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Magnetoresistance effect in Gd-doped Cu–Co alloys
Journal of Alloys and Compounds 492 (2010) 56–60
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Magnetoresistance effect in Gd-doped Cu–Co alloys Jacek Jaworski a,∗ , Alicja Strzała a,1 , Oh-Jib Kwon b , Eric Fleury b a b
The Henryk Niewodnicza´ nski Institute of Nuclear Physics PAN, ul. Radzikowskiego 152, 31-342 Kraków, Poland Korea Institute of Science and Technology, Center for Advanced Functional Materials, Hawolgok-dong, Seongbuk-gu, Seoul 136-791, Republic of Korea
a r t i c l e
i n f o
Article history: Received 31 July 2009 Received in revised form 27 November 2009 Accepted 2 December 2009 Available online 16 December 2009 Keywords: Giant magnetoresistance Copper Gadolinium Cobalt Granular alloy Multilayer system Ternary alloy
a b s t r a c t The present paper reports results on the Gd-doped Co–Cu granular alloys and metallic glasses prepared by different processing methods. The main aim of this research was to produce a ternary alloy which radically changes its magnetoresistance (MR) properties below critical temperature. A detailed study on the evolution of the structure and phase separation is presented basing on X-ray diffractometry (XRD) analysis and scanning electron microscopy (SEM) observation. MR was measured by means of a two point probe method in magnetic field using a new experimental device which was patented thereafter. © 2009 Elsevier B.V. All rights reserved.
1. Introduction For the first time, GMR was found in thin film structures composed of alternating ferromagnetic (Fe) and antiferromagnetic (Cr) layers [1,2], next in place of antiferromagnetic layers there were used non-magnetic layers (Cu, Ag, Au). The effect manifests itself as significant decrease in resistance under the influence of magnetic field growing from the zero-field state, when the magnetization of adjacent ferromagnetic layers are antiparallel due to a weak antiferromagnetic coupling between the layers, to a lower level of resistance when the magnetization of the adjacent crystals align due to applied external field. Analogical situation occurs in granular alloys and metallic glasses containing ferromagnetic nano-inclusions. Non-magnetic matrix is doped by a ferromagnetic material, transition metal with well determined magnetic moment. The localized magnetic moments of transition metals interact with themselves by polarizing spins of conduction electrons of nonmagnetic metal. The polarization of conduction electrons changes oscillatory with the growth of the distance from the ferromagnetic crystallite. The antiferromagnetic coupling dependence on the distance between magnetic crystallites is also well described by a RKKY theory [3,4]. Classical multilayer systems and alloys, mani-
∗ Corresponding author. Tel.: +48 12 6628350. E-mail addresses: [email protected] (J. Jaworski), [email protected] (A. Strzała), [email protected] (O.-J. Kwon), efl[email protected] (E. Fleury). 1 Tel.: +48 12 6628350. 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.12.008
festing the GMR effect, consist of immiscible elements. The study of granular alloys consisting of ferromagnetic particles in a paramagnetic matrix became a topic of interest after the discovery of GMR effect in this kind of alloys in 1992 [5,6]. In general, the effect was smaller than in magnetic multilayer systems. GMR in granular alloys is expected to be a function of different parameters such as the distance between magnetic grains, their shape and size and the magnitude of phase segregation. These parameters can be changed using different processing methods, mechanical and/or thermal [7] treatment. Many aspects of the magnetic origin of the magnetoresistance (MR) effect remain unclear. Using sputtering technique to produce thin films [8,9] or rapid solidification processing for the preparation of melt-spun ribbons [10,11] there are obtained ferromagnetic nano-inclusions in non-magnetic matrix and because it several authors proposed the superparamagnetism as the basic magnetic behavior instead of GMR in granular materials. This work was thus undertaken to produce ternary alloys using casting techniques. The alloy compositions were designed in order to obtain structure composed of three phases. The continuous phase should be paramagnetic while the other two should be ferromagnetic phases characterized by different Curie points enabling different magnetic behavioral dependencies on the temperature. The present paper reports results on the Gd-doped Co–Cu granular alloys prepared by different processing methods. The main aim of this research was to produce a ternary alloy which radically changes its magnetoresistant (MR) properties below critical temperature. A detailed study on the evolution of the structure and phase separation is presented basing on X-ray diffractometry (XRD)
J. Jaworski et al. / Journal of Alloys and Compounds 492 (2010) 56–60
57
Fig. 2. SEM picture of squeeze-cast CoCu4 . Two phases are visible: (a) Co12.45 Cu87.55 ; (b) Co83.37 Cu16.63.
Fig. 1. X-ray diffraction spectrum taken in 300 K for Gdx Coy Cu4y granular sample. For compositions with small amount of Gd (a) reflections from Co and Cu dominate and Gd reflections are invisible practically. For alloys Gd-rich (b) reflection from Co and Cu practically disappear. The picture b suggests possible inter-metallic compounds.
analysis and scanning electron microscopy (SEM) observation. MR was measured by means of a two point probe method in magnetic field using a new experimental device [12]. 2. Experimental The investigation involved two main groups of alloys: CoCux (where x = 2, 4, 8, 16) and CoCu4 doped with Gd. The Gdx Coy C4y alloys were designed with a Gd/Co ratio defined as 1/n (where n = 2m and m = 0, 1, 2, 3, 4, 5, 6). The Co–Cu and Co–Cu–Gd alloys were produced in two steps. The mixture of elements has been firstly melted in an electrical arc under argon atmosphere to produce ingots of about 10 g. The ingots have been re-melted and either solidified onto a copper wheel to produce thin ribbons of about 30 m thick and 6 mm wide, or squeeze-cast between two copper dies to produce flat discs of the following dimensions: about 40 mm of diameter and 2 mm thick. For the measurement of the magnetoresistance properties, the thin ribbons were used in the as-spun condition. However, the discs prepared by the squeeze-casting technique were rolled at room temperature using a conventional rolling mill with a reduction rate of about 80%. The samples were observed by optical microscopy and scanning electron microscopy (SEM), the structure was analyzed by X-ray diffraction (XRD), and the composition of the phases was determined using energy dispersive X-ray spectroscopy (EDS). The magnetoresistance (MR) was measured at the temperature range between −196 ◦ C and room temperature in a field ranging from 11 mT to 350 mT. The measurement in a field range from −350 mT to 0 mT and from 0 mT to 350 mT did not show MR amplitude significantly higher than in the range from 11 mT to 350 mT.
