation. In the case of magnetic insulators a description of ion-induced tracks is proposed which accounts for the experimental data over the whole range of electronic energy losses. Two rates of damage, correlated with a change of the shape of induced defects. are shown by Mijssbauer spectroscopy and high resolution electron microscopy observations.
1. In~~uction Slowing down in insulator targets, a swift heavy ion is known to induce, in many cases, important structural changes called latent tracks [l]. Recently, important progress has been made by the use of heavy ions accelerated by powerful machines such as GANIL (Caen, France), UNILAC (Da~stadt, FRG) and VICKSI (Berlin, Germany). A more precise description of the induced structural changes is now available. Nevertheless, the successive elementary processes which lead to the observed effects are far from being elucidated. The problem is to understand how the high-density electronic excitation which occurs in the wake of the incoming ion is converted into kinetic energy of the target atoms. As has already been discussed in an earlier article [2], there is at least a crucial question of time scale comparison between the duration of the electronic interaction (- lo-l6 s) and the target atom vibration period (- lo-l3 s). None of the models which have been proposed so far to describe the mechanisms involved in these radiation effects is satisfactory. This situation arises mainly from the lack of experimental data. It is only recently that the effects induced in matter by the slowing down of fast heavy ions have been investigated rather systematically on a large scale of stopping powers and in numerous different materials (electrical conductors, semiconductors and insulators). Hereafter we shall describe the experimental results obtained with the low temperature irradiation facility (IRABAT) of CIRIL for two types of fast heavy-ionirradiated materials: electrical conductors and insulators. It is of interest to point out that all the irradiated materials described hereafter are not damaged during photon or electron irradiation, that is when a low density of electronic excitations is generated. To the con0168-583X/89/$03.50 0 Elsevier Science Publishers (North-HolIand Physics Publishing Division)
B.V.
trary, the effects induced by fast heavy-ion irradiations will be attributed to high-density electronic excitations [3] and shown to occur above a threshold value. In this article we shall briefly describe the case of irradiated amorphous metallic alloys (which is presented in details in this conference) and the case of high T, superconductors. We shah emphasize on the results with insulating magnetic oxides and especially the Ys FesO,,. In this case we shall propose a description for the morphology of the damaged region in this material which applies for the large range of electronic stopping powers (dE/dx) of 100 MeV/u heavy ions coming to rest in a target, namely 0.1-3.5 keV/A.
2. Experimental results 2.1. Experimental facility All the irradiation experiments at GANIL make use of the IRABAT facility. The main features of IRABAT are: - temperature-controlled systems which allow irradiation from 10 up to 600 K at fluxes around 10’ ions/cm* s without any warming up larger than a few degrees; _ a beam handling facility made of horizontal and vertical sweeping magnets, slits and monitors of various intensities: samples of a few cm’ can be irradiated by homogeneous beams and the flux measured with a relative accuracy of 10%; - physical parameters, such as electrical resistance or lenght, can be measured in situ either during beam stops or along the irradiation. All the experiments described in this article have been performed at 77 K except when it is specified otherwise. 1. BEAM-SOLID
INTERACTIONS
M. Toulemonde et al. / Modifications during the slowing down of
2
ions in matter
2.2. Electrical conductors One of the most surprising results is the observation of damage induced by heavy-ion irradiations in amorphous metallic alloys. This damage was known to create a huge change of dimensions, without change of volume, and was put in evidence by the HMI group of Berlin. They first attributed this “growth effect”, to elastic collisions [4], then to the electronic energy losses [5]. On the other hand, important changes in electrical resistance during heavy-ion irradiation (U fission fragments) were put in evidence by D. Lesueur, then by A. Audouard, and they were attributed partly to elastic collisions, partly to electronic energy losses [6]. The experiments performed at GANIL have shown unambiguously that the whole phenomenon is due to electronic energy losses. This growth effect observed at 77 K is highly anisotropic: perpendicularly to the beam direction the sample expands and along the beam direction it shrinks.
I
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I
I
I
A
. .. . -’ . . .* .**
6_
I
. . .. ...... .. a *..,
y.._. *.. ; *.
C
. . *.
hence
[lOI Xe /cm21
Temperature(K)
Fig. 2. Variation of the resistance as a function of the temperature for a sample of La,CuO, irradiated by 2.9 GeV krypton ions. The different curves correspond to the following fluences: (a) pure, (b) 4.8~10” cme2, (c) lOI cm-*, (d) 1.9~10’~ cme2 , (e) 3.1 X lOI cmm2 and (f) 3.9 x lOI cmm2 [lo].
A. Audouard et al. have consequently irradiated a Fes,B,, amorphous alloy with different angles of incidence in the same sample holder, i.e. in the same experimental conditions [7]. Their results are presented in fig. 1. Two interesting features appear: - at low fluences there is no effect of the incidence angles; _ at high fluences the effect of the incidence angles is obvious. The low-fluence isotropic effect has been shown to be strongly dependent upon the electronic stopping power and assumed to be due to the creation of point defects in the short range order of the amorphous alloy [8]. Recently, under Xe irradiation the effect of an externally applied uniaxial stress has been shown to induce an important plastic deformation. This effect has been analysed in terms of radiation-induced creep [9].
06 IF P 2 01 LI 0 am 0.2
Fhence (1012Xe/cm2)
Fig. 1. Relative electrical resistance variation A R/R, measured at 77 K vs Xe fluence for Fe,,&, amorphous alloys irradiated with different tilting angle 19 between the ion beam and the normal to the sample surface [7]. Due to the fact that the samples are not totally free to grow in the sample holder, significant changes of the tilting angle occur during irradiation. Therefore, 0 varies as follows: A = (0-11)O; B = (29-31)O; C = (48-44) O; D = (58-61) O.
