Structural and magnetic studies of sputtered Fe 1−x Cr x thin films N H Duc, A Fnidiki, J Teillet, J Ben Youssef, and H Le Gall Citation: Journal of Applied Physics 88, 4778 (2000); doi: 10.1063/1.1289780 View online: http://dx.doi.org/10.1063/1.1289780 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/88/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Structural and magnetic properties of quaternary Co2Mn1-xCrxSi Heusler alloy thin films J Appl Phys 110, 053903 (2011); 10.1063/1.3626055 A structural, magnetic, and Mössbauer spectral study of the TbCo − x Fe x B compounds with x = , 1, and J Appl Phys 105, 113908 (2009); 10.1063/1.3138808 Magnetic and Mössbauer spectral study of ErFe 11 Ti and ErFe 11 TiH J Appl Phys 93, 3414 (2003); 10.1063/1.1544087 Magnetic and conversion electron Mössbauer spectral study of amorphous thin films of Dy x Fe 100−x and Dy 20 Fe 80−y Co y J Appl Phys 90, 1934 (2001); 10.1063/1.1385574 Magnetic, Mössbauer and magnetostrictive studies of amorphous Tb(Fe 0.55 Co 0.45 ) 1.5 films J Appl Phys 87, 7208 (2000); 10.1063/1.372970 [This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to ] IP: 129.24.51.181 On: Fri, 28 Nov 2014 10:32:36 JOURNAL OF APPLIED PHYSICS VOLUME 88, NUMBER 15 OCTOBER 2000 Structural and magnetic studies of sputtered Fe1À x Crx thin films N H Duca) Cryogenic Laboratory, Faculty of Physics, National University of Hanoi, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam A Fnidiki and J Teillet Laboratoire de Magne´tisme et Applications, GPM-UMR 6634, Universite´ de Rouen, 76821 Mont-Saint-Aignan, France J Ben Youssef and H Le Gall Groupe des Laboratoires de Bellevue, CNRS, 92195 Meudon Cedex, France ͑Received March 2000; accepted for publication 28 June 2000 X-ray diffraction, magnetization, and Moăssbauer effect investigations have been performed for sputtered Fe1Ϫx Crx (0рxр0.54) thin films The body-centered-cubic ͑bcc͒ phase appears for x Ͻ0.32, while the phase is formed for 0.38рxр0.44 For 0.32рxϽ0.38 and 0.44Ͻxр0.54, the samples are composed of both bcc and phases As the Cr concentration increases, the ferromagnetic fraction, magnetization, and magnitude of the hyperfine field decrease, whereas the magnitude of the isomer shift increases The Fe and Cr magnetic moments, isomer shift, and hyperfine field of the bcc Fe–Cr phase have been deduced and are discussed consistently in terms of charge and spin distributions, as well as magnetic valence © 2000 American Institute of Physics ͓S0021-8979͑00͒08419-X͔ I INTRODUCTION Electronic and magnetic investigations of bulk bodycentered-cubic ͑bcc͒ Fe–Cr alloys have been performed extensively for many years.1–9 This is one of only two Febased alloy series ͑the other is Fe–V͒ in which a wide solidsolution range occurs In thin films, the magnetism of Cr atoms has recently became the focus of interest because of its mediating role in exchange coupled superlattice and giant magnetoresistance materials.10–12 Substitution of Fe by Cr atoms strongly changes not only the magnetic properties but also the structural and mechanical properties.13–15 This is an interesting aspect in magnetism as well as in technical applications Magnetism of bcc Fe–Cr can be described in terms of the Cr-nearest neighbor environment.4,6 This is directly related to the charge and spin distributions In this context, information on hyperfine parameters deduced from Moăssbauer spectrometry studies is worth considering Indeed, a linear correlation between the hyperfine field (B hf) and the isomer shift ͑IS͒ was found for bulk bcc Fe–Cr