Journal of Magnetism and Magnetic Materials 262 (2003) 361–367 Microstructure and magnetic studies of magnetostrictive Terfecohan/YFeCo multilayers D.T Huong Gianga,*, N.H Duca, F Richommeb, S Schulzec a Faculty of Physics, Cryogenic Laboratory, Vietnam National University, 334 Nguyen Trai Road, Thanh Xuan, Hanoi, Viet Nam b GPM-UMR CNRS 6634, Universit!e de Rouen, Site Universitaire du Madrillet, B.P 12, 76801 Saint-Etienne-Du-Rouvray Cedex, France c Institute of Physics, Chemnitz University of Technology, D-09107, Chemnitz, Germany Abstract Sputtered [Tb0.4(Fe0.55Co0.45)0.6/(Y0.2Fe0.63Co0.17)]40 multilayers were investigated by means of energy-dispersive X-ray spectroscopy, high-resolution transmission electron microscope and SQUID magnetometer measurements The results show that the amorphous state exists in the whole as-deposited sample For the sample annealed at 350 C, the amorphous state still remains in the TbFeCo layers, whereas a fine grain (7–10 nm size) structure was formed in the YFeCo ones Magnetisation analysis indicates the existence of a non-collinear magnetic structure and a field-induced magnetic phase transition, in which the TbFeCo magnetisation tends to rotate along the YFeCo magnetisation direction The magnetic coercivity is discussed in terms of the magnetoelastic interactions r 2003 Elsevier Science B.V All rights reserved PACS: 75.60.Jk; 75.70.Cn; 81.07.Bc Keywords: Microstructure; Field-induced magnetic phase transition; Non-collinear magnetic structure; Magnetic coercivity Introduction During the last few years, there has been a great interest in magnetostrictive thin films for various microelectromechanical systems (MEMS) Highperformance magnetostrictive materials have been realised in the form of amorphous a-TbFeCo single layer films and/or of magnetostrictive spring-magnet type multilayers (MSMMs) of aTbFeCo/FeCo For more insight on the magne- *Corresponding author E-mail address: giangdth79@yahoo.com (D.T Huong Giang) toelastic effect and its applications, we refer to Refs [1–3] and references therein Applications require not only a large magnetostriction, but also a large magnetostrictive susceptibility at low magnetic fields The idea of preparing MSMMs is to combine large room-temperature magnetostriction layers (e.g TbFeCo) with softmagnetic layers with a high magnetisation (e.g Fe, Co), in order to enhance the average saturation magnetisation (MS ) This leads to the reduction of the saturation eld m0 HS ẳ 2K=MS ị instead of decreasing the anisotropy constant (K) [4] In such MSMMs, the FeCo individual layers are usually formed in the crystalline state They exhibit a magnetic coercivity as small as mT [5] An 0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V All rights reserved doi:10.1016/S0304-8853(03)00063-5 362 D.T Huong Giang et al / Journal of Magnetism and Magnetic Materials 262 (2003) 361–367 improvement of the soft-magnetic properties of MSMMs was achieved on the TbFe/FeCoBSi and TbDyFe/FeSiBNbCu multilayers, in which both magnetostrictive and soft magnetic layers are amorphous [6,7] Recently, we have reported an excellent magnetic softness in magnetostrictive TbFeCo/YFeCo multilayers [3,8] These multilayers show a record magnetostrictive susceptibility wlJ ẳ dlJ =dHị ¼ 13  10À2 /TÀ1 and a rather small magnetic coercivity m0 HC ¼ 0:6 mT The observed novel magnetic behaviour has been attributed to the nanostructure of the YFeCo layers In this paper, such a microstructure is confirmed by high-resolution transmission electron microscopy In addition, the magnetic structure and a field-induced collinear–non-collinear structure phase transition will be reported The low magnetic coercivity is discussed in terms of the magnetoelastic interactions Experimental [TbFeCo/(YFeCo)]n multilayers with individual layer thicknesses tTbFeCo ¼ tYFeCo ¼ 12:5 nm and with number of periods n ¼ 40 were prepared by RF-magnetron sputtering The typical power during sputtering was 400 W and the Ar pressure was 10À2 mbar A composite target, consisting of 18 segments of about 20 , of different elements, here Tb, Fe, Co and Y, has been used The substrates were glass microscope cover-slips with a nominal thickness