peaks of an inter-metallic compounds, which have similar structure as the GdCu6 , orthorhombic structure and space group Pnma, can be detected. For the Gd16.7 Co16.7 Cu66.6 (m = 0) alloy, the peaks ascribed to the Co and Cu phases are not visible, but another inter-metallic compounds, similar to the structure of Gd3 Co, orthorhombic structure and space group Pnma, appeared (Fig. 1b). The structure of these alloys and constituent phases composition were observed and determined by SEM and EDS. The CoCu4 alloy consisting of two phases is shown in Fig. 2: (a) copper rich CoCu7 bright grey matrix and (b) cobalt rich Co5 Cu dark grains. For alloys with a low gadolinium content, as expected from the XRD pattern, three phases could be identified (Fig. 3): the Cuand Co-solid solutions of composition Co7.74 Cu92.26 (grey – c) and Co83.21 Cu16.79 (black – b), respectively, and a Cu-rich phase in light grey of composition Gd13.01 Co7.53 Cu79.46 (a) thus containing a small amount of Co but with all the Gd present in the alloy. The gadolinium rich Gd16.7 Co16.7 Cu66.6 alloy consisted of four phases (Fig. 4) where three of them were represented by Cu-rich alloys. The most abundant element of Cu-rich phases of composition was Gd24.4 Co9.32 Cu66.2 (a), next Gd6.94 Co5.08 Cu87.98 (c) and the least frequent phase was Gd11.06 Co9.12 Cu79.27 (d). The fourth phase was represented by a phase with an equiatomic composition of Co and Cu and relatively high content of Gd–Gd19.48 Co42.16 Cu38.36 (b). On the micrographs in Fig. 5 it is shown that, excepting alloys rich in Gd, the gadolinium element dissolves in the copper phase only. The white phase consisting of Gd–Cu decreases as the Gd content was reduced. This result indicates that Gd has a higher tendency to mix with Cu to form inter-metallic compounds than with Co. The variation of the measured Gd atomic concentration in Gddoped Cu–Co alloys as a function of the Co/Gd ratio is shown in
3. Results and discussion 3.1. Microstructure Typical X-ray spectra obtained for Gdx Coy Cu4y alloys prepared by squeeze-casting technique are shown in Fig. 1. Alloys with low Gd content consist of two phases: Cu-rich and Co-rich phases, both of face-centered cubic structure and space group Fm3m (Fig. 1a). The intensity of the Co and Cu peaks decreased with increasing Gd content in the ternary alloys. From concentration of about 2 at.%,
Fig. 3. SEM picture of squeeze-cast Gd1.24 Co19.75 Cu79.01 . Three phases are visible: (a) Gd13.01 Co7.53 Cu79.46 ; (b) Co83.21 Cu16.79 ; (c) Co7.74 Cu92.26 .
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Fig. 4. SEM picture of squeeze-cast Gd16.7 Co16.7 Cu66.6 . Four phases are visible: (a) Gd24.4 Co9.32 Cu66.28 ; (b) Gd19.48 Co42.16 Cu38.36 ; (c) Gd6.94 Co5.08 Cu87.98 ; (d) Gd11.6 Co9.12 Cu79.27 . Fig. 6. Atomic concentration of Gd in white phase of SEM pictures Gd–Cu–Co alloys as a function of Co/Gd ratio in an alloy.
Fig. 6. Except from the Gd-rich alloy Gd16.7 Co16.7 Cu66.6 , the Gd atomic ratio oscillates around 12.5% which could correspond to the Cu7 Gd high temperature inter-metallic compound [11] owing to the high solidification rate involved in the squeeze-casting process used for the alloy preparation. 3.2. Magnetoresistance properties The magnetoresistance was measured in a magnetic field ranging from 11 mT to 350 mT. These MR measurements indicated that before rolling the Co–Cu and Gd–Co–Cu alloys did not possessed measurable magnetoresistance properties in that range of magnetic field. However, the MR effect could be measured in cold rolled specimens. But not every alloy could be rolled. Particularly alloys with a large amount of gadolinium, such as Gd16.7 Co16.7 Cu66.6 , Gd9.1 Co18.2 Cu72.7 , Gd4.76 Co19.04 Cu76.2 , could not be rolled because of their high brittleness. The measurements of rolled samples were taken in geometry where magnetic field and current were in plane of rolled grains.
For Co–Cu alloys without Gd, the MR was found to decrease with decreasing amount of cobalt in CoCux alloys following a 1/x like function (Fig. 7). Interesting coincidence is that for multilayer systems Co/Cu similar result also exists. In an experiment with changing thickness of non-magnetic spacer [3,13] in Co/Cu thin films structures, for 1 nm Co film thick maxima of GMR occurred for 1, 2, 3 and 4.8 nanometers of Cu layer thickness. Minima of the effect were for copper thickness 0.5, 1.5, 2.5, 3.8, but values of the minimum of the GMR effect for 3.8 nm and maximum for 4.8 nm were very much the same. About 12 nm the coupling between ferromagnetic layers disappeared but magnetoresistance decreased still until 17 nm, where experiment was stopped. Heights of these maxima decreased almost linear for 1, 2, 3 nm, but the trend changed for 4.8 nm. The fourth maximum was the last and from it the GMR effect only decreased until 17 nm. Diameters of Cu and Co atoms are similar and numbers of atoms of Co and Cu layers with equal thickness are almost equal, so Cu/Co atomic ratio for
Fig. 5. SEM photos of trinary alloys of Gdx Coy Cu4y (magnification 2000×); (a) Gd9.1 Co18.2 Cu72.7 ; (b) Gd4.76 Co19.04 Cu76.2 ; (c) Gd2.44 Co19.51 Cu78.05 ; (d) Gd0.62 Co19.88 Cu79.5 . White phase, consisted of Gd–Cu, decreases with reduction of Gd amount.
