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Transformation of amorphous alloy surface and thin film under impact of slow heavy ions
Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Transformation of amorphous alloy surface and thin film under impact of slow heavy ions Romuald Brzozowski a,b, Marek Moneta a,⇑ a b
Uniwersytet Łódzki, Katedra Fizyki Ciała Stałego, Pomorska 149, PL 90-236 Łódz´, Poland Krajowe Centrum Ochrony Radiologicznej w Ochronie Zdrowia, Smugowa 6, 91-433 Łódz´, Poland
a r t i c l e
i n f o
Article history: Received 20 October 2016 Received in revised form 11 April 2017 Accepted 11 April 2017 Available online xxxx Keywords: Amorphous alloys Ion implantation Surface sputtering Surface crystallisation Conversion electrons Mössbauer spectroscopy – CEMS Particle induced X-ray emission – PIXE
a b s t r a c t In this work thin foils of amorphous alloys VP800 (Fe73 Si16 B7 Cu1 Nb3 ) and VV8025X (Fe4 Co66 B14 Cu1 Nb2 Mo1 ) maintained at the onset point temperature for primary crystallisation were irradiated with slow heavy ions (100–300 keV Ar and Xe) at the fluence changing from 1010 to 1013 ions/cm2. The preferential surface modification during ion implantation-sputtering was analysed with SRIM and PIXE. With the use of CEMS Fe and Fe(Si) clusters accompanied by Fe3Si and even Fe23 B6 nanocrystals were found in the films irradiated at a lower fluence, whereas rather amorphous structure was found in surfaces more heavily implanted. Ó 2017 Published by Elsevier B.V.
1. Introduction Amorphous alloys, far from thermodynamic equilibrium, undergo stress relaxation and partial crystallisation if some amount of energy is supplied [1–4]. These transformations under a single ion impact are related to thresholds, either in the potential energy, 10 keV deposited by a highly charged ion (HCI) or with the threshold in electronic energy loss, 5 keV/nm transferred by a swift heavy ion (SHI). This energy is 102 times faster deposited to the electronic system of the material through electronic excitations (se 1015 s), than subsequently transferred to the lattice
through the electron–phonon coupling (sa 1013 s) [5–9]. The damage creation can be described within Thermal Spike [5,6] or Coulomb Explosion models [8,9]. The pulse of energy and shock wave can cause rearrangement of target atoms and relaxation of internal stresses. The process is governed by mobility, or by thermal conductivity of electrons, which may be substantially reduced in amorphous alloys, in comparison to metals, due to some clustering in the alloys, which introduces borders. An elemental composition of the alloy is also important for structural and magnetic transformations, but preferential sputtering under the impact of heavy ions (HI) can change the content of ⇑ Corresponding author.
the surface. In case of Fe73:5 Si13:5 B9 Cu1 Nb3 (called Finemet) Cu drives crystallisation acting as a nucleation centre, whereas Nb retards a crystal growth process, producing a large number of small crystallites instead of a small number of larger ones [1]. An important task is to produce nano-crystals of a radius smaller than the exchange–correlation length. Recently, a crystallization induced by GeV Pb ions at low fluences in amorphous alloys, which exhibit a two steps thermal crystallisation (like Finemet) was reported [10], as opposed to the absence of this crystallisation in alloys going through a single step thermal crystallisation, like Fe40 Ni35 Si10 B15 . In this case, only the secondary (without primary) crystallisation phase was observed, probably in some correlation with the absence of Cu [10]. Crystallities of 1–4 nm were formed around an amorphous ion track of 6–8 nm in diameter, thus a single ion converts material from initially amorphous to other amorphus and crystalline structures roughly within 100 nm2 area. The fluence of 1011 ionscm2 gives on average 1 ion per 1000 nm2 area, so the places where ions hit the surface are well separated. The energy deposition from 5 GeV Pb ions to the Finemet through electronic stopping is 40 keV/nm [12] which is sufficient for activation of crystallisation [2,3]. The two-step crystallisation process (Fe(Si) and Fe(B)), which is characteristic for a thermally treated bulk Finemet, after the irradiation was converted into a single-step crystallisation, where the only one became the secondary crystallisation (Fe(B)). The alloys
E-mail address: [email protected] (M. Moneta). http://dx.doi.org/10.1016/j.nimb.2017.04.039 0168-583X/Ó 2017 Published by Elsevier B.V.
