Journal of Magnetism and Magnetic Materials 242–245 (2002) 1425–1427 Excellent magnetic softness in magnetostrictive TbFeCo/YFe multilayers N.H Duca, F Richommeb, N.A Tuana, D.T Huong Gianga, T Verdierb, J Teilletb,* a b Faculty of Physics, Cryogenic Laboratory, Vietnam National University, Hanoi, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam Groupe de Physique des Materiaux, Faculte des Sciences de Rouen, Universit!e de Rouen, UMR CNRS 6634, 76821 Mont-Saint-Aignan, France Abstract The {Tb(Fe0.55Co0.45)1.5/Y0.2Fe0.8}40 multilayers are investigated by X-ray diffraction and SQUID measurements The results show that in the as-deposited multilayer, the TbFeCo and YFe layers are amorphous An excellent magnetic softness with a coercivity of 0.3 mT was achieved after annealing at 2501C These novel magnetic properties are associated to the evolution of a nanocrystalline structure in the YFe layers Different temperature dependences of the coercive field are described for the amorphous and nanocrystalline multilayers r 2002 Elsevier Science B.V All rights reserved Keywords: Multilayers; Magnetostrictive materials; Soft magnetic materials; Coercivity The need for microactuator applications has stimulated the development of thin films with a large magnetostriction and a large magnetostrictive susceptibility For this reason, spring-magnet-type magnetostrictive multilayers (MSMMs), e.g TbFeCo/FeCo, which combine layers with a large room-temperature magnetostriction and soft magnetic layers with a high magnetisation were investigated [1–2] In the absence of a long-range anisotropy in amorphous TbFeCo layers, along with negligible magneto-crystalline anisotropy in (FeCo) layers, the coercivity of MSMMs ranges between and 10 mT Recently, attempts to improve the soft magnetic properties of MSMMs were performed by Quandt and Ludwig [3] and Farber and Kronmuller [4] on TbFe/FeCoBSi and TbDyFe/FeSiBNbCu multilayers, respectively In these as-deposited multilayers, both magnetostrictive and soft magnetic layers are in an amorphous state, from which a nanocrystalline structure can be controlled Then, a better magnetic softness was expected to be obtained Practically, an almost negligible *Corresponding author Tel.: +33-2-35-14-66-19; fax: +332-35-14-66-52 E-mail address: jacques.teillet@univ-rouen.fr (J Teillet) magnetic hysteresis, but at the expense of the magnetostriction, was observed [4] In this paper, we study the {Tb(Fe0.55Co0.45)1.5/Y0.2Fe0.8} multilayers The [Tb(Fe0.55Co0.45)1.5/(Y0.2Fe0.8)]n multilayers with individual layer thicknesses tTbFeCo ¼ 12 nm, tYFe ¼ 13 nm and with n ¼ 40 were prepared by rfmagnetron sputtering from composite targets The substrates were glass microscope cover slips with a nominal thickness of 150 mm Both target and sample holder were water-cooled Samples were annealed at temperatures from TA¼ 2501C to 5001C for h in a vacuum of  10À5 mbar The X-ray diffraction (XRD) results are presented in Fig Besides a broad peak corresponding to Tb2O3 oxide, the as-deposited TbFeCo/YFe multilayer does not show any clear Bragg peaks, indicating the existence of the amorphous state in the TbFeCo as well as in the YFe layers The nanostructure associated with the BCCFe or iron-rich BCC-Fe(Y) phase in the YFe layers seems to occur for the samples annealed at TA ¼ 2501C: It is well evidenced at TA ¼ 3501C and is further confirmed by the presence of three expected peaks of BCC-Fe(Y) in the XRD parttens at TA ¼ 5001C: This argument is consistent with the results of the Mossbauer 0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V All rights reserved PII: S - 8 ( ) 0 9 - X N.H Duc et al / Journal of Magnetism and Magnetic Materials 242–245 (2002) 1425–1427 1426 1.0 TA = 500 oC M/Ms Intensity (arb units) As-deposited 0.0 TA = 350 oC (a) -1.0 -15 TA = 250 oC 15 1.0 TA = 250 oC 10 20 30 40 50 M/Ms As-deposited 60 θ (degree) Fig X-ray diffraction patterns of TbFeCo/YFe multilayers: BCC-Fe(Y) (closed triangles), and R2O3, Fe2O3 (open triangles) 0.0 (b) -1.5 -15 15 15 1.0 M/M(t = 0) TA = 350 oC M/MS 1.08 0.0 (c) -1.0 -15 1.04 1.0 1.00 10 20 30 t (minutes) 40 50 Fig Time dependence of the magnetisation measured at T ¼ 400 K for the TbFeCo/YFe multilayer (B ¼ T) M/MS TA = 500 oC 0.0 (d) -1.0 -15 15 µ H (mT) studies [5] It is worth mentioning here that, at this TA ; no reflection corresponding to the crystallisation of the TbFeCo layers is observed The crystallisation process of the BCC-Fe(Y) phase, therefore, occurred at rather low temperatures The reason may be related