J. Jaworski et al. / Journal of Alloys and Compounds 492 (2010) 56–60
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Fig. 7. Magnetoresistance dependence on atomic ratio Cu/Co in Co–Cu alloy measured in room temperature.
first three GMR maxima changed almost as integer numbers. The results could be transferred to rolled granular alloys. The rolling of alloys changed shapes of Co grains making them flat and wide in plane of sample. Flat Co grains were parallel to each other in perpendicular direction to the plane of sample and separated by thin copper films of alloy matrix which made them similar to multilayer system. The result suggests that, as described in literature, GMR effect occurs more significantly in thin film structures. Comparing results from the paper [13] and the book [14] with results shown in Fig. 7, it is observable similar behavior of GMR effect for thin film structures and rolled granular alloys. A ratio of magnitudes of GMR effect taken for Cu/Co atomic ratio 2, 4, 8 and 16 is around 2 for neighboring points both multilayer systems and rolled granular alloys. Parkin in his paper [15] showed results of measurements of Ru(1.5 nm)/[Co(1.1 nm)/Cu(tCu )]6 /Ru(5 nm)/Si(1 1 1) and Ru(1.5 nm)/[Co(1.1 nm)/Cu(tCu )]20 /Ru(5 nm)/Si(1 1 1) multilayer systems in RT and and 4.2 K temperature. Magnitudes of GMR changed as (1/tCu ) exp[−(tCu /320)] in RT and in temperature 4.2 K – as 1/tCu , where tCu is proportional to x because tCo constancy. These results for multilayer systems are in good agreement with the result for the rolled Co–Cu granular alloys. It indirectly confirms the suggestion that cold rolling transforms granular alloys to a kind of thin film structure. Both, at room and liquid nitrogen temperatures, the MR of rolled Gd–Co–Cu alloys containing a small amount of Gd initially grows with increasing Gd concentration up to a Gd/Co atomic ratio of about 0.06. Then the magnetoresitance diminishes below the value demonstrated by pure Co–Cu alloy (Fig. 8a and b). The behavior could be explained by big hardness of ferromagnetic Co–Gd or Cu–Gd grains with high Gd content for the last alloy and rolling procedure did not flatten it, so proper structures for GMR effect appearance were not created. It could be two additional reasons for such result. According to González et al. [16] researches on Co–Gd multilayer systems, Curie temperature depends on gadolinium content in Co–Gd alloy; the more Gd the lower Curie temperature is. It could be possible that Curie temperature for the Gd-richest alloy in Fig. 8 was below temperature of liquid nitrogen. The second explanation, to the results described in this section, is a conjecture that both alloys Co–Gd and Cu–Gd became ferromagnetic and coupled with themselves creating a new ferromagnetic structure manifesting anisotropic magnetoresistance (AMR) for bulk materials. The biggest magnetoresistance effect was obtained for ribbons of alloys prepared using melt-spinning technique. During tapeproduction using this method the nanograins are created with big
Fig. 8. Magnetoresistance dependence on atomic ratio Gd/Co in room temperature (a) and temperature of liquid nitrogen (b).
length and width comparatively to their thickness and, because of it, the ribbons could be treated, like rolled granular alloys, as some kind of multilayer system. Fig. 9 shows for the CoCu4 ribbon the MR effect increases almost linearly as the temperature is reduced. For Gd0.62 Co19.88 Cu79 ribbon, the magnetoresistance increases with decreasing temperature in the same way as the MR of CoCu4 ribbon until the temperature reaches the level of −70 ◦ C. Between −70 and −100 ◦ C there is observed a jump of the magnetoresistance. At the temperature of liquid nitrogen, the value of the MR for Gd0.62 Co19.88 Cu79 is almost twice that of the CoCu4 ribbon. The “jump” of magnetoresistance for the alloy containing Gd could be explained by reaching Curie point by Gd13.01 Co7.53 Cu79.46 inter-metallic compound for Gd-poor Gd–Co–Cu alloys. As Camley [17,18] and Altuncevahira with Koymen [19] have shown in their works gadolinium and transition metals (TM) coupling was antiferromagnetic. Theoretical works of Camley [18] has indicated that magnetization of Gd in a zero field is set antiparallel to magnetization of TM like Fe, Co, Ni. The coupling exists above Gd Curie temperature, which for gadolinium thin films is lower than for a bulk [20] and strong depends on thickness. In multilayer systems while TM and Gd layers couple antiferromagnetically at their
Fig. 9. Temperature dependence of magnetoresistance for tapes of CoCu4 and GdCo32 Cu128 (Gd0.62 Co19.88 Cu79 ) ribbons.
