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Home Search Collections Journals About Contact us My IOPscience Reaction Mechanism of Chemical Elements (Co, Fe, Mn) Existing in Spin Valves Containing Oxide Layers This content has been downloaded from IOPscience Please scroll down to see the full text 2006 Jpn J Appl Phys 45 88 (http://iopscience.iop.org/1347-4065/45/1R/88) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 139.184.14.159 This content was downloaded on 06/09/2015 at 23:33 Please note that terms and conditions apply Japanese Journal of Applied Physics Vol 45, No 1A, 2006, pp 88–92 #2006 The Japan Society of Applied Physics Reaction Mechanism of Chemical Elements (Co, Fe, Mn) Existing in Spin Valves Containing Oxide Layers Hoang-Duc Q UANG1;2 , Nguyen-Huy SINH2 , Suhk-Kun O H1 , Thanh-Nhan H UYNH1 , Nguyen The H IEN3 , Jerry ZIDANIC1 and Seong-Cho Y U1 à Applied Physics Laboratory, Department of Physics, Chungbuk National University, Cheongju 361-763, Korea Cryogenic Laboratory, Department of Physics, College of Natural Science, Vietnam National University, 334-Nguyen Trai, Hanoi, Vietnam College of Technology, Vietnam National University, Hanoi, Building E3, 114 Xuan Thuy Road, Cau Giay, Hanoi, Vietnam (Received April 21, 2004; revised June 20, 2005; accepted June 28, 2005; published online January 10, 2006) On the basis of chemical properties of the elements (Mn, Fe, Co, etc.) contained in the pinned layers of spin valves (SVs) with oxide layers (OXLs), we attempted to clarify the effect of reaction mechanism of these elements on the thermal properties of SVs The thermal degradation of MMn-based (M ¼ Fe, Co, Ir, Pt, etc.) specular SVs at high temperatures (> 250 C) is thought to be caused by the diffusion of the Mn component in the structure The role of Mn oxide as a diffusion barrier of Mn migration was clarified by the secondary-ion-mass spectroscopy (SIMS) and X-ray photoelectron spectroscopy (XPS) depth profile analyses Mn diffusion in the free layer was inhibited at up to 300 C in a SV with an OXL, where MnO formation occurs in the OXL, which is confirmed by the MnO 2p3=2 and 2p1=2 peaks As Mn diffused from FeMn (without an OXL), there appeared to be no significant oxide formation in the pinned layer The details of the possible reaction mechanism concerning the chemical elements that exist in the OXLs can be understood by utilizing the standard Gibbs free energy change (ÁG ) as a function of temperature for all chemical elements (Co, Fe, Mn, etc.) [DOI: 10.1143/JJAP.45.88] KEYWORDS: GMR, specular spin valve, thermal stability, diffusion barrier terization of the OXL was carried out only on the asdeposited [CoFe/OXL]n (n is the number of CoFe/OXL unit) multilayers without annealing However, these studies did not clearly show the effect of the OXL on the thermal stabilization of the SV structure Thus, the change in the OXL/magnetic layer interface with annealing and the thermal stability of the OXL itself are still open questions, and the chemical structure of the OXL after annealing is not completely resolved In this paper, we demonstrate the existence of Mn oxides in the OXL after annealing, and the role of the OXL as a diffusion barrier in Mn transmigration by secondary-ion-mass spectroscopy (SIMS) and X-ray photoelectron spectroscopy (XPS) depth profile analyses Introduction Spin transportation and magnetism in nanostructured materials has attracted a great deal of interest in recent years, as the reduction in physical dimensions is comparable to certain characteristic length scales.1–3) Recently, we have successfully fabricated spin valves (SVs) with attractive magnetoresistance (MR) properties In these structures, the post-annealing treatment and oxidation process, which affect both the magnetoresistance and thermal properties of SVs with oxide layers (OXLs), have been investigated In previous work by other researchers, it was confirmed that the insertion of an OXL could extend thermal stability at temperatures of 250 C or higher in antiferromagnet (AF)based spin valve structures (e.g., FeMn, IrMn, PtMn, PtPdMn, and NiMn).3–8) However, neither the insertion effect of an OXL