I
0 Fig. 3. Variation
I
Id2
I
2 Ill2
I
31012
of the transition temperature of the Kr ion fluence [ll].
lxx/cm ’ ;
T, as a function
Fig. 4. MSsbauer spectra of sintered samples of Y3Fe,012 garnet irradiated by Mo ions of 2.3 GeV total energy. (a) Reference spectrum. (b) Irradiated spectrum d E/dx = 17 MeV/gm; #r = 2 x lO’* ions/cm’. The magnetic disordered fraction is 37% of the total volume. The arrows specify the position of the paramagnetic doublet. The decrease of the intensity of the lines B, as compared to that of the lines A, is a direct observation of an anisotropy of the orientation of the hyperfine magnetic field 1281.
High-T,
[lO,ll]
superconductors have also been irradiated and the main feature was, for La,CuO,, a de-
crease of the electrical resistance measured in the metallic phase at 50 K, associated with an increase of T, (figs. 2 and 3). It is the only copper oxide superconductor where irradiation induced a significant increase of the critical temperature [11,12]. In all the other compounds of the type YBa,Cu,O,_, irradiated at GANJL, heavy-ion irradiations led to decreased T, and increased electrical resistivity.
In magnetic insulators like Y,Fe,O,, [13-171, BaFe,,O,, [l&-20] and spinels (Fe&,, ZnFe,Q, NiFe,O,, MgFe,O,) [19,X] the magnetic properties are very sensitive to the irradiation-induced disorder which is observed as a decrease in the saturation magnetization [13,14,17,18]. In MZissbauer spectrometry [16] the effect of ion bombardment consists of a vanishing of the magnetic sextuplet and the appearance of a paramagnetic doublet in the center of the spectrum (fig. 4). A more detaiied analysis of the Massbauer spectrum suggests several new features. In the case of Y,Fe,O,,, for the high values of the electronic energy losses the hyperfine magnetic field becomes parallel to the heavy ions path (fig. 4) since the relative intensity of the lines B decreases compared to that of lines A. This effect is also clearly observed in the spinels MgFe,O, and
NiFe& [21]. For moderate values of the electronic stopping power [15,16] irradiation induces no change in the orientation of the hyperfine magnetic field in a sintered material which has an isotropic distribution of the hyperfine magnetic field orientation, while in a single crystal the initial completely anisotropic orientation of the hyperfine magnetic field is lost. Moreover, in BaFe,,O,, the irradiation induces a continuous distribution of the hyperfine magnetic Field, supe~mpos~d to the appearance of a parama~eti~ phase. This continuous distribution is linked to a change in the ratio of occupancy of the iron crystallographic sites [ZO]. Whatever the irradiation temperature was, liquid nitrogen or room temperature, the defect production was identical within the experimental errors [16]. All these numerous experimental results can be analysed as atomic displacements or structure transformations around the ion path. In the next paragraph it is shown that all these features can be related to the electronic stopping power effect induced during the slowing down of the heavy ions in materials.
3. Is this an effect of the high density electronic excitation? At high energy the elastic collisions (nuclear stopping power) are still efficient in defect generation 122,231, In order to separate the effects due to nuclear elastic coilisicms from those induced by the electronic energy losses, the usual procedure is to normalize the experimental data by the calculated yield of displaced atams per atoms of the target (dpa = number of displacements per atoms) [13,24]. This procedure has been used successfully in the case of amorphous metallic alloys to demonstrate that for a value of dE,/dx > 14 MeV/pm nuclear collisions are far from accounting for the defect production [7].
Fig. 5. Variation of ST, for irradiated YBa$u,O,_s for electron, neutron and heavy iok irradiation [lO,ll]. Xe and Kr points correspond to the GeV irradiations.
1. BEAM-SOLID
INTERACTIONS
M. Toulemonde et al. / Modifications during the slowing down of ions in matter
Fig. 6. F,, the disordered material Ar(0) - dE/dx=4 MeV/pm,
dE/dx=4 MeV/pm, dE/dx = 25 MeV/Pm.
fraction or paramagnetic Kr(0) - dE/dx ~10
fraction, MeV/pm,
Kr~l.1 GeV (0) - dE/dx =7 MeV/pm, Krzl.1 GeV (a) - dE/dx=ll MeV/Fm, Xc(o) The lines are only drawn to guide the eye. The relative error of N,/N, has been estimated to be f0.15 owing to the measurement
For the case of high-T, superconductors the variation of the critical temperature versus dpa is plotted in fig. 5. It is shown that, whatever the incident particles, the variation of r, scales with the dpa except for the heaviest ions (high-energy Kr and Xe ions [ll]), where a one order of magnitude faster variation is observed. Thus, for YBa,Cu,O,_, the damage is enhanced by the high-density electronic excitation. For the case of insulating magnetic oxides the normalization of the results [6] by the dpa is shown in fig. 6, where it can be observed that the fraction of disordered material depends strongly on the value of the electronic stopping power. Moreover, these results can be compared to neutron irradiation experiments [25,26]: the defect creation induced by neutron irradiation is at least by two orders of magnitude less efficient than that observed during heavy-ion irradiation. This is a strong support of the assumption of an electronic energy losses
3
I-
-Y3Fe50,2
I
I
I 3-
vs dpa induced by Ar, Kr and Xe irradiations for (a) Y,Fe,O,,: Xc(o) - dE/dx = 25 MeV/pm; (b) BaFe,,O,,: Ar(0) -
of the fluence [16].
damage creation in the case of Ar in Y,Fe,O,, which corresponds to a value of d E/dx = 6 MeV/pm. Consequently it is now possible to normalize the results by the dose, i.e. the rate of damage per unit of deposited energy versus the electronic stopping power (fig. 7). It is observed that, above a critical value found to be equal to 8 MeV/l.tm for Y,Fe,O,, the damage creation yield depends strongly on dE/dx. At this critical value, which depends on the materials [16], a change in the orientation of the hyperfine magnetic field is observed for Y,Fe,O,,.
4. Defect morphology and physical property changes in magnetic insulator A systematic study of the defect morphology versus the electronic stopping power has been done using high
R (nm)
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/
/
0
0
,’
.