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interfaces hence forming an artificial ferrimagnet, the different temperature dependencies of the TM and Gd magnetizations lead to a true antiferromagnet at the compensation temperature TComp , where the Gd and TM magnetizations are equal in magnitude but opposite in direction [21]. When an external field is applied in the plane of surface, the equilibrium structure is the result of the competition between the Zeeman effect, which tends to align all the magnetic moments in the field direction, and exchange energy, which tends to maintain gadolinium moments parallel to each other, transition metal parallel, and, at the interface, gadolinium and TM moments antiparallel [22]. At a critical field Hc transition to a twisted state is observed. This transition is a kind of spin-flop transition with a large canting inside the layers. As it was shown above and in Massalski’s handbook [11] gadolinium makes alloys with cobalt very easy, so probably researchers have got Gd–TM alloys on interfaces, not the sharp Gd–Co interface, what implies thesis that ferromagnetic alloys of gadolinium couple antiferromagnetically. In Gd-poor Gd–Co–Cu alloys the Gd13.01 Co7.53 Cu79.46 alloy phase occurs, which composition is similar to ferromagnetic inter-metallic compound Cu7 Gd with Curie point below −70 ◦ C [11]. Potentially Gd13.01 Co7.53 Cu79.46 is that agent, which drastically increases GMR effect in low temperature by antiferromagnetic coupling with Co nanograins in Gd–Co–Cu metallic glasses by exchange interaction. The explanation of the phenomenon needs further researches. 4. Conclusions In summary, ternary Gd–Co–Cu alloys below certain Gd concentration all Gd elements mix with Cu to form three phases: a Co-rich and a Cu-rich phase without gadolinium and a third phase Cu-rich containing about 12.5 at.% of Gd. In these alloys gadolinium dissolves only in copper rich alloy. For alloys with big amount of Gd, it was observed that several phases possessed gadolinium and some with a large concentration of Gd up to about 25 at.%. It suggests that below a saturation point of Cu-rich phase for gadolinium only Cu participate in Gd alloys creation but about that point and new class of alloys Co containing are being created. Magnetoresistance measurements showed practical absence of MR effect in classical granular alloys with spherical ferromagnetic grains. The cold rolling of the alloys flattened ferromagnetic grains making a structure of alloy similar to micro multilayer system. Analogical situation occurred during metallic glasses production where rotary roller stretched grains, flattening them. The MR investigations showed that below −70 ◦ C, the Gd-poor Gd–Co–Cu compounds exhibited drastic growth of MR effect in comparison with pure Co–Cu alloy, which MR effect increased almost linearly with the decrease in temperature. It could be
suggested that Cu-rich alloy phase containing about 12.5 at.% Gd became ferromagnetic and coupled antiferromagnetic to Co grains increasing number of antiferromagnetic coupled thin films in that quasi-multilayer system. If further research shows that small temperature changes around Curie point of one of alloys ingredients have strong influence on GMR of Gd-poor Gd–Co–Cu metallic glasses, it would be possible to use these composites for production of materials for multi-value logics. With a miniaturization of logical gates technical solutions and recording devices, dimension effects stronger influence their work. Below certain boundary dimension further miniaturization is impossible. Acknowledgments This work has been supported by a grant (code #: 07K150100412) from the ‘Center for Nanostructured Materials Technology’ under ‘21st Century Frontier R&D Program’ of the Ministry of Science and Technology, Korea. References [1] M.N. Baibich, J.M. Broto, A. Fert, N. Van Dau, P. Etienne, G. Creauzet, A. Friedrich, A. Chazelas, Phys. Rev. Lett. 61 (1988) 2472. [2] G. Binasch, P. Grünberg, F. Saurenbach, W. Zinn, Phys. Rev. B 39 (1989) 4828. [3] J.-G. Kim, J.-G. Ha, Mater. Chem. Phys. 96 (2006) 307. [4] S.-F. Hao, B. Fan, L.-M. Wang, Z.-G. Zhang, T. Yu, X.-Q. Li, D.-G. Li, Q.-L. Li, P. Chen, JMMM 320 (2008) 2062. [5] J.Q. Xiao, J.S. Jiang, C.L. Chien, Phys. Rev. Lett. 68 (1992) 3749. [6] P. de Azevedo, M.S. Rogalski, J.B. Sousa, Solid State Commun. 100 (1996) 639. [7] A.E. Berkowitz, Bull. Am. Phys. Soc. 39 (1994) 864. [8] H. Wan, A. Tsoukatos, G.C. Hadjipanayis, Z.G. Li, J. Liu, Phys. Rev. B 49 (1994) 1524. [9] V. Madurga, R.J. Ortega, V. Korenivski, K.V. Rao, March Meeting of the American Physical Society, Pittsburgh, 1994. [10] V. Madurga, R.J. Ortega, V. Korenivski, H. Medeliu, K.V. Rao, Proceedings of the ICM 94, Warsaw, JMMM 465 (1995) 140–144. [11] T.B. Massalski, et al. (Eds.), Binary Alloy Phase Diagram, vol. 1, American Society for Metals, Metals Park, OH, 1986, p. 918. ´ atkowska, ˙ Swi ˛ S. Maranda, M. Marszałek, Z. ˛ [12] J. Jaworski, M. Kac, E. Flaury, O.-J. ˛ do wytwarzania pola magnetycznego o regulowanej Kwon, J.-J. Lee, Przyrzad indukcji (Device for magnetic field generation with controlled induction), P384311, 23.01.2008. [13] S.S.P. Parkin, Annu. Rev. Mater. Sci. 25 (1995) 357. [14] B. Heinrich, J.A.C. Bland, Ultrathin Magnetic Structures II; Measurement Techniques and Novel Magnetic Properties, Springer-Verlag, Berlin, Heidelberg, 1994. [15] S.S.P. Parkin, Phys. Rev. B 47 (1993) 9136. [16] J.A. González, J.P. Andrès, M.A. Arranz, M.A. Lòpez de la Torre, J.M. Riveiro, J. Appl. Phys. 92 (2002) 914. [17] R.E. Camley, Phys. Rev. B 35 (1987) 3608. [18] R.E. Camley, Phys. Rev. B 39 (1989) 12316. [19] B. Altuncevahira, A.R. Koymen, J. Appl. Phys. 90 (2001) 2939. [20] J.S. Jiang, D. Davidivic, D.H. Reich, C.L. Chien, Phys. Rev. Lett. 74 (1995) 314. [21] Y. Choi, D. Haskel, A. Cady, J.C. Lang, D.R. Lee, G. Srajer, J.S. Jiang, S.D. Bader, Phys. Rev. B 73 (2006) 174401. [22] M. Vaezzadeh, B. George, G. Marchal, Phys. Rev. B 50 (1994) 6113.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Magnetoresistance effect in Gd-doped Cu–Co alloys Jacek Jaworski a,∗ , Alicja Strzała a,1 , Oh-Jib Kwon b , Eric Fleury b a b
The Henryk Niewodnicza´ nski Institute of Nuclear Physics PAN, ul. Radzikowskiego 152, 31-342 Kraków, Poland Korea Institute of Science and Technology, Center for Advanced Functional Materials, Hawolgok-dong, Seongbuk-gu, Seoul 136-791, Republic of Korea
a r t i c l e
i n f o
Article history: Received 31 July 2009 Received in revised form 27 November 2009 Accepted 2 December 2009 Available online 16 December 2009 Keywords: Giant magnetoresistance Copper Gadolinium Cobalt Granular alloy Multilayer system Ternary alloy
a b s t r a c t The present paper reports results on the Gd-doped Co–Cu granular alloys and metallic glasses prepared by different processing methods. The main aim of this research was to produce a ternary alloy which radically changes its magnetoresistance (MR) properties below critical temperature. A detailed study on the evolution of the structure and phase separation is presented basing on X-ray diffractometry (XRD) analysis and scanning electron microscopy (SEM) observation. MR was measured by means of a two point probe method in magnetic field using a new experimental device which was patented thereafter. © 2009 Elsevier B.V. All rights reserved.