nor the reaction mechanisms for these structures at high temperatures are well known so far Thus, in this work, we tried to find the reason behind the outstanding thermal stability of the MMn-based (M ¼ Fe, Co, Ni, etc.) bottom-pinned specular SVs In SVs utilizing metallic Mn-based AF layers, a strong exchange bias with a high blocking temperature was found, although the MR ratio was reduced to a value comparable to that found in SVs with an oxide AF layer Nevertheless, these spin valves show marked thermal degradation of the multilayer structures caused by Mn diffusion at temperatures above 250 C.2,4,6) In the IrMn-based SV, the main cause of the thermal degradation of MR change at 250 C was the interdiffusion of Mn atoms along the grain boundary, so that the control of the grain boundary diffusion of Mn is the key to improving the thermal stability of SVs, which have Mnbased AF layers.2) Therefore, in a SV with an OXL, the control of Mn diffusion is very important The enhanced GMR effect of SVs with an OXL annealed at around 300 C was investigated,5,7,8) wherein the charac- Experimental # Bottom-pinned spin valve structures of Ta (48 A)/NiFe # # # # (18 A)/FeMn (78 A)/CoFe (18 A)/OXL/CoFe (18 A)/Cu # # # # (24 A)/CoFe (13 A)/NiFe (43 A)/Ta (48 A) without or with an OXL, called sample and sample 2, respectively, were fabricated by dc magnetron sputtering onto Si(100)/SiO2 # substrates Here, we used targets of Ta, Cu, CoFe, (1300 A) NiFe, FeMn, where CoFe, NiFe, and FeMn stand for Co90 Fe10 , Ni81 Fe19 , and Fe50 Mn50 , respectively The base pressure of the main chamber was below 2:8  10À6 Pa, whereas the working pressure of argon was 0.14 – 0.28 Pa and deposition rates were about 0.2 – 0.4 nm/min The OXL was formed in the pinned layer through natural oxidation to form the structure CoFe/oxide/CoFe After completing the deposition procedure, the samples were annealed for 30 in a high-vacuum furnace (6:6  10À4 Pa) at temperatures ranging from 150 to 350 C, followed by furnace cooling to room temperature in a magnetic field of 100 mT, which was applied along the SV easy axis Ramp-up and cooling-down times were about 40 and 140 min, respectively MR ratios for the SVs were measured by the dc four-point probe method by applying a magnetic field of 50 – 500 mT in the film plane The chemical structure of the OXL was studied by SIMS and XPS depth profile analyses These à Corresponding author E-mail: scyu@chungbuk.ac.kr 88 Jpn J Appl Phys., Vol 45, No 1A (2006) H.-D QUANG et al the scattering effect of the OXL.1,9,10) Figure shows a SIMS depth profile analysis carried out on samples and to elucidate the degradation of thermal properties of SVs at high temperatures (> 250 C) The SIMS depth profiles of Mn and all other elements were measured simultaneously Here, we see no significant difference between the samples without an OXL at different annealing temperatures The gradient of Mn content at the interface of CoFe/FeMn is small, and the boundary of CoFe/FeMn is not clear However, in the SV without an OXL annealed at 250 C, a steeper gradient of Mn content in the CoFe layer, which is adjacent to the FeMn layer, was obtained This suggests that intermixing by Mn atom diffusion has some influence on the degradation of MR properties, because CoFe and Cu mixed with Mn cause strong diffuse scattering in the magnetic layer In other words, for the SV with an OXL, a distinct Mn peak was observed in the region of the pinned layer (CoFe/oxide/ CoFe), and the position of this peak corresponds to the OXL region This distinct Mn peak suggests that Mn is segregated in the OXL region Figure shows the depth profiles of XPS spectra of the SV with an OXL (sample 2), which were measured to clarify the chemical properties of the OXL itself, Mn segregation, and Mn diffusion depth in SVs annealed at 300 C In Fig 3(a), we can see that the intensities of all elements, which contain the information for all regions in the SV with an OXL, are not high enough to be compared with the SV without an OXL; moreover, they not show any sharp peaks In this spectrum, the logarithm of the maximum-peak intensity is about 20  103 (in arbitrary units) In particular, in