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Exchange correlation and magnetism in bcc Fe0.8 Ni0.2 alloy
Journal Pre-proof Exchange correlation and magnetism in bcc Fe0.8 Ni0.2 alloy S.S. Acharya, K. Bapna, K. Ali, D. Biswas, R. Rawat, V.R.R. Medicherla, Kalobaran Maiti
PII:
S0368-2048(20)30001-3
DOI:
https://doi.org/10.1016/j.elspec.2020.146933
Reference:
ELSPEC 146933
To appear in:
Journal of Electron Spectroscopy and Related Phenomena
Received Date:
14 June 2019
Revised Date:
16 January 2020
Accepted Date:
17 January 2020
Please cite this article as: S.S. Acharya, K. Bapna, K. Ali, D. Biswas, R. Rawat, V.R.R. Medicherla, Kalobaran Maiti, Exchange correlation and magnetism in bcc Fe0.8Ni0.2 alloy, (2020), doi: https://doi.org/10.1016/j.elspec.2020.146933
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier.
Exchange correlation and magnetism in bcc Fe0.8 Ni0.2 alloy S. S. Acharya1 , K. Bapna2 , K. Ali2 , D. Biswas2 , R. Rawat3 , V. R. R. Medicherla1∗ and Kalobaran Maiti2 1
Department of Physics, ITER, Siksha ’O’ Anusandhan Deemed to be University, Bhubaneswar 751030, India 2 Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Homi Bhabha Road,Colaba, Mumbai 400005, India and 3 UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452017, India (Dated: January 16, 2020)
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We investigate the magnetism and the electronic structure of body centred cubic, Fe0.8 Ni0.2 alloy. While the magnetic moment of pure Fe and the alloy in bcc phase are almost similar, the hysteresis loop exhibits soft magnetic behavior. Transport measurements exhibit hysteresis in cooling and heating cycles indicating possibilities of structural change in the alloy. Photoemission results exhibit signature of correlation induced effect in the core level spectra. The interaction parameters associated to the Ni electronic states are found to be influenced by the alloying presumably due to disorder induced effect. Fe 2s core level spectra exhibit three distinct peaks due to exchange splitting and correlation of Fe 3d electrons. The exchange splitting seems to remain unchanged in the temperature range studied in the alloy. Evidently, electron correlation and disorder is important to derive the complex electronic properties of this material. Our results establish that the magnetism and transport properties of Fe can be influenced significantly keeping the Fe moment unchanged via alloying with Ni, which is important for the applications of this material. Further studies are required to understand the resistivity anomaly and soft magnetism of this alloy. PACS numbers: 71.20.-b, 71.10.Hf, 79.60.-i
INTRODUCTION
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Fe-Ni alloys exhibit unique magnetic, elastic and structural properties and play a key role in cutting edge microelectronic and construction materials. The Fe1−x Nix alloy system exhibits complicated magnetic phase diagram1–5 . Fe-Ni alloys with Ni concentrations up to 25% form in bcc structure (α phase) and the alloys transform to fcc structure (γ phase) above 25% Ni concentration. Both α and γ phases form disordered alloys by metastable quenching. The fcc phase exhibits complex magnetic behavior and is known to exist in different magnetic states1 similar to pure fcc Fe6,7 . fcc Fe-Ni alloys have been extensively studied due to the invar property8 exhibited by the alloys with nickel concentration around 35%. Recently many researchers focused their attention on Fe-Ni invar alloys as it was suggested that the invar property is related to non collinear magnetic structures in the alloys9–11 . In Fe-Ni alloys, the Ni magnetic moment is about 0.6-0.7µB and that of Fe is about 2.5-2.7µB 12,13 .
The physical properties of Fe1−x Nix alloys are highly sensitive to method of preparation and temperature of annealing19 . Keeping this in mind, we study the electronic structure of FeNi alloy formed in bcc structure employing x-ray photoemission spectroscopy (XPS) and compare it with the electronic structure of pure Fe and Ni.
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Fe0.8 Ni0.2 alloy was prepared by co-melting high purity (99.99%) Fe and Ni metals in an arc furnace under high purity argon atmosphere. The alloy was homogenized by vacuum annealing at 350 o C for two days; the temperature of annealing was kept purposefully low to avoid the formation of compounds. The strtucture of the alloy was studied using x-ray diffraction (XRD) technique employing Cu Kα x-ray radiation. The temperature dependence of resistivity of the alloy was measured in both cooling and heating cycles using standard four probe method down to 10K. The field and temperature dependance of magnetization was measured down to 10K using SQUID magnetometer. The XPS measurements were carried out using a PHOIBOS 150 Analyser and monochromatic Al Kα x-ray (1486.6 eV) from specs GmbH at an experimental pressure better than 4.0×10−10 Torr. The energy resolution of the spectrometer was set in the range of 0.4 - 0.7 eV depending on the signal strength and signal to noise ratio. The melt grown alloys were extremely hard and was impossible to cleave and/or fracture inside the ultrahigh vacuum chamber. Therefore to produce a clean surface, the sample was polished in ultrahigh vacuum condition with a fine grain diamond file. Cleanliness of the surface was ensured by the intensity of O 1s and C 1s signals. Reproducibility of the spectra were confirmed after each cycle of scraping. The XPS spectra were recorded at temperatures 300 K
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Though the γ phase Fe-Ni alloys have been extensively studied, the studies on α phase alloys are scarce14–16 . The inner core of the earth is known to predominantly consist of α (bcc) phase of Fe-Ni alloy. The investigations on iron meteorites and cosmochemical data suggested 515% Ni concentration in the inner core of earth17,18 . At high pressures (above 225 GPa) and high temperatures (over 3400 K), Fe0.9 Ni0.1 alloy possesses bcc structure17 . Thus, the understanding of the electronic structure of bcc Fe-Ni alloy can help us to reveal the underlying physics of these materials which form the core of earth as well as extraterrestrial materials. Moreover, Fe-Ni alloys have extensive application in technology such as in magnetic recording, making watches and cryogenic dewars, etc.