, /’
.
be’ . /’ ., ,
2
I
,
/
./
/
/
/’
9’
electron microscopy Rutherford ha&scattering Miissbauer spectrometry
;
dE/dx
[MeV/pml
Fig. 7. Plot of [ln(l- T,)/($d dE/dx)] versus dE/dx for Ar, Kr, MO and Xe irradiations. Td is the magnetic disorder deduced from MGssbauer spectrometry [2]. The quoted value corresponds to the electronic energy loss at which the easy chemical etching appears [M. Toulemonde et al., private communication].
OJ 3.2
/
.
i
i
4.3 (dE/dx)“’
I
I
5.4
.O .O .
f 6.5
( MeV /em ,“*
Fig. 8. A plot of the track radius as a function of the square root of the electronic stopping power (in MeV/pm) [17]. All the black markers are from ref. [28] while the others are quoted there.
M. Toulemonde et al. / Modifications during the slowing down
resolution electron microscopy (HREM) [28-301. The defect morphology will be described as a function of the electronic stopping power. The emphasis will be put mainly on the results obtained on Y,Fe,O,,. Some other results for several different materials will also be reported. At high values of the electronic stopping power the damage track is seen by HREM as a long continuous cylinder [19,28]. The lattice resolution allows a direct measurement of the cylinder radii which vary around 4 nm. These radii have been compared to other experimental determinations such as Rutherford backscattering (RBS) and Mossbauer spectrometry (fig. 8). There is a rather good overall agreement between the topological disorder observed by HREM, RBS as well as the disordered paramagnetic phase fraction deduced from Miissbauer spectrometry. Moreover, in Ys Fe, O,, the long cylinder is associated with an orientation of the hyperfine magnetic field parallel to the heavy ion path. This long cylinder morphology was first put in evidence by small-angle X-ray diffraction patterns [31,32] observed on mica irradiated by Ar, Kr, Xe and U beams. The same results are obtained for Ni Fe,O,, MgFe,O, [19], BaFe,,O,, [19,20] and Bi,Fe,Og [19,29]. From all these results, obtained with Y,Fe,O,,, a detailed description of the latent track has been given. The model assumes that a track is made of three shells: an amorphous core, a transition region and an external crystalline stressed shell [28]. In the case of a Bi,Fe,O, compound a very sharp transition between the amorphous and the crystalline phase is observed by HREM [19,29]. In the intermediate region of electronic stoppingpower, medium-resolution electron mi8 - 17 MeV/um, croscopy observations [30] show long discontinuous cylinders. With increasing dE/dx, the length of these cylinders (fig. 9) increases and consequently the undamaged regions along the ion path decrease. The hy-
Fig. 9. Medium-resolution electron microscopy of Y,Fe,0,2 irradiated by krypton ions for a dE/dx =lO MeV/pm [30].
ofionsin matfer
5
perfine magnetic field is still parallel to the heavy ion path [16]. At moderate values of the electronic stopping power (i.e. d E/dx = 8 MeV/um for Y,Fe,O,,) HREM shows isolated extended defects with a spheroidal shape [30]. This change in the shape of the defects was suggested previously [16] by the change in the orientation of the hyperfine magnetic field which is then isotropic. It is interesting to notice that the existence of these extended defects has been suggested previously by Dartyge et al. [33,35] for mica observed by small-angle X-ray scattering. A change in the damage creation yield is observed when a transition of shape occurs, i.e. at the overlap of the spheroidal defects. At high values of dE/dx the radius variation of the continuous cylinder cannot be interpreted by a percolation of the defects as in the Dartyge and Sigmund’s model [35]. In summary, from all these experimentals results, obtained by three different experimental methods, a general model, available over the whole range of (dE/dx), can be assumed: at moderate values, separated extended defects are created; (2) in a transition region, discontinuous cylinders could be a result of an overlap of these extended defects; and (3) at high values of electronic energy losses, long continuous cylinders are created. Their radii increase with d E/dx.
il)
5. Conclusion In materials which are not sensitive to single localized electronic excitation it is demonstrated that highdensity electronic excitations lead to important effects such as defect creation and structure modifications. Typical results have been presented for high-T, superconductors, amorphous metallic alloys and insulating magnetic oxides. In high-T, superconductors, the electronic stopping power induces a more rapid decrease of T, than in those observed when only the elastic collisions processes are efficient. In amorphous metallic alloys above a threshold value the electronic energy losses induce a damage two orders of magnitude higher than the elastic collisions. A twostep process is observed in the damage production: at low fluences defects in the short range order are created while at high fluences sample growth appears. In insulating magnetic oxides a similar behaviour is observed for the damage production by electronic energy losses. Above a critical value for dE/dx, the damage production is enhanced. Different damage yields have been put in evidence which correspond to changes I. BEAM-SOLID
INTERACTIONS
6
M. Toulemonde et al. / Modifications during the slowing down of ions in matter
in the morphology of the induced defects. The shape transition appears at the overlap of the spheroidal defects which are created at moderate values of dE/dx leading to cylindrical defects at high values of dE/dx. In conclusion, all the results reported here lead to a better description of the damage induced by high density electronic excitations. An important question remains: what are the elementary processes by which the relaxation of the deposited energy leads to such effects?
