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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TMAG.2014.2313590, IEEE Transactions on Magnetics

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The studied sample of Si(Substrate)/Co/Fe /Fe/Al bilayers were prepared by ion beam sputtering technique. Using Ar ion beam, the high quality Fe and Co sputtering targets were sputtered over Si(100) substrate. Si substrates were ultrasonically cleaned with acetone and subsequently with isopropanol prior to deposition. A base pressure of about 10-6 Torr was achieved in the ultra high vacuum sputtering chamber before the commencement of sputtering. Firstly, Co layer of 32 nm were deposited followed by 5 nm of Fe57 layer. Subsequently, a 37 nm of Fe layer was deposited on top of the Fe57 layer. The Fe57 layer was deposited at the interface for Mössbauer studies. A capping layer of 5 nm aluminium was deposited on top to protect the bilayers from oxidation. Phase characterization of the sample was performed by Rigaku’s SmartLab X-ray diffractometer using Cu Kα lines in the parallel beam with θ - 2θ geometry. The X-ray diffraction (XRD) pattern was obtained by scanning the samples from 20° to 60° with the step size of 0.02°. The anode power was kept fixed at 9 kW throughout the measurement. Room temperature Mössbauer spectroscopy was investigated by collecting Fe57 Conversion Electronic Mössbauer Spectrum (CEMS) using a flowing gas 95% He, 5% CH4 proportional counter with a 25 mC Co-57 source in Rh matrix. NORMOS code was used to analyze the CEMS spectrum [20]. A Quantum Design (QD) Magnetic Property Measurement System (MPMS) was used to acquire the magnetic hysteresis loops. The magnetization data were taken at different temperatures (5, 100 and 300K). Before commencing the isothermal magnetization measurement at 5K and 300K, a pinning magnetic field of -5T and +7T were applied. A systematic minimization of trapped magnetic field in the superconducting magnet of MPMS was performed before every measurement. It is worth mentioning here that all the measurements are repeatable. III. RESULTS AND DISCUSSION The Mössbauer spectrum taken at room temperatures explicitly depicts a sextet as shown in the Fig. 1. NORMOS code was employed to fit the experimental data. The fitting yielded sextet corresponds to FeCo (Hyperfine field Bhf = 34.78 T, Isomer shift δ = 0.04 mm/s). The obtained value of Bhf field is close enough to α -Fe (Bhf =33 T) indicating the diffusion of Fe into Co during deposition, at least at the interface. Please place Figure.1 here In the Co/Fe bilayers deposited by MBE technique, Carbuchhio et al. have also observed two magnetic component attributed to FeCo alloy (Bhf = 35 T) and to a solid solution of Fe in to Co [21]. The inset of Fig. 1 shows the XRD spectrum of the as-deposited multilayer. A good match of the peak positions was observed with the earlier reported data by Gupta et. al. on similar system [22]. They found peaks at 44.05 degree corresponding to bcc Fe(110), fcc Co(111), hcp Co(002) and at 46.82 degree corresponding to hcp Co(101). In the present case, these peaks are clearly visible with a minimal shift of 0.5° in peak positions.