1. Introduction For the first time, GMR was found in thin film structures composed of alternating ferromagnetic (Fe) and antiferromagnetic (Cr) layers [1,2], next in place of antiferromagnetic layers there were used non-magnetic layers (Cu, Ag, Au). The effect manifests itself as significant decrease in resistance under the influence of magnetic field growing from the zero-field state, when the magnetization of adjacent ferromagnetic layers are antiparallel due to a weak antiferromagnetic coupling between the layers, to a lower level of resistance when the magnetization of the adjacent crystals align due to applied external field. Analogical situation occurs in granular alloys and metallic glasses containing ferromagnetic nano-inclusions. Non-magnetic matrix is doped by a ferromagnetic material, transition metal with well determined magnetic moment. The localized magnetic moments of transition metals interact with themselves by polarizing spins of conduction electrons of nonmagnetic metal. The polarization of conduction electrons changes oscillatory with the growth of the distance from the ferromagnetic crystallite. The antiferromagnetic coupling dependence on the distance between magnetic crystallites is also well described by a RKKY theory [3,4]. Classical multilayer systems and alloys, mani-
∗ Corresponding author. Tel.: +48 12 6628350. E-mail addresses: [email protected] (J. Jaworski), [email protected] (A. Strzała), [email protected] (O.-J. Kwon), efl[email protected] (E. Fleury). 1 Tel.: +48 12 6628350. 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.12.008
festing the GMR effect, consist of immiscible elements. The study of granular alloys consisting of ferromagnetic particles in a paramagnetic matrix became a topic of interest after the discovery of GMR effect in this kind of alloys in 1992 [5,6]. In general, the effect was smaller than in magnetic multilayer systems. GMR in granular alloys is expected to be a function of different parameters such as the distance between magnetic grains, their shape and size and the magnitude of phase segregation. These parameters can be changed using different processing methods, mechanical and/or thermal [7] treatment. Many aspects of the magnetic origin of the magnetoresistance (MR) effect remain unclear. Using sputtering technique to produce thin films [8,9] or rapid solidification processing for the preparation of melt-spun ribbons [10,11] there are obtained ferromagnetic nano-inclusions in non-magnetic matrix and because it several authors proposed the superparamagnetism as the basic magnetic behavior instead of GMR in granular materials. This work was thus undertaken to produce ternary alloys using casting techniques. The alloy compositions were designed in order to obtain structure composed of three phases. The continuous phase should be paramagnetic while the other two should be ferromagnetic phases characterized by different Curie points enabling different magnetic behavioral dependencies on the temperature. The present paper reports results on the Gd-doped Co–Cu granular alloys prepared by different processing methods. The main aim of this research was to produce a ternary alloy which radically changes its magnetoresistant (MR) properties below critical temperature. A detailed study on the evolution of the structure and phase separation is presented basing on X-ray diffractometry (XRD)
J. Jaworski et al. / Journal of Alloys and Compounds 492 (2010) 56–60
57
Fig. 2. SEM picture of squeeze-cast CoCu4 . Two phases are visible: (a) Co12.45 Cu87.55 ; (b) Co83.37 Cu16.63.
Fig. 1. X-ray diffraction spectrum taken in 300 K for Gdx Coy Cu4y granular sample. For compositions with small amount of Gd (a) reflections from Co and Cu dominate and Gd reflections are invisible practically. For alloys Gd-rich (b) reflection from Co and Cu practically disappear. The picture b suggests possible inter-metallic compounds.
analysis and scanning electron microscopy (SEM) observation. MR was measured by means of a two point probe method in magnetic field using a new experimental device [12]. 2. Experimental The investigation involved two main groups of alloys: CoCux (where x = 2, 4, 8, 16) and CoCu4 doped with Gd. The Gdx Coy C4y alloys were designed with a Gd/Co ratio defined as 1/n (where n = 2m and m = 0, 1, 2, 3, 4, 5, 6). The Co–Cu and Co–Cu–Gd alloys were produced in two steps. The mixture of elements has been firstly melted in an electrical arc under argon atmosphere to produce ingots of about 10 g. The ingots have been re-melted and either solidified onto a copper wheel to produce thin ribbons of about 30 m thick and 6 mm wide, or squeeze-cast between two copper dies to produce flat discs of the following dimensions: about 40 mm of diameter and 2 mm thick. For the measurement of the magnetoresistance properties, the thin ribbons were used in the as-spun condition. However, the discs prepared by the squeeze-casting technique were rolled at room temperature using a conventional rolling mill with a reduction rate of about 80%. The samples were observed by optical microscopy and scanning electron microscopy (SEM), the structure was analyzed by X-ray diffraction (XRD), and the composition of the phases was determined using energy dispersive X-ray spectroscopy (EDS). The magnetoresistance (MR) was measured at the temperature range between −196 ◦ C and room temperature in a field ranging from 11 mT to 350 mT. The measurement in a field range from −350 mT to 0 mT and from 0 mT to 350 mT did not show MR amplitude significantly higher than in the range from 11 mT to 350 mT.