of 150 mm Both target and (a) 20.00 nm sample holder were water-cooled Samples were annealed at temperatures up to TA ¼ 350 C for h in a vacuum of  10À5 mbar Film composition is determined by energy-dispersive X-ray (EDX) spectroscopy The obtained composition is Tb0.4(Fe0.55Co0.45)0.6/Y0.2Fe0.63Co0.17 (denoted as Terfecohan/Y0.2Fe0.63Co0.17), instead of Terfecohan/Y0.2Fe0.8 as reported previously [3,8] The microstructure was investigated using a high-resolution transmission electron microscope (HRTEM) The magnetisation was measured by means of a SQUID magnetometer in magnetic fields up to T and at temperatures ranging from to 298 K Experimental results and discussion 3.1 Microstructure Figs 1(a) and (b) show the cross section TEM micrographs of the as-deposited and 350 Cannealed Terfecohan/(Y0.2Fe0.63Co0.17) multilayers, named M0 and M350, respectively It can be clearly seen that, for sample M0, the amorphous state exists in the whole sample, i.e in both the magnetostrictive and the soft-magnetic layers For sample M350, the amorphous state still remains in the Terfecohan layers, whereas a fine grain (7–10 nm size) structure was formed in the YFeCo ones The corresponding TEM electron diffraction patterns are presented in Figs 2(a) and (b) This finding supports the X-ray diffraction (b) 20.00 nm Fig Cross-section HRTEM micrographs of multilayers M0 (a) and M350 (b) D.T Huong Giang et al / Journal of Magnetism and Magnetic Materials 262 (2003) 361–367 363 (b) (a) Fig TEM electron diffraction patterns of multilayers M0 (a) and M350 (b) 1000 800 600 900 M (kA/m) M (kA/m) 5K 5K 700 77 K 298 K 500 400 (a) 77 K 700 298 K 600 300 200 800 µoH (T) 500 (b) µoH (T) Fig In-plane field-down magnetisation curves of multilayers M0 (a) and M350 (b) results obtained earlier [8] and corroborates the nanostructure nature of the excellent magnetic softness of the films under consideration 3.2 Magnetisation and magnetic structure The in-plane magnetisation curves measured in decreasing field at 5, 77 and 298 K are presented in Figs 3(a) and (b) for the samples M0 and M350 Their magnetisation data at different temperatures and applied magnetic fields are listed in Table All magnetisation curves show a large remanence The zero-field magnetisation Mt (0 T) listed in Table 1, however, is the value obtained by extrapolating to m0 H ¼ 0: For the as-deposited sample, at room temperature Mt Tị ẳ 480 kA/m only After annealing at TA ¼ 350 C, Mt (0 T) increases to 630 kA/m In accordance with the Mossbauer studies [3], this magnetisation enhancement is related to the magnetic evolution in nanostructured YFeCo layers As the temperature is decreased, Mt (0 T) slightly increases in both samples M0 and M350 The (FeCo)-rich YFeCo alloys usually exhibit a weakly temperature dependent magnetisation However, the magnetisation of the Terfecohan film shows a rather strong temperature dependence (see Fig 4) Hence, the observed variation of Mt (0 T) can not be described by combining the two magnetisation contributions of the individual Terfecohan and YFeCo layers In addition, it is interesting to note that the magnetisation curves show a rather large high-field magnetic susceptibility (whf ) For instance, for the film M0, whf takes a value of 22.5 (kA/m)/T at T ¼ 298 K and reaches to 25.2 and 52.7 (kA/m)/T at T ¼ 77 and K, respectively A similar result is obtained for the sample annealed at 350 C These findings may imply a field-induced magnetic phase transition in the investigated samples À409 À30 110 À353 13 129 800 Terfecohan µoH =1 T M (kA/m) À206 84 106 mo H ¼ 5T mo H ¼ 4T 1200 À267 57 147 À124 126 126 mo H ¼ 3T 400 À155 99 165 À16 182 146 mo H ¼ 2T 0 50 100 150 200 T (K) 250 300 350 141 183 209 215 219 944 754 684 *The extrapolated value to mo H ¼ T for the magnetisation 873 711 666 77 298 M350 635* 632* 630* 738 669 648 817 690 657 916 733 675 148 234 172 236 250 214 716 566 542 649 524 524 77 298 M0 469* 462* 480* 513 470 501 595 496 514 690 545 534 mo H ¼ 1T mo H ¼ 0T mo H ¼ 3T mo H ¼ 0T T (K) Mt (kA/m) mo H ¼ 1T mo H ¼ 2T mo H ¼ 4T mo H ¼ 5T MTbFeCo (kA/m) Fig Temperature dependence of magnetisation of Terfecohan film in mo H ¼ T Sample Table The magnetisation