2 cooled (FC) cycles is shown in figure 1(d) and exhibits a marginal difference at low temperatures. Evidently, the magnetism and therefore, the electron interaction parameters have changed significantly with the alloying.
and 10 K using an open cycle helium cryostat. 0.2
300
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FIG. 2. The survey scans of Fe, Fe0.8 Ni0.2 and Ni recorded at 10 K and 300 K temperatures using monochromatic Al Kα radiation. All the features correspond to the energy levels of the constituent elements.
In order to investigate the underlying physics behind such phenomenon, we have studied the electronic structure of this material employing photoemission spectroscopy. The high quality of the sample and the surface cleanliness is demonstrated in the survey scans shown in Fig. 2; the spectra recorded on Fe0.8 Ni0.2 alloy at 10 K and 300 K temperatures are shown in figures 2(a) and 2(b), respectively. The survey scans of pure Fe and Ni are also shown for reference. The binding energy was calibrated using the Fermi level derived from the photoemission spectra of clean Ag. All the features observed in the spectra have been identified and indexed as shown in the figures - they all correspond to the core level spectra of the sample. The survey scans at both the temperatures exhibit insignificant intensity for C 1s and O 1s core levels establishing a clean surface - the impurity level is below the detection level. All the core levels of Fe and Ni, valence band and some of the intense Auger features like L3 M23 M23 and L3 M45 M45 of both Fe and Ni are marked in the figure. In Figs. 3(a) and 3(b), we show the XPS spectra of Ni 2p and Fe 2s regions recorded at 10 K and 300 K temperatures for Fe0.8 Ni0.2 alloy. In pure Ni sample, the Ni 2p3/2 peak appears at a binding energy of 852.7 eV and Ni 2p1/2 at 870.0 eV with a spin orbit splitting of about 17.3 eV. The splitting remains nearly same for pure Ni and the alloy. Each of the features exhibits a distinct satellite peak at higher binding energies (∼ 5.7 eV away from the main peak), which is often called poorly screened feature while the intense lower binding energy feature is called the well screened feature. These distinct features appear due to finite electron-electron Coulomb
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Binding Energy (eV)
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In Fig. 1(a), we show the powder x-ray diffraction (XRD) pattern of Fe0.8 Ni0.2 alloy obtained using Cu Kα radiation; the experimental data has been indexed considering bcc structure. We observe that all the peaks could be captured using bcc structure and the refined lattice parameter is found to be 2.864˚ A; this value is consistent with the reported structural data20 . The resistivity of Fe0.8 Ni0.2 alloy as a function of temperature is shown in Fig. 1(b). Experimental data exhibits a large hysteresis with temperature. It was found that the low Ni concentration (10% to 30%) Fe1−x Nix alloys often exhibit martensitic transition21 . Thus, the hysteresis observed here can be attributed to structural transition from Austenite (bcc structure) to martensite (fcc structure) phase during cooling cycle. From our data, the Ms value for Fe0.8 Ni0.2 alloy is estimated to be about 54 K, while Mf is much above room temperature. M-H loops of Fe0.8 Ni0.2 alloy recorded at 300K and 10K are shown in Fig. 1(c). The observed saturation magnetization is about 130 emu/gm. The M-H loops at 300 K and 10 K temperatures almost overlap on each other; the increase in magnetization from 300 K to 10 K is marginal (about 5 emu/gm). Such insignificant temperature dependence of magnetism in this large temperature range studied indicates Curie temperature significantly higher than the room temperature as also observed in pure Fe. However, the hysteresis loop observed in the alloy appear to be very similar to a soft ferromagnetic material. The temperature dependent magnetization measured in both zero field cooled (ZFC) and field
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FIG. 1. (a) XRD pattern of Fe0.8 Ni0.2 recorded using Cu Kα radiation. No impurity peak is found in the data. (b)Temperature dependent resistivity in both cooling and heating cycles exhibiting hysteresis. (c) Field dependent magnetization - the hysteresis loop is similar to that of a soft ferromagnet. (d) Temperature dependent magnetization in both zero field cooled (ZFC) and field cooled (FC) conditions.