Please cite this article in press as: R. Brzozowski, M. Moneta, Transformation of amorphous alloy surface and thin film under impact of slow heavy ions, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.04.039
2
R. Brzozowski, M. Moneta / Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
with one-step crystallisation did not crystallise in response to irradiation [10]. In this paper it was shown that the impact of relatively slow and relatively heavy ions can also cause structural and magnetic transformations of the amorphous alloys surface, monitored by Mössbauer spectroscopy, the phenomenona up to now reserved only for slow HCI and fast HI and restricted by the energy thresholds. 2. Theoretical
Y¼
The impact of an energetic ion on the surface transfers locally an enormous amount of energy to the electron–phonon system. The description of the process within the thermal spike model [5,6] is based on the differential equations:
1 @ r ½rDe @ r T e gðT e T i Þ þ S r 1 @ t T i ¼ @ r ½rDi @ r T i gðT i T e Þ; r
ð1Þ
where T’s are the temperature distributions for electrons e and ions i respectively, D’s are diffusivities, g is the electron–phonon coupling constant of the order of 5 1018 W/K/m3 [11] and S is the source of energy. In the simplest case describing the time evolution of thermal spike temperature distribution T e ðr; tÞ the Eqs. (1) can be replaced by the heat diffusion equations in the cylindrical reference frame
k 1 @ ðr@ Tðr; tÞÞ; @ t Tðr; tÞ ¼ qc r r r
ð2Þ
where: r is the distance from z-axis selected along the ion track, c is the specific heat, k is the heat conductance and q is the volume density. Subjected to the initial condition: jdx EjdðrÞ ¼ qcTðr; 0Þ, where jdx Ej is the energy loss per unit distance, the analytical solution of Eq. (2) is
Tðr; tÞ ¼
qcr2 jdx Ej exp : 4pkt 4kt
nm at depth of Rrange ¼ 47nm in t stop ¼ 3 1013 s. It shows that boiling of Fe is possible within a cylinder of the 3.5 nm radius and melting occurs within 4.5 nm. The sputtering yield Y from the thermal spike is determined by the evaporation flux jðr; tÞ of m-mass atoms with a Maxwellian energy distribution, the energy of which exceeds the surface binding energy U s
Z
2.1. Analytical approach – Thermal Spike Model
@t T e ¼
In Fig. (1) is presented the distribution of temperature Tðr; tÞ induced by 300 keV Xe ion which stops in Fe with dx E ¼ 6 keV/
ð3Þ
2
d r
Z dtjðr; tÞ ¼
Cð3=2Þ
2
jdx Ej kB pffiffiffi pffiffiffiffiffi 24 2p3=2 k mU 3=2 s
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi kB Tðr; tÞ Us ; jðr; tÞ ¼ n Þ exp 2p m kB Tðr; tÞ
ð4Þ
ð5Þ
provided that qc 3nkB , where kB is the Boltzmann constant. The yield Y is thus inversely proportional to the surface binding energy, U 3=2 (1 eV, different for atomic species in multicomponent mates rial) which causes preferential sputtering. Initially amorphous alloys exposed to energetic heavy ions suffer modification of subsurface properties. This is caused by the deposition of a large amount of energy accompanied by selective sputtering of surface elements and unavoidable implantation of the beam ions, leading to a local formation of new structural and magnetic phases. In simple terms, the 200 keV Ar ion, which statistically stops in 2 1013 s after travelling in an alloy a projected length equal to 100 nm, transfers to electrons the power of 0.15 W, M i =Nme times more effectively than to ions, where N is the ionization degree. If we accept that the thermal conductivity of the alloy is smaller than that of the iron, 80 W/m/K, due to lower mobility of electrons caused by the presence of grains and boundaries closing atomic clusters, the temperature in the track centre will jump well above 3000 K, as shown in Fig. (1). This is higher than the melting points of all the components in the alloy, even if the sample, as a whole, is maintained in thermal contact with LN2 cryostat. The track center is cooled at 1011 K/s and solidified in the secondary amorphous phase by transferring heat to a
Fig. 1. Temperature as a function of distance r and time t from the ion track induced by impact of 300 keV Xe ion on Fe surface. The iron specific heat c = 440 J/kg/K, heat conductance k ¼ 80 W/m/K and volume density q ¼ 7870 kg/m3 are the data for bulk Fe. The Fe melting 1811 K and vaporizing 3134 K temperatures are shown by horizontal markers. The ion stops in 3 1013 s after passage of 50 nm [12].
Please cite this article in press as: R. Brzozowski, M. Moneta, Transformation of amorphous alloy surface and thin film under impact of slow heavy ions, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.04.039
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R. Brzozowski, M. Moneta / Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
2.2. Simulation – binary collisions model Simulation with SRIM [12] based on the binary collision model confirms that the composition of VP800 and VV8025X surfaces is not stable against irradiation with HI. Sputtering with 200 keV Ar ions at the normal incidence shows that a relative number of atoms removed from VP800 for instance, with one Ar ion impact, ranges from 3.1%-Cu, 2.6%-Fe and 2.1%-Si to 1.3%-Nb and 1.1%-B. At a nearly grazing incidence of 5°, the corresponding figures range from 37%-Cu, 30%-Fe and 24%-Si to 17%-Nb and 13%-B. At the same time the average energy absorbed by the atoms is: 450 eV/Fe, 440 eV/Si, 390 eV/Si, 720 eV/Nb and 330 eV/Cu, which is much more than the energy required for the initialization of crystallization 5 eV/atom. Even if the desorption energy (mean 5.2 eV), which quantifies the sputtering may be subjected to a discussion, this result means that the preferential sputtering enriches the VP surface with B and Nb, and at the same time causes deficiency in Cu and Fe with consequences on the reactions in the solid. Moreover, the density distribution of incident Ar ions reaches a maximum of 0.1 ppm (fluence 1011 ions/cm2) at the most probable penetration depth of 100 nm.