References [l] R.L. Fleischer, P.B. Price and R.M. Walker, in: Nuclear
Tracks in Solids: Principles and Applications (University of California Press, 1975); B.E. Fischer and R. Spohr, Rev. Mod. Phys. 55 (1983) 907. [2] E. Balanzat, J.C. Jousset and M. Toulemonde, Nucl. Instr. and Meth. B32 (1988) 368 (Proc. 4th Int. Conf. Radiation Effects in Insulators). [3] K. Tanimura and N. Itoh, Nucl. Instr. and Meth. B32 (1988) 211 (Proc. 4th Int. Conf. Radiation Effects in Insulators). [4] S. Klaumunzer, G. Schumacher, S. Rentzsch, G. Vogel, L. SGldner and H. Bieger, Acta Metall. 30 (1982) 1493;S. KIaumunzer and G. Schumacher, Phys. Rev. Lett. 51 (1983) 987. Hou, S. KIaumunzer and G. Schumacher, [51 Ming-Dong Nucl. Instr. and Meth. B19/20 (1987) 16. Radiat. Eff. 24 (1975) 101; 161 D. Lesueur and A. Audouard, A. Audouard, J. Dural and J.C. Jousset, J. Phys. (Paris) 41, C8 (1980) 835; A. Audouard, Rapport CEA R-5204 (1983). E. Balanzat, G. Fuchs, J.C. Jousset, D. 171 A. Audouard, Lesueur and L. Thorn&, Europhys. Lett. 5 (1988) 241. E. BaIanzat, G. Fuchs, J.C. Jousset, D. P31 A. Audouard, Lesueur and L. Thorn&, Europhys. Lett. 3 (1987) 327. E. Balanzat, G. Fuchs, J.C. Jousset, D. [91 A. Audouard, Lesueur and L. Thorn& J. Phys. Lett. F18 (1988) LlOl; A. Audouard, E. Balanzat, G. Fuchs, J.C. Jousset, D. Lesueur and L. Thorn&, these Proceedings (IBMM ‘88), Nucl. Instr. and Meth. B39 (1989) 18. WI D. Groult, J. Provost, B. Raveau, F. Studer, J.C. Jousset, S.I. Lewandowski, M. Toulemonde and F. RulIier-Albenque, Europhys. Lett. 6 (1988) 151. 1111F. Studer, J. Provost, D. Groult, B. Raveau, S. Bouffard, M. Toulemonde, J.C. Jousset and F. RuUier-Albenque, High-Temperature Superconductors, eds. M.B. Brodsky, R.C. Dynes, K. Kitazawa and H.L. Tuller, MRS Proc. vol. 99 (1988) p. 877.
[12] A.E. White, K.T. Short, D.C. Jacobson, J.M. Poate, R.C. Dynes, P.M. Mankiewich, W.J. Skocpol, R.E. Howard, M. AnzIowar, K.W. Baldwin, A.F.J. Levi, J.R. Kwo, T. Hsieh and M. Hong, to be published in Phys. Rev. B. [13] G. Fuchs, F. Studer, E. BaIanzat, D. Groult, M. Toulemonde and J.C. Jousset, Europhys. Lett. 3 (1987) 3. [14] G. Fuchs, F. Studer, E. Balanzat, D. Groult, J.C. Jousset and B. Raveau, Nucl. Instr. and Meth. B12 (1985) 471. [15] M. Toulemonde, D. Groult, N. Nguyen and F. Studer, Les Editions de Physique 1986, Magnetic Thin Films, ed. R. Krishnan, p. 121. [16] M. Toulemonde, G. Fuchs, N. Nguyen, F. Studer and D. Groult, Phys. Rev. B35 (1987) 6560. [17] P. Hansen, H. Heitman and P.M. Srnit, Phys. Rev. B26 (1982) 3539. [18] D. Groult, M. Hervieu, N. Nguyen, B. Raveau, G. Fuchs and E. Balanzat, Radiat. Eff. 90 (1982) 19. (191 C. Houpert, M. Hervieu, D. Groult, F. Studer and M. Toulemonde, Nucl. Instr. and Meth. B32 (1988) 393 (Proc. 4th Int. Conf. Radiation Effects in Insulators). [20] C. Houpert, N. Nguyen, F. Studer, D. Groult and M. Toulemonde, Nucl. Instr. and Meth. B34 (1988) 228. [21] F. Studer, H. Pascard, D. Groult, C. Houpert, N. Nguyen and M. Toulemonde, Nucl. Instr. and Meth. B32 (1988) 389 (Proc. 4th Int. Conf. Radiation Effects in Insulators). [22] A. Barbu, G. Martin, M. Toulemonde and J.C. Jousset, C.R. Acad. Sci. 299 (1984) 409. [23] A. Dunlop, L. Boulanger, D. Lesueur, N. Lorenzelli and M. Toulemonde, Mater. Sci. For. 15-18 (1987) 1117. [24] A. Iwase, S. Sasaki, I. Iwata and T. Nihira, Phys. Rev. Lett. 58 (1987) 2450. [25] A.K. Podsekin and V.N. Zaitsev, Sov. Phys. Tech. Phys. 24 (1982) 342. [26] Y.G. Chukalkin, V.V. Petrov and B.N. Goschitzkii, Phys. Status Solid A67 (1981) 421. [27] G. Fuchs, These, Rapport CEA-R-5343 (1986). [28] M. Toulemonde and F. Studer, to be published in Philos. Mag. [29] D. Groult, M. Hervieu, N. Nguyen, and B. Raveau, to be published in J. Solid State Chem. 1301 C. Houpert, F. Studer, D. Groult and M. Toulemonde, these Proceedings (IBMM ‘88) Nucl. Instr. and Meth. B39 (1989) 720. [31] D. AIbrecht, P. Armbruster, R. Spohr, M. Roth, K. Schaupert and H. Stuhrmann, Appl. Phys. A37 (1985) 37. [32] D. Albrecht, E. BaIanzat and K. Schaupert, Nucl. Tracks Radiat. Meas. 11 (1986) 93. [33] E. Dartyge, M. Lambert and M. Maurette, J. Phys. (Paris) 37 (1976) 939. [34] E. Dartyge, J.P. Duraud, Y. Langevin and M. Maurette, Phys. Rev. B23 (1981) 5213. [35] E. Dartyge and P. Sigmund, Phys. Rev. B32 (1985) 5249.
in Physics
Research
B39 (1989) l-6
STRUCTURAL MODIFICATIONS INDUCED BY ELECTRONIC ENERGY DEPOSITION DURING THE SLOWING DOWN OF HEAVY IONS IN MATTER M. TOULE~ONDE, CYRIL (CEA, CNRSj,
E. BALANZAT,
S. BOUFFARD
and J.C. JOUSSET
Rue Claude Bloch, BP 5133, 14040 Cam Cedex, France
The defects or the phase transformations induced by high density electronic excitation are studied in thick targets. Two kinds of materials have been studied: electrical conductors and magnetic insulators. These materials are insensitive to individual electronic excitations such as those created by photon or electron irradiations. In amorphous metallic alloys a two step process of damage creation has been observed: defect pr~uction followed by a change of dimensions of the samples. In high-ir, superconductors large electronic excitations enhance the rate of decrease of the critical temperature except in La,CuO, where Y& increases upon ion irradiation. In the case of magnetic insulators a description of ion-induced tracks is proposed which accounts for the experimental data over the whole range of electronic energy losses. Two rates of damage, correlated with a change of the shape of induced defects. are shown by Mijssbauer spectroscopy and high resolution electron microscopy observations.