of the Terfecohan layers (MTbFeCo ) estimated from the experimental magnetisation data (Mt ) measured at different temperatures (T) and different magnetic applied field (mo H) À258 42 90 D.T Huong Giang et al / Journal of Magnetism and Magnetic Materials 262 (2003) 361–367 364 In the multilayer under consideration, the individual layers are thick enough for magnetic coupling but they are thinner than the magnetic exchange length, for which domain walls cannot be formed at the interface In this state, the 3d–3d exchange interactions ensure that parallel coupling of the (Fe,Co)-magnetic moments persists throughout the entire multilayer Without creating domain walls at the interfaces, the multilayer behaves as one piece of material Then, magnetisation processes result from the average of the magnetic characteristics of each individual layer Assuming a collinear magnetic structure, the (experimental) total magnetisation Mt of the multilayer, which is dominated by the YFeCo contribution, can be described as a function of thickness ti and magnetisation Mi of the individual layers as follows: Mt ¼ tYFeCo MYFeCo tTbFeCo MTbFeCo : tYFeCo ỵ tTbFeCo 1ị The total magnetisation Mt ; measured at different temperatures and in different applied magnetic fields, is listed in Table In Ref [3], a hyperfine field of Bhf ¼ 27 and 34 T was reported for the as-deposited and 350 C-annealed films, respectively By scaling the BCC-Fe hyperfine field (33 T) with its magnetisation (1740 kA/m), the FeCo-magnetisation and then the magnetisation of the YFeCo layer (MYFeCo ) can be deduced from the strength of the corresponding hyperfine field It turns out that MYFeCo equals 1140 and 1450 kA/m in the as-deposited and 350 C-annealed films, D.T Huong Giang et al / Journal of Magnetism and Magnetic Materials 262 (2003) 361–367 300 300 200 MTbFeCo (kA/m) MTbFeCo (kA/m) 200 100 -100 5K 77 K 298 K -200 100 -100 -200 -300 5K 77 K 298 K -400 -500 -300 (a) 365 µoH (T) (b) µoH (T) Fig Plot of MTbFeCo as a function of applied magnetic field respectively Below, this magnetisation value is assumed to be temperature independent Inserting the experimental value of the structural and magnetic parameters, e.g tTbFeCo ; tFe ; MYFeCo and Mt into Eq (1), the magnetisation of the individual TbFeCo layer (MTbFeCo ) can be derived on the basis of the collinear situation The obtained results are also listed in Table 1, and are plotted as a function of applied field in Figs 5(a) and (b) The results show that at room temperature MTbFeCo (0 T) is 236 and 209 kA/m for the films M0 and M350, respectively At low temperatures, MTbFeCo (0 T) is slightly enhanced in M0, however, it is weakly decreased in M350 The positive sign of MTbFeCo (0 T) confirms the antiparallel orientation of TbFeCo and YFeCo magnetisation In addition, the obtained MTbFeCo (0 T) value of the as-deposited and 350 C-annealed samples is comparable to the room temperature value of 259 kA/m of the Terfecohan film (see Fig 4) The decrease of MTbFeCo (0 T) with decreasing temperature does not mean a reduction of the magnitude of the TbFeCo magnetisation, but a reduction of its contribution along the applied-field direction It may be associated to the existence of a zero-field non-collinear magnetic structure of the TbFeCo and YCoFe magnetisation In this context, it is interesting to mention that, while Mt increases strongly with increasing applied fields, MTbFeCo ðm0 HÞ is decreased (Fig 5) Specially, in the investigated magnetic fields we observe also a change in sign of MTbFeCo m0 Hị at T ẳ K for the sample M0 (Fig 5a) and at T ¼ and 77 K for the sample M350 (Fig 5b) This finding supports the above mentioned picture of a non-collinear magnetic structure This argument seems to be strengthened at low temperatures At T ¼ K, however, jMTbFeCo ð5 TÞj reaches 406 kA/m only, which is still lower than the value of 1000 kA/m observed for Terfecohan film at low temperature (see Fig 4) This implies that the rotation is not completed in the investigated magnetic fields (mo Hp5 T) 3.3 Magnetic coercivity The in-plane magnetic hysteresis loops measured at 298, 77 and K are presented in Figs 6(a)–(c) for the samples M0 and M350 All