ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 282 (2004) 44–48 Nanostructure and magnetization reversal process in TbFeCo/Yx(FeCo)1Àx spring-magnet type multilayers N.H Duca,*, D.T Huong Gianga, N Chaub b a Cryogenic Laboratory, Faculty of Physics, Vietnam National University, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam Center for Materials Science, Faculty of Physics, Vietnam National University, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam Available online 28 April 2004 Abstract Studies of the naturally formed nanostructure and magnetization reversal process were performed for the sputtered Tb(Fe0.55Co0.45)1.5/Yx(Fe0.7Co0.3)1Àx multilayers (0pxp0.2) with a TbFeCo layer thickness tTbFeCo=12 nm and YFeCo layer thickness tYFeCo=10 nm The structural investigations showed that nanocrystals are naturally formed and coexist within the amorphous matrix in Y0.1(FeCo)0.9 layers In this state, low magnetic coercivity and large parallel magnetostrictive susceptibility are observed The results are discussed in terms of the crystalline discontinuity of the soft YFeCo layers r 2004 Elsevier B.V All rights reserved PACS: 75.60.Jk; 75.70.Cn; 81.07.Bc Keywords: Spring-exchange multilayers; Nanocrystalline structure; Magnetization reversal; Giant magnetostriction Introduction The exchange-spring concept [1] opened an alternative route towards new high-performance hard magnetic materials By associating a coercive hard magnetic phase with a large magnetization soft phase, it was expected that new high-energy product materials could be prepared Exchangespring behavior was found in various systems However, as far as our knowledge, no material has been found with properties clearly superior to those of usual hard magnetic materials Mean*Corresponding author Tel.: +84-8-7680978; fax: +84-48340724 E-mail address: ducnh@vnu.edu.vn (N.H Duc) while, this concept has successfully been applied to the so-called giant magnetostrictive spring-exchange multilayers, where high magnetostrictive layers and soft magnetic layers alternate [2] Indeed, Quandt and Ludwig [3] obtained a magnetostriction as large as 890  10À6 and a huge parallel magnetostrictive susceptibility (wlJ=dlJ/dH) of  10À2 TÀ1 in an applied field of about a few 10 mT for magnetostrictive/soft magnetic TbFeCo/FeCo multilayers In this case, magnetization reversal is thought to be nucleated within the soft layer in a low applied field and propagates from the soft layers into the magnetostrictive layers [4] In the TbFeCo/FeCo multilayers, the soft FeCo-layer is continuous (Fig 1a), thus the nucleation of reversal occurs at some 0304-8853/$ - see front matter r 2004 Elsevier B.V All rights reserved doi:10.1016/j.jmmm.2004.04.010 ARTICLE IN PRESS N.H Duc et al / Journal of Magnetism and Magnetic Materials 282 (2004) 44–48 Fig Illustration of the magnetostrictive exchange-spring multilayers with a structural continuous (a) and discontinuous (b) soft magnetic layer defect points on the sample surface and interfaces In this context, one expects that the reversal can be nucleated in an easier way in discontinuous soft phase, i.e in layers in which the FeCo nanograins are embedded within a non-magnetic matrix (Fig 1b) Practically, an excellent magnetic softness has recently been reported for Tb(Fe0.55Co0.45)1.5/ Y0.2(Fe0.8Co0.2)0.8 (denotes as Terfecohan/ Y0.2(Fe0.8Co0.2)0.8 multilayers, in which the nanostructure of YFeCo layers was formed from an amorphous phase by heat treatments [5–6] As far as our knowledge, the transformation of the amorphous state in the RCoFe (R = rare earths and/or light transition metals) layer is shown to be dependent on the R-concentration [7] At a critical R-concentration, the nanostructured RFeCo layer is expected to be naturally formed in as-deposited multilayers By this way, an optimization of the magnetostriction and magnetostrictive softness can be reached right after depositing or/and annealing at low temperatures In this paper, a direct approach to the natural nanostructure and large parallel magnetostrictive susceptibility will be applied for the Terfecohan/ Yx(Fe0.7Co0.3)1Àx multilayers The results are discussed in terms of the structural discontinuity caused by the formation of the nanostructure in the soft magnetic layers Experimental {Terfecohan/Yx(Fe0.7Co0.3)1Àx}n multilayers with x=0, 0.1, 0.2, n=50 and the individual layer 45 thicknesses are tTbFeCo=12 nm and tFeCo=10 nm were fabricated by RF-magnetron sputtering The typical power during sputtering was 200 W and the Ar pressure was 10À2 mbar Composite targets were used which consisted of segments of different elements (here Tb, Y, Fe, Co) The substrates were glass microscope cover slips with a nominal thickness of 150 mm Both target and sample holders were water-cooled Samples were annealed at different temperatures TA=200 C, 300 C, 350 