1. In~~uction Slowing down in insulator targets, a swift heavy ion is known to induce, in many cases, important structural changes called latent tracks [l]. Recently, important progress has been made by the use of heavy ions accelerated by powerful machines such as GANIL (Caen, France), UNILAC (Da~stadt, FRG) and VICKSI (Berlin, Germany). A more precise description of the induced structural changes is now available. Nevertheless, the successive elementary processes which lead to the observed effects are far from being elucidated. The problem is to understand how the high-density electronic excitation which occurs in the wake of the incoming ion is converted into kinetic energy of the target atoms. As has already been discussed in an earlier article [2], there is at least a crucial question of time scale comparison between the duration of the electronic interaction (- lo-l6 s) and the target atom vibration period (- lo-l3 s). None of the models which have been proposed so far to describe the mechanisms involved in these radiation effects is satisfactory. This situation arises mainly from the lack of experimental data. It is only recently that the effects induced in matter by the slowing down of fast heavy ions have been investigated rather systematically on a large scale of stopping powers and in numerous different materials (electrical conductors, semiconductors and insulators). Hereafter we shall describe the experimental results obtained with the low temperature irradiation facility (IRABAT) of CIRIL for two types of fast heavy-ionirradiated materials: electrical conductors and insulators. It is of interest to point out that all the irradiated materials described hereafter are not damaged during photon or electron irradiation, that is when a low density of electronic excitations is generated. To the con0168-583X/89/$03.50 0 Elsevier Science Publishers (North-HolIand Physics Publishing Division)
B.V.
trary, the effects induced by fast heavy-ion irradiations will be attributed to high-density electronic excitations [3] and shown to occur above a threshold value. In this article we shall briefly describe the case of irradiated amorphous metallic alloys (which is presented in details in this conference) and the case of high T, superconductors. We shah emphasize on the results with insulating magnetic oxides and especially the Ys FesO,,. In this case we shall propose a description for the morphology of the damaged region in this material which applies for the large range of electronic stopping powers (dE/dx) of 100 MeV/u heavy ions coming to rest in a target, namely 0.1-3.5 keV/A.
2. Experimental results 2.1. Experimental facility All the irradiation experiments at GANIL make use of the IRABAT facility. The main features of IRABAT are: - temperature-controlled systems which allow irradiation from 10 up to 600 K at fluxes around 10’ ions/cm* s without any warming up larger than a few degrees; _ a beam handling facility made of horizontal and vertical sweeping magnets, slits and monitors of various intensities: samples of a few cm’ can be irradiated by homogeneous beams and the flux measured with a relative accuracy of 10%; - physical parameters, such as electrical resistance or lenght, can be measured in situ either during beam stops or along the irradiation. All the experiments described in this article have been performed at 77 K except when it is specified otherwise. 1. BEAM-SOLID
INTERACTIONS
M. Toulemonde et al. / Modifications during the slowing down of
2
ions in matter
2.2. Electrical conductors One of the most surprising results is the observation of damage induced by heavy-ion irradiations in amorphous metallic alloys. This damage was known to create a huge change of dimensions, without change of volume, and was put in evidence by the HMI group of Berlin. They first attributed this “growth effect”, to elastic collisions [4], then to the electronic energy losses [5]. On the other hand, important changes in electrical resistance during heavy-ion irradiation (U fission fragments) were put in evidence by D. Lesueur, then by A. Audouard, and they were attributed partly to elastic collisions, partly to electronic energy losses [6]. The experiments performed at GANIL have shown unambiguously that the whole phenomenon is due to electronic energy losses. This growth effect observed at 77 K is highly anisotropic: perpendicularly to the beam direction the sample expands and along the beam direction it shrinks.
I
I
I
I
I
A
. .. . -’ . . .* .**
6_
I
. . .. ...... .. a *..,
y.._. *.. ; *.
C
. . *.
hence
[lOI Xe /cm21
Temperature(K)
Fig. 2. Variation of the resistance as a function of the temperature for a sample of La,CuO, irradiated by 2.9 GeV krypton ions. The different curves correspond to the following fluences: (a) pure, (b) 4.8~10” cme2, (c) lOI cm-*, (d) 1.9~10’~ cme2 , (e) 3.1 X lOI cmm2 and (f) 3.9 x lOI cmm2 [lo].
A. Audouard et al. have consequently irradiated a Fes,B,, amorphous alloy with different angles of incidence in the same sample holder, i.e. in the same experimental conditions [7]. Their results are presented in fig. 1. Two interesting features appear: - at low fluences there is no effect of the incidence angles; _ at high fluences the effect of the incidence angles is obvious. The low-fluence isotropic effect has been shown to be strongly dependent upon the electronic stopping power and assumed to be due to the creation of point defects in the short range order of the amorphous alloy [8]. Recently, under Xe irradiation the effect of an externally applied uniaxial stress has been shown to induce an important plastic deformation. This effect has been analysed in terms of radiation-induced creep [9].
06 IF P 2 01 LI 0 am 0.2
Fhence (1012Xe/cm2)
Fig. 1. Relative electrical resistance variation A R/R, measured at 77 K vs Xe fluence for Fe,,&, amorphous alloys irradiated with different tilting angle 19 between the ion beam and the normal to the sample surface [7]. Due to the fact that the samples are not totally free to grow in the sample holder, significant changes of the tilting angle occur during irradiation. Therefore, 0 varies as follows: A = (0-11)O; B = (29-31)O; C = (48-44) O; D = (58-61) O.