peaks of an inter-metallic compounds, which have similar structure as the GdCu6 , orthorhombic structure and space group Pnma, can be detected. For the Gd16.7 Co16.7 Cu66.6 (m = 0) alloy, the peaks ascribed to the Co and Cu phases are not visible, but another inter-metallic compounds, similar to the structure of Gd3 Co, orthorhombic structure and space group Pnma, appeared (Fig. 1b). The structure of these alloys and constituent phases composition were observed and determined by SEM and EDS. The CoCu4 alloy consisting of two phases is shown in Fig. 2: (a) copper rich CoCu7 bright grey matrix and (b) cobalt rich Co5 Cu dark grains. For alloys with a low gadolinium content, as expected from the XRD pattern, three phases could be identified (Fig. 3): the Cuand Co-solid solutions of composition Co7.74 Cu92.26 (grey – c) and Co83.21 Cu16.79 (black – b), respectively, and a Cu-rich phase in light grey of composition Gd13.01 Co7.53 Cu79.46 (a) thus containing a small amount of Co but with all the Gd present in the alloy. The gadolinium rich Gd16.7 Co16.7 Cu66.6 alloy consisted of four phases (Fig. 4) where three of them were represented by Cu-rich alloys. The most abundant element of Cu-rich phases of composition was Gd24.4 Co9.32 Cu66.2 (a), next Gd6.94 Co5.08 Cu87.98 (c) and the least frequent phase was Gd11.06 Co9.12 Cu79.27 (d). The fourth phase was represented by a phase with an equiatomic composition of Co and Cu and relatively high content of Gd–Gd19.48 Co42.16 Cu38.36 (b). On the micrographs in Fig. 5 it is shown that, excepting alloys rich in Gd, the gadolinium element dissolves in the copper phase only. The white phase consisting of Gd–Cu decreases as the Gd content was reduced. This result indicates that Gd has a higher tendency to mix with Cu to form inter-metallic compounds than with Co. The variation of the measured Gd atomic concentration in Gddoped Cu–Co alloys as a function of the Co/Gd ratio is shown in
3. Results and discussion 3.1. Microstructure Typical X-ray spectra obtained for Gdx Coy Cu4y alloys prepared by squeeze-casting technique are shown in Fig. 1. Alloys with low Gd content consist of two phases: Cu-rich and Co-rich phases, both of face-centered cubic structure and space group Fm3m (Fig. 1a). The intensity of the Co and Cu peaks decreased with increasing Gd content in the ternary alloys. From concentration of about 2 at.%,
Fig. 3. SEM picture of squeeze-cast Gd1.24 Co19.75 Cu79.01 . Three phases are visible: (a) Gd13.01 Co7.53 Cu79.46 ; (b) Co83.21 Cu16.79 ; (c) Co7.74 Cu92.26 .
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Fig. 4. SEM picture of squeeze-cast Gd16.7 Co16.7 Cu66.6 . Four phases are visible: (a) Gd24.4 Co9.32 Cu66.28 ; (b) Gd19.48 Co42.16 Cu38.36 ; (c) Gd6.94 Co5.08 Cu87.98 ; (d) Gd11.6 Co9.12 Cu79.27 . Fig. 6. Atomic concentration of Gd in white phase of SEM pictures Gd–Cu–Co alloys as a function of Co/Gd ratio in an alloy.
Fig. 6. Except from the Gd-rich alloy Gd16.7 Co16.7 Cu66.6 , the Gd atomic ratio oscillates around 12.5% which could correspond to the Cu7 Gd high temperature inter-metallic compound [11] owing to the high solidification rate involved in the squeeze-casting process used for the alloy preparation. 3.2. Magnetoresistance properties The magnetoresistance was measured in a magnetic field ranging from 11 mT to 350 mT. These MR measurements indicated that before rolling the Co–Cu and Gd–Co–Cu alloys did not possessed measurable magnetoresistance properties in that range of magnetic field. However, the MR effect could be measured in cold rolled specimens. But not every alloy could be rolled. Particularly alloys with a large amount of gadolinium, such as Gd16.7 Co16.7 Cu66.6 , Gd9.1 Co18.2 Cu72.7 , Gd4.76 Co19.04 Cu76.2 , could not be rolled because of their high brittleness. The measurements of rolled samples were taken in geometry where magnetic field and current were in plane of rolled grains.
For Co–Cu alloys without Gd, the MR was found to decrease with decreasing amount of cobalt in CoCux alloys following a 1/x like function (Fig. 7). Interesting coincidence is that for multilayer systems Co/Cu similar result also exists. In an experiment with changing thickness of non-magnetic spacer [3,13] in Co/Cu thin films structures, for 1 nm Co film thick maxima of GMR occurred for 1, 2, 3 and 4.8 nanometers of Cu layer thickness. Minima of the effect were for copper thickness 0.5, 1.5, 2.5, 3.8, but values of the minimum of the GMR effect for 3.8 nm and maximum for 4.8 nm were very much the same. About 12 nm the coupling between ferromagnetic layers disappeared but magnetoresistance decreased still until 17 nm, where experiment was stopped. Heights of these maxima decreased almost linear for 1, 2, 3 nm, but the trend changed for 4.8 nm. The fourth maximum was the last and from it the GMR effect only decreased until 17 nm. Diameters of Cu and Co atoms are similar and numbers of atoms of Co and Cu layers with equal thickness are almost equal, so Cu/Co atomic ratio for
Fig. 5. SEM photos of trinary alloys of Gdx Coy Cu4y (magnification 2000×); (a) Gd9.1 Co18.2 Cu72.7 ; (b) Gd4.76 Co19.04 Cu76.2 ; (c) Gd2.44 Co19.51 Cu78.05 ; (d) Gd0.62 Co19.88 Cu79.5 . White phase, consisted of Gd–Cu, decreases with reduction of Gd amount.