(b) Ni 3p
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(210)
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(200)
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(b) XRD Pattern
Fe 2p
Ni
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Fe Auger
Fe (a)
3 Pure Ni 3/2
(a)
Fe
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Ni 2p
T= 10K
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in the figure) due to Fe 2s photoemission. Signature of three peaks are also found in Fe but the peaks are much distinct in the spectrum of the alloy.
Pure Fe Expt
Fe Ni 0.8 0.2
Fit
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FIG. 4. Fe 2s spectra of (a) pure Fe at 10 K, (b) Fe0.8 Ni0.2 alloy at 10 K, (c) pure Fe at 300 K, and (d) Fe0.8 Ni0.2 alloy at 300 K. Fe 2s spectra are extracted via subtraction of Shirley background and Ni 2p part. Symbols are the experimental data exhibiting three distinct features A, B and C. Lines represent the simulated spectra using Lorentzian-Gaussian line shape; fitting is done using least square error method. Individual peaks are also shown.
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Fe 2s structure has been deconvoluted by fitting the experimental spectra with three Lorentzian-Gaussian peaks representing the features, A, B and C; we have considered the Shirley background to take care of the background intensities. The background subtracted and deconvoluted Fe 2s spectra are shown in Fig. 4; Fig. 4(a) and 4(b) show the Fe 2s spectra of pure Fe and Fe0.8 Ni0.2 alloy, respectively at 10 K temperature, and Fig. 4(c) and 4(d) show the respective Fe 2s spectra at 300 K. The simulation provide a good description of the experimental spectra and all the spectra could be simulated by three peaks A, B, C located at 834.0 eV, 838.4 eV and 844.0 eV binding energies, respectively for all the four spectra shown in the figure. The full width at half maximum (FWHM) of the features A, B and C are found to be 3.5 eV, 5.0 eV and 5.4 eV respectively. The position and FWHM of the features A, B, and C are found to be same for the four spectra shown in the figure. Such reproduction of the spectra obtained from different samples at different conditions provide confidence to the simulation process. Production of core hole due to photoexcitation leaves an unbalanced charge at the photoemission site, which is equivalent to an enhancement of the nuclear charge at the site. Thus, the energy of various electronic states will be pulled down (binding energy will increase). To compensate the situation electrons in the valence orbital will populate itself near the photoemission site leading
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FIG. 3. Ni 2p and Fe 2s core level spectra of pure Ni, Fe0.8 Ni0.2 alloy and pure Fe at temperatures (a) 10 K (b) 300 K, respectively.
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repulsion between the valence electrons, and the corehole created due to photoemission. The calculation of the experimental spectral functions of nickelates have been carried out extensively in the literature22,23 . Interestingly, the energy separation between the satellite and main peak is quite similar to the Ni 2p peaks in the divalent nickel oxides although the intensity of the satellite peak is much weaker in the present case with the major contribution for the screened peak. This suggests that the electron correlation and other electron interaction parameters are similar in these systems and the high metallicity of pure Ni provides higher degree of screening. In the alloy, the energy separation between the main peak and the satellite increases slightly (by about 0.5 eV) and the relative intensity of the satellite with the main peak also increases. Such changes suggests enhancement of the interaction parameters and higher degree of electron localization in the alloy as expected due to disorder in the system. Interestingly, the lower binding energy side of the Ni 2p3/2 peak exhibits a broad feature consisting of three distinct peaks (marked by A, B and C
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CONCLUSION
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The high purity Fe0.8 Ni0.2 alloy has been prepared by arc melting method and was investigated using x-ray Photoelectron Spectroscopy. Ni 2p core level spectra exhibit multiple features due to correlation induced effect and the interaction parameters seem to enhance with the alloying with Fe. Fe 2s core level spectra exhibit distinct peaks due to exchange splitting of the 3d valence band and electron correlation. We do not see significant change in exchange splitting due to temperature and alloying as manifested in the magnetization data. While change in magnetic behaviour due to alloying in the bcc phase of Fe could be important for applications, further studies are required to reveal the underlying physics of the anomalies in electronic properties.