I
0 Fig. 3. Variation
I
Id2
I
2 Ill2
I
31012
of the transition temperature of the Kr ion fluence [ll].
lxx/cm ’ ;
T, as a function
Fig. 4. MSsbauer spectra of sintered samples of Y3Fe,012 garnet irradiated by Mo ions of 2.3 GeV total energy. (a) Reference spectrum. (b) Irradiated spectrum d E/dx = 17 MeV/gm; #r = 2 x lO’* ions/cm’. The magnetic disordered fraction is 37% of the total volume. The arrows specify the position of the paramagnetic doublet. The decrease of the intensity of the lines B, as compared to that of the lines A, is a direct observation of an anisotropy of the orientation of the hyperfine magnetic field 1281.
High-T,
[lO,ll]
superconductors have also been irradiated and the main feature was, for La,CuO,, a de-
crease of the electrical resistance measured in the metallic phase at 50 K, associated with an increase of T, (figs. 2 and 3). It is the only copper oxide superconductor where irradiation induced a significant increase of the critical temperature [11,12]. In all the other compounds of the type YBa,Cu,O,_, irradiated at GANJL, heavy-ion irradiations led to decreased T, and increased electrical resistivity.
In magnetic insulators like Y,Fe,O,, [13-171, BaFe,,O,, [l&-20] and spinels (Fe&,, ZnFe,Q, NiFe,O,, MgFe,O,) [19,X] the magnetic properties are very sensitive to the irradiation-induced disorder which is observed as a decrease in the saturation magnetization [13,14,17,18]. In MZissbauer spectrometry [16] the effect of ion bombardment consists of a vanishing of the magnetic sextuplet and the appearance of a paramagnetic doublet in the center of the spectrum (fig. 4). A more detaiied analysis of the Massbauer spectrum suggests several new features. In the case of Y,Fe,O,,, for the high values of the electronic energy losses the hyperfine magnetic field becomes parallel to the heavy ions path (fig. 4) since the relative intensity of the lines B decreases compared to that of lines A. This effect is also clearly observed in the spinels MgFe,O, and
NiFe& [21]. For moderate values of the electronic stopping power [15,16] irradiation induces no change in the orientation of the hyperfine magnetic field in a sintered material which has an isotropic distribution of the hyperfine magnetic field orientation, while in a single crystal the initial completely anisotropic orientation of the hyperfine magnetic field is lost. Moreover, in BaFe,,O,, the irradiation induces a continuous distribution of the hyperfine magnetic Field, supe~mpos~d to the appearance of a parama~eti~ phase. This continuous distribution is linked to a change in the ratio of occupancy of the iron crystallographic sites [ZO]. Whatever the irradiation temperature was, liquid nitrogen or room temperature, the defect production was identical within the experimental errors [16]. All these numerous experimental results can be analysed as atomic displacements or structure transformations around the ion path. In the next paragraph it is shown that all these features can be related to the electronic stopping power effect induced during the slowing down of the heavy ions in materials.
3. Is this an effect of the high density electronic excitation? At high energy the elastic collisions (nuclear stopping power) are still efficient in defect generation 122,231, In order to separate the effects due to nuclear elastic coilisicms from those induced by the electronic energy losses, the usual procedure is to normalize the experimental data by the calculated yield of displaced atams per atoms of the target (dpa = number of displacements per atoms) [13,24]. This procedure has been used successfully in the case of amorphous metallic alloys to demonstrate that for a value of dE,/dx > 14 MeV/pm nuclear collisions are far from accounting for the defect production [7].
Fig. 5. Variation of ST, for irradiated YBa$u,O,_s for electron, neutron and heavy iok irradiation [lO,ll]. Xe and Kr points correspond to the GeV irradiations.
1. BEAM-SOLID
INTERACTIONS
M. Toulemonde et al. / Modifications during the slowing down of ions in matter
Fig. 6. F,, the disordered material Ar(0) - dE/dx=4 MeV/pm,
dE/dx=4 MeV/pm, dE/dx = 25 MeV/Pm.
fraction or paramagnetic Kr(0) - dE/dx ~10
fraction, MeV/pm,
Kr~l.1 GeV (0) - dE/dx =7 MeV/pm, Krzl.1 GeV (a) - dE/dx=ll MeV/Fm, Xc(o) The lines are only drawn to guide the eye. The relative error of N,/N, has been estimated to be f0.15 owing to the measurement
For the case of high-T, superconductors the variation of the critical temperature versus dpa is plotted in fig. 5. It is shown that, whatever the incident particles, the variation of r, scales with the dpa except for the heaviest ions (high-energy Kr and Xe ions [ll]), where a one order of magnitude faster variation is observed. Thus, for YBa,Cu,O,_, the damage is enhanced by the high-density electronic excitation. For the case of insulating magnetic oxides the normalization of the results [6] by the dpa is shown in fig. 6, where it can be observed that the fraction of disordered material depends strongly on the value of the electronic stopping power. Moreover, these results can be compared to neutron irradiation experiments [25,26]: the defect creation induced by neutron irradiation is at least by two orders of magnitude less efficient than that observed during heavy-ion irradiation. This is a strong support of the assumption of an electronic energy losses
3
I-
-Y3Fe50,2
I
I
I 3-
vs dpa induced by Ar, Kr and Xe irradiations for (a) Y,Fe,O,,: Xc(o) - dE/dx = 25 MeV/pm; (b) BaFe,,O,,: Ar(0) -
of the fluence [16].
damage creation in the case of Ar in Y,Fe,O,, which corresponds to a value of d E/dx = 6 MeV/pm. Consequently it is now possible to normalize the results by the dose, i.e. the rate of damage per unit of deposited energy versus the electronic stopping power (fig. 7). It is observed that, above a critical value found to be equal to 8 MeV/l.tm for Y,Fe,O,, the damage creation yield depends strongly on dE/dx. At this critical value, which depends on the materials [16], a change in the orientation of the hyperfine magnetic field is observed for Y,Fe,O,,.
