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Structural modifications of crystalline and amorphous Ni3B irradiated with high-energy heavy ions
414
Nuclear Instruments and Methods in Physics Research B59/60 (1991) 414-417 Noah-Holland
Structural modifications of crystalline and amorphous Ni 3B irradiated with high-energy heavy ions A. Audouard Seruice des Champs Magnitudes
Intenses, Iaboratoire
de Physique des Solides, INSA, Avenue de Rangueil, 31077 Toulouse, France
E. Balanzat, S. Bouffard and J.C. Jousset CIRIL,
Rue Claude Bloch R. I? 5133, 14040 Caen Cedex, France
A. Chamberod Centre &Etudes Nucleaires de Grenoble, Departement
de Recherche Fondamentale,
38041 Grenoble, France
A. Dunlop and D. Lesueur IRDI/DMECN/DTECH-SESI,
Laboratoire
des Solides Irradies, Ecole Polytechnique,
91 I28 Palaiseau
Cedex, France
G. Fuchs Departement
de Physique des Matkriaux,
Universite Claude Bernard, Bd du Ii Novembre
1918, 69622 Villeurbanne,
France
R. Spohr and J. Vetter Gesellschaft fur Schwerionenforschung
Postfach 110552, 6100 Darmstadt
11, German,y
L. Thorn6 Centre de SpectromPtrie
Nucleaire et de Specirometrie
de Masse, IN2P3-CNRS,
BcEIiment 108, 91405 &say
Campus, France
Crystalline and amorphous Ni,B ribbons have been irradiated at low temperature with GeV heavy ions in order to study the structural modifications induced in metallic alloys by ion electronic energy loss. The atomic rearrangements produced by irradiation were characterized via in situ electrical resistance measurements and room-temperature X-ray and electron diffraction. Amorphous Ni,B behaves as the other amorphous alloys already investigated: creation of disorder with a cross section increasing with the ion electronic stopping power, followed by an anisotropic growth of the sample dimensions. Crystalline Ni,B shows a new and very interesting behavior: a huge electrical resistivity increase above an electronic energy loss threshold attributed to amo~~zation of the irradiated alloy.
Recent electrical resistance and length measurements performed on a large variety of metallic systems irradiated with high-energy heavy ions [1] have demonstrated that ion electronic energy loss [(d Efdn),] leads in these materials to important atomic rearrangements strongly depending on the structure of the target. Amorphous metallic alloys exhibit [2-S]: fi) a damage production with a cross section up to two orders of magnitude higher than that of elastic collisions: (ii) a huge anisotropic growth of the target dimensions perpendicular to 0168-583X/91/$03.50
the ion beam direction. Crystalline pure metals or metallic alloys do not present the plastic deformation occurring in glasses, but are nevertheless more or less sensitive to ion electronic energy loss depending on the ability of their electronic system to couple to the lattice atoms [9-141. For instance, an enhancement of the damage rate has been observed at high electronic excitation densities in pure Fe [10,13] and Ga [ll] and in Ni,Fe [12-141. It was then of major interest to study the effect of (d E/dx), in a system which (i) offers the possibility of being synthesized in both the crystalline and the
0 1991 - Elsevier Science Publishers B.V. (Noah-Holl~d)
A. Audouard et al. / Structural modifications of c- and a-Ni, B
amorphous state at the same nominal composition, (ii) is known to be easily upset by ion bombardment. The metal-metalloid Ni,B alloy presents the above characteristics. The disordering behavior of Ni,B by elastic collisions was widely investigated in the last decade [15-171. The results revealed that low-energy ion irradiation induces radiation damage in a-Ni,B [16], while c-Ni,B undergoes a crystalline-to-amorphous transition above - 0.1 displacement per atom [15,17). On the other hand, recent electrical resistance experiments on c-Ni,B irradiated with 3 GeV Xe ions showed only a weak effect of (dE/dx), up to a value of the electronic stopping power of 3.7 keV/A (181. This latter result can be accounted for by considering that a critical (dE/dx), threshold exists for the damaging process. Irradiation with ions of various atomic numbers from Kr to U, which provide a wide range of (dE/dx),, was performed to better underst~d the disorde~ng behavior caused in metallic alloys by swift heavy-ion irradiation.
