Physica B 327 (2003) 328–333 Magnetisation process and magnetostriction in Fe/TerfecoHan/Fe sandwich films with perpendicular magnetic anisotropy N.H Duca,*, D.T Huong Gianga, V.N Thuca, N.T Minh Honga, N Chaub b a Cryogenic Laboratory, Faculty of Physics, Vietnam National University, Nguyen Trai, Thanh Xuan, Hanoi 334, Viet Nam Center for Materials Science, Faculty of Physics, Vietnam National University, Hanoi 334 Nguyen Trai Road, Thanh Xuan, Hanoi, Viet Nam Abstract A new magnetostrictive film with out-of-plane magnetic anisotropy is realised in sandwich type of Fe/ Tb(Fe0.55Co0.45)1.5/Fe (denoted as Fe/TerfecoHan/Fe) films with a fixed individual TerfecoHan-layer thickness tTbFeCo ¼ 600 nm and variable Fe layer thickness tFe ¼ 0; 10, 15 and 60 nm As-deposited Fe/TerfecoHan/Fe films exhibit a magnetostriction as large as 10À3 in an applied field of m0 H ¼ 0:6 T This magnetostriction is exclusively observed in the field direction parallel to the sample length The development of giant low-field magnetostriction has been performed by heat treatment A maximal magnetostrictive susceptibility wl8 ¼ 2:3  10À2 TÀ1 is reached in m0 H ¼ mT for the TerfecoHan film annealed at TA ¼ 4501C: For Fe/TerfecoHan/Fe sandwich films, however, the magnetic softness is just slightly improved with TA ¼ 2501C and 3501C At TA ¼ 4501C; both the perpendicular magnetic anisotropy and the magnetic coercivity are reinforced These magnetic behaviours are associated to the fact that the additional Fe-layers accelerated the crystallisation process in the TerfecoHan layer The sandwich films are favourable for making the Fe/TerfecoHan/Fe a low-field magnetostrictive material for microelectromechanical systems as well as a perpendicular recording medium Mechanisms of the magnetisation process and huge parallel magnetostriction are discussed for the films under investigation r 2002 Elsevier Science B.V All rights reserved Keywords: Thin films; Microstructure; Magnetisation process; Magnetostriction Introduction Magnetostriction of thin films has been studied intensively in the last decade First microelectromechanical devices using magnetostrictive films have been realised For a more detailed review of the present state of theory and experiments of the *Corresponding author E-mail address: duc@netnam.org.vn (N.H Duc) magnetostriction of rare earth–transition metal thin films, we refer the reader to the chapters written by Duc [1] and more recently by Duc and Brommer [2] As a tradition, research of giant magnetostrictive thin films has also been based on amorphous (a) rare earth–iron alloys, in particular a-Tb0.27Dy0.73Fe2 (known as a-Terfenol-D) Practically, a record magnetostriction of 1020  10À6 has been achieved on the a-Tb(Fe0.55Co0.45)2.1 thin film [3] Enhancement of the low-field 0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V All rights reserved PII: S - ( ) - N.H Duc et al / Physica B 327 (2003) 328–333 Experimental The Fe/TerfecoHan/Fe sandwiches with a fixed individual TerfecoHan-layer thickness tTbFeCo ¼ 600 nm and a variable Fe layer thickness tFe ¼ 0; 10, 15 and 60 nm (denoted as samples S0, S10, S15 and S60, respectively) were prepared by RF-magnetron sputtering The TerfecoHan layer was sputtered under a power of 200 W, while the sputtering power of the Fe-layers was 100 W A composite target has been used for the TerfecoHan layer, a high-purity metal plate for the Fe layers The substrates were glass microscope cover slips Both target and sample holder were water-cooled Samples were annealed in the temperature range from TA ¼ 2501C to 4501C for h in a vacuum of  10À5 mbar Film structure was investigated with X-ray diffraction (XRD) and high-resolution transmission electron microscope (HRTEM) The magnetisation was measured using a vibrating sample magnetometer (VSM) in magnetic fields up to 1.4 T The magnetostriction was measured using an optical deflectometer (with a resolution of  10À6 rad.) Experimental results and discussions 3.1 Microstructure Fig presents the XRD spectra for the asdeposited Fe/TerfecoHan/Fe samples S0, S10 and S60 No diffraction peaks indicating crystalline TbFeCo phases are observed Moreover, the existence of the BCC-Fe layers is weekly evidenced It may relate to the fact that the Fe-layer is too thin (for sample S10) and/or the grain size of the crystalline phase is too small (for sample S60) to be detected by the XRD However, the BCC-Fe crystalline phase was well determined by crosssection TEM micrographs (Figs 2a,b) An average BCC Fe grain size of about nm can be deduced for sample S10 (Fig 2a) The Fe grain size increases with increasing tFe, e.g it equals nm in sample S60 (figure not shown) After annealing at TA ¼ 4501C, fine grain (5–7 nm size) structure is observed in the TerfecoHan film S0 In the sample S10, the fine-grain (3–4 nm size) structure still remains in the Fe-layers, while a grain size of the crystalline phase of about 25 nm was formed in the TerfecoHan layer (Fig 2b) The transmission bcc-Fe S60 Intensity (arb unit) magnetostrictive