4. Defect morphology and physical property changes in magnetic insulator A systematic study of the defect morphology versus the electronic stopping power has been done using high
R (nm)
-
/
/
/
0
0
,’
.
, /’
.
be’ . /’ ., ,
2
I
,
/
./
/
/
/’
9’
electron microscopy Rutherford ha&scattering Miissbauer spectrometry
;
dE/dx
[MeV/pml
Fig. 7. Plot of [ln(l- T,)/($d dE/dx)] versus dE/dx for Ar, Kr, MO and Xe irradiations. Td is the magnetic disorder deduced from MGssbauer spectrometry [2]. The quoted value corresponds to the electronic energy loss at which the easy chemical etching appears [M. Toulemonde et al., private communication].
OJ 3.2
/
.
i
i
4.3 (dE/dx)“’
I
I
5.4
.O .O .
f 6.5
( MeV /em ,“*
Fig. 8. A plot of the track radius as a function of the square root of the electronic stopping power (in MeV/pm) [17]. All the black markers are from ref. [28] while the others are quoted there.
M. Toulemonde et al. / Modifications during the slowing down
resolution electron microscopy (HREM) [28-301. The defect morphology will be described as a function of the electronic stopping power. The emphasis will be put mainly on the results obtained on Y,Fe,O,,. Some other results for several different materials will also be reported. At high values of the electronic stopping power the damage track is seen by HREM as a long continuous cylinder [19,28]. The lattice resolution allows a direct measurement of the cylinder radii which vary around 4 nm. These radii have been compared to other experimental determinations such as Rutherford backscattering (RBS) and Mossbauer spectrometry (fig. 8). There is a rather good overall agreement between the topological disorder observed by HREM, RBS as well as the disordered paramagnetic phase fraction deduced from Miissbauer spectrometry. Moreover, in Ys Fe, O,, the long cylinder is associated with an orientation of the hyperfine magnetic field parallel to the heavy ion path. This long cylinder morphology was first put in evidence by small-angle X-ray diffraction patterns [31,32] observed on mica irradiated by Ar, Kr, Xe and U beams. The same results are obtained for Ni Fe,O,, MgFe,O, [19], BaFe,,O,, [19,20] and Bi,Fe,Og [19,29]. From all these results, obtained with Y,Fe,O,,, a detailed description of the latent track has been given. The model assumes that a track is made of three shells: an amorphous core, a transition region and an external crystalline stressed shell [28]. In the case of a Bi,Fe,O, compound a very sharp transition between the amorphous and the crystalline phase is observed by HREM [19,29]. In the intermediate region of electronic stoppingpower, medium-resolution electron mi8 - 17 MeV/um, croscopy observations [30] show long discontinuous cylinders. With increasing dE/dx, the length of these cylinders (fig. 9) increases and consequently the undamaged regions along the ion path decrease. The hy-
Fig. 9. Medium-resolution electron microscopy of Y,Fe,0,2 irradiated by krypton ions for a dE/dx =lO MeV/pm [30].
ofionsin matfer
5
perfine magnetic field is still parallel to the heavy ion path [16]. At moderate values of the electronic stopping power (i.e. d E/dx = 8 MeV/um for Y,Fe,O,,) HREM shows isolated extended defects with a spheroidal shape [30]. This change in the shape of the defects was suggested previously [16] by the change in the orientation of the hyperfine magnetic field which is then isotropic. It is interesting to notice that the existence of these extended defects has been suggested previously by Dartyge et al. [33,35] for mica observed by small-angle X-ray scattering. A change in the damage creation yield is observed when a transition of shape occurs, i.e. at the overlap of the spheroidal defects. At high values of dE/dx the radius variation of the continuous cylinder cannot be interpreted by a percolation of the defects as in the Dartyge and Sigmund’s model [35]. In summary, from all these experimentals results, obtained by three different experimental methods, a general model, available over the whole range of (dE/dx), can be assumed: at moderate values, separated extended defects are created; (2) in a transition region, discontinuous cylinders could be a result of an overlap of these extended defects; and (3) at high values of electronic energy losses, long continuous cylinders are created. Their radii increase with d E/dx.
il)
5. Conclusion In materials which are not sensitive to single localized electronic excitation it is demonstrated that highdensity electronic excitations lead to important effects such as defect creation and structure modifications. Typical results have been presented for high-T, superconductors, amorphous metallic alloys and insulating magnetic oxides. In high-T, superconductors, the electronic stopping power induces a more rapid decrease of T, than in those observed when only the elastic collisions processes are efficient. In amorphous metallic alloys above a threshold value the electronic energy losses induce a damage two orders of magnitude higher than the elastic collisions. A twostep process is observed in the damage production: at low fluences defects in the short range order are created while at high fluences sample growth appears. In insulating magnetic oxides a similar behaviour is observed for the damage production by electronic energy losses. Above a critical value for dE/dx, the damage production is enhanced. Different damage yields have been put in evidence which correspond to changes I. BEAM-SOLID
INTERACTIONS
6
M. Toulemonde et al. / Modifications during the slowing down of ions in matter
in the morphology of the induced defects. The shape transition appears at the overlap of the spheroidal defects which are created at moderate values of dE/dx leading to cylindrical defects at high values of dE/dx. In conclusion, all the results reported here lead to a better description of the damage induced by high density electronic excitations. An important question remains: what are the elementary processes by which the relaxation of the deposited energy leads to such effects?