0.15
.015
0.10
.01
AR R,
0.05
00s
0
2.0
.4
1.5
.3
1.0
.2
0.5
.i
0 0
1 ION FLUENCE
Amorphous and polycrystalline 20 pm thick Ni,B ribbons were prepared by sputtering and (for the crystalline ones) annealing for 12 h at 470°C. They were irradiated at low temperature (10 or 80 K) in the IBABAT facility with GeV heavy ions, either at GANIL-Caen (Kr and Xe) or at GSI-Darmstadt (U). Stacks of total thicknesses much smaller than the range of the irradiating ions allow to vary the mean value of (dE/dx), inside the samples from 1.2 to 7.2 keV/.&. Electrical resistance measurements were performed in situ during beam stops all along the irradiation using a standard four-probe technique. At the end of irradiation, the samples were heated up to room temperature and analyzed by X-ray and electron diffraction.
415
2
3
4
~10'*ions/cm2)
Fig. 1. Relative electrical resistance variation AR/R, vs ion fluence for (a) amorphous and (b) crystalline NisB ribbons irradiated at low temperature with various high-energy heavy ions. Full lines are best fits to experimental electrical resistance data using eq. (1) (a-NisB) or eq. (2) (c-NisB) of the text. Note that the ordinate scale has been divided by a factor of 10 for Kr irradiation data in the case of a-NisB and by a factor of 5 for Kr and Xe irradiation data in the case of c-Ni sB.
equation, representing the relative resistance variation of a ribbon irradiated at a fluence @, was derived: AR -= Ro
$$$&, f-
a
+ ra[l - exp( --s&i)]
sd sd +$a
'd-'a-+) (
X[l-exP[-(Sd+%j@~]y
3. Results and discussion Fig. 1 presents typical electrical resistance variations of the irradiated ribbons with the ion fluence for the various irradiations performed. The data registered on amorphous Ni,B can be accounted for by considering that the ribbon resistance increase is due to both a resistivity variation and an anisotropic growth as in the case of a-Fe,,B,, irradiated with high-energy heavy ions [4,5,7]. In a previous paper [18], a quantitative description of the effect observed during GeV Xe irradiation was attempted, based on the assumption that: (i) an incoming ion creates a cylindrical volume of damaged matter along its path with a cross section sd; (ii) part of the created disorder can be annealed by a subsequent ion impact with a cross section s,, leading to sample growth. The following
(1)
where r, and r, are the relative resistivity variations [);.= (p, - po)/pO] of the damaged and annealed regions, respectively, and n is a parameter which accounts for the degree of anisotropy of the atomic movements leading to the growth. Fits of eq. (1) to the expe~mental data of fig. la were attempted keeping the values of the parameters intrinsic to the target r,, r, and q constant from one irradiation to the other. However, due to the large number of parameters entering eq. (l), different sets of values were found to well reproduce the experimental data. As a consequence, only the quantities ( rdsd) and [ qsds.J(sd + s,)], which represent respectively the initial and final slopes of the damage production curves AR( @)/R,, can be unambiguously determined. It has in this respect to be noted that these two quantities (reported in table 1 and fig. 2a) are nothing but the resistivity increase and growth crosssections. The results show that these cross-sections preIV. CRYSTALLIZATION/AMORPHIZATION
A. A&ward et aI. / Structural modifications of c- and a-Ni,B
416 sent a similar behaviour
as a function of the ion electronic energy loss, especially a threshold at - 2 lceV/A. An observation of the crystalline Ni,B ribbons at the end of irradiation indicates that the huge deformation (due to the anisotropic growth) observed in a-Ni,B has not occurred in its crystalline counterpart, in agreement with previous results reported on crystalline metal-metal alloys [lP]_ The large value of AR/R, obtained following U irradiation (> 2 at a fluence of 4 X 101* ions cmv2 ) can consequently be essentially ascribed to a resistivity increase due to creation of disorder by the ion beam. The general eq. (1) used to describe the resistance increase of a-Ni,B ribbons can be simplified in the case of c-Ni,B by considering that (i) no growth occurs (n = 0) during irradiation, and (ii) r, = 0 for crystalline alloys (181, leading to: AR -z-z
Ro
Ap PO
%[I-exp[-(Sd+~,)gll.