alloys by Dubiel and Zukrowski.6 It has successfully been used to confirm the charge and spin transfers in this system These experimental results were well reproduced by electronic structure calculations.9 Structure, magnetization, and the Moăssbauer effect have been studied for nonequilibrium FeCr thin films.13 In Ref 13, the difference in local atomic configurations of the phase and A15 phase in the paramagnetic state was shown, however, the Fe and Cr magnetic moments, charge, and spin transfer were not mentioned In this article, we present our experimental investigations of the crystal structure, magnetization, and Moăssbauer a Corresponding author; Electronic mail: duc@cryolab.edu.vn effect for sputtered Fe1Ϫx Crx thin films Charge and spin transfer in the bcc Fe–Cr phase are discussed by considering the configuration of the Fe and Cr magnetic moments, the relationship between the hyperfine field and magnetization, the relationship between the hyperfine field and the isomer shift, as well as the magnetic valence II EXPERIMENT The Fe1Ϫx Crx (0рxр0.54) thin films were deposited onto a glass substrate at 300 K using a triode rf-sputtering system To avoid corrosion and oxidation, the film stacks were covered with a 10 nm thick Nb layer on top The film thickness ranged from 0.5 to 1.5 m The composition was analyzed using energy dispersive x-ray ͑EDX͒ analysis The structure of the samples was investigated by high-angle x-ray diffraction ͑XRD͒ a using a cobalt anticathode ( CoK ␣ ϭ0.1790 nm) The magnetization was measured with a vibrating sample magnetometer ͑VSM͒ in magnetic fields up to 1.4 T applied in the filmplane directions Conversion-electron Moăssbauer spectra CEMS at room temperature were recorded using a conventional spectrometer equipped with a homemade helium–methane proportional counter The source was 57Co in a rhodium matrix The film was set perpendicular to the incident ␥ beam The spectra were fitted with a least-squares technique using a histogram method with respect to discrete distributions, thereby constraining the linewidths of each elementary spectrum to be the same Isomer shifts are given relative to ␣-Fe at 300 K The average ‘‘cone angle’’  between the incident ␥-ray direction ͑the film–normal direction͒ and that of the hyperfine field B hf ͑or the Fe magnetic moment direction͒ is 0021-8979/2000/88(8)/4778/5/$17.00 4778 © 2000 American Institute of Physics [This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to ] IP: 129.24.51.181 On: Fri, 28 Nov 2014 10:32:36 Duc et al J Appl Phys., Vol 88, No 8, 15 October 2000 4779 FIG Magnetic moment of the Fe1Ϫx Crx thin films: ͑᭹͒ bcc, and ͑᭺͒ phase FIG Typical x-ray diffraction patterns of several Fe1Ϫx Crx thin films: x ϭ͑a͒ 0.28, ͑b͒ 0.32, and ͑c͒ 0.44 estimated from the line-intensity ratios 3:x:1:1:x:3 of the six Moăssbauer lines, where x is related to  by sin2  ϭ2x/(4ϩx) III EXPERIMENTAL RESULTS AND DISCUSSION The crystal structure of the Fe1Ϫx Crx (0рxр0.54) thin films is clearly detected by the x-ray diffraction investigation The bcc phase appears for xϽ0.32 ͓Fig 1͑a͔͒ The phase, which is classified as the topologically closed packed one, is formed for 0.38рxр0.44 ͓Fig 1͑c͔͒ For 0.32рx р0.38 and 0.44рxр0.54, samples are composed of both the bcc and phase ͓Fig 1͑b͔͒ The phase diagram is presented in Fig 2͑a͒ It is usually obtained for Fe1Ϫx Crx thin films prepared by the sputter-deposition method.13 In Ref 13, however, the composition range for the formation of the phase is 0.45ϽxϽ0.55 For comparison, these data are also included in Fig 