References [l] R.L. Fleischer, P.B. Price and R.M. Walker, in: Nuclear
Tracks in Solids: Principles and Applications (University of California Press, 1975); B.E. Fischer and R. Spohr, Rev. Mod. Phys. 55 (1983) 907. [2] E. Balanzat, J.C. Jousset and M. Toulemonde, Nucl. Instr. and Meth. B32 (1988) 368 (Proc. 4th Int. Conf. Radiation Effects in Insulators). [3] K. Tanimura and N. Itoh, Nucl. Instr. and Meth. B32 (1988) 211 (Proc. 4th Int. Conf. Radiation Effects in Insulators). [4] S. Klaumunzer, G. Schumacher, S. Rentzsch, G. Vogel, L. SGldner and H. Bieger, Acta Metall. 30 (1982) 1493;S. KIaumunzer and G. Schumacher, Phys. Rev. Lett. 51 (1983) 987. Hou, S. KIaumunzer and G. Schumacher, [51 Ming-Dong Nucl. Instr. and Meth. B19/20 (1987) 16. Radiat. Eff. 24 (1975) 101; 161 D. Lesueur and A. Audouard, A. Audouard, J. Dural and J.C. Jousset, J. Phys. (Paris) 41, C8 (1980) 835; A. Audouard, Rapport CEA R-5204 (1983). E. Balanzat, G. Fuchs, J.C. Jousset, D. 171 A. Audouard, Lesueur and L. Thorn&, Europhys. Lett. 5 (1988) 241. E. BaIanzat, G. Fuchs, J.C. Jousset, D. P31 A. Audouard, Lesueur and L. Thorn&, Europhys. Lett. 3 (1987) 327. E. Balanzat, G. Fuchs, J.C. Jousset, D. [91 A. Audouard, Lesueur and L. Thorn& J. Phys. Lett. F18 (1988) LlOl; A. Audouard, E. Balanzat, G. Fuchs, J.C. Jousset, D. Lesueur and L. Thorn&, these Proceedings (IBMM ‘88), Nucl. Instr. and Meth. B39 (1989) 18. WI D. Groult, J. Provost, B. Raveau, F. Studer, J.C. Jousset, S.I. Lewandowski, M. Toulemonde and F. RulIier-Albenque, Europhys. Lett. 6 (1988) 151. 1111F. Studer, J. Provost, D. Groult, B. Raveau, S. Bouffard, M. Toulemonde, J.C. Jousset and F. RuUier-Albenque, High-Temperature Superconductors, eds. M.B. Brodsky, R.C. Dynes, K. Kitazawa and H.L. Tuller, MRS Proc. vol. 99 (1988) p. 877.
[12] A.E. White, K.T. Short, D.C. Jacobson, J.M. Poate, R.C. Dynes, P.M. Mankiewich, W.J. Skocpol, R.E. Howard, M. AnzIowar, K.W. Baldwin, A.F.J. Levi, J.R. Kwo, T. Hsieh and M. Hong, to be published in Phys. Rev. B. [13] G. Fuchs, F. Studer, E. BaIanzat, D. Groult, M. Toulemonde and J.C. Jousset, Europhys. Lett. 3 (1987) 3. [14] G. Fuchs, F. Studer, E. Balanzat, D. Groult, J.C. Jousset and B. Raveau, Nucl. Instr. and Meth. B12 (1985) 471. [15] M. Toulemonde, D. Groult, N. Nguyen and F. Studer, Les Editions de Physique 1986, Magnetic Thin Films, ed. R. Krishnan, p. 121. [16] M. Toulemonde, G. Fuchs, N. Nguyen, F. Studer and D. Groult, Phys. Rev. B35 (1987) 6560. [17] P. Hansen, H. Heitman and P.M. Srnit, Phys. Rev. B26 (1982) 3539. [18] D. Groult, M. Hervieu, N. Nguyen, B. Raveau, G. Fuchs and E. Balanzat, Radiat. Eff. 90 (1982) 19. (191 C. Houpert, M. Hervieu, D. Groult, F. Studer and M. Toulemonde, Nucl. Instr. and Meth. B32 (1988) 393 (Proc. 4th Int. Conf. Radiation Effects in Insulators). [20] C. Houpert, N. Nguyen, F. Studer, D. Groult and M. Toulemonde, Nucl. Instr. and Meth. B34 (1988) 228. [21] F. Studer, H. Pascard, D. Groult, C. Houpert, N. Nguyen and M. Toulemonde, Nucl. Instr. and Meth. B32 (1988) 389 (Proc. 4th Int. Conf. Radiation Effects in Insulators). [22] A. Barbu, G. Martin, M. Toulemonde and J.C. Jousset, C.R. Acad. Sci. 299 (1984) 409. [23] A. Dunlop, L. Boulanger, D. Lesueur, N. Lorenzelli and M. Toulemonde, Mater. Sci. For. 15-18 (1987) 1117. [24] A. Iwase, S. Sasaki, I. Iwata and T. Nihira, Phys. Rev. Lett. 58 (1987) 2450. [25] A.K. Podsekin and V.N. Zaitsev, Sov. Phys. Tech. Phys. 24 (1982) 342. [26] Y.G. Chukalkin, V.V. Petrov and B.N. Goschitzkii, Phys. Status Solid A67 (1981) 421. [27] G. Fuchs, These, Rapport CEA-R-5343 (1986). [28] M. Toulemonde and F. Studer, to be published in Philos. Mag. [29] D. Groult, M. Hervieu, N. Nguyen, and B. Raveau, to be published in J. Solid State Chem. 1301 C. Houpert, F. Studer, D. Groult and M. Toulemonde, these Proceedings (IBMM ‘88) Nucl. Instr. and Meth. B39 (1989) 720. [31] D. AIbrecht, P. Armbruster, R. Spohr, M. Roth, K. Schaupert and H. Stuhrmann, Appl. Phys. A37 (1985) 37. [32] D. Albrecht, E. BaIanzat and K. Schaupert, Nucl. Tracks Radiat. Meas. 11 (1986) 93. [33] E. Dartyge, M. Lambert and M. Maurette, J. Phys. (Paris) 37 (1976) 939. [34] E. Dartyge, J.P. Duraud, Y. Langevin and M. Maurette, Phys. Rev. B23 (1981) 5213. [35] E. Dartyge and P. Sigmund, Phys. Rev. B32 (1985) 5249.