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corresponding author: [email protected] E. F. Wassermann, in Ferromagnetic Materials, edited by H. J.Buschow and E. P. Wohlfarth Elsevier, Amsterdam, Vol. 5 (1990). O. Kubaschewski, Iron Binary Phase Diagrams (Springer, Berlin, 1982); H. Masumoto, K. Watanabe, Japan Inst. Met. 42, (1978) 256 K. B. Reuter, D. B. Williams, and J. I. Goldstein, Metall. Trans. 20A, 719 (1988); . W.Xiong, H.Zhangc, L.Vitos, M.Selleby, Acta Materialia 59, 521 (2011). C. -W. Yang, D. B. Williams, J. I. Goldstein, J. Phase Equilibeia, 17, 522 ((1996) W. A. A. Macedo and W. Keune, Phys. Rev. Lett. 61, 475 (1988). A. Dallmeyer, K. Maiti, O. Rader, L. Pasquali, C. Carbone, and W. Eberhardt, Phys. Rev. B 63, 104413 (2001); K. Maiti et al., Applied Surface Science 182, 302 (2001). C. Guillaume, C. R. Acad. Sci. Paris 125, 235 (1897). M. V. Schiltgaarde, R.A.Abrikosov, and B.Tohansson, Nature 400, 46 (1999). I.A.Abrikosov, A.E.Kissavos, F.Liot, B.Alling, S.I.Simak, O.Piel, and A.V.Ruban, Phys.Rev. B 76, 014434 (2007). S. S. Acharya, V. R. R. Medicherla, R. Rawat, K. Bapna, K. Ali, D. Biswas, and K. Maiti, J. Elect. Spect. Relat. Phenom. 212, 1 (2016). B. Glaubitz, S. Bucshhorn, F. Brussing, R. Abrudan and H. Zabel, J. Phys. Condes. Matter [23, 254210 (2011)
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Interestingly, we find one additional distinct and intense peak at 844 eV (defined as C) in all our spectra of Fe 2s . This can be attributed to the correlation induced effect as also found in other systems25 . Observed exchange splitting is found to be similar in magnitude in Fe metal and the alloy suggesting no drastic change in Fe magnetic moment with composition of Fe-Ni alloys13 . The observed exchange splitting is same at 300 K and
10 K for the alloy indicating no change in Fe local magnetic moment with temperature which is also supported by the temperature dependent magnetization shown in Fig. 1(d). While the magnetic moment and its temperature dependence is well captured by the features in the photoemission spectra, further studies are required to understand the soft magnetic behaviour and the resistivity anomaly found in the alloy.
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to multiple features in the core level spectra depending on the level of screening of the core-hole. It has been observed24 that the exchange splitting of the partially filled valence d levels in transition metals and their compounds is reflected in the 2s and 3s core level spectra as the energy of the final state will depend on the j (=l + s) value of the electron screening the core hole. Thus, one gets an estimate of the exchange splitting from the splitting of the core s-level spectra. For elemental transition metals, this splitting should increase with the filling of d level from d0 to d5 and then gradually reduce for the filling from d6 to d10 . Comparing the experimental results with the reported theoretical and experimental results, it appears that the features A and B corresponds to screening due to exchange split 3d electrons of Fe; the splitting is found to be about 4.4 eV which is very close to the estimates found in other Fe-based materials24 .
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19
20
21
22
23 24
25
J.Kudrnovsks, V.Drchad, and P.Brano, Phys. Rev. B 77, 224422 (2008). C. Paduani, Phys. Stat. Sol. 240, 634-639 (2003). T. Kakeshita, K. Shimuzu, S. Funada and M. Date, Acta Metall 33, 1381 (1985). H. Hasegawa and J. Kanamori, J. Phys. Soc. Japan, 33, 1607 (1972). L. Dubrovinsky, N. Dubrovinskaia, O. Narygina, I. Kantor, A. Kuznetzov, V. B. Prakapenka, L. Vitos, B. Johansson, A. S. Mikhaylushkin, S. I. Simak, and I. A. Abrikosov, Science 316, 1880 (2007). W. F. Bottke, D. Nesvorny, R. E. Grimm, A. Morbidelli, ´ and D. P. OBrien, Nature 439, 821 (2006). D. G. Rancourt, P. Hargrave, G. Lamarche, and R. A. Dunlap, J. Magn. Magn. Mater. 87, 71 (1990). R. P. Reed and R. E. Schramm, J. Appl. Phys. 40, 3453 (1969) Werner Pepperhoff, Mehmet Acet, Constitution and Magnetism of Iron and its Alloys,ISBN 978-3-642-07630-5, Springer (2001) 107. K. Maiti, P. Mahadevan and D.D. Sarma, Phys. Rev. B 59, 12457 (1999). K. Maiti and D. D. Sarma, Phys. Rev. B 58, 9746 (1998). S. H¨ ufner and G. K. Wertheim, Phys. Rev. B 7, 2333 (1973). B. W. Veal and A. P. Paulikas, Phys. Rev. Lett. 21, 1995 (1983).
PII:
S0368-2048(20)30001-3
DOI:
https://doi.org/10.1016/j.elspec.2020.146933
Reference:
ELSPEC 146933
To appear in:
Journal of Electron Spectroscopy and Related Phenomena
Received Date:
14 June 2019
Revised Date:
16 January 2020
Accepted Date:
17 January 2020
Please cite this article as: S.S. Acharya, K. Bapna, K. Ali, D. Biswas, R. Rawat, V.R.R. Medicherla, Kalobaran Maiti, Exchange correlation and magnetism in bcc Fe0.8Ni0.2 alloy, (2020), doi: https://doi.org/10.1016/j.elspec.2020.146933
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier.