2͑b͒ For the Fe–Cr films prepared by thermal coevaporation, an amorphous phase can be formed in the composition range of 0.4уxу0.75.15 These distinctions reflect a strong effect of the deposition process on the formation of Fe–Cr alloys The spontaneous magnetization at 300 K is determined by linearly extrapolating the magnetization curve from high field to zero field The spontaneous magnetization (M S ) referring to the volume of the bcc phase is shown in Fig Note that, as Fe is substituted by Cr, the room temperature M S value of the bcc alloys decreases and seems to tend to zero at xϾ0.6 with a slope of approximately Ϫ3.7 B /at This result is comparable with that reported in Ref 13 At low temperature, however, a slope of Ϫ2.4 B /at, which is rather close to that given from a simple Slater–Pauling analysis, was found.4,13 This difference between the low and room temperature M S variations may be related to the change in film ordering temperature with Cr concentration However, we not know the T C values for this system so far The spontaneous magnetization of the alloys does not follow the tendency found for the bcc alloys, but is almost zero This is due to the fact that the phase is ferromagnetic with low ordering temperature and very small magnetic moment.13 Figure shows typical CEM spectra at 300 K for several investigated Fe1Ϫx Crx thin films A ferromagnetic sextet is observed for the bcc phase with xр0.32 ͓Figs 4͑a͒ and 4͑b͔͒ With increasing x, however, the Moăssbauer lines broaden due to a random distribution of the hyperfine field Moreover, a center contribution superimposed on these ferromagnetic sextets is enhanced and becomes prominent for the bcc Fe–Cr films with xу0.32 ͓Figs 3͑c͒–3͑e͔͒ This paramagnetic contribution is attributed to the phase For the pure alloys ͑i.e., for 0.38рxϽ0.44͒, only quadrupole splitting characterized for the paramagnetic contribution is observed This is, however, not presented here The CEM spectra of the Fe1Ϫx Crx thin films were fitted with a wide distribution of hyperfine field P(B hf) to take into account all the environments experienced by the 57Fe nucleus FIG Phase diagram of sputtered Fe1Ϫx Crx thin films: ͑a͒ present study and ͑b͒ from Ref 13 [This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to ] IP: 129.24.51.181 On: Fri, 28 Nov 2014 10:32:36 4780 Duc et al J Appl Phys., Vol 88, No 8, 15 October 2000 FIG Moăssbauer spectra and hyperfine field distributions in bcc Fe1Ϫx Crx thin films: xϭ͑a͒ 0.08, ͑b͒ 0.28, ͑c͒ 0.32, ͑d͒ 0.52, and ͑e͒ 0.54 ͓Figs 4͑a͒–4͑e͔͒ This provides average values of the hyperfine field ( ͗ B hf͘ ), the isomer shift ͑͗IS͒͘, the cone angle between the Fe moment direction and the film–normal direction ͑͗͒͘, and the relative ferromagnetic fraction (A ferro) The results of A ferro ͑in the Fe1Ϫx Crx alloys͒ and of ͗ B hf͘ , ͗IS͘, ͗͘ ͑referred to the bcc Fe–Cr phase͒ obtained are plotted in Figs 5͑a͒–5͑d͒, respectively Here, ͗ B hf͘ and ͗IS͘ are negative Note that the accuracy of the isomer shift values of the two compounds with xϭ0.52 and 0.54 is low Their reported ͗IS͘ values correspond to a minimum of test in fitting It can be seen from Fig that, although A ferro , ͉ ͗ B hf͘ ͉ and ͗͘ decrease with increasing the Cr concentration, ͉͗IS͉͘ increases In accordance with the XRD results, the decrease of A ferro is associated with the appearance of the phase The change of hyperfine field is related to a change of spin density and the change of the isomer shift indicates a change of charge density The increase of the ͉͗IS͉͘ value can be described