3. Experiment In this study we used samples of VP800 and VV8025X in the form of 20lm thick amorphous ribbons which were prepared by a rapid cooling (107 K/s) with the melt-spinning technique, by throwing the melt on a rotating Cu wheel in the Ar atmosphere. It was checked with an X-ray diffraction (XRD), Mössbauer spectrometry (CEMS and TMS) that the foils were initially fully amorphous. It was shown with differential scanning calorimetry (DSC) and scanning magnetometry (SM) that the alloys exhibit a two step crystallization and complicated magnetic properties [2–4]. Prior to the proper irradiation with HI, the stability of surface composition against the ion impact was analysed with PIXE based on the Cockroft-Walton accelerator with Thonemann ion source [13,14]. The X-ray spectra induced by different energy (200 keV) Ar6+ and Xe4+ ions were measured in time sequence with a spectrometer of 120 eV energy resolution at 6.4 keV in varying geometry, specifically in the grazing angle incidence-exit geometry. The samples were transferred through the air to the reaction chamber of 106 h Pa, fixed on the heater at the temperature raised to 760 K which is slightly is below the onset point for primary crystallisation and irradiated at a low current with the fluence ranging from 1010 to 1013 ions/cm2 and performed at the normal incidence under a controlled ion flux of less than 109 ions/cm2 s. A penetration depth for 200 keV Ar in VP800 is 100 nm and the rates of energy deposition into electronic and nuclear processes are Se ’ Sn ’ 778 eV/nm [12], on a time scale of 1013 s. The Xe ion of the same energy stops at 40 nm losing energy with the rates of Se ’638 eV/nm and Sn ’ 4657 eV/nm. Subsequently, the samples were again transferred through the air to the Mössbauer chamber of 109 h Pa for the ex situ CEMS measurements [2–4]. The setup enables a detection of the 6.4 keV conversion electrons from Fe contained in the alloy with a channeltron placed at 90° in reference to the incident 14.4 keV c-rays and at 45° to the sample surface.
3.1. Dynamics of surface composition under HI irradiation analysed with PIXE In order to analyse the influence of preferential sputtering on the composition of the surface subjected to HI irradiation [13,14], PIXE characteristic X-ray spectra of elements contained in the surface of amorphous alloys, were measured in time sequence, with varying geometry and in the energy range from 100 to 250 keV. The corresponding penetration depth, at the normal ion incidence, ranges approximately from 50 nm to 120 nm for Ar and from 20 nm to 40 nm for Xe ions [12]. The process of a selective surface modification during implantation was in-situ monitored with PIXE in the grazing-exit-angle geometry which suppresses the background radiation. Intensities of the characteristic X-rays emitted by elements in the surface in dependence on the dose of implanted Ar ions, sampled in time sequence were measured and presented in Fig. (2). Radiation from B was too soft to be registered by SDD detector. It was found that below the implanted fluence of about 31013 ions/cm2 there is a balanced PIXE response from all the atoms building the surface, whereas above this fluence we can see an increasing response from the accumulated Ar and Si atoms. It is suspected that other species are preferentially removed from the surface by direct sputtering or by implantation as recoils into the bulk, thus enriching the surface with elements, like Si or Ar. From the previous study [1–4] Cu is known to catalyse crystallisation, acting as nucleation centre, whereas Nb retards the crystal growth process producing a large number of small crystallites instead of a small number of larger ones. This change of the surface composition is expected to exert an influence on parallel running structural modification of the surface, due to the transfer of energy from the projectile to the electrons system. If the amount of energy exceeds locally the level of initialization, a crystallisation begins.
100.0 PIXE ka x-ray intensity from surface elements [%]
distant region through the electron–electron coupling and then by heating a lattice through the electron–phonon coupling.
220keV Ar+6 =>
Co
Fe4Co66Si12B14Nb1Mo2Cu1 in=5o out=85o Si 10.0 Ar Fe
1.0 Mo Nb Cu 0.1 0
5
10
15
20
25
30
35
40
45
50
implanted dose [1012 Ar/cm2] Fig. 2. Intensity of X-rays from elements (except for B) in the 100 nm thick subsurface region of Fe4Co66Si12B14Nb1Mo2Cu1 alloy as a function of dose of 220 keV energy Ar ions. The /in /out angles measured in respect to the surface normal. From [13].