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Fig. 7. Magnetoresistance dependence on atomic ratio Cu/Co in Co–Cu alloy measured in room temperature.
first three GMR maxima changed almost as integer numbers. The results could be transferred to rolled granular alloys. The rolling of alloys changed shapes of Co grains making them flat and wide in plane of sample. Flat Co grains were parallel to each other in perpendicular direction to the plane of sample and separated by thin copper films of alloy matrix which made them similar to multilayer system. The result suggests that, as described in literature, GMR effect occurs more significantly in thin film structures. Comparing results from the paper [13] and the book [14] with results shown in Fig. 7, it is observable similar behavior of GMR effect for thin film structures and rolled granular alloys. A ratio of magnitudes of GMR effect taken for Cu/Co atomic ratio 2, 4, 8 and 16 is around 2 for neighboring points both multilayer systems and rolled granular alloys. Parkin in his paper [15] showed results of measurements of Ru(1.5 nm)/[Co(1.1 nm)/Cu(tCu )]6 /Ru(5 nm)/Si(1 1 1) and Ru(1.5 nm)/[Co(1.1 nm)/Cu(tCu )]20 /Ru(5 nm)/Si(1 1 1) multilayer systems in RT and and 4.2 K temperature. Magnitudes of GMR changed as (1/tCu ) exp[−(tCu /320)] in RT and in temperature 4.2 K – as 1/tCu , where tCu is proportional to x because tCo constancy. These results for multilayer systems are in good agreement with the result for the rolled Co–Cu granular alloys. It indirectly confirms the suggestion that cold rolling transforms granular alloys to a kind of thin film structure. Both, at room and liquid nitrogen temperatures, the MR of rolled Gd–Co–Cu alloys containing a small amount of Gd initially grows with increasing Gd concentration up to a Gd/Co atomic ratio of about 0.06. Then the magnetoresitance diminishes below the value demonstrated by pure Co–Cu alloy (Fig. 8a and b). The behavior could be explained by big hardness of ferromagnetic Co–Gd or Cu–Gd grains with high Gd content for the last alloy and rolling procedure did not flatten it, so proper structures for GMR effect appearance were not created. It could be two additional reasons for such result. According to González et al. [16] researches on Co–Gd multilayer systems, Curie temperature depends on gadolinium content in Co–Gd alloy; the more Gd the lower Curie temperature is. It could be possible that Curie temperature for the Gd-richest alloy in Fig. 8 was below temperature of liquid nitrogen. The second explanation, to the results described in this section, is a conjecture that both alloys Co–Gd and Cu–Gd became ferromagnetic and coupled with themselves creating a new ferromagnetic structure manifesting anisotropic magnetoresistance (AMR) for bulk materials. The biggest magnetoresistance effect was obtained for ribbons of alloys prepared using melt-spinning technique. During tapeproduction using this method the nanograins are created with big
Fig. 8. Magnetoresistance dependence on atomic ratio Gd/Co in room temperature (a) and temperature of liquid nitrogen (b).
length and width comparatively to their thickness and, because of it, the ribbons could be treated, like rolled granular alloys, as some kind of multilayer system. Fig. 9 shows for the CoCu4 ribbon the MR effect increases almost linearly as the temperature is reduced. For Gd0.62 Co19.88 Cu79 ribbon, the magnetoresistance increases with decreasing temperature in the same way as the MR of CoCu4 ribbon until the temperature reaches the level of −70 ◦ C. Between −70 and −100 ◦ C there is observed a jump of the magnetoresistance. At the temperature of liquid nitrogen, the value of the MR for Gd0.62 Co19.88 Cu79 is almost twice that of the CoCu4 ribbon. The “jump” of magnetoresistance for the alloy containing Gd could be explained by reaching Curie point by Gd13.01 Co7.53 Cu79.46 inter-metallic compound for Gd-poor Gd–Co–Cu alloys. As Camley [17,18] and Altuncevahira with Koymen [19] have shown in their works gadolinium and transition metals (TM) coupling was antiferromagnetic. Theoretical works of Camley [18] has indicated that magnetization of Gd in a zero field is set antiparallel to magnetization of TM like Fe, Co, Ni. The coupling exists above Gd Curie temperature, which for gadolinium thin films is lower than for a bulk [20] and strong depends on thickness. In multilayer systems while TM and Gd layers couple antiferromagnetically at their
Fig. 9. Temperature dependence of magnetoresistance for tapes of CoCu4 and GdCo32 Cu128 (Gd0.62 Co19.88 Cu79 ) ribbons.