(2)
With this equation, the product r,s, and the sum .sd+ s, can be determined by fitting the data as far as the irradiation fluence is high enough for the quantity (sd + s,)@ to be non-negligible when compared to unity, i.e. as the resistance variation presents an exponential saturation at high fluence (Xe irradiation). In the case where this condition is not fulfilled (Kr and U irradiation), only r,sd (which corresponds to the initial slope of the resistance increase) and an upper bound of sd + s, (considering that (s, + s,)@ * 1) can be provided. Table 1 collects the values of the parameters thus obtained. Fig. 2b presents the variation of rdsd with the ion electronic stopping power. The fact that the value of rdsd sharply increases by one order of magnitude at 7.2 keV/& while the value of sd + s, strongly decreases at the same (dE/dx),, provides a clear indication that a different disordering mechanism takes place above an electronic energy loss threshold located between 3.7 and
0
0
2 fdE/dXf,
4
6 (k&/i1
Table 1 Parameters extracted from the fits to experimental electrical resistance data using eq. (1) (a-Ni,B) or eq. (2) (c-NisB) of the text Ion a-Ni,B Kr Xe Xe Xe U c-Ni,B Kr Xe Xe Xe U
(dv=), [keV/A]
‘dsd
1.28 2.80 3.20 3.70 7.22
Loxlo-’ 1.0x 10-14 2.1x10-‘4 4.3x10-14 7.9x10-14
1.23 2.80 3.20 3.70 7.22
7.6x10-16 1.0x10-‘4 1.6x10-‘4 5.7x10-14 5.6x10-13
[cm2l
3
[cm21
1.3x10-‘6 1.1 x 10-14 1.9x10-‘4 2.7x10-l4 3.3 x lo- I4
7.2 keV/A. The conclusion of amorphous phase formation at high (d E/dx), is then suggested by the observation that the resistivity of the U-irradiated ribbon at the end of irradiation (44 u&Jcm) is not far from that of its amorphous counterpart (80 l& cm) and is still linearly increasing with the U fluence. This conclusion is reinforced by the measurement of the temperature coefficient of the resistivity [TCR = (l/~s~~)(Ap/AhT)] between 80 and 300 K which exhibits a strong decrease from 2.4 x 10e3 to 1.3 x lop3 K-’ before and after irradiation with U ions. The resistivity results reported above are fully confirmed by X-ray and electron diffraction experiments performed on the irradiated samples after annealing at room temperature. While no amorphous regions can be detected on unirradiated samples or on c-Ni,B irradiated with GeV Xe ions [18], diffraction spectra recorded
-0
(cJE/dxf,
rkaV/‘il
Fig. 2. Variation of rdsd (circles) and ns&/(rd + sa) (triangles) extracted from the fits of experimental data presented in fig. 1 versus the ion electronic energy loss for (a) amorphous and (b) crystalline Ni,B. Lines serve only the purpose to guide the eye.
A. Audouurd et al. / Structural modifications of c- and a-Ni,B
417
in agreement with the results obtained on the other crystalline systems investigated [19]. Nevertheless, damage creation by (d E/dn), occurs as in a-NisB. The combination of electrical resistance, X-ray and electron diffraction results allows to conclude that amorphization of the irradiated layer is achieved at high electronic excitation (above - 4 keV/A). This unexpected result, owing to the idea widely held a few years ago that free electrons should very rapidly and efficiently smear out the perturbation caused by the high-energy ion beam in metallic systems, merits further in-depth studies.
References [l] For a review, see: L. Thorn& Condensed Matter Studies
by Nuclear Methods, Roe. 25th Zakopane School on Physics, ed. J. Stanek and T. Pedziwiatr (World Scientific, Singapore, 1990) p. 178. [2) S. Klaumtinzer and G. Schumacher, Phys. Rev. Lett. 51 (1983) 1987. [3]_ S. Klaumbnxer, M.-D. Hou and G. Schumacher, Phys. _
Fig. 3. Transmission-electron diffraction patterns of (a) virgin crystalline NisB; (b) the same ribbon irradiated at 80 K with 3 GeV U ions at a fluence of 4 X lo’* ions cm-’ and annealed at room temperature. on the U-irradiated sample (see fig. 3) exhibit patterns characteristic of a mixture of crystalline and amorphous phases.