Please cite this article in press as: R. Brzozowski, M. Moneta, Transformation of amorphous alloy surface and thin film under impact of slow heavy ions, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.04.039
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R. Brzozowski, M. Moneta / Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
3.2. Surface structure and magnetism of bulk VP800 foil before and after HI ions irradiation, analysed with CEMS The conversion electron Mössbauer spectroscopy (CEMS) spectra obtained from the primary foil and the foils irradiated with the HI fluence ranging from 1010 ions/cm2 to 1013 ions/cm2 were measured. The spectrum from the untreated foil reveals an amorphous structure which is characterised by a set of hyperfine parameters and structure factors. They were previously analysed in detail [2–4] in dependence on the temperature ranging from 70 K up to 1200 K and backwards. With the use of differential scanning calorimetry (DSC) and X-ray diffraction (XRD) after annealing at a high temperature and TMS [2] it was shown that during conventional thermal treatment a bulk VP800 experiences a set of structural and magnetic transformations correlated with each other. They begin and end at definite temperatures and are characterised by specific parameters, like activation energy for crystallisation, the Curie temperatures or hyperfine fields and isomer shifts [3,4]. The surfaces irradiated at the fluence of 1010 ions/cm2 are statistically within 1% covered with approximately 100 nm2-size spots, where ions hit the surface, whereas the spots overlapping begins at 1012 ions/cm2 causing a massive structural and magnetic destruction. This latter implanted fluence, distributed over 100 nm penetration depth, results in the average implants concentration of 1 ppm, which can influence phase transformations. A nearly half of 200 keV energy of each HI within 1013 s is transferred to electrons contained in 8 103 nm3 and subsequently shared among 5 105 atoms. It results in the increase of the lattice temperature to 3 103 K, which in turn induces magnetic and structural phase transitions of the subsurface region. The penetration depth of 100 nm for 200 keV Ar ion is comparable to the range of conversion electrons 1:5 Ee ðeVÞ0:5 130 nm, which allows for CEMS analysis of the whole irradiation region. An example of the measured CEMS spectra from VP800, in dependence on the ion fluence is shown in Fig. (3). For the
untreated foil and for the foil irradiated with Xe at 1010 ions/cm2, which corresponds to 1% coverage of the surface with ion spots, the spectra reveal only a broad Zeeman sextet ascribed to the primary amorphous phase. This phase (and amorphous remainder) is characterised mainly by distribution of the hyperfine field which reveals weak (10 T) and strong (22 T) field components related to two fundamentally different magnetic surroundings of Fe nuclei. A set of small sharp sextets, which can be seen in the Fig. (3), appears at the fluence of 1011 ions/cm2. They lose intensity at 1012 ions/cm2 and completely disappear at 1013 ions/cm2, leaving only the Zeeman sextet from amorphous remainder. The sextets are related to crystalline parts of the alloy produced under the impact of Ar ions. They were identified with CEMS coming mostly from Fe3B and Fe23B6 nanocrystals, but also from small amounts of Fe clusters and Fe3Si nanocrystals and Fe(Si) clusters of a solid solution. 4. Discussion and conclusion The previous analysis of the bulk VP800 showed that structural and magnetic properties of the alloy depend on temperature in a complicated way [3,4]. In the initially amorphous material, after thermal treatment, two principal crystallisation phases [13,15] can be determined with the use of DSC and XRD: Fe3Si which begins at 800 K and requires 460 kJ/mol of activation crystallisation energy, and Fe3B which begins at 900 K and requires 580 kJ/mol crystallisation energy. As determined by transmission Mössbauer spectroscopy (TMS) and scanning magnetometry (SM), the initial ferromagnetic properties of the amorphous bulk VP800 disappear at 600 K and appear again at 780 K as magnetism associated with crystalline structures [3,4]. It was shown that sharp CEMS sextets superimposed on the initial broad one, appear at the fluence of 1011 ions/cm2, which corresponds to 10% coverage of the surface with ion spots. They lose intensity at 1012 ions/cm2 and completely disappear at 1013 ions/ cm2, leaving only the Zeeman sextet describing the amorphous
Fig. 3. Conversion electron Mössbauer spectra CEMS, taken at RT, from the pristine surface of amorphous VP800 (black) and from the surface after irradiation with 180 keV Xe at 1011 ions/cm2 (red). The samples were present in the reaction chamber during irradiation, fixed to the sample holder and heated to 760 K. For fluences 1010 and 1012 ions/ cm2 plots of the spectra follow the black line. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Please cite this article in press as: R. Brzozowski, M. Moneta, Transformation of amorphous alloy surface and thin film under impact of slow heavy ions, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.04.039
R. Brzozowski, M. Moneta / Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
remainder. The sextets were ascribed to crystalline parts of the alloy produced under the impact of Ar ions. They were identified to come from Fe3B and Fe23B6 nanocrystals and also from Fe clusters, Fe3Si nanocrystals and Fe(Si) clusters of a solid solution embedded in a considerable amount of the amorphous matrix. It can be seen that the actual structural and magnetic composition of the surface is a result of at least two concurrent processes: the creation of structures caused by heating–cooling pulses due to energy loss by the ion and disintegration of the structures due to kinematic amorphization by HI impact. It was shown previously that certain types of amorphous metal alloys which are thermodynamically unstable, subjected to ion irradiation crystallise locally around the place of each ion impact even if the amount of energy deposited by the ion may not be sufficient to heat the spot up to the bulk crystallisation temperature. Even if the mechanism is not fully explained, the effect can be used for a controlled creation of a matrix composed of magnetic nanocrystals.