C, 400 C and 450 C for h in a vacuum of  10À5 mbar The crystal structure of the sample was investigated by X-ray diffraction using the D5005 Siemens with a copper anticathode The magnetization was measured with a vibrating magnetization magnetometer (VSM) in a magnetic field upto 1.4 T at room temperature The magnetostriction was measured by using an optical deflectometer (resolution of  10À6 rad), in which the bending of the substrate due to the magnetostriction in the film was determined Experimental results and discussion In the x=0 sample, the large X-ray diffraction intensity at 2y=45 is characteristic of the (1 0) reflection of BCC-Fe (Fig 2) No other diffraction peaks are observed indicating that the TbFeCo layer is amorphous The intensity of the BCC-Fe reflection is strongly reduced in the x=0.1 sample This is attributed to the formation of BCC-Fe nanocrystals in the YFeCo layers The crosssectional HRTEM image shown in Fig reveals the coexistence of nanograins (with an average grain size of about 10 nm) and of an amorphous phase in the Y0.1(Fe,Co)0.9 layers This transformation to the nanostructure was associated with the reduction of the thermodynamic driving force for the crystallization caused by substitution [8] Finally, the (1 0) reflection almost disappears at x=0.2 reflecting the fact that the whole layer is now amorphous Similar phenomenon was observed for the Terfecohan/YxFe1Àx multilayers [4] Low-temperature annealing (at TAp350 C) is usually performed to relieve the stress induced during the sputtering process At present, as ARTICLE IN PRESS 46 N.H Duc et al / Journal of Magnetism and Magnetic Materials 282 (2004) 44–48 magnetization By, increasing x from to 0.1, the (magnetic) coercivity (MHC) decreases from 4.8 to 3.1 mT then it increases again to 6.2 mT for x=0.2 In these samples, the YFeCo composition is ferromagnetic at room temperature [7] It is that the FeCo (and/or YFeCo)/TbFeCo coupling imposes in-plane magnetization The smallest coercivity value found in the sample with x=0.1 may be attributed to the specific nanostructure of this sample as observed by X-ray diffraction results at room temperature In the TbFeCo/FeCo multilayer, the soft FeCo-layer is continuous We thus expect that the nucleation of reversal occurs at some defect points on the sample surface In Fig X-ray diffraction patterns of as-deposited Terfecohan/ Yx(Fe,Co)1Àx multilayers Fig X-ray diffraction patterns of 350 -annealed Terfecohan/ Yx(Fe,Co)1Àx multilayers Fig Cross-sectional HR-TEM image of the as-deposited Terfecohan/Y0.1(Fe,Co)0.9 multilayer illustrated in Fig 4, the microstructure of the 350 C-annealed samples with x=0 and 0.1 is almost the same as that of the corresponding asdeposited ones However, the modification of the amorphous state to form BCC-Fe nanostructured phase is observed in the Y0.2(Fe,Co)0.8 layers The magnetic hysteresis loops measured as a function of the magnetic fields applied in the film plane are presented in Fig For all samples, the observed curves are characteristic of in-plane Fig Magnetic hysteresis loops of as-deposited Terfecohan/ Yx(Fe,Co)1Àx multilayers ARTICLE IN PRESS N.H Duc et al / Journal of Magnetism and Magnetic Materials 282 (2004) 44–48 TbFeCo/Y0.1(FeCo)0.9, the FeCo grains are nanocrystallized and embedded within an amorphous matrix Each FeCo nanocrystals are decoupled from each other Soft phase reversal can then be nucleated at any of the nanocrystals, on a defect position Statistically, it is expected that nucleation will be easier than in FeCo-pure system This explains qualitatively the observed difference in coercive field values between the samples with x=0 and 0.1 For x=0.2, the whole Y0.2(FeCo)0.8 layer becomes continuous in the amorphous state, then the magnetic coercivity is enhanced again Magnetic softness improvement due to stress releasing effects is clearly provided by the reduction of the magnetic coercivity with the same factor of in the 350 C-annealed samples with x=0 and 0.1 This coercivity decreasing factor increases up to in the x=0.2 sample Moreover, it is worthwhile to note that the coercivity in the 350 C-annealed samples with x=0.1 and 0.2 is almost comparable (i.e MHC equals to 1.7 and 1.6 mT) In this context, it is possible to argue that, besides the stress releasing effects, the nanostructure formed in Y0.2(FeCo)0.8 layers due to heat treatment must be the reason for the low magnetic coercivity mechanism Magnetostriction l (=lJ-l>) data are determined For all samples, the magnetostriction obtained is comparable to the value deduced from the data of the single-layer Terfecohan samples, e.g lTbFeCoB10À3 [9–12] using the following expression [23,6]: /lS ẳ lYFeCo tYFeCo ỵ lTbFeCo tTbFeCo : tYFeCo ỵ tTbFeCo Low-eld parallel magnetostriction lJ data are presented in Fig 6(a) for the as-deposited and 350 C-annealed films with