in terms of the increase in the density of s and p electrons at Fe nuclei due to the substituted Cr atoms, whose electronegativity is lower than that of Fe atoms With regard to the charge and spin transfer, Dubiel and Zukrowski6 considered the relationship between the hyperfine field and the isomer shift in bulk Fe–Cr alloys They found a linear correlation between ͗ B hf͘ and ͗IS͘ This plot is presented in Fig for the bcc Fe1Ϫx Crx phase Here, the data are also consistent with a linear relation, although they not prove it With regard to the Dubiel and Zukrowski approach, a linear dependence can be expressed as ͗ B hf͘ ϭ32ϩ165͗ IS͘ The slope of this line ͓ d ͗ B hf͘ /d/ ͗ IS͘ ϭ165 T/͑mm/s)] is close to that of 172.5 T/͑mm/s͒ reported for bulk Fe–Cr alloys.6 Such a strong correlation means that the charge transfer is directly related to the spin transfer We FIG Ferromagnetic fraction (A ferro), hyperfine field ( ͗ B hf͘ ), isomer shift ͑͗IS͒͘, and cone angle ͗͘ in Fe1Ϫx Crx thin films: ͑᭹͒ bcc, ͑᭺͒ , and ͑ࡗ͒ mixed bccϩ phase will come back to details of the charge and spin distribution later The variation of ͗͘ reflects a random orientation of the Fe magnetic moments in high Cr concentration alloys The hyperfine magnetic field is not proportional the 3d(Fe) magnetic moment, but can be empirically given by4,6 ͉ ͗ B hf͘ ͉ ϭaM FeϩbM S , ͑1͒ FIG Correlation between ͗ B hf͘ and ͗IS͘ in the bcc Fe1Ϫx Crx phase [This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to ] IP: 129.24.51.181 On: Fri, 28 Nov 2014 10:32:36 Duc et al J Appl Phys., Vol 88, No 8, 15 October 2000 FIG Correlation between ͗ B hf͘ and M s in the bcc Fe1Ϫx Crx phase where the magnetic moments of Fe atoms (M Fe) and of the alloy (M S ) could themselves be concentration dependent, but a and b are assumed to be constant The correlation between ͗ B hf͘ and M S is presented in Fig Clearly, good linear variation ͗ B hf͘ (M S ) is observed, except for a few points at high Cr concentrations ͑i.e., up to xϭ0.3 only͒ Similar behavior with almost the same values of a and b was found for Fe–Cr and Fe–Al alloys.4,16 At low temperature (Tϭ5 K), Shiga and Nakamura4 found that the B hf vs M S curve shows good linearity for the Fe1Ϫx Crx up to xϭ0.7 They proposed that in this concentration range the Fe magnetic moment remains almost constant and the decrease in ͗ B hf͘ is mainly due to a reduction in M S The magnitude of the Fe magnetic moment is simply determined from the value of the aM Fe term in Eq ͑1͒ The procedure is as follows As a guide to the eye, the variation of experimental points can be described by the equation ͗ B hf͘ ϭ22ϩ5M S ͑Fig 7͒ In comparison with Eq ͑1͒, it turns out that aM Fe ϭ22 T and bϭ5T/ B Taking M Feϭ2.2 B /at for bcc Fe, one can derive that aϭ10T/ B This result is comparable with that reported for the bulk Fe–Cr alloys.4 These deduced a and b values are assumed to be constant for all the investigated Fe–Cr alloys For the detail of each composition, from Eq ͑1͒, M Fe(x) is estimated as M Fe(x)ϭ ͓ B hf(x) ϪbM S (x) ͔ /a The results of M Fe(x) obtained are plotted in Fig This result is supported by a neutron diffraction study.3 From these values of M Fe and the measured sponta- 4781 neous magnetization data, the Cr magnetic moment (M Cr), which is oriented antiparallel with respect to the Fe moment, is estimated by applying the expression ͉ M Cr͉ ϭ ͉ ͓ M S Ϫ(1 Ϫx)M Fe͔ ͉ /x It results in with increasing Cr concentration, ͉ M Cr͉ decreasing from the value of 1.9 B /Cr at and being annulled at xϾ0.32 ͑Fig 8͒ This finding is comparable