5
References [1] G. Herzer, IEEE Trans. Magn. 26 (1990) 1397. [2] R. Brzozowski, M. Wasiak, P. Uznan´ski P, P. Sovak, M. Moneta, J. Alloys Compd. 470 (2009) 5. [3] M. Antoszewska, M. Wasiak, T. Gwizdałła, P. Sovak, M. Moneta, J. Therm. Anal. Calorim. 115 (2014) 1381. [4] R. Brzozowski, M. Moneta, Nucl. Instr. Meth. Phys. Res. B297 (2012) 208. [5] J.G. Fujimoto, J.M. Liu, E.P. Ippen, N. Blomberg, Phys. Rev. Lett. 53 (1984) 1837. [6] M. Toulemonde, C. Dufour, E. Paumier, Phys. Rev. B 46 (1992) 14362. [7] F. Aumayr, S. Facsko, A.S. El-Said, C. Trautmann, M. Schleberger, J. Phys. Condens Matter 23 (2011) 393001. [8] A. Dunlop, D. Lesueur, Radiat. Eff. Defects Solids 126 (1993) 123. [9] E.M. Bringa, R.E. Johndon, Phys. Rev. Lett. 88 (2002) 165501-1. [10] A. Dunlop, G. Jaskierowicz, G. Rizza, M. Kopcewicz, Phys. Rev. Lett. 90 (2003) 015503-1. [11] C. Dufour, Z.G. Wang, E. Paumier, M. Toulemonde, Bull. Mater. Sci. 22 (1999) 671. [12] J.F. Ziegler, J.P. Biersack, SRIM2013, www.srim.org, The Stopping and Ranges of Ions in Solids, J.F. Ziegler, J.P. Biersack, M.D. Ziegler, 2008. [13] M. Antoszewska, R. Brzozowski, J. Balcerski, K. Dolecki, E. Frtczak, B. Pawłowski, M. Moneta, Nucl. Instr. Meth. Phys. Res. B310 (2013) 27. [14] M. Antoszewska-Moneta, R. Brzozowski, M. Moneta, Eur. Phys. J. D 69 (2015) 77. [15] W.Z. Chen, P.L. Ryder, Mater. Sci. Eng. B34 (1995) 204.
Please cite this article in press as: R. Brzozowski, M. Moneta, Transformation of amorphous alloy surface and thin film under impact of slow heavy ions, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.04.039
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Transformation of amorphous alloy surface and thin film under impact of slow heavy ions Romuald Brzozowski a,b, Marek Moneta a,⇑ a b
Uniwersytet Łódzki, Katedra Fizyki Ciała Stałego, Pomorska 149, PL 90-236 Łódz´, Poland Krajowe Centrum Ochrony Radiologicznej w Ochronie Zdrowia, Smugowa 6, 91-433 Łódz´, Poland
a r t i c l e
i n f o
Article history: Received 20 October 2016 Received in revised form 11 April 2017 Accepted 11 April 2017 Available online xxxx Keywords: Amorphous alloys Ion implantation Surface sputtering Surface crystallisation Conversion electrons Mössbauer spectroscopy – CEMS Particle induced X-ray emission – PIXE
a b s t r a c t In this work thin foils of amorphous alloys VP800 (Fe73 Si16 B7 Cu1 Nb3 ) and VV8025X (Fe4 Co66 B14 Cu1 Nb2 Mo1 ) maintained at the onset point temperature for primary crystallisation were irradiated with slow heavy ions (100–300 keV Ar and Xe) at the fluence changing from 1010 to 1013 ions/cm2. The preferential surface modification during ion implantation-sputtering was analysed with SRIM and PIXE. With the use of CEMS Fe and Fe(Si) clusters accompanied by Fe3Si and even Fe23 B6 nanocrystals were found in the films irradiated at a lower fluence, whereas rather amorphous structure was found in surfaces more heavily implanted. Ó 2017 Published by Elsevier B.V.
1. Introduction Amorphous alloys, far from thermodynamic equilibrium, undergo stress relaxation and partial crystallisation if some amount of energy is supplied [1–4]. These transformations under a single ion impact are related to thresholds, either in the potential energy, 10 keV deposited by a highly charged ion (HCI) or with the threshold in electronic energy loss, 5 keV/nm transferred by a swift heavy ion (SHI). This energy is 102 times faster deposited to the electronic system of the material through electronic excitations (se 1015 s), than subsequently transferred to the lattice
through the electron–phonon coupling (sa 1013 s) [5–9]. The damage creation can be described within Thermal Spike [5,6] or Coulomb Explosion models [8,9]. The pulse of energy and shock wave can cause rearrangement of target atoms and relaxation of internal stresses. The process is governed by mobility, or by thermal conductivity of electrons, which may be substantially reduced in amorphous alloys, in comparison to metals, due to some clustering in the alloys, which introduces borders. An elemental composition of the alloy is also important for structural and magnetic transformations, but preferential sputtering under the impact of heavy ions (HI) can change the content of ⇑ Corresponding author.
the surface. In case of Fe73:5 Si13:5 B9 Cu1 Nb3 (called Finemet) Cu drives crystallisation acting as a nucleation centre, whereas Nb retards a crystal growth process, producing a large number of small crystallites instead of a small number of larger ones [1]. An important task is to produce nano-crystals of a radius smaller than the exchange–correlation length. Recently, a crystallization induced by GeV Pb ions at low fluences in amorphous alloys, which exhibit a two steps thermal crystallisation (like Finemet) was reported [10], as opposed to the absence of this crystallisation in alloys going through a single step thermal crystallisation, like Fe40 Ni35 Si10 B15 . In this case, only the secondary (without primary) crystallisation phase was observed, probably in some correlation with the absence of Cu [10]. Crystallities of 1–4 nm were formed around an amorphous ion track of 6–8 nm in diameter, thus a single ion converts material from initially amorphous to other amorphus and crystalline structures roughly within 100 nm2 area. The fluence of 1011 ionscm2 gives on average 1 ion per 1000 nm2 area, so the places where ions hit the surface are well separated. The energy deposition from 5 GeV Pb ions to the Finemet through electronic stopping is 40 keV/nm [12] which is sufficient for activation of crystallisation [2,3]. The two-step crystallisation process (Fe(Si) and Fe(B)), which is characteristic for a thermally treated bulk Finemet, after the irradiation was converted into a single-step crystallisation, where the only one became the secondary crystallisation (Fe(B)). The alloys
E-mail address: [email protected] (M. Moneta). http://dx.doi.org/10.1016/j.nimb.2017.04.039 0168-583X/Ó 2017 Published by Elsevier B.V.