x=0.1 Like in magnetic hysteresis loops, there is a so-called (magnetostrictive) coercive field (lHC), where l=0 in the magnetostrictive hysteresis loops Experimentally, the lHC is observed to be equal to the MHC value obtained from the magnetization measurements In addition, it is in good agreement with that already reported in Ref [12] that the magnetostrictive response to applied fields is always strongest in the magnetizing fields just above the coercivity Because the performance of microsys- 47 Fig Low-field magnetostriction (a) and parallel magnetostrictive susceptibility (b) data of the as-deposited and 350 Cannealed x=0.1 multilayer tems is determined by the parallel magnetostrictive response to an applied field, the observed behavior is an important factor to consider the working point for the magnetostrictive films in microsystems In this case, the value of the parallel magnetostrictive susceptibility wlJ is usually discussed The low-field dependence of the parallel magnetostrictive susceptibility is shown in Fig 6(b) for the corresponding films It can be seen from this figure that the as-deposited x=0.1 sample with the natural nanostructure exhibits already a wlJ value as large as 3.2  10À2 TÀ1 This value is almost times higher than that of asdeposited x=0 sample and is comparable with that of 350 C-annealed x=0 one The magnetostrictive softness, in particular, is strongly improved after annealing at 350 C: wlJ reaches a maximal value of 14.5  10À2 TÀ1 at the magnetic field ARTICLE IN PRESS 48 N.H Duc et al / Journal of Magnetism and Magnetic Materials 282 (2004) 44–48 conjunction with large wlJ This is because magnetostriction is intimately associated with magnetocrystalline anisotropy and it is well known that the coercivity tends to be high in large anisotropy systems The obtained spectacular result illustrates the significance of the approach, which we have developed in view of optimizing both magnetostriction and magnetostrictive susceptibility in the spring-exchange magnet type multilayers with structurally discontinuous soft layers Acknowledgements This work was granted by the State Program for Natural Scientific Researches of Vietnam The authors acknowledge discussions with D Givord References Fig Low-field magnetostriction (a) and parallel magnetostrictive susceptibility (b) data of the 400 C- annealed x=0.2 multilayer 1.9 mT This wlJ value is almost 15 times higher than that obtained in Terfenol-D [13] and times higher than that obtained in multilayers by Quandt et al [2,3] Low-field lJ and wlJ data are presented in Fig for the x=0.2 films annealed at 400 C (i.e for the thermally induced nanostructured film) It turns out that the maximal wlJ value equals 15.6  10À2 TÀ1 This is in good accordance with the coercivity data that the excellent magnetic as well as magnetostrictive softness can be obtained either in naturally or in thermally formed nanostructured multilayers Concluding remarks For conclusion, it is worthwhile to mention that, in general, a large l value is not obtained in [1] R Coehoorn, D.B de Mooij, J.P.W.B Duchteau, K.H.J Buschow, J Phys (Paris) Colloq 49 (1988) C8; R Coehoorn, D.B de Mooij, D de Waard, J Magn Magn Mater 88 (1989) 101 [2] E Quandt, A Ludwig, J Betz, K Mackay, D Givord, J App Phys 81 (1997) 5420 [3] A Ludwig, E Quandt, J Appl Phys 87 (2000) 4691 [4] N.H Duc, D.T Huong Giang, V.N Thuc, L.V Vu, N Chau, Appl Phys Lett., submitted [5] N.H Duc, J Magn Magn Mater 242–245 (2002) 1411 [6] N.H Duc, P.E Brommer, in: K.H.J Buschow (Ed.), Handbook of Magnetic Materials, Vol 14, Elsevier Science, North-Holland, Amsterdam, 2002, p 89 [7] P Hansen, in: K.H.J Buschow (Ed.), Handbook of Magnetic Materials, Vol 6, Elsevier Science, NorthHolland, Amsterdam, 1991, p 289 [8] U Herr, H Geisler, H Ippach, K Samwer, Phys Rev B 59 (1999) 13719 [9] N.H Duc, K Mackay, J Betz, D Givord, J Appl Phys 79 (1996) 973 [10] T.M Danh, N.H Duc, H.N Thanh, J Teillet, J Appl Phys 87 (2000) 7208 [11] N.H Duc, D.T Huong Giang, V.N Thuc, N.T Minh Hong, N Chau, Physica B 327 (2003) 328 [12] N.H Duc, T.M Danh, N.A Tuan, J Teillet, Appl Phys Lett 76 (2001) 3848 [13] E Tr!emolet de Lacheisserise, D Gignoux, M Schlenker, Magnetism, Vol 2, Kluwer Academic Publisher, Dordrecht, 2002, p 227 ... and discontinuous (b) soft magnetic layer defect points on the sample surface and interfaces In this context, one expects that the reversal can be nucleated in an easier way in discontinuous soft... ferromagnetic at room temperature [7] It is that the FeCo (and/ or YFeCo) /TbFeCo coupling imposes in- plane magnetization The smallest coercivity value found in the sample with x=0.1 may be attributed... Terfecohan/YxFe1Àx multilayers [4] Low-temperature annealing (at TAp350 C) is usually performed to relieve the stress induced during the sputtering process At present, as ARTICLE IN PRESS 46 N.H