Fig 3(b), there is no distinct Mn 2p peak in the CoFe layers (pinned and free) or in the free NiFe layer in the case of the SV with an OXL Thus, Mn atom diffusion into the free layer was inhibited at up to 300 C in the SV with an OXL We should also note that there is some MnO formation in the OXL, i.e., MnO 2p3=2 and 2p1=2 peaks are seen in the OXL region In Fig 4(a), we can see that the intensities of all elements, corresponding to all regions in the SV without an OXL (sample 1), are sufficiently high and have sharp peaks when compared with the SV with an OXL discussed experiments were performed with the aim of comparing the specular spin valve having an OXL with a conventional SV without an OXL inside the pinned layer From these analyses, we demonstrated the existence of Mn oxides in the OXL after annealing, and the role of the OXL as a diffusion barrier in Mn transmigration In addition, thermodynamic properties of some chemical elements (e.g., Mn, Fe, and Co) existing in this structure containing an OXL were investigated Results and Discussion The variations in MR ratios (ÁR=R) with the annealing temperatures (Ta ) for samples and are shown in Fig Up to an annealing temperature of 300 C, MR ratios of 6.35 – 7.15% were obtained in the case of as-deposited SVs The MR ratio of the SV without an OXL shows a slight increase with annealing until 250 C and then, a rapid degradation to 5.7% at 300 C, whereas the MR ratio of the spin valve with an OXL increases with annealing temperatures above 150 C, and it reaches the maximum values of 10.3% at 250 C and 9.8% at 300 C These results show that the SV with an OXL is more stable than the conventional one This enhancement in the MR ratio is presumably due to Fig MR ratio for each SV with and without OXL as function of annealing temperature (sample and sample 2) Fig SIMS depth profiles of Mn in SV with OXL annealed at 300 C (solid circles and dot-dash line) and SV without OXL annealed at 250 C (open circles and solid line) and 300 C (solid squares and solid line) 89 Jpn J Appl Phys., Vol 45, No 1A (2006) H.-D QUANG et al Fig (a) Depth profiles of Mn atoms in 2p XPS spectra for the spin valve with OXL, (b) Mn 2p XPS spectra in free layer region (NiFe/CoFe) and Mn 2p XPS spectra in OXL layer region Fig (a) Depth profiles of Mn 2p XPS spectra in SVs without OXL, (b) Mn 2p XPS spectra in free layer region (NiFe/CoFe) and Mn 2p XPS spectra in pinned layer (CoFe) region Vertical lines denote Mn 2p1=2 position equal to 653.0 eV and Mn 2p3=2 equal to 641.1 eV Insert bars denote, for comparison, line positions of Mn 2p3=2 determined for MnO (Mn2ỵ ions placed within 640.4 641.5 eV), for Mn2 O3 (Mn3ỵ ions, within 641.1 641.7 eV), and for MnO2 (Mn4ỵ ions, within 641.1 642.3 eV) For metallic manganese the Mn 2p1=2 and Mn 2p3=2 lines are located at the center of 650 and 639 eV, respectively.15) above The logarithm of the maximum-peak intensity is almost threefold higher than that of the structure containing an OXL Furthermore, Fig 4(b) shows that Mn atoms, diffused from FeMn, exist as elemental Mn in the SV without an OXL, and no significant oxide formation occurs in the pinned layer In the pinned layer region, there are distinct Mn 2p3=2 and 2p1=2 peaks In the previous work on Co/SiO2 and Co–Mn/SiO2 bimetallic systems, we found the following: (1) There was 60% dispersion of Co without Mn, but more than 90% dispersion when Mn was included (2) Extended X-ray absorption fine structure (EXAFS) results suggested that Mn was in an oxidized state on the SiO2 surface, whereas much of Co was in a metallic state In fact, Mn behaved as sacrificial metal scavenging oxidized sites on SiO2 , thereby enabling Co to remain metallic (3) The ensemble effect of Mn on Co particles did not appear to be important.11,12) These results, combined with the intriguing catalytic activities of Co–Mn systems, encouraged us to carry out further investigations The binding energies of the Co 2p and O 1s region are given for Co metal, CoO, and Co3 O4 (Table I) The values for Mn agree very well with the earlier data of Carver and Schweitzer13) (Table II) The binding energies of Mn 2p3=2 and Mn 2p1=2 for oxides