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On short-range ordering in Fe–Ni alloys
Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 401–404
On short-range ordering in Fe–Ni alloys N.V. Lomova a , I.N. Shabanova a,b,∗ , A.G. Chirkov b , A.G. Ponomaryov a a
Physical–Technical Institute, Ural Branch of RAS, 132 Kirov Str., Izhevsk 426001, Russia b Udmurt State University, Izhevsk, Russia Available online 22 December 2006
Abstract In the work, the X-ray electron spectroscopy and molecular dynamics methods were used for investigating the electron and atomic structure of Fe–Ni alloys in order to explain the variations in their short-range order. The comparative study was carried out for the Fe–Ni system with the invar (Fe65 Ni35 ) and non-invar (Fe25 Ni75 ) compositions at two different temperatures (with heating). The presence of the invar effect in the Fe–Ni alloys can be related to strong local distortions in the fcc lattice leading to the redistributions of atoms in the nearest environment. © 2007 Elsevier B.V. All rights reserved. Keywords: X-ray photoelectron spectroscopy (XPS); Invar alloys; Electronic structure; Core level; Multiplet splitting; Valence band
1. Introduction There are hardly any other substances with such strong anomalies in most physical properties like the alloys of the invar group. They have got their name owing to a very low temperature coefficient of linear expansion [1]. At present, there are many hypotheses explaining the nature of the invar state. However, despite the attempts of researchers, the question concerning the invar nature has not been solved yet. The important task is to establish the connection between the physical properties and the electron structure of the invar alloys. The invar anomaly of properties takes place in the temperature range lower than Curie point. Consequently, the invar nature should be connected with magnetism. Since the anomalies in the lattice structure are related to specific features of the electronic structure, the anomalies in the magnetic and other properties are due to the peculiarity of the electronic structure. The X-ray photoelectron spectroscopy method was used for investigating the specific features of the 3s-core level spectra and the valence band spectra in order to obtain data on the electron structure, the nearest environment of atoms and the changes of the spin magnetic moment of metal atoms in the invar alloys. The investigations were carried out on the X-ray electron 30-cm magnetic spectrometer with double focusing by the non-uniform magnetic field of the axis symmetry [2]. The
∗
Corresponding author. Tel.: +7 3412432539; fax: +7 3412250614. E-mail address: [email protected] (I.N. Shabanova).
0368-2048/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2006.12.034
Al K␣-line (1486.6 eV) is used as exciting radiation; the spectrometer resolution is 10−4 ; the accuracy of the peak position determination is ∼0.1 eV. The spectrometer is equipped with a special adapter allowing to perform mechanical cleaning of samples in the spectrometer chamber and to change a sample temperature. To obtain the clean sample surface, the mechanical cleaning (with a tungsten brush) was carried out in vacuum of 10−8 Pa. The quality of cleanness was checked by the spectra of oxygen and carbon (O 1s and C 1s) and the 2p-spectra of metals. In the Fe 2p and Ni 2p spectra, oxide maxima are absent. The samples are prepared from the mixture of pure components by the method of induction smelting in argon atmosphere. Fe25 Ni75 and Fe65 Ni35 are ferromagnetic alloys (TC = 873 K for Fe25 Ni75 and TC = 500 K for Fe65 Ni35 ). The Fe–Ni alloy samples are polycrystalline. To interpret the spectra obtained from the binary alloys, the reference samples of pure Fe and Ni metals are used. The additional information on the atomic magnetic moment and the localization of the d-electron density can be obtained from studying the multiplet splitting of the X-ray electron 3s-spectra of d-metals, which appears due to the exchange interaction between the electron spins of the unfilled 3d-shell and ionized 3s-shell. After a 3s-electron has been emitted, another unpaired electron appears. The exchange interaction takes place only for the electrons (3s–3d) with parallel spins, which leads to the energy that is lower than in the case of anti-parallel spins. As the result, in the 3s-spectra of d-metals there are two final states. Their maxima intensities (I1 , I2 ) are determined by the number of uncompensated 3d-electrons [3]. We have established the dependence of the ratio of the maxima intensities (I2 /I1 ) of the
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N.V. Lomova et al. / Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 401–404