susceptibility, however, has been found in TbCo/FeCo multilayers [4,5] Moreover, an excellent magnetostrictive softness was reported for a-Tb(Fe0.55Co0.45)1.5 (denoted as a-TerfecoHan) single layer, Fe/a-TerfecoHan/Fe sandwich and a-TerfecoHan/n-YFeCo multilayer films [6] The giant magnetostriction observed in these materials was explained by the enhancement of the 4f–3d exchange, leading to the diminishing of the Tb-sperimagnetic cone-angle, whereas their huge magnetostrictive susceptibility is connected to the nanostructure of the n-YFeCo layer Giant magnetostriction originates from the rotation of the magnetic moments It is usually observed in uniaxially anisotropic magnetic systems, one of which are thin films with out-of-plane magnetic anisotropy Such a giant magnetostriction, however, requires a high magnetic field In Ref [6], it was reported that the Fe layers played an important role in improving the magnetic softness of the Fe/TerfecoHan/Fe sandwiches with parallel magnetic anisotropy In this paper, a new magnetostrictive material with perpendicular magnetic anisotropy is realised in sandwich type Fe/TerfecoHan/Fe films Their microstructure, magnetisation process and magnetostriction will be described in comparison with those of the TerfecoHan-single layer films 329 S10 S0 40 42 44 46 48 50 2θ θ (degree) Fig XRD spectra of as-deposited S0, S1, S60 films 330 N.H Duc et al / Physica B 327 (2003) 328–333 Fig TEM bright field images of the as-deposited (a) and 4501C-annealed (b) sample S10 TEM diffraction patterns of the corresponding TerfecoHan phases are shown in the insets S0 M /M1T electron diffraction patterns of corresponding TerfecoHan nanocrystalline phases are also illustrated in images inset in Fig The observed different annealing effects could be attributed to the nucleus formed in TerfecoHan/Fe interfaces // -1 3.2 Magnetic hysteresis and magnetisation orientation M /M1 T S15 // -1 S60 M /M1T Fig shows the magnetic hysteresis loops measured in magnetic fields applied parallel and perpendicular to the film plane at room temperature for several as-deposited films Note that, the in-plane magnetisation requires a magnetic field higher than 0.35 T to saturate and that the remanence is almost zero (e.g in samples S0, S10 and S15) These features suggest the existence of a perpendicular anisotropy, in addition to the usual shape anisotropy For sample S60, the magnetisation seems to consist of both perpendicular and parallel magnetic components Indeed, it is found from Mossbauer studies [7] that, in this sample, the Fe-magnetic moments in TerfecoHan layer orient along the out-of-plane direction, whereas those in the Fe-layers lie in the film plane In this context, it is possible that a 901-domain wall may be created in the interfaces By annealing, the in-plane (soft) magnetic state is quickly established and improved in the sample S0 (see Fig 4a) For the samples S10 and S15, however, the in-plane magnetic anisotropy can be formed with annealing at TAp2501C only At // -1 -1 µ oH (T) Fig Magnetic hysteresis loops of the as-deposited samples S0, S15 and S60 higher-temperature annealing, one observes not only the re-establishment of the out-of-plane magnetic anisotropy, but also a strong enhancement of the magnetic coercivity (see e.g Fig 4b for S15) Finally, besides the establishment of the in-plane magnetic anisotropy, the sample S60 annealed at 4501C exhibits also a field-induced magnetic transition at m0 HE200 mT (Fig 4c) N.H Duc et al / Physica B 327 (2003) 328–333 M/M1T // M/M1T S15 TA = 450 oC -1 M/M1T length l> is small, whereas a l8 value of 10À3 was achieved for the films S0, S10 and S15 As far as our knowledge, such behaviour is usually observed in the in-plane induced uniaxial magnetic anisotropic films [3] The observed phenomenon will be tackled below The l8 data, measured in an applied magnetic field m0H = 0.6 T are shown in Fig and listed in Table for the as-deposited sandwiches It is clearly seen that the parallel magnetostriction initially increases almost linearly A saturation tendency can be evidenced in magnetic fields higher than 0.3 T This implies that it is rather difficult to rotate spins into the film-plane An almost constant low-field magnetostrictive susceptibility wl8 ¼ ql8 =qðm0 HÞE0:25 10À2 TÀ1 can be deduced for the samples S0, S10 and S15 For the sample S60, wl8 ¼ 0:06 10À2 TÀ1 only (see Table for more details) Annealing at temperatures TA p4501C reduces the saturation magnetostriction but enhances the low-field magnetostriction in the single layer TerfecoHan films This is due to the disappearance S0 TA = 450 oC -1 // S60 TA = 450 oC // 331 -1 -1 µoH (T) Fig Magnetic hysteresis loops of the 4501C-annealed samples S0, S15 and S60 Such a magnetic behaviour is usually observed in sandwich films made by stacking coupled layers with typical thicknesses around 100 nm [8] It was associated with the different magnetisation reversal of individual layers, leading to the formation of the so-called extended domain wall at the interfaces The magnetisation