Exchange correlation and magnetism in bcc Fe0.8 Ni0.2 alloy S. S. Acharya1 , K. Bapna2 , K. Ali2 , D. Biswas2 , R. Rawat3 , V. R. R. Medicherla1∗ and Kalobaran Maiti2 1
Department of Physics, ITER, Siksha ’O’ Anusandhan Deemed to be University, Bhubaneswar 751030, India 2 Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Homi Bhabha Road,Colaba, Mumbai 400005, India and 3 UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452017, India (Dated: January 16, 2020)
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of
We investigate the magnetism and the electronic structure of body centred cubic, Fe0.8 Ni0.2 alloy. While the magnetic moment of pure Fe and the alloy in bcc phase are almost similar, the hysteresis loop exhibits soft magnetic behavior. Transport measurements exhibit hysteresis in cooling and heating cycles indicating possibilities of structural change in the alloy. Photoemission results exhibit signature of correlation induced effect in the core level spectra. The interaction parameters associated to the Ni electronic states are found to be influenced by the alloying presumably due to disorder induced effect. Fe 2s core level spectra exhibit three distinct peaks due to exchange splitting and correlation of Fe 3d electrons. The exchange splitting seems to remain unchanged in the temperature range studied in the alloy. Evidently, electron correlation and disorder is important to derive the complex electronic properties of this material. Our results establish that the magnetism and transport properties of Fe can be influenced significantly keeping the Fe moment unchanged via alloying with Ni, which is important for the applications of this material. Further studies are required to understand the resistivity anomaly and soft magnetism of this alloy. PACS numbers: 71.20.-b, 71.10.Hf, 79.60.-i
INTRODUCTION
re
Fe-Ni alloys exhibit unique magnetic, elastic and structural properties and play a key role in cutting edge microelectronic and construction materials. The Fe1−x Nix alloy system exhibits complicated magnetic phase diagram1–5 . Fe-Ni alloys with Ni concentrations up to 25% form in bcc structure (α phase) and the alloys transform to fcc structure (γ phase) above 25% Ni concentration. Both α and γ phases form disordered alloys by metastable quenching. The fcc phase exhibits complex magnetic behavior and is known to exist in different magnetic states1 similar to pure fcc Fe6,7 . fcc Fe-Ni alloys have been extensively studied due to the invar property8 exhibited by the alloys with nickel concentration around 35%. Recently many researchers focused their attention on Fe-Ni invar alloys as it was suggested that the invar property is related to non collinear magnetic structures in the alloys9–11 . In Fe-Ni alloys, the Ni magnetic moment is about 0.6-0.7µB and that of Fe is about 2.5-2.7µB 12,13 .
The physical properties of Fe1−x Nix alloys are highly sensitive to method of preparation and temperature of annealing19 . Keeping this in mind, we study the electronic structure of FeNi alloy formed in bcc structure employing x-ray photoemission spectroscopy (XPS) and compare it with the electronic structure of pure Fe and Ni.
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Fe0.8 Ni0.2 alloy was prepared by co-melting high purity (99.99%) Fe and Ni metals in an arc furnace under high purity argon atmosphere. The alloy was homogenized by vacuum annealing at 350 o C for two days; the temperature of annealing was kept purposefully low to avoid the formation of compounds. The strtucture of the alloy was studied using x-ray diffraction (XRD) technique employing Cu Kα x-ray radiation. The temperature dependence of resistivity of the alloy was measured in both cooling and heating cycles using standard four probe method down to 10K. The field and temperature dependance of magnetization was measured down to 10K using SQUID magnetometer. The XPS measurements were carried out using a PHOIBOS 150 Analyser and monochromatic Al Kα x-ray (1486.6 eV) from specs GmbH at an experimental pressure better than 4.0×10−10 Torr. The energy resolution of the spectrometer was set in the range of 0.4 - 0.7 eV depending on the signal strength and signal to noise ratio. The melt grown alloys were extremely hard and was impossible to cleave and/or fracture inside the ultrahigh vacuum chamber. Therefore to produce a clean surface, the sample was polished in ultrahigh vacuum condition with a fine grain diamond file. Cleanliness of the surface was ensured by the intensity of O 1s and C 1s signals. Reproducibility of the spectra were confirmed after each cycle of scraping. The XPS spectra were recorded at temperatures 300 K
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Though the γ phase Fe-Ni alloys have been extensively studied, the studies on α phase alloys are scarce14–16 . The inner core of the earth is known to predominantly consist of α (bcc) phase of Fe-Ni alloy. The investigations on iron meteorites and cosmochemical data suggested 515% Ni concentration in the inner core of earth17,18 . At high pressures (above 225 GPa) and high temperatures (over 3400 K), Fe0.9 Ni0.1 alloy possesses bcc structure17 . Thus, the understanding of the electronic structure of bcc Fe-Ni alloy can help us to reveal the underlying physics of these materials which form the core of earth as well as extraterrestrial materials. Moreover, Fe-Ni alloys have extensive application in technology such as in magnetic recording, making watches and cryogenic dewars, etc.