multiplet splitting on the spin moment of d-electrons (s/(s + 1)), which is close to linear I2 /I1 = 0.9s/(s + 1) for most systems [4]. The exception is the systems, where strong relaxation effects and configurational interactions take place. The probability of these effects is high for the systems with a large portion of the covalent component in the chemical bond [5]. The distance between the peaks of the multiplet splitting E is the energy of the exchange interaction and it is characterized by the exchange integral depending on the overlapping of the 3s–3d-shells. In contrast to pure metals, in alloys, the changes in the multiplet splitting parameters of the 3s-spectra of d-metals gives the information on the tendencies of the changes in the atomic magnetic moments and the localization of the 3d-electron density. The structure of the multiplet splitting of the 3s-spectra of metals reflects the nearest environment of the metal atoms. 2. Results and discussion Fig. 1 shows the photoelectron 3s-spectra of iron in the Fe65 Ni35 alloy (2) and of pure Fe (1) at room temperature and after heating to the temperature of T ∼ 450 K (Fig. 1(3)). The investigation of the specific features of the M 3s-spectrum structure (M is metal) in the Fe–Ni systems shows the presence of three maxima in the Fe 3s-spectra in the Fe65 Ni35 system in contrast to the pure iron and the Fe25 Ni75 alloy (Fig. 1(4)), where there are two maxima in the Fe 3s-spectrum. This can be explained either by the presence of two multiplet splittings (a and b maxima – the first multiplet splitting, and a and b maxima – the second multiplet splitting) in the Fe 3s spectrum or by two spin states of the Fe atoms in the invar alloy. The main maximum (a) contains contributions from two multiplet splittings. The maximum (b) is the second maximum of the first multiplet splitting. Two multiplet splittings appear in the Fe 3sspectrum because the iron atoms in the Fe65 Ni35 invar alloy are in two non-equivalent positions. These atoms differ in their nearest environments and localization of the electron density of the d-states, and, consequently, they have different magnetic moments. Once E has been determined by the Fe 3s-spectrum of the Fe65 Ni35 system, one can say that one state is formed by the Fe atoms and has more localized d-states (E1 = 3.9 eV, close to pure ␣-Fe), and the other is formed due to d–d interaction with the neighboring atoms and has less localized d-states (E2 = 6.5 eV). This confirms the hypothesis of Tauer and Weiss [6] on the influence of the local environment on the magnetic moments of the iron atoms in the FeNi invar alloys and on the existence of two positions or two magnetic states for the Fe atoms in the fcc lattice of the invar alloys. The first multiplet splitting reflects high-spin state of the iron atoms, which is intrinsic to ␣-Fe and similar to the splitting in the Fe 3s-spectrum of pure iron in its parameters. Same atoms are presumably present in the environment of these Fe atoms. The second multiplet splitting (a , b ) is characteristics of those iron atoms, which have the smaller distance between the neighboring atoms and the interatomic interaction between them is larger, therefore, low-spin state of the Fe atoms appears (with a small magnetic moment) as in the case of ␥-Fe. In the environment of these atoms, there are Ni and Fe atoms.
Fig. 1. X-ray electronic Fe 3s-spectra of: 1, metallic Fe; 2, Fe65 Ni35 alloy at Troom ; 3, Fe65 Ni35 at T ∼ 450 K; 4, Fe25 Ni75 alloy at Troom ; 5, Fe25 Ni75 at T ∼ 450 K.
For the Fe65 Ni35 invar alloy, with heating (Fig. 1(3)), the intensity of the third component in the Fe 3s spectrum is growing due to a decrease of the intensity of the second peak, i.e., the number of the micro-regions grows, where the distance between the atoms is smaller and the interaction between the neighboring atoms is larger. In other words, the number of the iron atoms in the low-spin state is growing. This is related to the transition of a portion of the iron atoms from a high-spin state into a lowspin state. An iron atom in a high-spin state has a larger volume than in a low-spin state [7]. An anomaly low thermal expansion of alloys occurs due to this fact. An increase in the disordering region is characteristic of the invar alloys. In the Ni 3s spectrum (Fig. 2) for the Fe65 Ni35 invar alloy, no significant changes due to heating are found. In Figs. 1(4 and 5) and 2(4 and 5), one can see the Fe 3s and Ni 3s spectra for the Fe25 Ni75 non-invar alloy. One multiplet splitting is characteristic of the Fe 3s spectrum of this system (a, b), which does not practically change at heating.