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The magnetic hysteresis loop measurements, performed at different temperatures (5, 100, 300K) with applied magnetic field along the plane of the film, has revealed PEB, with EB field of 8.1, 6.7 and 6.1Oe. Presumably, due to establishment of short range exchange interaction between Fe and Co layers, a single magnetic phase behavior is observed in the magnetic hysteresis loops as shown in Fig. 2. Earlier, Gupta et al. have found two fold uniaxial anisotropy because of strong texture in similar multilayered samples [22]. Films deposited by RF sputtering have also revealed similar anisotropic behavior, where they reported the squareness of hysteresis curve as an indicator of such anisotropic behavior [23]. Thus, in our case the two fold uniaxial anisotropy, which is also regarded as one of the prerequisites for EB [24], can be assumed from the observed squareness of the curve as can be seen in Fig. 2. On the other hand, during deposition the bottom Co layer has magnetization direction aligned in-plane and owing to strong coupling, the top Fe layer tends to align along with it. Thus, the bilayer is supposed to has low anisotropy dispersion which results in easy domain wall motion. The observed exchange bias is very small and may have contribution from remnant field in the superconducting coil of MPMS. In order to show that in our measurement the contribution of remnant field is significantly low, we performed hysteresis loop measurement in our MOKE (magneto-optic Kerr effect) setup which has an electromagnet as source of magnetic field. The data is shown in the inset of Fig. 2. The observed EB in MOKE measurement is found to be 8Oe which is close to the value obtained through MPMS setup. This clearly demonstrates very low contribution from remnant field of superconducting coils of MPMS. In order to explore the effect of pinning magnetic field on EB, we measured the magnetization loops at 300K and 5K with a pinning field of 7T and -5T, as shown in Fig. 3. First, we pinned the high coercive Co film with the pinning field and then measured the hysteresis loop by varying the magnetic field in the range of ±500Oe. Upon applying the positive pinning field of 7T, we observed enhancement in the PEB with complete shift of the hysteresis loop in the pinning field direction. The EB field values increases to 17.6Oe at 300K and 19.6Oe at 5K, as compare to EB field values measured without pinning field. Interestingly, application of -5T pinning field results in shifting of the hysteresis curve towards the negative field direction with EB field values of -15Oe at 300K and -9.5Oe at 5K. In an earlier work, Nogués et al. reported PEB in FeF2/Fe which is a AMFFM system [25]. The coupling between large cooling field and AFM surface spins above Neèl temperature and development of AFM interaction at the interface have been established as two main reasons behind the observed exchange bias. However, in our case Please place Figure.2 here owing to ferromagnetic nature of the bilayers and intermixing at the interface, the magnetic interaction seems to be different and more complex. Recently, Ke et al., have shown the existence of PEB between two ferromagnetically ordered layers [11]. But in their case PEB is investigated at low temperature

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> FOR CONFERENCE-RELATED PAPERS, REPLACE THIS LINE WITH YOUR SESSION NUMBER, E.G., AB-02 (DOUBLE-CLICK HERE) < and at the maximum pinning field of 4T. Exchange bias in FM layers is also studied exclusively by Binek et al. [13]. The observation of positive exchange bias in our system is due to the establishment of AFM exchange interaction at the interface. In our all ferromagnet bilayer sample, the development of unusual AFM interactions at the interface might be owing to the non-uniform spread of Fe spins at the interface and intermixing of Fe and Co at the interface. The ion beam sputtering technique induces several local regions of interfacial spin configurations which are relevant for positive exchange bias [24]. In similar systems, it is already reported that interface mixing can cause modification of exchange coupling [26]. In the present case, Mössbauer data has revealed the fact that there is diffusion of Fe into Co layer at the interface. Development of FeCo alloy, at the interface of such a system is already well established in literature [20, 27]. The non-uniform spread of atoms at the interface induces roughness. The development of rough surface brings in the demagnetization factor because of finite shape of the material.

Please place figure.3 here

In case of single layers, the local roughness causes generation of local in-plane magnetic poles, resulting in demagnetization field [28]. It is because of these magnetic poles or uncompensated spins, the incoherent rotation of spins is favored. For Fe/Co system, such a non uniform response of spins to the applied magnetic field is already reported in Ref. [29]. We argue that these rigid moments act as the pinning centers and hence respond incoherently to the applied magnetic field. These pinned centers are collectively responsible for the establishment of AFM coupling at the interface, resulting in positive EB. However, the actual interaction at the interface may be more complicated. The relation between positive EB and AFM coupling at the interface can be easily understood through simple model considering reversal of soft Fe layer through rigid rotation. The large positive magnetic (pinning) field would be able to align both the layers towards positive direction. While performing hysteresis loop measurement, as the magnetic field decreases the soft Fe layer rotates whereas the hard Co layer remains fixed because of strong anisotropy. As a result of AFM coupling at the interface, which favors antiparallel alignment, the response of soft layer to the reversing magnetic field occurs earlier in loops (not at zero field), at a positive applied magnetic field. Hence, a shift in magnetization loops towards positive magnetic field direction is observed. AFM exchange coupling between Fe and Co bilayers through Cu spacer layer has been reported [30], but no confirmed result on AFM coupling between Fe/Co has been presented so far. We argue that this can be achieved by carefully choosing the deposition condition. IV. CONCLUSION In summary, we have shown the existence of positive EB in exchange coupled soft/hard Fe/Co spring magnet system both