lie within a narrow range, which caused difficulty in the unambiguous identification of our catalyst samples These results of SIMS and XPS analyses illustrate how the OXL reacts with Mn, which diffused from FeMn during annealing, and how segregated MnO is formed within the OXL Probable reactions are FeO ỵ Mn ! Fe ỵ MnO and CoO ỵ Mn ! Co ỵ MnO We calculated, thermodynamically, the standard Gibbs free energy changes (ÁG ) as a linear function of temperature (TG ) for oxide formation reactions of Fe, Co, and Mn The reaction mechanism for this process can be expressed by the general equation: ÁG ¼ ÁH  À TÁS , where H  is the standard molar enthalpy (heat) at 298.15 K in kJ/mol, and S is the standard molar entropy at 298.15 K in kJ/mol Using the enthalpy and entropy values of all elements and compounds (i.e., Co, CoO, Fe, FeO, Fe3 O4 , Fe2 O3 , Mn, 90 Jpn J Appl Phys., Vol 45, No 1A (2006) H.-D QUANG et al Table I XPS binding energies (eV) for Co reference compounds (MP = binding energy of main peak, SS = binding energy of satellite peak, ÁS = energy separation between main peak and satellite peak, and Á = spin–orbit splitting; error bounds are Ỉ0:2 eV Inside the parentheses, we indicated the theoretical atomic ratio on the basis of anhydrous materials) Co 2p3=2 Sample MP Co metal SS Co 2p/O 1s 2p ẳ 2p1=2 ỵ 2p3=2 Co 2p1=2 S 777.3 MP SS ÁS 792.4 Á O 1s 15.1 Co3 O4 779.6 787.1 7.5 794.8 804.1 9.3 15.2 529.3 530.7 0.60 (0.75) CoO 780.1 788.0 7.9 795.3 804.6 9.3 15.2 529.8 531.6 0.53 (1.00) Table II XPS binding energies (eV) for Mn reference compounds (Peak widths are full width at half-maximum (fwhm) and are listed inside the parentheses Á = spin–orbit splitting Inside the parentheses in the right-hand column, we indicated the theoretical atomic ratio on the basis of anhydrous materials) Mn 2p/O 1s 2p ẳ 2p1=2 ỵ 2p3=2 Mn 2p3=2 Mn 2p1=2 Á O 1s MnO2 641.9 (3.4) 653.6 (3.5) 11.7 529.2 0.29 (0.50) Mn3 O4 MnO 641.3 (3.2) 641.4 (3.2) 652.9 (3.3) 653.1 (3.9) 11.6 11.7 529.1 529.8 0.48 (0.75) 0.53 (1.00) 11.7 529.8 0.46 (0.67) Compound Mn2 O3 641.7 (3.2) 653.4 (4.0) Mn 638.6 (1.9) 649.7 (2.5) MnO, and O2 ),16) we calculated the standard Gibbs free energy changes (ÁG ) as a linear function of temperature (TG ) as follows: is about À110:137 to À150:034 kJ/mol This confirms that Mn atoms are trapped by the OXL, and that the diffusion can be inhibited during the annealing process It has already been shown in Fig that the OXL, which plays the role of a diffusion barrier for Mn migration, increases the thermal stability of the specular SV structure, whereas it maintains a high MR ratio on annealing at up to 300 C In addition to MnO formation in the OXL during annealing, the chemical composition of the OXL in the pinned layer includes MnO as well as CoO and FeO.14,15) We can identify the CoO signal (780.2 – 780.4 eV as 2p3=2 , 795.6 – 796 eV as 2p1=2 ) as well as Co and Fe peaks in the OXL region from the 2p XPS spectra of Co and Fe shown in Fig However, intensities of FeO (709.2 – 709.5 eV as 2p3=2 , 722.5 – 723 eV as 2p1=2 ) in the OXL region are negligible, and the existence of FeO is not clear Furthermore, comparing the hysteresis loops of the SVs, the degradation of the magnetic moment in the OXL is due to the loss of ferromagnetic Co and Fe The effective thickness of the pinned layer with the magnetic moment was # in the SV with an OXL, which can be compared 35 – 37 A # for the SV without an OXL This implies the with 36 A existence of nonmagnetic Co and Fe oxides in the OXL From these results, the composition of the OXL contributing to specular electron scattering can be expressed as (CoO)x1 (FeO)x2 (MnO)1Àx1Àx2 (x1 > x2 , x2 % 0), which was annealed at 300 C We believe that the formation of MnO within the OXL during annealing may help form a smoother OXL/ CoFe interface, which is related to the specular reflection of electrons To confirm this, in Figs 7(a) and 7(b), we show the sheet resistance of the SV with and without an OXL, which was measured for samples and 1, respectively It decreased from 11.71 /square at Ta ¼ 150 C to 11.23 / square at Ta ¼ 300 C (OXL), whereas it increased rapidly from 11.22 /square at Ta ¼ 150 