N.V. Lomova et al. / Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 401–404
Fig. 2. X-ray electronic Ni 3s-spectra of: 1, metallic Ni; 2, Fe65 Ni35 alloy at Troom ; 3, Fe65 Ni35 at T ∼ 450 K; 4, Fe25 Ni75 alloy at Troom ; 5, Fe25 Ni75 at T ∼ 450 K.
In order to determine what atoms are in the environment of the iron atoms in various groupings, the valence band spectra were studied. Fig. 3 displays the valence band spectra of pure metals and of the Fe65 Ni35 and Fe25 Ni75 alloys at Troom and at heating. The structure of the X-ray electron valence-band spectrum of the alloy reflects the distribution of the d-state density of metals (due to the photo-ionization section of d-electrons that is more than one order larger compared with that of s- and p-electrons). Therefore, the Fe65 Ni35 valence band should mainly reflect the distribution of the density of the iron d-states since the number
403
Fig. 3. The valence band XPS spectra of: 1, Fe65 Ni35 at Troom ; 2, Fe65 Ni35 at T ∼ 450 K; 3, Fe25 Ni75 at Troom ; 4, Fe25 Ni75 at T ∼ 450 K; 5, metallic Fe; 6, metallic Ni.
of the Fe atoms is nearly twice as large as the number of the Ni atoms in the Fe65 Ni35 composition. However, from the comparison of the valence band spectrum of the invar alloy (Fig. 3(1)) with the valence band spectra for pure Fe (Fig. 3(5)) and Ni (Fig. 3(6)) it is seen that in the spectrum shape, in addition to the well-expressed maximum from pure Fe, there are d-states of Ni (first and third maxima). The electron structure of the Fe65 Ni35 alloy indicates the presence of Fe–Fe and Fe–Ni bonds at room temperature. At heating, the valence band and 3s-core level spectra of metals in the Fe65 Ni35 invar alloy undergo certain changes. In the
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N.V. Lomova et al. / Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 401–404
valence-band spectrum shape of the Fe65 Ni35 alloy (Fig. 3(2)), the Ni structure becomes more expressed on the background of the less expressed Fe structure at heating. This is possible when there is an overlapping of the d-wave functions of Fe and Ni and the Ni atoms are involved into a hybridized bond. Consequently, in the invar alloy some changes take place in the nearest environment of atoms at heating, which indicate the formation of the predominant Fe–Ni bond. For the Fe25 Ni75 non-invar alloy, at room temperature and at T ∼ 450 K (Fig. 3(3 and 4)) the spectra are identical and they show that only the Ni atoms are in the environment of the Fe atoms. The close similarity between the valence-band spectrum shape of the Fe25 Ni75 alloy and the valence-band spectrum shape of pure Ni indicates the presence of the Ni atoms being surrounded with the same atoms. The offered interpretation of the reasons causing the changes in the XPS-spectra is in agreement with conclusions drawn during the analysis of the atomic structure, which was obtained by the molecular dynamics modelling of the investigated Fe–Ni alloys. The calculation was performed using the program complex “MDSEAM” and many-particle potentials of the interatomic interaction. Earlier in work [8], within the framework of the embedded atom method, the potentials Fe1−x Nix have been calculated, which we use for the fcc Fe–Ni solutions up to Curie temperature and above it. Molecular dynamics modelling shows that at room temperature, the fcc lattice of the solid solution of Fe65 Ni35 is strongly distorted and disordered, which leads to the appearance of different types of short-range ordering: Fe–Fe and Fe–Ni. It seems that it is due to a strong anharmonicity of the system, which is characterized by a low-frequency (soft) mode in the alloy phonon spectrum that corresponds to 0.01 eV (0.03 × 1014 Hz) (Fig. 4). At heating, the lattice relaxation takes place, strong local distortions are removed, which leads to the change of the nearest environment. Low-frequency oscillations disappear at 450 K. The partial functions of the radial distribution of atoms
Fig. 5. The density of the vibrational states of atoms in Fe65 Ni35 at 1, 300 K; 2, 450 K.