Please cite this article in press as: R. Brzozowski, M. Moneta, Transformation of amorphous alloy surface and thin film under impact of slow heavy ions, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.04.039
2
R. Brzozowski, M. Moneta / Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
with one-step crystallisation did not crystallise in response to irradiation [10]. In this paper it was shown that the impact of relatively slow and relatively heavy ions can also cause structural and magnetic transformations of the amorphous alloys surface, monitored by Mössbauer spectroscopy, the phenomenona up to now reserved only for slow HCI and fast HI and restricted by the energy thresholds. 2. Theoretical
Y¼
The impact of an energetic ion on the surface transfers locally an enormous amount of energy to the electron–phonon system. The description of the process within the thermal spike model [5,6] is based on the differential equations:
1 @ r ½rDe @ r T e gðT e T i Þ þ S r 1 @ t T i ¼ @ r ½rDi @ r T i gðT i T e Þ; r
ð1Þ
where T’s are the temperature distributions for electrons e and ions i respectively, D’s are diffusivities, g is the electron–phonon coupling constant of the order of 5 1018 W/K/m3 [11] and S is the source of energy. In the simplest case describing the time evolution of thermal spike temperature distribution T e ðr; tÞ the Eqs. (1) can be replaced by the heat diffusion equations in the cylindrical reference frame
k 1 @ ðr@ Tðr; tÞÞ; @ t Tðr; tÞ ¼ qc r r r
ð2Þ
where: r is the distance from z-axis selected along the ion track, c is the specific heat, k is the heat conductance and q is the volume density. Subjected to the initial condition: jdx EjdðrÞ ¼ qcTðr; 0Þ, where jdx Ej is the energy loss per unit distance, the analytical solution of Eq. (2) is
Tðr; tÞ ¼
qcr2 jdx Ej exp : 4pkt 4kt
nm at depth of Rrange ¼ 47nm in t stop ¼ 3 1013 s. It shows that boiling of Fe is possible within a cylinder of the 3.5 nm radius and melting occurs within 4.5 nm. The sputtering yield Y from the thermal spike is determined by the evaporation flux jðr; tÞ of m-mass atoms with a Maxwellian energy distribution, the energy of which exceeds the surface binding energy U s
Z
2.1. Analytical approach – Thermal Spike Model
@t T e ¼
In Fig. (1) is presented the distribution of temperature Tðr; tÞ induced by 300 keV Xe ion which stops in Fe with dx E ¼ 6 keV/
ð3Þ
2
d r
Z dtjðr; tÞ ¼
Cð3=2Þ
2
jdx Ej kB pffiffiffi pffiffiffiffiffi 24 2p3=2 k mU 3=2 s
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi kB Tðr; tÞ Us ; jðr; tÞ ¼ n Þ exp 2p m kB Tðr; tÞ
ð4Þ
ð5Þ
provided that qc 3nkB , where kB is the Boltzmann constant. The yield Y is thus inversely proportional to the surface binding energy, U 3=2 (1 eV, different for atomic species in multicomponent mates rial) which causes preferential sputtering. Initially amorphous alloys exposed to energetic heavy ions suffer modification of subsurface properties. This is caused by the deposition of a large amount of energy accompanied by selective sputtering of surface elements and unavoidable implantation of the beam ions, leading to a local formation of new structural and magnetic phases. In simple terms, the 200 keV Ar ion, which statistically stops in 2 1013 s after travelling in an alloy a projected length equal to 100 nm, transfers to electrons the power of 0.15 W, M i =Nme times more effectively than to ions, where N is the ionization degree. If we accept that the thermal conductivity of the alloy is smaller than that of the iron, 80 W/m/K, due to lower mobility of electrons caused by the presence of grains and boundaries closing atomic clusters, the temperature in the track centre will jump well above 3000 K, as shown in Fig. (1). This is higher than the melting points of all the components in the alloy, even if the sample, as a whole, is maintained in thermal contact with LN2 cryostat. The track center is cooled at 1011 K/s and solidified in the secondary amorphous phase by transferring heat to a
Fig. 1. Temperature as a function of distance r and time t from the ion track induced by impact of 300 keV Xe ion on Fe surface. The iron specific heat c = 440 J/kg/K, heat conductance k ¼ 80 W/m/K and volume density q ¼ 7870 kg/m3 are the data for bulk Fe. The Fe melting 1811 K and vaporizing 3134 K temperatures are shown by horizontal markers. The ion stops in 3 1013 s after passage of 50 nm [12].