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interfaces hence forming an artificial ferrimagnet, the different temperature dependencies of the TM and Gd magnetizations lead to a true antiferromagnet at the compensation temperature TComp , where the Gd and TM magnetizations are equal in magnitude but opposite in direction [21]. When an external field is applied in the plane of surface, the equilibrium structure is the result of the competition between the Zeeman effect, which tends to align all the magnetic moments in the field direction, and exchange energy, which tends to maintain gadolinium moments parallel to each other, transition metal parallel, and, at the interface, gadolinium and TM moments antiparallel [22]. At a critical field Hc transition to a twisted state is observed. This transition is a kind of spin-flop transition with a large canting inside the layers. As it was shown above and in Massalski’s handbook [11] gadolinium makes alloys with cobalt very easy, so probably researchers have got Gd–TM alloys on interfaces, not the sharp Gd–Co interface, what implies thesis that ferromagnetic alloys of gadolinium couple antiferromagnetically. In Gd-poor Gd–Co–Cu alloys the Gd13.01 Co7.53 Cu79.46 alloy phase occurs, which composition is similar to ferromagnetic inter-metallic compound Cu7 Gd with Curie point below −70 ◦ C [11]. Potentially Gd13.01 Co7.53 Cu79.46 is that agent, which drastically increases GMR effect in low temperature by antiferromagnetic coupling with Co nanograins in Gd–Co–Cu metallic glasses by exchange interaction. The explanation of the phenomenon needs further researches. 4. Conclusions In summary, ternary Gd–Co–Cu alloys below certain Gd concentration all Gd elements mix with Cu to form three phases: a Co-rich and a Cu-rich phase without gadolinium and a third phase Cu-rich containing about 12.5 at.% of Gd. In these alloys gadolinium dissolves only in copper rich alloy. For alloys with big amount of Gd, it was observed that several phases possessed gadolinium and some with a large concentration of Gd up to about 25 at.%. It suggests that below a saturation point of Cu-rich phase for gadolinium only Cu participate in Gd alloys creation but about that point and new class of alloys Co containing are being created. Magnetoresistance measurements showed practical absence of MR effect in classical granular alloys with spherical ferromagnetic grains. The cold rolling of the alloys flattened ferromagnetic grains making a structure of alloy similar to micro multilayer system. Analogical situation occurred during metallic glasses production where rotary roller stretched grains, flattening them. The MR investigations showed that below −70 ◦ C, the Gd-poor Gd–Co–Cu compounds exhibited drastic growth of MR effect in comparison with pure Co–Cu alloy, which MR effect increased almost linearly with the decrease in temperature. It could be
suggested that Cu-rich alloy phase containing about 12.5 at.% Gd became ferromagnetic and coupled antiferromagnetic to Co grains increasing number of antiferromagnetic coupled thin films in that quasi-multilayer system. If further research shows that small temperature changes around Curie point of one of alloys ingredients have strong influence on GMR of Gd-poor Gd–Co–Cu metallic glasses, it would be possible to use these composites for production of materials for multi-value logics. With a miniaturization of logical gates technical solutions and recording devices, dimension effects stronger influence their work. Below certain boundary dimension further miniaturization is impossible. Acknowledgments This work has been supported by a grant (code #: 07K150100412) from the ‘Center for Nanostructured Materials Technology’ under ‘21st Century Frontier R&D Program’ of the Ministry of Science and Technology, Korea. References [1] M.N. Baibich, J.M. Broto, A. Fert, N. Van Dau, P. Etienne, G. Creauzet, A. Friedrich, A. Chazelas, Phys. Rev. Lett. 61 (1988) 2472. [2] G. Binasch, P. Grünberg, F. Saurenbach, W. Zinn, Phys. Rev. B 39 (1989) 4828. [3] J.-G. Kim, J.-G. Ha, Mater. Chem. Phys. 96 (2006) 307. [4] S.-F. Hao, B. Fan, L.-M. Wang, Z.-G. Zhang, T. Yu, X.-Q. Li, D.-G. Li, Q.-L. Li, P. Chen, JMMM 320 (2008) 2062. [5] J.Q. Xiao, J.S. Jiang, C.L. Chien, Phys. Rev. Lett. 68 (1992) 3749. [6] P. de Azevedo, M.S. Rogalski, J.B. Sousa, Solid State Commun. 100 (1996) 639. [7] A.E. Berkowitz, Bull. Am. Phys. Soc. 39 (1994) 864. [8] H. Wan, A. Tsoukatos, G.C. Hadjipanayis, Z.G. Li, J. Liu, Phys. Rev. B 49 (1994) 1524. [9] V. Madurga, R.J. Ortega, V. Korenivski, K.V. Rao, March Meeting of the American Physical Society, Pittsburgh, 1994. [10] V. Madurga, R.J. Ortega, V. Korenivski, H. Medeliu, K.V. Rao, Proceedings of the ICM 94, Warsaw, JMMM 465 (1995) 140–144. [11] T.B. Massalski, et al. (Eds.), Binary Alloy Phase Diagram, vol. 1, American Society for Metals, Metals Park, OH, 1986, p. 918. ´ atkowska, ˙ Swi ˛ S. Maranda, M. Marszałek, Z. ˛ [12] J. Jaworski, M. Kac, E. Flaury, O.-J. ˛ do wytwarzania pola magnetycznego o regulowanej Kwon, J.-J. Lee, Przyrzad indukcji (Device for magnetic field generation with controlled induction), P384311, 23.01.2008. [13] S.S.P. Parkin, Annu. Rev. Mater. Sci. 25 (1995) 357. [14] B. Heinrich, J.A.C. Bland, Ultrathin Magnetic Structures II; Measurement Techniques and Novel Magnetic Properties, Springer-Verlag, Berlin, Heidelberg, 1994. [15] S.S.P. Parkin, Phys. Rev. B 47 (1993) 9136. [16] J.A. González, J.P. Andrès, M.A. Arranz, M.A. Lòpez de la Torre, J.M. Riveiro, J. Appl. Phys. 92 (2002) 914. [17] R.E. Camley, Phys. Rev. B 35 (1987) 3608. [18] R.E. Camley, Phys. Rev. B 39 (1989) 12316. [19] B. Altuncevahira, A.R. Koymen, J. Appl. Phys. 90 (2001) 2939. [20] J.S. Jiang, D. Davidivic, D.H. Reich, C.L. Chien, Phys. Rev. Lett. 74 (1995) 314. [21] Y. Choi, D. Haskel, A. Cady, J.C. Lang, D.R. Lee, G. Srajer, J.S. Jiang, S.D. Bader, Phys. Rev. B 73 (2006) 174401. [22] M. Vaezzadeh, B. George, G. Marchal, Phys. Rev. B 50 (1994) 6113.