with those deduced from the magnetization,2,4 neutron scattering diffraction,3 and electronic structure calculations9 and those reported for the Cr magnetic moment in Fe/Cr interfaces.11,12 The magnetic and hyperfine properties can globally be described as a result of the hybridization between the 3d(Fe) states and 3d ͑early transition metal͒ states.9–17 In this case, it is worth recalling here the main arguments given by Li and Luo.9 In the Fe–Cr alloys, the Cr moment is negative with respect to the Fe moment This is due to the fact that the majority ͑spin-up͒ 3d(Cr) states are located near the Fermi level (E F ), while the minority ͑spin-down͒ states are below E F 9,17 The minority 3d(Cr) band thus strongly overlaps the minority 3d(Fe) band This leads to enhancement of spindown 3d(Cr) – 3d(Fe) interactions Consequently, the potential energies of the minority 3d band may be lowered relative to those of the corresponding majority 3d band Finally, spin-up 3d(Fe) electrons transfer to the spin-down 3d(Cr) subband, resulting in the reduction of the 3d(Fe) magnetic moment This decrease of the number of the 3d(Fe) electrons leads to less shielding of the Fe nucleus, hence to a larger s, p electron density at the Fe origin and then to the observed increase of ͗IS͘ The total magnetic moment of the Fe atoms, however, also comes from polarization of the 4s and 4p magnetic moments Indeed, in pure iron metal, due to 3d(Fe)-(s,p) hybridization, a negative s, p magnetic moment relative to the 3d(Fe) moment is usually formed.9,17 With substitution of the Cr atom, the hybridization between the ͑spin-up͒ 3d(Cr) and ͑s, p͒ states is stronger than that between the ͑spin-down͒ 3d(Fe) and ͑s, p͒ states This leads to a change in the direction of the 4s, 4p polarization from negative ͑for pure Fe͒ to positive ͑for Fe–Cr alloys͒ This increase of the s, p spin-up density was confirmed by the electronic structure calculations.9 It was the assumed mechanism for the maintenance of the high magnetic moment and the reduction of the magnitude of the hyperfine field at Fe sites in the Fe–Cr alloys Magnetic properties of the transition metal alloys are described by the well-known Slater–Pauling curve.18 However, a better description of the role of the elements when they appear as solutes in Fe-, Co-, and Ni-based alloys is a magnetic valence, but not a chemical one.19 Within the simple concept of the magnetic valence Z m , not only the magnetic atoms but also the nonmagnetic ones are considered In this case, the average magnetic moment per atom is written as19 ↑ M ϭZ m ϩ2N sp , ͑2͒ ↑ is the number of s, p electrons in the spin band; where 2N sp ↑ ϭ0.3.19 in the late transition metals, N sp For the Fe1Ϫx Crx thin films, Z m is determined by the chemical values Z Fe(ϭ8) and Z Cr(ϭ6) and by the number N ↑d of d electrons in the spin-up subband ͑N ↑d ϭ5 in the strong ferromagnets͒: FIG Fe and Cr magnetic moments in the bcc Fe1Ϫx Crx phase [This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to ] IP: 129.24.51.181 On: Fri, 28 Nov 2014 10:32:36 4782 Duc et al J Appl Phys., Vol 88, No 8, 15 October 2000 Fe1Ϫx Crx thin films The main results have been considered consistently in terms of the Fe and Cr magnetic moment configuration, the correlation between the hyperfine field and the magnetization, the correlation between the hyperfine field and the isomer shift, as well as the magnetic valence ACKNOWLEDGMENTS The stay of one of the authors ͑N.H.D.