(RDF) are obtained. The RDF for Fe–Fe and Ni–Ni do not significantly change at heating to 450 K. At heating above 300 K, in the RDF of Fe–Ni, additional peaks appear at the distances of ´˚ which correspond to the first and the second coor3.5 and 5.2 A, dination spheres (Fig. 5). It evidences the change in the nearest environment of the Fe atoms. For the Fe25 Ni75 non-invar alloy, no changes are observed in the phonon spectrum and in the RDF shape at heating. 3. Conclusion Based on the results obtained, the following conclusions can be drawn: 1. In the Fe65 Ni35 invar alloy, two non-equivalent states of iron atoms differing from each other in their magnetic moments are found in the fcc lattice. It is due to different nearest environments and chemical bonds of the iron atoms. 2. At heating, a certain part of atoms in the high-spin state, surrounding mainly the Fe atoms, passes into the low-spin state, and there are mainly the Ni-atoms in their surrounding. 3. In the Fe–Ni alloys, the presence of the invar effect can be related to strong local distortions in the fcc lattice leading to the redistributions of atoms in the nearest environment. References
Fig. 4. The partial function of radial distribution of atoms Fe–Ni in the Fe65 Ni35 alloy at 1, 300 K; 2, 450 K.
[1] R.A. Reck, Phys. Rev. B 9 (1974) 2381. [2] I.N. Shabanova, V.A. Trapeznikov, L.V. Dobysheva, et al., Izvestia AN SSSR: seria fizicheskaya 50 (1986) 16. [3] D. Briggs, M.P. Seach (Eds.), Practical Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy, John Wiley & Sons, 1983, p. 598. [4] I.N. Shabanova, N.V. Keller, Surf. Interface Anal. 32 (2001) 114. [5] B.W. Veah, A.P. Paulikas, Phys. Rev. Lett. 51 (1983) 1995. [6] K.J. Tauer, R.J. Weiss, Bull. Am. Phys. Soc. 6 (1961) 125. [7] S. Chikazumi, Magn. Magn. Mater. 15–18 (1980) 1130. [8] R. Meyer, P. Entel, Phys. Rev. B 57 (1998) 5140.
On short-range ordering in Fe–Ni alloys N.V. Lomova a , I.N. Shabanova a,b,∗ , A.G. Chirkov b , A.G. Ponomaryov a a
Physical–Technical Institute, Ural Branch of RAS, 132 Kirov Str., Izhevsk 426001, Russia b Udmurt State University, Izhevsk, Russia Available online 22 December 2006
Abstract In the work, the X-ray electron spectroscopy and molecular dynamics methods were used for investigating the electron and atomic structure of Fe–Ni alloys in order to explain the variations in their short-range order. The comparative study was carried out for the Fe–Ni system with the invar (Fe65 Ni35 ) and non-invar (Fe25 Ni75 ) compositions at two different temperatures (with heating). The presence of the invar effect in the Fe–Ni alloys can be related to strong local distortions in the fcc lattice leading to the redistributions of atoms in the nearest environment. © 2007 Elsevier B.V. All rights reserved. Keywords: X-ray photoelectron spectroscopy (XPS); Invar alloys; Electronic structure; Core level; Multiplet splitting; Valence band
1. Introduction There are hardly any other substances with such strong anomalies in most physical properties like the alloys of the invar group. They have got their name owing to a very low temperature coefficient of linear expansion [1]. At present, there are many hypotheses explaining the nature of the invar state. However, despite the attempts of researchers, the question concerning the invar nature has not been solved yet. The important task is to establish the connection between the physical properties and the electron structure of the invar alloys. The invar anomaly of properties takes place in the temperature range lower than Curie point. Consequently, the invar nature should be connected with magnetism. Since the anomalies in the lattice structure are related to specific features of the electronic structure, the anomalies in the magnetic and other properties are due to the peculiarity of the electronic structure. The X-ray photoelectron spectroscopy method was used for investigating the specific features of the 3s-core level spectra and the valence band spectra in order to obtain data on the electron structure, the nearest environment of atoms and the changes of the spin magnetic moment of metal atoms in the invar alloys. The investigations were carried out on the X-ray electron 30-cm magnetic spectrometer with double focusing by the non-uniform magnetic field of the axis symmetry [2]. The
∗
Corresponding author. Tel.: +7 3412432539; fax: +7 3412250614. E-mail address: [email protected] (I.N. Shabanova).