C to 11.91 /square at Ta ¼ 300 C (no OXL) ÁG ¼ 467;800 ỵ 143:77TG 2Cosị ỵ O2gị ẳ 2CoOsị ị G ẳ 519;200 ỵ 125:1TG 2Fesị ỵ O2gị ẳ 2FeOsị ị G ẳ 545;530 ỵ 156:4TG 3=2Fesị ỵ O2gị ẳ 1=2Fe3 O4sịị G ẳ 540;347 ỵ 169:3TG 4=3Fesị ỵ O2gị ẳ 2=3Fe2 O3sị ị G ẳ 769;400 ỵ 145:6TG 2Mnsị ỵ O2gị ẳ 2MnOsị ị: Figure presents the Gibb free energy change (ÁG ) as a linear function of temperature using all equations given above When the reaction is (FeO or CoO) ỵ Mn ! (Fe or Co) + MnO, the standard Gibbs free energy changes (ÁG ) Fig Representative Gibb’s free energy changes (ÁG ) of several elements (e.g., Co, Fe, and Mn) existing in SV with OXL as function of temperature (CoO, solid line; FeO, dash line; Fe3 O4 , dash dot–dot; Fe2 O3 , short dot; MnO, short dash dot) 91 Jpn J Appl Phys., Vol 45, No 1A (2006) H.-D QUANG et al Fig OXL layer region was presented by (a) Fe 2p XPS spectra and (b) Co 2p XPS spectra Fig Sheet resistance of SV (a) with or (b) without OXL as function of annealing temperature 2) A T Saito, H Iwasaki, Y Kamiguchi, H N Fuke and M Sahashi: IEEE Trans Magn 34 (1998) 1420 3) H Iwasaki, A T Saito, A Tsutai and M Sahashi: Dig INTERMAG 97, 1997, HA-04 4) M Takiguchi, S Ishii, E Makino and A Okabe: J Appl Phys 87 (2000) 2469 5) H Li, P P Freitas, Z Wang, J B Sousa, P Gogol and J Chapman: J Appl Phys 89 (2001) 6904 6) Y K Kim, S R Lee, S A Song, G S Park, H S Yang and K I Min: J Appl Phys 89 (2001) 6907 7) G W Anderson, Y Huai and M Pakala: J Appl Phys 87 (2000) 5726 8) S Colis, G Guth, J Arabski, A Dinia and D Muller: J Appl Phys 91 (2002) 2172 9) H Sakakima, M Satomi, Y Sugita and Y Kawawake: J Magn Magn Mater 210 (2000) L20 10) A Veloso, P P Freitas, P Wei, N P Barradas, J C Soares, B Almeida and J B Sousa: Appl Phys Lett 77 (2000) 1020 11) Y Imizu and K J Klabunde: in Catalysts of Organic Reactions, ed R L Augustine (Marcel Dekker, New York, 1985) p 421 12) B J Tan, K J Klabunde, T Tanaka, H Kanai and S Yoshida: J Am Chem Soc 110 (1988) 5951 13) J C Carver, G K Schweitzer and T A Carlson: J Chem Phys 57 (1972) 973 14) K Li, Y Wu, J Qui, G Han, Z Guo, H Xie and T Chong: Appl Phys Lett 79 (2001) 3663 15) J F Moulder, W F Stickle, P E Sobol and K Bomben: Handbook of X-ray Photoelectron Spectroscopy (Perkin-Elmer, Eden Praire, 1995) p 468 16) D R Lide: in Handbook of Chemistry and Physics, ed D R Lide (CRC Press, Boca Raton, 2003) 84th ed., p 5-1 Conclusions In this work, Mn–O formation within the OXL was demonstrated, and its role in the thermal stability of SVs during the annealing process was investigated using the depth profiles of SIMS and XPS of SVs with or without an OXL, in order to clarify the chemical properties of the elements existing in the SV with an OXL We note that the OXL plays the role not only of a specular reflective plane for electrons to enhance the GMR effect, but also of a Mn atom diffusion barrier for the improved thermal stability of the SV structure The improved thermal stability of the SV with an OXL is strongly dependent on MnO formation within the OXL following Mn diffusion from the FeMn layer during annealing SIMS and 2p XPS spectra characterized the chemical structure and thickness of the OXL after annealing We also clarified the nature of the thermodynamic properties of elements (e.g., Co, Fe, and Mn) existing in the bottompinned SV structure Acknowledgements This research, carried out at Chungbuk National University, was supported by the Korea Research Foundation Grant (KRF-2003-005-C00018) 1) Y Kamigushi, H Yuasa, H Fukuzawa, K Koui, H Iwasaki and M Sahashi: Dig INTERMAG 99, 1999, DB-01 92 ... Journal of Applied Physics Vol 45, No 1A, 2006, pp 88–92 #2006 The Japan Society of Applied Physics Reaction Mechanism of Chemical Elements (Co, Fe, Mn) Existing in Spin Valves Containing Oxide Layers. .. published online January 10, 2006) On the basis of chemical properties of the elements (Mn, Fe, Co, etc.) contained in the pinned layers of spin valves (SVs) with oxide layers (OXLs), we attempted... (MP = binding energy of main peak, SS = binding energy of satellite peak, ÁS = energy separation between main peak and satellite peak, and Á = spin orbit splitting; error bounds are Ỉ0:2 eV Inside