4. Conclusions The experiments on amorphous and crystalline Ni,B alloys irradiated with high-energy heavy ions reported in this article allow to draw the main conclusion that electronic energy loss can lead to important atomic rearrangements in metallic systems. It was also demonstrated that the structure of the irradiated target (crystalline or amorphous) plays a fundamental role regard-
ing the structural modifications induced by (dE/dx),. In a-Ni,B, electronic excitation leads to defect production at the beginning of the irradiation, followed by the large anisotropic growth of the sample dimensions perpendicular to the ion beam direction already observed in many amorphous metallic or nonmetallic systems [20]. The amplitude of both the initial disordering and the growth increases with the ion electronic energy loss. No growth phenomenon has been detected in c-Ni,B,
Rev. Lett. 57 (1986) 850. [4] A. Audouard, E. Balanzat, G. Fuchs, J.C. Jousset, D. Lesueur and L. Thorn&, Europhys. Lett. 3 (1987) 327. [5] A. Audouard, E. Balanzat, G. Fuchs, J.C. Jousset, D. Lesueur and L. Thorn&, Europhys. Lett. 5 (1988) 241. [6] S. Klaumiinzer, Radiat. Eff. Def. Sol. 110 (1989) 79. [7] A. Audouard, E. Balanzat, J.C. Jousset, G. Fuchs, D. Lesueur and L. Thorn& Radiat. Eff. Def. Sol. 110 (1989) 109. [8] M.-D. Hou, S. Klaumlinzer and G. Schumacher, Phys. Rev. B41 (1990) 1444. [9] A. Iwase, S. Sasaki, T. Iwata and T. Nihira, Phys. Rev. Lett. 58 (1987) 2450. [lo] A. Dunlop, D. Lesueur, J. Morillo, J. Dural, R. Spohr and J. Vetter, C.R. Acad. Sci. Paris 309 (1989) 1277. [ll] E. Parker, M. Toulemonde, J. Dural, F. Rullier-Albenque, J.P. Girard and P. Bogdanski, Europhys. Lett. 10 (1989) 555. 1121A. Audouard et al., Radiat. Eff. Def. Sol. 110 (1989) 113. [13] A. Dunlop, D. Lesueur, J. Morillo, J. Dural, R. Spohr and J. Vetter, Nucl. Instr. and Meth. B48 (1990) 419. [14] A. Dunlop, D. Lesueur, N. Lorenzelli, A. Audouard, C. Dimitrov, J. M. Ramillon and L. Thorn& J. Phys. Cond. Matt. 2 (1990) 1733. [15] A. Benyagoub, J.C. Pivin, F. Pons and L. Thorn&, Phys. Rev. B34 (1986) 4464. [16] A. Audouard, A. Benyagoub and L. Thorn& Phys. Rev. B3f (1987) 8308. [17] L. Thorn&, F. Pons, E. Ligeon, J. Fontenille and R. Danielou, J. Appl. Phys. 63 (1988) 722. 1181 A. Audouard, E. Balanzat, J.C. Jousset, A. Chamberod, G. Fuchs, D. Lcsucur and L. Thorn& Phil. Mag. A (1991) in press. [19] S. Klaumtinrer, M.-D. Hou, G. Schumachei and C. Li, Mater. Res. Symp. Proc. 93 (1987) 21. [20] S. Klaumtinzer, C. Li, S. Laffler, M. Rammensee, G. Schumacher and H.C. Neitzert, Radiat. Eff. Dcf. Sol. 108 (1989) 131. IV. CRYSTALLIZATION/AMORPHIZATlON
Nuclear Instruments and Methods in Physics Research B59/60 (1991) 414-417 Noah-Holland
Structural modifications of crystalline and amorphous Ni 3B irradiated with high-energy heavy ions A. Audouard Seruice des Champs Magnitudes
Intenses, Iaboratoire
de Physique des Solides, INSA, Avenue de Rangueil, 31077 Toulouse, France
E. Balanzat, S. Bouffard and J.C. Jousset CIRIL,
Rue Claude Bloch R. I? 5133, 14040 Caen Cedex, France
A. Chamberod Centre &Etudes Nucleaires de Grenoble, Departement
de Recherche Fondamentale,
38041 Grenoble, France
A. Dunlop and D. Lesueur IRDI/DMECN/DTECH-SESI,
Laboratoire
des Solides Irradies, Ecole Polytechnique,
91 I28 Palaiseau
Cedex, France
G. Fuchs Departement
de Physique des Matkriaux,
Universite Claude Bernard, Bd du Ii Novembre
1918, 69622 Villeurbanne,
France
R. Spohr and J. Vetter Gesellschaft fur Schwerionenforschung
Postfach 110552, 6100 Darmstadt
11, German,y
L. Thorn6 Centre de SpectromPtrie
Nucleaire et de Specirometrie
de Masse, IN2P3-CNRS,
BcEIiment 108, 91405 &say
Campus, France