samples show a rather low coercive field, e.g at room temperature, mo HC equals to 3.5 and 0.6 mT for samples M0 and M350, respectively As the temperature is decreased, mo HC is enhanced However, it is worthwhile to mention that, at a certain temperature, mo HC of M350 is always smaller than that of M0 This behaviour may also be described in the relation with the magnetic enhancement in the nanocrystalline YFeCo layers Indeed, the role of magnetoelastic interactions in controlling the magnitude of switching field and coercivity was investigated for TbFe/FeCo multilayers [9] Results showed that magnetoelastic constraints at the TbFe/FeCo interfaces, arising from different magnetostriction values (li ) in adjacent layers, lead to biaxial stress and, as a 366 D.T Huong Giang et al / Journal of Magnetism and Magnetic Materials 262 (2003) 361–367 800 600 M350 300 M (kA/m) M (kA/m) M0 400 -400 -300 298 K 298 K -600 -20 -10 µoH (mT) 10 -800 -20 20 µoH (mT) 10 µoH (mT) 10 M0 M350 M (kA/m) 300 400 -400 -300 77 K -600 -300 -200 -100 100 µoH (mT) -800 -20 200 300 600 900 M0 600 300 M (kA/m) M (kA/m) 20 800 600 M (kA/m) -10 -300 5K -600 -400 -200 200 77 K -10 M350 300 -300 5K -600 400 20 -900 -200 -100 µoH (mT) 100 200 Fig Magnetic hysteresis loops for the multilayers M0 and M350 at T ¼ 298; 77 and K consequence, to a corresponding stress-induced anisotropy Applying this to the present case, we write the coercivity as mo HC ¼ 3ðlTbFeCo À lYFeCo Þ2 E ; MYFeCo À MTbFeCo to the compensation of the (lTbFeCo À lYFeCo ) factor in Eq (2) Concluding remarks ð2Þ here, E is the Young modulus of TbFeCo This model was postulated on the basis of a magnetic collinear structure At present, however, it can be applied to discuss the observed magnetic softness improvement Indeed, the observed decrease of mo HC with increasing annealing temperature can be explained by the enhancement of the average magnetisation /MSð¼ MYFeCo F MTbFeCo Þ: The almost zero-coercivity observed in the 350 C-annealed film, on the other hand, may also be associated to the development of the magnetostriction in the nanostructured YFeCo layers with respect to its amorphous state and thus Besides the excellent magnetic softness and giant low-field magnetostriction, multilayers consisting of a periodic sequence of amorphous TbFeCo and fine grain structure YFeCo as soft-magnetic interlayers exhibit also the existence of a noncollinear magnetic structure and a field induced magnetic phase transition Their low magnetic coercivity can be described on the basis of the magnetoelastic interactions Acknowledgements This work was supported granted by the Vietnam National University, Hanoi within the project QG.02.06 and by KC.02.13 D.T Huong Giang et al / Journal of Magnetism and Magnetic Materials 262 (2003) 361–367 References [1] N.H Duc, in: K.A Gschneirdner Jr., L Eyring, G.H Lander (Eds.), Handbook on the Physics and Chemistry of Rare Earths, Vol 32, Elsevier, Amsterdam, 2001, p (Chapter 205) [2] N.H Duc, P.E Brommer, in: K.H.J Buschow (Ed.), Handbook on Magnetic Materials, Vol 14, Elsevier, Amsterdam, 2002, p 89 [3] N.H Duc, J Magn Magn Mater 242–245 (2002) 1411 367 [4] N.H Duc, T.M Danh, N.A Tuan, J Teillet, Appl Phys Lett 78 (2001) 3648 [5] E Quandt, A Ludwig, J Betz, K Mackay, D Givord, J App Phys 81 (1997) 5420 [6] P Farber, H Kronmuller, J Appl Phys 88 (2000) 2781 [7] A Ludwig, E Quandt, J Appl Phys 87 (2000) 4691 [8] N.H Duc, F Richomme, N.A Tuan, D.T Huong Giang, T Verdier, J Teillet, J Mag Magn Mater 242–245 (2002) 1425 [9] H.D Chopra, D.X Yang, P Wilson, J Appl Phys 87 (2000) 5780 ... magnetisation curves of multilayers M0 (a) and M350 (b) results obtained earlier [8] and corroborates the nanostructure nature of the excellent magnetic softness of the films under consideration... Journal of Magnetism and Magnetic Materials 262 (2003) 361–367 improvement of the soft -magnetic properties of MSMMs was achieved on the TbFe/FeCoBSi and TbDyFe/FeSiBNbCu multilayers, in which both magnetostrictive. .. Temperature dependence of magnetisation of Terfecohan film in mo H ¼ T Sample Table The magnetisation of the Terfecohan layers (MTbFeCo ) estimated from the experimental magnetisation data (Mt