0368-2048/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2006.12.034
Al K␣-line (1486.6 eV) is used as exciting radiation; the spectrometer resolution is 10−4 ; the accuracy of the peak position determination is ∼0.1 eV. The spectrometer is equipped with a special adapter allowing to perform mechanical cleaning of samples in the spectrometer chamber and to change a sample temperature. To obtain the clean sample surface, the mechanical cleaning (with a tungsten brush) was carried out in vacuum of 10−8 Pa. The quality of cleanness was checked by the spectra of oxygen and carbon (O 1s and C 1s) and the 2p-spectra of metals. In the Fe 2p and Ni 2p spectra, oxide maxima are absent. The samples are prepared from the mixture of pure components by the method of induction smelting in argon atmosphere. Fe25 Ni75 and Fe65 Ni35 are ferromagnetic alloys (TC = 873 K for Fe25 Ni75 and TC = 500 K for Fe65 Ni35 ). The Fe–Ni alloy samples are polycrystalline. To interpret the spectra obtained from the binary alloys, the reference samples of pure Fe and Ni metals are used. The additional information on the atomic magnetic moment and the localization of the d-electron density can be obtained from studying the multiplet splitting of the X-ray electron 3s-spectra of d-metals, which appears due to the exchange interaction between the electron spins of the unfilled 3d-shell and ionized 3s-shell. After a 3s-electron has been emitted, another unpaired electron appears. The exchange interaction takes place only for the electrons (3s–3d) with parallel spins, which leads to the energy that is lower than in the case of anti-parallel spins. As the result, in the 3s-spectra of d-metals there are two final states. Their maxima intensities (I1 , I2 ) are determined by the number of uncompensated 3d-electrons [3]. We have established the dependence of the ratio of the maxima intensities (I2 /I1 ) of the
402
N.V. Lomova et al. / Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 401–404
multiplet splitting on the spin moment of d-electrons (s/(s + 1)), which is close to linear I2 /I1 = 0.9s/(s + 1) for most systems [4]. The exception is the systems, where strong relaxation effects and configurational interactions take place. The probability of these effects is high for the systems with a large portion of the covalent component in the chemical bond [5]. The distance between the peaks of the multiplet splitting E is the energy of the exchange interaction and it is characterized by the exchange integral depending on the overlapping of the 3s–3d-shells. In contrast to pure metals, in alloys, the changes in the multiplet splitting parameters of the 3s-spectra of d-metals gives the information on the tendencies of the changes in the atomic magnetic moments and the localization of the 3d-electron density. The structure of the multiplet splitting of the 3s-spectra of metals reflects the nearest environment of the metal atoms. 2. Results and discussion Fig. 1 shows the photoelectron 3s-spectra of iron in the Fe65 Ni35 alloy (2) and of pure Fe (1) at room temperature and after heating to the temperature of T ∼ 450 K (Fig. 1(3)). The investigation of the specific features of the M 3s-spectrum structure (M is metal) in the Fe–Ni systems shows the presence of three maxima in the Fe 3s-spectra in the Fe65 Ni35 system in contrast to the pure iron and the Fe25 Ni75 alloy (Fig. 1(4)), where there are two maxima in the Fe 3s-spectrum. This can be explained either by the presence of two multiplet splittings (a and b maxima – the first multiplet splitting, and a and b maxima – the second multiplet splitting) in the Fe 3s spectrum or by two spin states of the Fe atoms in the invar alloy. The main maximum (a) contains contributions from two multiplet splittings. The maximum (b) is the second maximum of the first multiplet splitting. Two multiplet splittings appear in the Fe 3sspectrum because the iron atoms in the Fe65 Ni35 invar alloy are in two non-equivalent positions. These atoms differ in their nearest environments and localization of the electron density of the d-states, and, consequently, they have different magnetic moments. Once E has been determined by the Fe 3s-spectrum of the Fe65 Ni35 system, one can say that one state is formed by the Fe atoms and has more localized d-states (E1 = 3.9 eV, close to pure ␣-Fe), and the other is formed due to d–d interaction with the neighboring atoms and has less localized d-states (E2 = 6.5 eV). This confirms the hypothesis of Tauer and Weiss [6] on the influence of the local environment on the magnetic moments of the iron atoms in the FeNi invar alloys and on the existence of two positions or two magnetic states for the Fe atoms in the fcc lattice of the invar alloys. The first multiplet splitting reflects high-spin state of the iron atoms, which is intrinsic to ␣-Fe and similar to the splitting in the Fe 3s-spectrum of pure iron in its parameters. Same atoms are presumably present in the environment of these Fe atoms. The second multiplet splitting (a , b ) is characteristics of those iron atoms, which have the smaller distance between the neighboring atoms and the interatomic interaction between them is larger, therefore, low-spin state of the Fe atoms appears (with a small magnetic moment) as in the case of ␥-Fe. In the environment of these atoms, there are Ni and Fe atoms.