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at room temperature (300K) and low temperature (5K). The effect of pinning field of 7T and -5T results in noticeable shift of magnetic hysteresis loop towards the pinning field direction. We proposed that the existence of positive EB in our system occurs because of the establishment of AFM coupling at the interface caused by the non-uniform spread of Fe spins and due to intermixing of Fe and Co layers. With the application of pinning field, enhanced PEB has been observed. However, we do agree that actual interactions or couplings at the interface might be more complicated and advanced experimental techniques like inelastic neutron scattering can be used to probe the interfacial magnetic structure. Our reported results are in good agreement with previous results performed on other [10, 12] and similar [28] systems. REFERENCES [1] Li K, Wu Y, Guo Z, Zheng Y, Han G, Qiu J, Luo P, An L, and Zhou T, “Exchange coupling and its applications in magnetic data storage,” J. Nanosci. Nanotechnol., vol. 7, pp. 13-45, 2007. [2] W. H. Meiklejohn, “Exchange Anisotropy- A Review,” J. Appl. Phys., vol. 33, pp. 1328-1335, 1962. [3] J. Nogués, and I. K. Schuller, “Exchange Bias,” J. Magn. Magn Mater., vol. 192, pp. 203-232, 1999. [4] N. H. March, P. Lambin, and F. Herman, “Cooperative magnetic properties in single- and two-phase 3d metallic alloys relevant to exchange and magnetocrystalline anisotropy,” J. Magn. Magn. Mater., vol. 44, pp. 1-19, 1984. [5] R. Jungblut, R. Coehoorn, M. T. Johnson, J. aan de Stegge, and A. Reinders, “Orientational dependence of the exchange biasing in molecular‐beam‐epitaxy‐grown Ni80Fe20/Fe50Mn50 bilayers (invited),” J. Appl. Phys., vol. 75, pp. 6659-6664, 1994. [6] D. Niebieskikwiat, and M. B. Salamon, “Intrinsic interface exchange coupling of ferromagnetic nanodomains in a charge ordered manganite,” Phys. Rev. B, vol. 72, pp. 174422-1-174422-6, 2005. [7] S. Mangin, F. Montaigne, and A. Schuhl, “Interface domain wall and exchange bias phenomena in ferrimagnetic/ferrimagnetic bilayers,” Phys. Rev. B, vol. 68, pp. 140404(R)-1-140404(R)-4, 2005. [8] J. Nogués, J. Sort, V. Langlais, V. Skumryev, S. Suriñach, J. S. Muñoz, and M. D. Baró, “Exchange bias in nanostructurese,” Phys. Rep., vol. 422, pp. 65-117, 2005. [9] P. K. Manna, S. M. Yusuf, R. Shukla, and A. K. Tyagi, “Exchange bias in BiFe0.8Mn0.2O3 nanoparticles with an antiferromagnetic core and a diluted antiferromagnetic shell,” Phys. Rev. B, vol. 83, pp. 184412-1184412-5, 2011. [10] M. Kiwi, José Mejía-López, R. D. Portugal, and R. Ramírez, “Positive exchange bias model: Fe/FeF2 and Fe/MnF2 bilayers,” Solid State Comm., vol. 116, pp. 315319, 2000. [11] X. Ke, M. S. Rzchowski, L. J. Belenky and C. B. Eom, “Positive exchange bias in ferromagnetic La0.67Sr0.33MnO3 /SrRuO3 bilayers,” Appl. Phys. Lett., vol. 84, pp. 54585460, 2004 .



> FOR CONFERENCE-RELATED PAPERS, REPLACE THIS LINE WITH YOUR SESSION NUMBER, E.G., AB-02 (DOUBLE-CLICK HERE) < [12] A. Berger, D. T. Margulies, and H. Do, “Magnetic hysteresis loop tuning in antiferromagnetically coupled bilayer structures,” Appl. Phys. Lett., vol. 85, pp. 1571-1573, 2004 . [13] Ch. Binek, S. Polisetty, X. He, and A. Berger, “Exchange bias training effect in coupled all ferromagnetic bilayer structures,” Phys. Rev. Lett., vol. 96, pp. 067201-1-0672014, 2006. [14] E. Goto, N. Hayashi, T. Miyashita, and K. Nakagawa, “Magnetization and switching characteristics of composite thin magnetic films,” J. Appl. Phys., vol. 36, pp. 2951-2958, 1965. [15] E. E. Fullerton, J. S. Jiang, M. Grimsditch, C. H. Sowers, and S. D. Bader, “Exchange-spring behavior in epitaxial hard/soft magnetic bilayers,” Phys. Rev. B, vol. 58, pp. 12193-12200, 1998. [16] E. E. Fullerton, J. S. Jiang, and S. D. Bader, “Hard/soft magnetic heterostructures: model exchange-spring magnets,” J. Magn. Magn. Mater., vol. 200, pp. 392-404, 1999. [17] E. F. Kneller, and R. Hawig, “The exchange-spring magnet: a new material principle for permanent magnets,” IEEE Trans. Magn., vol. 27, pp. 3588-3600, 1999. [18] Z. J. Guo, J. S. Jiang, J. E. Pearson, S. D. Bader, and J. P. Liu, “Exchange-coupled Sm–Co/Nd–Co nanomagnets: correlation between soft phase anisotropy and exchange field,” Appl. Phys. Lett., vol. 81, pp. 2029-2031, 2002. [19]M. Estrader, A. López-Ortega, S. Estradé, I. V. Golosovsky, G. Salazar-Alvarez, M. Vasilakaki, K. N. Trohidou, M. Varela, D.C. Stanley, M. Sinko, M. J. Pechan, D.J. Keavney, F. Peiró, S. Suriñach, M.D. Baró and J. Nogués, “Robust antiferromagnetic coupling in hard-soft bi-magnetic core/shell nanoparticles” Nat. Commun., vol. 4, pp. 2960-1-2960-8, 2013. [20] R. A. Brand, “Improving the validity of hyperfine field distributions from magnetic alloys, part 1: unpolarized source,” Nucl. Instr. And Meth.B, vol. 28, pp. 398-416, 1987. [21] M. Carbuchhio, R. Ciprian, and L. Nasi, “Effects of deposition temperature and elemental layer thickness on the properties of Fe/Co multilayers grown by molecular beam epitaxy,” J. Phys. D: Appl. Phys., vol. 43, pp. 405001-1405001-7, 2010.