͒ at the Groupe de Physique des Mate´riaux, University of Rouen, was supported by the Ministe`re Franc¸ais de l’Education Nationale, de la Recherche et de la Technologie FIG Magnetic moment as a function of the magnetic valence for bcc Fe1Ϫx Crx thin films M V Nevitt and A T Aldred, J Appl Phys 34, 463 ͑1963͒ A T Aldred, Phys Rev B 14, 219 ͑1976͒ A T Aldred, B D Rainford, J S Kouvel, and T J Hicks, Phys Rev B 14, 228 ͑1976͒ M Shiga and N Nakamura, J Phys Soc Jpn 49, 528 ͑1980͒ S M Dubiel, Acta Phys Pol A 49, 619 ͑1976͒ S M Dubiel and J Zukrowski, J Magn Magn Mater 23, 214 ͑1981͒ H Hasegawa and J Kanamori, J Phys Soc Jpn 33, 1607 ͑1972͒ H Ebert, H Winter, D D Jhonson, and F J Pinski, J Phys.: Condens Matter 2, 443 ͑1990͒ Z Li and H Luo, J Phys.: Condens Matter 3, 9141 ͑1991͒ 10 J A C Bland and B Heirich, Ultrathin Magnetic Structure, Vol ͑Springer, Berlin, 1994͒ 11 H Zabel, J Phys.: Condens Matter 11, 9303 ͑1999͒ 12 T Asada, G Bihlmayer, H Handshuh, S Heinze, P Kurz, and S Bluăgel, J Phys.: Condens Matter 11, 9347 ͑1999͒ 13 K Sumiyama, N Ohshima, and Y Nakamura, Trans Jpn Inst Met 28, 699 ͑1987͒ 14 J Gwiazda, E Marianska, J Oleniacz, M Peryt, and W Zych, Hyperfine Interact 55, 973 ͑1990͒ 15 S K Xia, E Baggio-Saitovitch, and C Larica, Phys Rev 49, 927 ͑1994͒ 16 M Shiga and N Nakamura, J Phys Soc Jpn 40, 1295 ͑1976͒ 17 N H Duc, in Handbook of the Physics and Chemistry of Rare Earths, edited by K A Gschneidner and L Eyring ͑Elsevier Science, Amsterdam, 1997͒, Vol 24, p 339 18 S Chikazumi, Physics of Magnetism ͑Wiley, New York, 1964͒, p 73 19 A R William, V L Moruzzi, A P Malozemoff, and K Terakura, IEEE Trans Magn 19, 1983 ͑1983͒ 20 J P Gavigan, D Givord, H S Li, and J Voiron, Physica B149, 345 ͑1988͒ Z m ϭ ͑ 2N ↑d ϪZ Fe͒͑ 1Ϫx ͒ ϪxZ Cr ͑3͒ The experimental and calculated magnetic moments are presented in Fig as a function of Z m Starting from pure Fe, the experimental magnetization falls below the M ϭZ m ϩ0.6 line This was assumed to reveal the weak ferromagnetic character of Fe.19,20 As the Cr content increases, the experimental magnetization data initially approach the calculated line and finally excess this plot Indeed, the magnetization data of the bcc Fe–Cr with low Cr concentration can be ↑ described by the equation of M ϭZ m ϩ0.9 ͑i.e., with N sp ↑ ϭ0.45͒ This increase of N sp is usually observed in the alloys of Fe and Co with early-transition metals and with rareearth elements.17,19,20 It agrees with the above discussion that the number of the spin-up s, p electrons increases in the Fe–Cr alloys Moreover, this departure of the experimental data from the M ϭZ m ϩ0.6 line may also relate to the effect of removing electrons from the d band of the host, i.e., the 3d electrons transfer from Fe to the neighboring Cr atoms This results in a reduction of the number of the spin-up 3d electrons N ↑d In conclusion, we have studied the crystal structure, magnetization, hyperfine field, and isomer shift for sputtered [This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to ] IP: 129.24.51.181 On: Fri, 28 Nov 2014 10:32:36 ...JOURNAL OF APPLIED PHYSICS VOLUME 88, NUMBER 15 OCTOBER 2000 Structural and magnetic studies of sputtered Fe1À x Crx thin films N H Duca) Cryogenic Laboratory, Faculty of Physics, National University... FIG Magnetic moment of the Fe1Ϫx Crx thin films: ͑᭹͒ bcc, and ͑᭺͒ phase FIG Typical x-ray diffraction patterns of several Fe1Ϫx Crx thin films: x ϭ͑a͒ 0.28, ͑b͒ 0.32, and ͑c͒ 0.44 estimated... back to details of the charge and spin distribution later The variation of ͗͘ reflects a random orientation of the Fe magnetic moments in high Cr concentration alloys The hyperfine magnetic field