Fig. 1. X-ray electronic Fe 3s-spectra of: 1, metallic Fe; 2, Fe65 Ni35 alloy at Troom ; 3, Fe65 Ni35 at T ∼ 450 K; 4, Fe25 Ni75 alloy at Troom ; 5, Fe25 Ni75 at T ∼ 450 K.
For the Fe65 Ni35 invar alloy, with heating (Fig. 1(3)), the intensity of the third component in the Fe 3s spectrum is growing due to a decrease of the intensity of the second peak, i.e., the number of the micro-regions grows, where the distance between the atoms is smaller and the interaction between the neighboring atoms is larger. In other words, the number of the iron atoms in the low-spin state is growing. This is related to the transition of a portion of the iron atoms from a high-spin state into a lowspin state. An iron atom in a high-spin state has a larger volume than in a low-spin state [7]. An anomaly low thermal expansion of alloys occurs due to this fact. An increase in the disordering region is characteristic of the invar alloys. In the Ni 3s spectrum (Fig. 2) for the Fe65 Ni35 invar alloy, no significant changes due to heating are found. In Figs. 1(4 and 5) and 2(4 and 5), one can see the Fe 3s and Ni 3s spectra for the Fe25 Ni75 non-invar alloy. One multiplet splitting is characteristic of the Fe 3s spectrum of this system (a, b), which does not practically change at heating.
N.V. Lomova et al. / Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 401–404
Fig. 2. X-ray electronic Ni 3s-spectra of: 1, metallic Ni; 2, Fe65 Ni35 alloy at Troom ; 3, Fe65 Ni35 at T ∼ 450 K; 4, Fe25 Ni75 alloy at Troom ; 5, Fe25 Ni75 at T ∼ 450 K.
In order to determine what atoms are in the environment of the iron atoms in various groupings, the valence band spectra were studied. Fig. 3 displays the valence band spectra of pure metals and of the Fe65 Ni35 and Fe25 Ni75 alloys at Troom and at heating. The structure of the X-ray electron valence-band spectrum of the alloy reflects the distribution of the d-state density of metals (due to the photo-ionization section of d-electrons that is more than one order larger compared with that of s- and p-electrons). Therefore, the Fe65 Ni35 valence band should mainly reflect the distribution of the density of the iron d-states since the number
403
Fig. 3. The valence band XPS spectra of: 1, Fe65 Ni35 at Troom ; 2, Fe65 Ni35 at T ∼ 450 K; 3, Fe25 Ni75 at Troom ; 4, Fe25 Ni75 at T ∼ 450 K; 5, metallic Fe; 6, metallic Ni.