Crystalline and amorphous Ni,B ribbons have been irradiated at low temperature with GeV heavy ions in order to study the structural modifications induced in metallic alloys by ion electronic energy loss. The atomic rearrangements produced by irradiation were characterized via in situ electrical resistance measurements and room-temperature X-ray and electron diffraction. Amorphous Ni,B behaves as the other amorphous alloys already investigated: creation of disorder with a cross section increasing with the ion electronic stopping power, followed by an anisotropic growth of the sample dimensions. Crystalline Ni,B shows a new and very interesting behavior: a huge electrical resistivity increase above an electronic energy loss threshold attributed to amo~~zation of the irradiated alloy.
Recent electrical resistance and length measurements performed on a large variety of metallic systems irradiated with high-energy heavy ions [1] have demonstrated that ion electronic energy loss [(d Efdn),] leads in these materials to important atomic rearrangements strongly depending on the structure of the target. Amorphous metallic alloys exhibit [2-S]: fi) a damage production with a cross section up to two orders of magnitude higher than that of elastic collisions: (ii) a huge anisotropic growth of the target dimensions perpendicular to 0168-583X/91/$03.50
the ion beam direction. Crystalline pure metals or metallic alloys do not present the plastic deformation occurring in glasses, but are nevertheless more or less sensitive to ion electronic energy loss depending on the ability of their electronic system to couple to the lattice atoms [9-141. For instance, an enhancement of the damage rate has been observed at high electronic excitation densities in pure Fe [10,13] and Ga [ll] and in Ni,Fe [12-141. It was then of major interest to study the effect of (d E/dx), in a system which (i) offers the possibility of being synthesized in both the crystalline and the
0 1991 - Elsevier Science Publishers B.V. (Noah-Holl~d)
A. Audouard et al. / Structural modifications of c- and a-Ni, B
amorphous state at the same nominal composition, (ii) is known to be easily upset by ion bombardment. The metal-metalloid Ni,B alloy presents the above characteristics. The disordering behavior of Ni,B by elastic collisions was widely investigated in the last decade [15-171. The results revealed that low-energy ion irradiation induces radiation damage in a-Ni,B [16], while c-Ni,B undergoes a crystalline-to-amorphous transition above - 0.1 displacement per atom [15,17). On the other hand, recent electrical resistance experiments on c-Ni,B irradiated with 3 GeV Xe ions showed only a weak effect of (dE/dx), up to a value of the electronic stopping power of 3.7 keV/A (181. This latter result can be accounted for by considering that a critical (dE/dx), threshold exists for the damaging process. Irradiation with ions of various atomic numbers from Kr to U, which provide a wide range of (dE/dx),, was performed to better underst~d the disorde~ng behavior caused in metallic alloys by swift heavy-ion irradiation.
0.15
.015
0.10
.01
AR R,
0.05
00s
0
2.0
.4
1.5
.3
1.0
.2
0.5
.i
0 0
1 ION FLUENCE
Amorphous and polycrystalline 20 pm thick Ni,B ribbons were prepared by sputtering and (for the crystalline ones) annealing for 12 h at 470°C. They were irradiated at low temperature (10 or 80 K) in the IBABAT facility with GeV heavy ions, either at GANIL-Caen (Kr and Xe) or at GSI-Darmstadt (U). Stacks of total thicknesses much smaller than the range of the irradiating ions allow to vary the mean value of (dE/dx), inside the samples from 1.2 to 7.2 keV/.&. Electrical resistance measurements were performed in situ during beam stops all along the irradiation using a standard four-probe technique. At the end of irradiation, the samples were heated up to room temperature and analyzed by X-ray and electron diffraction.