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2.2. Simulation – binary collisions model Simulation with SRIM [12] based on the binary collision model confirms that the composition of VP800 and VV8025X surfaces is not stable against irradiation with HI. Sputtering with 200 keV Ar ions at the normal incidence shows that a relative number of atoms removed from VP800 for instance, with one Ar ion impact, ranges from 3.1%-Cu, 2.6%-Fe and 2.1%-Si to 1.3%-Nb and 1.1%-B. At a nearly grazing incidence of 5°, the corresponding figures range from 37%-Cu, 30%-Fe and 24%-Si to 17%-Nb and 13%-B. At the same time the average energy absorbed by the atoms is: 450 eV/Fe, 440 eV/Si, 390 eV/Si, 720 eV/Nb and 330 eV/Cu, which is much more than the energy required for the initialization of crystallization 5 eV/atom. Even if the desorption energy (mean 5.2 eV), which quantifies the sputtering may be subjected to a discussion, this result means that the preferential sputtering enriches the VP surface with B and Nb, and at the same time causes deficiency in Cu and Fe with consequences on the reactions in the solid. Moreover, the density distribution of incident Ar ions reaches a maximum of 0.1 ppm (fluence 1011 ions/cm2) at the most probable penetration depth of 100 nm.
3. Experiment In this study we used samples of VP800 and VV8025X in the form of 20lm thick amorphous ribbons which were prepared by a rapid cooling (107 K/s) with the melt-spinning technique, by throwing the melt on a rotating Cu wheel in the Ar atmosphere. It was checked with an X-ray diffraction (XRD), Mössbauer spectrometry (CEMS and TMS) that the foils were initially fully amorphous. It was shown with differential scanning calorimetry (DSC) and scanning magnetometry (SM) that the alloys exhibit a two step crystallization and complicated magnetic properties [2–4]. Prior to the proper irradiation with HI, the stability of surface composition against the ion impact was analysed with PIXE based on the Cockroft-Walton accelerator with Thonemann ion source [13,14]. The X-ray spectra induced by different energy (200 keV) Ar6+ and Xe4+ ions were measured in time sequence with a spectrometer of 120 eV energy resolution at 6.4 keV in varying geometry, specifically in the grazing angle incidence-exit geometry. The samples were transferred through the air to the reaction chamber of 106 h Pa, fixed on the heater at the temperature raised to 760 K which is slightly is below the onset point for primary crystallisation and irradiated at a low current with the fluence ranging from 1010 to 1013 ions/cm2 and performed at the normal incidence under a controlled ion flux of less than 109 ions/cm2 s. A penetration depth for 200 keV Ar in VP800 is 100 nm and the rates of energy deposition into electronic and nuclear processes are Se ’ Sn ’ 778 eV/nm [12], on a time scale of 1013 s. The Xe ion of the same energy stops at 40 nm losing energy with the rates of Se ’638 eV/nm and Sn ’ 4657 eV/nm. Subsequently, the samples were again transferred through the air to the Mössbauer chamber of 109 h Pa for the ex situ CEMS measurements [2–4]. The setup enables a detection of the 6.4 keV conversion electrons from Fe contained in the alloy with a channeltron placed at 90° in reference to the incident 14.4 keV c-rays and at 45° to the sample surface.
3.1. Dynamics of surface composition under HI irradiation analysed with PIXE In order to analyse the influence of preferential sputtering on the composition of the surface subjected to HI irradiation [13,14], PIXE characteristic X-ray spectra of elements contained in the surface of amorphous alloys, were measured in time sequence, with varying geometry and in the energy range from 100 to 250 keV. The corresponding penetration depth, at the normal ion incidence, ranges approximately from 50 nm to 120 nm for Ar and from 20 nm to 40 nm for Xe ions [12]. The process of a selective surface modification during implantation was in-situ monitored with PIXE in the grazing-exit-angle geometry which suppresses the background radiation. Intensities of the characteristic X-rays emitted by elements in the surface in dependence on the dose of implanted Ar ions, sampled in time sequence were measured and presented in Fig. (2). Radiation from B was too soft to be registered by SDD detector. It was found that below the implanted fluence of about 31013 ions/cm2 there is a balanced PIXE response from all the atoms building the surface, whereas above this fluence we can see an increasing response from the accumulated Ar and Si atoms. It is suspected that other species are preferentially removed from the surface by direct sputtering or by implantation as recoils into the bulk, thus enriching the surface with elements, like Si or Ar. From the previous study [1–4] Cu is known to catalyse crystallisation, acting as nucleation centre, whereas Nb retards the crystal growth process producing a large number of small crystallites instead of a small number of larger ones. This change of the surface composition is expected to exert an influence on parallel running structural modification of the surface, due to the transfer of energy from the projectile to the electrons system. If the amount of energy exceeds locally the level of initialization, a crystallisation begins.
100.0 PIXE ka x-ray intensity from surface elements [%]
distant region through the electron–electron coupling and then by heating a lattice through the electron–phonon coupling.
220keV Ar+6 =>
Co
Fe4Co66Si12B14Nb1Mo2Cu1 in=5o out=85o Si 10.0 Ar Fe
1.0 Mo Nb Cu 0.1 0
5
10
15
20
25
30
35
40
45
50
implanted dose [1012 Ar/cm2] Fig. 2. Intensity of X-rays from elements (except for B) in the 100 nm thick subsurface region of Fe4Co66Si12B14Nb1Mo2Cu1 alloy as a function of dose of 220 keV energy Ar ions. The /in /out angles measured in respect to the surface normal. From [13].