2 cooled (FC) cycles is shown in figure 1(d) and exhibits a marginal difference at low temperatures. Evidently, the magnetism and therefore, the electron interaction parameters have changed significantly with the alloying.
and 10 K using an open cycle helium cryostat. 0.2
300
600
Magnetic Filed (Oe)
130
128
50
100
150
200
250
RT
Temperature(K)
400
200
Ni 3p
0
-p
FIG. 2. The survey scans of Fe, Fe0.8 Ni0.2 and Ni recorded at 10 K and 300 K temperatures using monochromatic Al Kα radiation. All the features correspond to the energy levels of the constituent elements.
In order to investigate the underlying physics behind such phenomenon, we have studied the electronic structure of this material employing photoemission spectroscopy. The high quality of the sample and the surface cleanliness is demonstrated in the survey scans shown in Fig. 2; the spectra recorded on Fe0.8 Ni0.2 alloy at 10 K and 300 K temperatures are shown in figures 2(a) and 2(b), respectively. The survey scans of pure Fe and Ni are also shown for reference. The binding energy was calibrated using the Fermi level derived from the photoemission spectra of clean Ag. All the features observed in the spectra have been identified and indexed as shown in the figures - they all correspond to the core level spectra of the sample. The survey scans at both the temperatures exhibit insignificant intensity for C 1s and O 1s core levels establishing a clean surface - the impurity level is below the detection level. All the core levels of Fe and Ni, valence band and some of the intense Auger features like L3 M23 M23 and L3 M45 M45 of both Fe and Ni are marked in the figure. In Figs. 3(a) and 3(b), we show the XPS spectra of Ni 2p and Fe 2s regions recorded at 10 K and 300 K temperatures for Fe0.8 Ni0.2 alloy. In pure Ni sample, the Ni 2p3/2 peak appears at a binding energy of 852.7 eV and Ni 2p1/2 at 870.0 eV with a spin orbit splitting of about 17.3 eV. The splitting remains nearly same for pure Ni and the alloy. Each of the features exhibits a distinct satellite peak at higher binding energies (∼ 5.7 eV away from the main peak), which is often called poorly screened feature while the intense lower binding energy feature is called the well screened feature. These distinct features appear due to finite electron-electron Coulomb
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Binding Energy (eV)
re
In Fig. 1(a), we show the powder x-ray diffraction (XRD) pattern of Fe0.8 Ni0.2 alloy obtained using Cu Kα radiation; the experimental data has been indexed considering bcc structure. We observe that all the peaks could be captured using bcc structure and the refined lattice parameter is found to be 2.864˚ A; this value is consistent with the reported structural data20 . The resistivity of Fe0.8 Ni0.2 alloy as a function of temperature is shown in Fig. 1(b). Experimental data exhibits a large hysteresis with temperature. It was found that the low Ni concentration (10% to 30%) Fe1−x Nix alloys often exhibit martensitic transition21 . Thus, the hysteresis observed here can be attributed to structural transition from Austenite (bcc structure) to martensite (fcc structure) phase during cooling cycle. From our data, the Ms value for Fe0.8 Ni0.2 alloy is estimated to be about 54 K, while Mf is much above room temperature. M-H loops of Fe0.8 Ni0.2 alloy recorded at 300K and 10K are shown in Fig. 1(c). The observed saturation magnetization is about 130 emu/gm. The M-H loops at 300 K and 10 K temperatures almost overlap on each other; the increase in magnetization from 300 K to 10 K is marginal (about 5 emu/gm). Such insignificant temperature dependence of magnetism in this large temperature range studied indicates Curie temperature significantly higher than the room temperature as also observed in pure Fe. However, the hysteresis loop observed in the alloy appear to be very similar to a soft ferromagnetic material. The temperature dependent magnetization measured in both zero field cooled (ZFC) and field
800
ro
1000
FIG. 1. (a) XRD pattern of Fe0.8 Ni0.2 recorded using Cu Kα radiation. No impurity peak is found in the data. (b)Temperature dependent resistivity in both cooling and heating cycles exhibiting hysteresis. (c) Field dependent magnetization - the hysteresis loop is similar to that of a soft ferromagnet. (d) Temperature dependent magnetization in both zero field cooled (ZFC) and field cooled (FC) conditions.
(b) Ni 3p
FC
LT
VB
-100
(d)
Ni 3s
0
ZFC
132
Fe 3p
10K
0
250
C 1s
300K
-300
200
of
(c)
-600
150
Temperature (K)
Mnetization (emu/gm)
Magnetization (emu/gm)
100
100
O 1s
50
Ni Auger
90
Fe 2p
80
Ni Auger
70
Fe Auger
60
Ni 2p
50
Scattering Angle (2 )
Ni 2s
40
Intensity (arb. units)
30
VB
Ni 3s Fe 3s
Ni
0.8
Fe 3p
Fe Ni 0.8 0.2
Fe 3s
1.2
(a)
Fe
C 1s
Ni 2p
1.6
O 1s
(210)
heating
2.0
Ni Auger
(200)
Cooling
2.4
Ni 2s
Intensity (arb. units)
T=300K
Resistance (m Ohm)
(b) XRD Pattern
Fe 2p
Ni
Ni Auger
0.8
(110)
Fe Auger
Fe (a)
3 Pure Ni 3/2
(a)
Fe
0.8
1/2
Ni 2p
T= 10K
Ni
0.2
in the figure) due to Fe 2s photoemission. Signature of three peaks are also found in Fe but the peaks are much distinct in the spectrum of the alloy.