to the influence of the rare earth (or yttrium) concentration As regards the magnetic evolution during the crystallisation process, we investigated the time dependence of the magnetisation during a field annealing The result obtained in the case of TA ¼ 400 K and m0 H ¼ T is presented in Fig It is clearly seen that a magnetisation enhancement followed by an exponential tendency was found The magnetic hysteresis loops measured in magnetic fields applied parallel to the film plane are presented in Figs 3(a–d) for the as-deposited, 2501C-, 3501C- and 5001C-annealed Tb(Fe0.55Co0.45)1.5/(Y0.2Fe0.8) samples, respectively For the as-deposited sample, the saturation magnetisation MS ¼ 426 kA/m only After annealing, MS increases to 580 kA/m at TA ¼ 2501C and reaches a saturation value of 620 kA/m at TA X3501C: The total Fig Magnetic hysteresis loops of TbFeCo/YFe multilayers (measured) magnetisation MS of the multilayer can be described in relation with thickness ti and magnetisation Mi of individual layers as follows [2]: MS ¼ tYFe MYFe À tTbFeCo MTbFeCo : tYFe ỵ tTbFeCo 1ị Introducing the experimental value of the structural and magnetic parameters, e.g tTbFeCo; tYFe ; MS and MTbFeCo ¼ 250 kA/m into Eq (1), the magnetisation of the individual YFe layer (MYFe ) can be derived The magnetisation value MYFe equals 1050 kA/m for the asdeposited sample At TA X3501C; MYFe reaches 1425 kA/m, which implies a Fe-magnetisation value of 1770 kA/m It is rather close to that of BCC-Fe (1740 kA/m) This result is consistent with that deduced from the Mossbauer experiments [5] The multilayers under consideration, in particular, show a rather low coercive field at room temperature: N.H Duc et al / Journal of Magnetism and Magnetic Materials 242–245 (2002) 1425–1427 µ o H c (mT) 100 80 25 0˚ C 60 35 0˚ C as deposited 40 20 (a) H c / H c(T = K) 1.0 25 0˚ C 35 0˚ C as deposited 0.5 -5 H c = - 0062 T + 10 T (b) Hc = T -0 0.5 100 200 T (K) 300 400 Fig Temperature dependence of the coercive field in TbFeCo/YFe multilayers m0 HC equals to 3.5, 0.3, 0.6 and 3.0 mT for the asdeposited, 2501C-, 3501C- and 5001C-annealed films, respectively In spring-magnet-type multilayers, the decrease of the coercivity is usually related to the enhancement of the (total) magnetisation At present, moreover, it is also associated to the relaxation of the amorphous state resulting in more stable and homogeneous TbFeCo layers, and to the formation of the nanostructure in the YFe layers The observed increase of m0 HC in the 5001C-annealed sample can be attributed to the growth of Fe(Y) grains 1427 As temperature decreases, the coercivity is enhanced (Fig 4a) At T ¼ K, m0 HC equals 91.7, 13.4 and 15 mT for the as-deposited, 2501C- and 3501C-annealed samples, respectively The variation of the normalised coercive eld (HC Tị=HC Tẳ Kị) as a function of the temperature is plotted in Fig 4b This figure shows different temperature dependences of the coercivity: while m0 HC follows a polynomial tendency m0 HC B120:0062T ỵ 105 T for the as-deposited multilayer, it follows a power law of m0 HC B5=T À0:9 for the films annealed at TA ¼ 2501C and 3501C The coercivity in these multilayers, therefore, must be described by two different mechanisms corresponding to the amorphous and nanostructure states This, however, is still open at present In conclusion, the optimum condition for the magnetic softness was found for the Tb(Fe0.55Co0.45)1.5/ (Y0.2Fe0.8) multilayers This excellent magnetic property is associated with the formation of the nanostructure in the YFe layers This paper is partly supported by the Fundamental Research Program of Vietnam under project 420301 References [1] E Quandt, A Ludwig, J Betz, K Mackay, D Givord, J App Phys 81 (1997) 5420 [2] A Ludwig, E Quandt, J Appl Phys 87 (2000) 4691 [3] E Quandt, A Ludwig, J Appl Phys 85 (1999) 6232 [4] P Farber, H Kronmuller, J Appl Phys 88 (2000) 2781 [5] N.H Duc, Magma, in this proceeding ... À tTbFeCo MTbFeCo : tYFe ỵ tTbFeCo 1ị Introducing the experimental value of the structural and magnetic parameters, e.g tTbFeCo; tYFe ; MS and MTbFeCo ¼ 250 kA/m into Eq (1), the magnetisation... of TbFeCo/ YFe multilayers (measured) magnetisation MS of the multilayer can be described in relation with thickness ti and magnetisation Mi of individual layers as follows [2]: MS ¼ tYFe MYFe... sample, the saturation magnetisation MS ¼ 426 kA/m only After annealing, MS increases to 580 kA/m at TA ¼ 2501C and reaches a saturation value of 620 kA/m at TA X3501C: The total Fig Magnetic hysteresis