415
2
3
4
~10'*ions/cm2)
Fig. 1. Relative electrical resistance variation AR/R, vs ion fluence for (a) amorphous and (b) crystalline NisB ribbons irradiated at low temperature with various high-energy heavy ions. Full lines are best fits to experimental electrical resistance data using eq. (1) (a-NisB) or eq. (2) (c-NisB) of the text. Note that the ordinate scale has been divided by a factor of 10 for Kr irradiation data in the case of a-NisB and by a factor of 5 for Kr and Xe irradiation data in the case of c-Ni sB.
equation, representing the relative resistance variation of a ribbon irradiated at a fluence @, was derived: AR -= Ro
$$$&, f-
a
+ ra[l - exp( --s&i)]
sd sd +$a
'd-'a-+) (
X[l-exP[-(Sd+%j@~]y
3. Results and discussion Fig. 1 presents typical electrical resistance variations of the irradiated ribbons with the ion fluence for the various irradiations performed. The data registered on amorphous Ni,B can be accounted for by considering that the ribbon resistance increase is due to both a resistivity variation and an anisotropic growth as in the case of a-Fe,,B,, irradiated with high-energy heavy ions [4,5,7]. In a previous paper [18], a quantitative description of the effect observed during GeV Xe irradiation was attempted, based on the assumption that: (i) an incoming ion creates a cylindrical volume of damaged matter along its path with a cross section sd; (ii) part of the created disorder can be annealed by a subsequent ion impact with a cross section s,, leading to sample growth. The following
(1)
where r, and r, are the relative resistivity variations [);.= (p, - po)/pO] of the damaged and annealed regions, respectively, and n is a parameter which accounts for the degree of anisotropy of the atomic movements leading to the growth. Fits of eq. (1) to the expe~mental data of fig. la were attempted keeping the values of the parameters intrinsic to the target r,, r, and q constant from one irradiation to the other. However, due to the large number of parameters entering eq. (l), different sets of values were found to well reproduce the experimental data. As a consequence, only the quantities ( rdsd) and [ qsds.J(sd + s,)], which represent respectively the initial and final slopes of the damage production curves AR( @)/R,, can be unambiguously determined. It has in this respect to be noted that these two quantities (reported in table 1 and fig. 2a) are nothing but the resistivity increase and growth crosssections. The results show that these cross-sections preIV. CRYSTALLIZATION/AMORPHIZATION
A. A&ward et aI. / Structural modifications of c- and a-Ni,B
416 sent a similar behaviour
as a function of the ion electronic energy loss, especially a threshold at - 2 lceV/A. An observation of the crystalline Ni,B ribbons at the end of irradiation indicates that the huge deformation (due to the anisotropic growth) observed in a-Ni,B has not occurred in its crystalline counterpart, in agreement with previous results reported on crystalline metal-metal alloys [lP]_ The large value of AR/R, obtained following U irradiation (> 2 at a fluence of 4 X 101* ions cmv2 ) can consequently be essentially ascribed to a resistivity increase due to creation of disorder by the ion beam. The general eq. (1) used to describe the resistance increase of a-Ni,B ribbons can be simplified in the case of c-Ni,B by considering that (i) no growth occurs (n = 0) during irradiation, and (ii) r, = 0 for crystalline alloys (181, leading to: AR -z-z
Ro
Ap PO
%[I-exp[-(Sd+~,)gll.