Please cite this article in press as: R. Brzozowski, M. Moneta, Transformation of amorphous alloy surface and thin film under impact of slow heavy ions, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.04.039
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3.2. Surface structure and magnetism of bulk VP800 foil before and after HI ions irradiation, analysed with CEMS The conversion electron Mössbauer spectroscopy (CEMS) spectra obtained from the primary foil and the foils irradiated with the HI fluence ranging from 1010 ions/cm2 to 1013 ions/cm2 were measured. The spectrum from the untreated foil reveals an amorphous structure which is characterised by a set of hyperfine parameters and structure factors. They were previously analysed in detail [2–4] in dependence on the temperature ranging from 70 K up to 1200 K and backwards. With the use of differential scanning calorimetry (DSC) and X-ray diffraction (XRD) after annealing at a high temperature and TMS [2] it was shown that during conventional thermal treatment a bulk VP800 experiences a set of structural and magnetic transformations correlated with each other. They begin and end at definite temperatures and are characterised by specific parameters, like activation energy for crystallisation, the Curie temperatures or hyperfine fields and isomer shifts [3,4]. The surfaces irradiated at the fluence of 1010 ions/cm2 are statistically within 1% covered with approximately 100 nm2-size spots, where ions hit the surface, whereas the spots overlapping begins at 1012 ions/cm2 causing a massive structural and magnetic destruction. This latter implanted fluence, distributed over 100 nm penetration depth, results in the average implants concentration of 1 ppm, which can influence phase transformations. A nearly half of 200 keV energy of each HI within 1013 s is transferred to electrons contained in 8 103 nm3 and subsequently shared among 5 105 atoms. It results in the increase of the lattice temperature to 3 103 K, which in turn induces magnetic and structural phase transitions of the subsurface region. The penetration depth of 100 nm for 200 keV Ar ion is comparable to the range of conversion electrons 1:5 Ee ðeVÞ0:5 130 nm, which allows for CEMS analysis of the whole irradiation region. An example of the measured CEMS spectra from VP800, in dependence on the ion fluence is shown in Fig. (3). For the
untreated foil and for the foil irradiated with Xe at 1010 ions/cm2, which corresponds to 1% coverage of the surface with ion spots, the spectra reveal only a broad Zeeman sextet ascribed to the primary amorphous phase. This phase (and amorphous remainder) is characterised mainly by distribution of the hyperfine field which reveals weak (10 T) and strong (22 T) field components related to two fundamentally different magnetic surroundings of Fe nuclei. A set of small sharp sextets, which can be seen in the Fig. (3), appears at the fluence of 1011 ions/cm2. They lose intensity at 1012 ions/cm2 and completely disappear at 1013 ions/cm2, leaving only the Zeeman sextet from amorphous remainder. The sextets are related to crystalline parts of the alloy produced under the impact of Ar ions. They were identified with CEMS coming mostly from Fe3B and Fe23B6 nanocrystals, but also from small amounts of Fe clusters and Fe3Si nanocrystals and Fe(Si) clusters of a solid solution. 4. Discussion and conclusion The previous analysis of the bulk VP800 showed that structural and magnetic properties of the alloy depend on temperature in a complicated way [3,4]. In the initially amorphous material, after thermal treatment, two principal crystallisation phases [13,15] can be determined with the use of DSC and XRD: Fe3Si which begins at 800 K and requires 460 kJ/mol of activation crystallisation energy, and Fe3B which begins at 900 K and requires 580 kJ/mol crystallisation energy. As determined by transmission Mössbauer spectroscopy (TMS) and scanning magnetometry (SM), the initial ferromagnetic properties of the amorphous bulk VP800 disappear at 600 K and appear again at 780 K as magnetism associated with crystalline structures [3,4]. It was shown that sharp CEMS sextets superimposed on the initial broad one, appear at the fluence of 1011 ions/cm2, which corresponds to 10% coverage of the surface with ion spots. They lose intensity at 1012 ions/cm2 and completely disappear at 1013 ions/ cm2, leaving only the Zeeman sextet describing the amorphous
Fig. 3. Conversion electron Mössbauer spectra CEMS, taken at RT, from the pristine surface of amorphous VP800 (black) and from the surface after irradiation with 180 keV Xe at 1011 ions/cm2 (red). The samples were present in the reaction chamber during irradiation, fixed to the sample holder and heated to 760 K. For fluences 1010 and 1012 ions/ cm2 plots of the spectra follow the black line. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Please cite this article in press as: R. Brzozowski, M. Moneta, Transformation of amorphous alloy surface and thin film under impact of slow heavy ions, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.04.039
R. Brzozowski, M. Moneta / Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
remainder. The sextets were ascribed to crystalline parts of the alloy produced under the impact of Ar ions. They were identified to come from Fe3B and Fe23B6 nanocrystals and also from Fe clusters, Fe3Si nanocrystals and Fe(Si) clusters of a solid solution embedded in a considerable amount of the amorphous matrix. It can be seen that the actual structural and magnetic composition of the surface is a result of at least two concurrent processes: the creation of structures caused by heating–cooling pulses due to energy loss by the ion and disintegration of the structures due to kinematic amorphization by HI impact. It was shown previously that certain types of amorphous metal alloys which are thermodynamically unstable, subjected to ion irradiation crystallise locally around the place of each ion impact even if the amount of energy deposited by the ion may not be sufficient to heat the spot up to the bulk crystallisation temperature. Even if the mechanism is not fully explained, the effect can be used for a controlled creation of a matrix composed of magnetic nanocrystals.
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Please cite this article in press as: R. Brzozowski, M. Moneta, Transformation of amorphous alloy surface and thin film under impact of slow heavy ions, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.04.039