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properties of nanoscale Fe/Co bilayers,” Surface and Coatings Tech., vol. 203, pp. 2717-2720, 2009. [23] M. H. Park, Y. K. Hong, S. H. Gee, M. L. Mottern, T. W. Jang, S. Burkett, “Difference in coercivity between Co/Fe and Fe/Co bilayers,” J. Appl. Phys., vol. 91, pp. 72187220, 2002. [24] H. Fulhara, S. Chaudhary, S. C. Kashyap, and D. K. Pandya, “Positive exchange bias in as-deposited ion-beam sputtered IrMn/CoFeB system,” J. Appl. Phys., vol. 110, pp. 093916-1-093916-4, 2011 . [25] J. Nogués, D. Lederman, T. J. Moran, and I. K. Schuller, “Positive Exchange Bias in FeF2-Fe Bilayers,” Phys. Rev. Lett., vol. 76, pp. 4624-4627, 1996. [26] Y. Choi, J. S. Jiang, J. E. Pearson, S. D. Bader, J. J. Kavich, J. W. Freeland, and J. P. Liu, “Controlled interface profile in Sm – Co/Fe exchange-spring magnets,” Appl. Phys. Lett., vol. 91, pp. 072509-1-072509-3, 2007. [27] F. Dirne and C. Denissen, “Interface missing in Fe/Co multilayers,” J. Magn. Magn. Mater., vol. 78, pp. 122-128, 1989. [28] Y. P. Zhao, R. M. Gamache, G. -C. Wang, T. -M. Lu, G. Palasantzas, and J. Hosson, “Surface/interface-roughnessinduced demagnetizing effect in thin magnetic films,” Phys. Rev. B, vol. 60, pp. 1216-1226, 1999. [29] D. Aurongzeb, K. B. Ram, and L. Menon, “Influence of surface/interface roughness and grain size on magnetic property of Fe/Co bilayer,” Appl. Phys. Lett., vol. 87, pp. 172509-1-172509-3, 2005. [30] W. F. Egelhoff, Jr. and M. T. Kief, “Antiferromagnetic coupling in Fe/Cu/Fe and Co/Cu/Co multilayers on Cu(111),” Phys. Rev. B, vol. 45, pp. 7795-7804, 1992.

[22] R. Gupta, A. Khandelwal, R. Ansari, A. Gupta and K. G. M. Nair, “Investigation of structural and magnetic



> FOR CONFERENCE-RELATED PAPERS, REPLACE THIS LINE WITH YOUR SESSION NUMBER, E.G., AB-02 (DOUBLE-CLICK HERE) < Fig. 1 Mössbauer measurement performed at Fe/Co sample at 300K. The inset shows the X-Ray diffraction pattern collected at room temperature for the same sample.

Fig. 2 Magnetic hysteresis loops measurement performed at 300K, 100K and 5K temperatures. The inset shows the MH loop measurement carried out in MOKE setup at 300K.

Fig. 3 Magnetic hysteresis loops measurement performed at 5K, 300K temperatures with -5T and 7T biasing field


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