(2)
With this equation, the product r,s, and the sum .sd+ s, can be determined by fitting the data as far as the irradiation fluence is high enough for the quantity (sd + s,)@ to be non-negligible when compared to unity, i.e. as the resistance variation presents an exponential saturation at high fluence (Xe irradiation). In the case where this condition is not fulfilled (Kr and U irradiation), only r,sd (which corresponds to the initial slope of the resistance increase) and an upper bound of sd + s, (considering that (s, + s,)@ * 1) can be provided. Table 1 collects the values of the parameters thus obtained. Fig. 2b presents the variation of rdsd with the ion electronic stopping power. The fact that the value of rdsd sharply increases by one order of magnitude at 7.2 keV/& while the value of sd + s, strongly decreases at the same (dE/dx),, provides a clear indication that a different disordering mechanism takes place above an electronic energy loss threshold located between 3.7 and
0
0
2 fdE/dXf,
4
6 (k&/i1
Table 1 Parameters extracted from the fits to experimental electrical resistance data using eq. (1) (a-Ni,B) or eq. (2) (c-NisB) of the text Ion a-Ni,B Kr Xe Xe Xe U c-Ni,B Kr Xe Xe Xe U
(dv=), [keV/A]
‘dsd
1.28 2.80 3.20 3.70 7.22
Loxlo-’ 1.0x 10-14 2.1x10-‘4 4.3x10-14 7.9x10-14
1.23 2.80 3.20 3.70 7.22
7.6x10-16 1.0x10-‘4 1.6x10-‘4 5.7x10-14 5.6x10-13
[cm2l
3
[cm21
1.3x10-‘6 1.1 x 10-14 1.9x10-‘4 2.7x10-l4 3.3 x lo- I4
7.2 keV/A. The conclusion of amorphous phase formation at high (d E/dx), is then suggested by the observation that the resistivity of the U-irradiated ribbon at the end of irradiation (44 u&Jcm) is not far from that of its amorphous counterpart (80 l& cm) and is still linearly increasing with the U fluence. This conclusion is reinforced by the measurement of the temperature coefficient of the resistivity [TCR = (l/~s~~)(Ap/AhT)] between 80 and 300 K which exhibits a strong decrease from 2.4 x 10e3 to 1.3 x lop3 K-’ before and after irradiation with U ions. The resistivity results reported above are fully confirmed by X-ray and electron diffraction experiments performed on the irradiated samples after annealing at room temperature. While no amorphous regions can be detected on unirradiated samples or on c-Ni,B irradiated with GeV Xe ions [18], diffraction spectra recorded
-0
(cJE/dxf,
rkaV/‘il
Fig. 2. Variation of rdsd (circles) and ns&/(rd + sa) (triangles) extracted from the fits of experimental data presented in fig. 1 versus the ion electronic energy loss for (a) amorphous and (b) crystalline Ni,B. Lines serve only the purpose to guide the eye.
A. Audouurd et al. / Structural modifications of c- and a-Ni,B
417
in agreement with the results obtained on the other crystalline systems investigated [19]. Nevertheless, damage creation by (d E/dn), occurs as in a-NisB. The combination of electrical resistance, X-ray and electron diffraction results allows to conclude that amorphization of the irradiated layer is achieved at high electronic excitation (above - 4 keV/A). This unexpected result, owing to the idea widely held a few years ago that free electrons should very rapidly and efficiently smear out the perturbation caused by the high-energy ion beam in metallic systems, merits further in-depth studies.
References [l] For a review, see: L. Thorn& Condensed Matter Studies
by Nuclear Methods, Roe. 25th Zakopane School on Physics, ed. J. Stanek and T. Pedziwiatr (World Scientific, Singapore, 1990) p. 178. [2) S. Klaumtinzer and G. Schumacher, Phys. Rev. Lett. 51 (1983) 1987. [3]_ S. Klaumbnxer, M.-D. Hou and G. Schumacher, Phys. _
Fig. 3. Transmission-electron diffraction patterns of (a) virgin crystalline NisB; (b) the same ribbon irradiated at 80 K with 3 GeV U ions at a fluence of 4 X lo’* ions cm-’ and annealed at room temperature. on the U-irradiated sample (see fig. 3) exhibit patterns characteristic of a mixture of crystalline and amorphous phases.
4. Conclusions The experiments on amorphous and crystalline Ni,B alloys irradiated with high-energy heavy ions reported in this article allow to draw the main conclusion that electronic energy loss can lead to important atomic rearrangements in metallic systems. It was also demonstrated that the structure of the irradiated target (crystalline or amorphous) plays a fundamental role regard-
ing the structural modifications induced by (dE/dx),. In a-Ni,B, electronic excitation leads to defect production at the beginning of the irradiation, followed by the large anisotropic growth of the sample dimensions perpendicular to the ion beam direction already observed in many amorphous metallic or nonmetallic systems [20]. The amplitude of both the initial disordering and the growth increases with the ion electronic energy loss. No growth phenomenon has been detected in c-Ni,B,
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