Pure Fe Expt
Fe Ni 0.8 0.2
Fit
Ni 2p
A
Fe 2s
C
Pure Fe
B
Fe 2s
C
Pure Fe
(a)
T=300K
T=10K
B
(c)
Fe 2s
Ni 2p
1/2
(b)
Fe0.8Ni0.2
Fe 2s
T=10K
T=300K
845
A
A
A
B
840
835
830
845
840
835
830
Binding Energy (eV)
FIG. 4. Fe 2s spectra of (a) pure Fe at 10 K, (b) Fe0.8 Ni0.2 alloy at 10 K, (c) pure Fe at 300 K, and (d) Fe0.8 Ni0.2 alloy at 300 K. Fe 2s spectra are extracted via subtraction of Shirley background and Ni 2p part. Symbols are the experimental data exhibiting three distinct features A, B and C. Lines represent the simulated spectra using Lorentzian-Gaussian line shape; fitting is done using least square error method. Individual peaks are also shown.
880
860
840
Fe 2s structure has been deconvoluted by fitting the experimental spectra with three Lorentzian-Gaussian peaks representing the features, A, B and C; we have considered the Shirley background to take care of the background intensities. The background subtracted and deconvoluted Fe 2s spectra are shown in Fig. 4; Fig. 4(a) and 4(b) show the Fe 2s spectra of pure Fe and Fe0.8 Ni0.2 alloy, respectively at 10 K temperature, and Fig. 4(c) and 4(d) show the respective Fe 2s spectra at 300 K. The simulation provide a good description of the experimental spectra and all the spectra could be simulated by three peaks A, B, C located at 834.0 eV, 838.4 eV and 844.0 eV binding energies, respectively for all the four spectra shown in the figure. The full width at half maximum (FWHM) of the features A, B and C are found to be 3.5 eV, 5.0 eV and 5.4 eV respectively. The position and FWHM of the features A, B, and C are found to be same for the four spectra shown in the figure. Such reproduction of the spectra obtained from different samples at different conditions provide confidence to the simulation process. Production of core hole due to photoexcitation leaves an unbalanced charge at the photoemission site, which is equivalent to an enhancement of the nuclear charge at the site. Thus, the energy of various electronic states will be pulled down (binding energy will increase). To compensate the situation electrons in the valence orbital will populate itself near the photoemission site leading
lP
Binding Energy (eV)
re
-p
B
(d)
C
Binding Energy (eV)
C
A
Fe0.8Ni0.2
Fe 2s
C
Fe 2s
B C
B
3/2
T= 300K
A
of
(b)
B C
ro
Ni 2p
Intensity (arb. units)
A
Ni 2p
Intensity (arb. units)
Intensity (arb. units)
Ni 2p
ur na
FIG. 3. Ni 2p and Fe 2s core level spectra of pure Ni, Fe0.8 Ni0.2 alloy and pure Fe at temperatures (a) 10 K (b) 300 K, respectively.
Jo
repulsion between the valence electrons, and the corehole created due to photoemission. The calculation of the experimental spectral functions of nickelates have been carried out extensively in the literature22,23 . Interestingly, the energy separation between the satellite and main peak is quite similar to the Ni 2p peaks in the divalent nickel oxides although the intensity of the satellite peak is much weaker in the present case with the major contribution for the screened peak. This suggests that the electron correlation and other electron interaction parameters are similar in these systems and the high metallicity of pure Ni provides higher degree of screening. In the alloy, the energy separation between the main peak and the satellite increases slightly (by about 0.5 eV) and the relative intensity of the satellite with the main peak also increases. Such changes suggests enhancement of the interaction parameters and higher degree of electron localization in the alloy as expected due to disorder in the system. Interestingly, the lower binding energy side of the Ni 2p3/2 peak exhibits a broad feature consisting of three distinct peaks (marked by A, B and C
4
3
4
5
6
7
8 9
10
11
12
CONCLUSION
ro
of
The high purity Fe0.8 Ni0.2 alloy has been prepared by arc melting method and was investigated using x-ray Photoelectron Spectroscopy. Ni 2p core level spectra exhibit multiple features due to correlation induced effect and the interaction parameters seem to enhance with the alloying with Fe. Fe 2s core level spectra exhibit distinct peaks due to exchange splitting of the 3d valence band and electron correlation. We do not see significant change in exchange splitting due to temperature and alloying as manifested in the magnetization data. While change in magnetic behaviour due to alloying in the bcc phase of Fe could be important for applications, further studies are required to reveal the underlying physics of the anomalies in electronic properties.