of the Fe atoms is nearly twice as large as the number of the Ni atoms in the Fe65 Ni35 composition. However, from the comparison of the valence band spectrum of the invar alloy (Fig. 3(1)) with the valence band spectra for pure Fe (Fig. 3(5)) and Ni (Fig. 3(6)) it is seen that in the spectrum shape, in addition to the well-expressed maximum from pure Fe, there are d-states of Ni (first and third maxima). The electron structure of the Fe65 Ni35 alloy indicates the presence of Fe–Fe and Fe–Ni bonds at room temperature. At heating, the valence band and 3s-core level spectra of metals in the Fe65 Ni35 invar alloy undergo certain changes. In the
404
N.V. Lomova et al. / Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 401–404
valence-band spectrum shape of the Fe65 Ni35 alloy (Fig. 3(2)), the Ni structure becomes more expressed on the background of the less expressed Fe structure at heating. This is possible when there is an overlapping of the d-wave functions of Fe and Ni and the Ni atoms are involved into a hybridized bond. Consequently, in the invar alloy some changes take place in the nearest environment of atoms at heating, which indicate the formation of the predominant Fe–Ni bond. For the Fe25 Ni75 non-invar alloy, at room temperature and at T ∼ 450 K (Fig. 3(3 and 4)) the spectra are identical and they show that only the Ni atoms are in the environment of the Fe atoms. The close similarity between the valence-band spectrum shape of the Fe25 Ni75 alloy and the valence-band spectrum shape of pure Ni indicates the presence of the Ni atoms being surrounded with the same atoms. The offered interpretation of the reasons causing the changes in the XPS-spectra is in agreement with conclusions drawn during the analysis of the atomic structure, which was obtained by the molecular dynamics modelling of the investigated Fe–Ni alloys. The calculation was performed using the program complex “MDSEAM” and many-particle potentials of the interatomic interaction. Earlier in work [8], within the framework of the embedded atom method, the potentials Fe1−x Nix have been calculated, which we use for the fcc Fe–Ni solutions up to Curie temperature and above it. Molecular dynamics modelling shows that at room temperature, the fcc lattice of the solid solution of Fe65 Ni35 is strongly distorted and disordered, which leads to the appearance of different types of short-range ordering: Fe–Fe and Fe–Ni. It seems that it is due to a strong anharmonicity of the system, which is characterized by a low-frequency (soft) mode in the alloy phonon spectrum that corresponds to 0.01 eV (0.03 × 1014 Hz) (Fig. 4). At heating, the lattice relaxation takes place, strong local distortions are removed, which leads to the change of the nearest environment. Low-frequency oscillations disappear at 450 K. The partial functions of the radial distribution of atoms
Fig. 5. The density of the vibrational states of atoms in Fe65 Ni35 at 1, 300 K; 2, 450 K.
(RDF) are obtained. The RDF for Fe–Fe and Ni–Ni do not significantly change at heating to 450 K. At heating above 300 K, in the RDF of Fe–Ni, additional peaks appear at the distances of ´˚ which correspond to the first and the second coor3.5 and 5.2 A, dination spheres (Fig. 5). It evidences the change in the nearest environment of the Fe atoms. For the Fe25 Ni75 non-invar alloy, no changes are observed in the phonon spectrum and in the RDF shape at heating. 3. Conclusion Based on the results obtained, the following conclusions can be drawn: 1. In the Fe65 Ni35 invar alloy, two non-equivalent states of iron atoms differing from each other in their magnetic moments are found in the fcc lattice. It is due to different nearest environments and chemical bonds of the iron atoms. 2. At heating, a certain part of atoms in the high-spin state, surrounding mainly the Fe atoms, passes into the low-spin state, and there are mainly the Ni-atoms in their surrounding. 3. In the Fe–Ni alloys, the presence of the invar effect can be related to strong local distortions in the fcc lattice leading to the redistributions of atoms in the nearest environment. References
Fig. 4. The partial function of radial distribution of atoms Fe–Ni in the Fe65 Ni35 alloy at 1, 300 K; 2, 450 K.
[1] R.A. Reck, Phys. Rev. B 9 (1974) 2381. [2] I.N. Shabanova, V.A. Trapeznikov, L.V. Dobysheva, et al., Izvestia AN SSSR: seria fizicheskaya 50 (1986) 16. [3] D. Briggs, M.P. Seach (Eds.), Practical Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy, John Wiley & Sons, 1983, p. 598. [4] I.N. Shabanova, N.V. Keller, Surf. Interface Anal. 32 (2001) 114. [5] B.W. Veah, A.P. Paulikas, Phys. Rev. Lett. 51 (1983) 1995. [6] K.J. Tauer, R.J. Weiss, Bull. Am. Phys. Soc. 6 (1961) 125. [7] S. Chikazumi, Magn. Magn. Mater. 15–18 (1980) 1130. [8] R. Meyer, P. Entel, Phys. Rev. B 57 (1998) 5140.