13
14
lP
2
corresponding author: [email protected] E. F. Wassermann, in Ferromagnetic Materials, edited by H. J.Buschow and E. P. Wohlfarth Elsevier, Amsterdam, Vol. 5 (1990). O. Kubaschewski, Iron Binary Phase Diagrams (Springer, Berlin, 1982); H. Masumoto, K. Watanabe, Japan Inst. Met. 42, (1978) 256 K. B. Reuter, D. B. Williams, and J. I. Goldstein, Metall. Trans. 20A, 719 (1988); . W.Xiong, H.Zhangc, L.Vitos, M.Selleby, Acta Materialia 59, 521 (2011). C. -W. Yang, D. B. Williams, J. I. Goldstein, J. Phase Equilibeia, 17, 522 ((1996) W. A. A. Macedo and W. Keune, Phys. Rev. Lett. 61, 475 (1988). A. Dallmeyer, K. Maiti, O. Rader, L. Pasquali, C. Carbone, and W. Eberhardt, Phys. Rev. B 63, 104413 (2001); K. Maiti et al., Applied Surface Science 182, 302 (2001). C. Guillaume, C. R. Acad. Sci. Paris 125, 235 (1897). M. V. Schiltgaarde, R.A.Abrikosov, and B.Tohansson, Nature 400, 46 (1999). I.A.Abrikosov, A.E.Kissavos, F.Liot, B.Alling, S.I.Simak, O.Piel, and A.V.Ruban, Phys.Rev. B 76, 014434 (2007). S. S. Acharya, V. R. R. Medicherla, R. Rawat, K. Bapna, K. Ali, D. Biswas, and K. Maiti, J. Elect. Spect. Relat. Phenom. 212, 1 (2016). B. Glaubitz, S. Bucshhorn, F. Brussing, R. Abrudan and H. Zabel, J. Phys. Condes. Matter [23, 254210 (2011)
ur na
1
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∗
II.
re
Interestingly, we find one additional distinct and intense peak at 844 eV (defined as C) in all our spectra of Fe 2s . This can be attributed to the correlation induced effect as also found in other systems25 . Observed exchange splitting is found to be similar in magnitude in Fe metal and the alloy suggesting no drastic change in Fe magnetic moment with composition of Fe-Ni alloys13 . The observed exchange splitting is same at 300 K and
10 K for the alloy indicating no change in Fe local magnetic moment with temperature which is also supported by the temperature dependent magnetization shown in Fig. 1(d). While the magnetic moment and its temperature dependence is well captured by the features in the photoemission spectra, further studies are required to understand the soft magnetic behaviour and the resistivity anomaly found in the alloy.
-p
to multiple features in the core level spectra depending on the level of screening of the core-hole. It has been observed24 that the exchange splitting of the partially filled valence d levels in transition metals and their compounds is reflected in the 2s and 3s core level spectra as the energy of the final state will depend on the j (=l + s) value of the electron screening the core hole. Thus, one gets an estimate of the exchange splitting from the splitting of the core s-level spectra. For elemental transition metals, this splitting should increase with the filling of d level from d0 to d5 and then gradually reduce for the filling from d6 to d10 . Comparing the experimental results with the reported theoretical and experimental results, it appears that the features A and B corresponds to screening due to exchange split 3d electrons of Fe; the splitting is found to be about 4.4 eV which is very close to the estimates found in other Fe-based materials24 .
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17
18
19
20
21
22
23 24
25
J.Kudrnovsks, V.Drchad, and P.Brano, Phys. Rev. B 77, 224422 (2008). C. Paduani, Phys. Stat. Sol. 240, 634-639 (2003). T. Kakeshita, K. Shimuzu, S. Funada and M. Date, Acta Metall 33, 1381 (1985). H. Hasegawa and J. Kanamori, J. Phys. Soc. Japan, 33, 1607 (1972). L. Dubrovinsky, N. Dubrovinskaia, O. Narygina, I. Kantor, A. Kuznetzov, V. B. Prakapenka, L. Vitos, B. Johansson, A. S. Mikhaylushkin, S. I. Simak, and I. A. Abrikosov, Science 316, 1880 (2007). W. F. Bottke, D. Nesvorny, R. E. Grimm, A. Morbidelli, ´ and D. P. OBrien, Nature 439, 821 (2006). D. G. Rancourt, P. Hargrave, G. Lamarche, and R. A. Dunlap, J. Magn. Magn. Mater. 87, 71 (1990). R. P. Reed and R. E. Schramm, J. Appl. Phys. 40, 3453 (1969) Werner Pepperhoff, Mehmet Acet, Constitution and Magnetism of Iron and its Alloys,ISBN 978-3-642-07630-5, Springer (2001) 107. K. Maiti, P. Mahadevan and D.D. Sarma, Phys. Rev. B 59, 12457 (1999). K. Maiti and D. D. Sarma, Phys. Rev. B 58, 9746 (1998). S. H¨ ufner and G. K. Wertheim, Phys. Rev. B 7, 2333 (1973). B. W. Veal and A. P. Paulikas, Phys. Rev. Lett. 21, 1995 (1983).