process, thus, depends not only on the crystallisation of the TerfecoHan phase, but also on the film structure (e.g thickness as well as the microstructure of the Fe-layers) and magnetisation configuration 3.3 Magnetostriction We measured two coefficients, l8 and l>, which correspond to the applied field, in the film plane, being, respectively, parallel and perpendicular to the measuring direction, which is along the sample Fig Parallel magnetostriction of as-deposited films Table Magnetostriction data (see text) for as-deposited Fe/TerfecoHan/Fe films Samples l8 (10À6) wl8 (10À2 TÀ1) S0 S10 S15 S60 1140 840 820 400 0.23 0.26 0.21 0.06 332 N.H Duc et al / Physica B 327 (2003) 328–333 of the perpendicular magnetic anisotropy leading to, on the one hand, an isotropic in-plane distribution of the magnetostriction (l8 =l> E À 1) and, on the other hand, to the reinforcement of the domain wall motion contributions to the magnetisation process For this film, the optimum annealing temperature is 4501C In this case, the magnetostriction l8 ¼ 290  10À6 is already developed in rather low applied magnetic fields of about 20 mT In addition, its coercive field is less than mT It is worthwhile to mention that in an applied field less than 10 mT, the (parallel) magnetostrictive susceptibility has reached its maximal value, wl8 ¼ 2:3  10À2 TÀ1 These results reproduce well those reported previously [9] For the sandwiches S10 and S15, the magnetostriction follows well the observed magnetic behaviours The initial magnetostrictive susceptibility reaches the maximal value wl8 ¼ 1:3  10À2 TÀ1 at TA ¼ 2501C This low-temperature heat treatment is favourable for making the {Fe/TerfecoHan/Fe} films a low-field giant-magnetostrictive material for MEMS At higher-temperature heat treatments, a perpendicular anisotropy type of magnetostriction is established again, so that these films become a perpendicular recording medium We usually associate the field dependence of the magnetostriction with different types of magnetisation processes [1,9] This approach has also been applied for the films under investigation It turns out that the magnetisation process is governed by the rotation of spins in samples S0, S10 and S15, whereas it takes place in two-steps in sample S60 First, the motion of domain walls leads to a magnetisation of M0 ¼ 12MS without any contribution to magnetostriction In the second step, the spins rotate into the direction of the applied magnetic field leading to the change of both magnetisation and magnetostriction Finally, let us now tackle the problem, mentioned above, to understand the disappearance of the perpendicular magnetostriction in the out-ofplane spin systems For simplicity, one may assume that the 4f-electron density distribution is a plate form as illustrated in Fig We now consider the position of the four atoms located in the film plane and along the x- and y-axis In a zero applied magnetic field, these four atoms are Fig Magnetostriction mechanism in films with out-of-plane magnetic anisotropy equivalent with respect to the distance from the reference atom (Fig 6a) The l8 -configuration, as already described above, corresponds to the applied field being parallel to the sample length (i.e along the y-axis, see Fig 6b) Due to the 4fmagnetic moment rotation into the field direction, the interatomic distance along the y-axis increases, whereas the interatomic distance along the x-axis remains the same As a consequence, a huge l8 is detected In contradistinction, in the l> configuration (Fig 6c), only the interatomic distance change in the x-direction is observed This explains the almost zero perpendicular magnetostriction in the out-of-plane anisotropic films Concluding remarks Magnetic and magnetostrictive properties of the out-of-plane magnetic anisotropic {Fe/TerfecoHan/Fe} films have been investigated and discussed in connection with their microstructure This new type of magnetic material appears to be rather promising for MEMS as well as for application as perpendicular recording medium N.H Duc et al / Physica B 327 (2003) 328–333 The magnetisation process and, in particular, the magnetostriction mechanism are proposed for films having perpendicular magnetic anisotropy Acknowledgements This work is supported by the Vietnam National University, Hanoi within project QG 02 06 [2] [3] [4] [5] [6] [7] References [8] [1] N.H Duc, in: K.A Gschneirdner, L Eyring, G.H Lander (Eds.), Handbook on the Physics and Chemistry of Rare [9] 333 Earths, Vol 32, Elsevier Science, Amsterdam, 2001, p Chapter 205 N.H Duc, P.E Brommer, in: K.H.J Buschow (Eds.), Handbook on Magnetic Materials, Vol 14, Elsevier Science, Amsterdam, 2002, p 89, Chapter N.H Duc, K Mackay, J Betz, D Givord, J Appl Phys 79 (1996) 973 E Quandt, J Appl Phys 75 (1994) 5653 N.H Duc, T.M Danh, N.A Tuan, J Teillet, Appl Phys Lett 78 (2001) 3648 N.H Duc, J Magn Magn Mater 242–245 (2002) 1411 F Richomme, N.H Duc, D.T Huong Giang, J Teillet, in preparation D Givord, J Betz, K Mackay, J.C Toussaint, J Voiron, S Wuchner, J Magn Magn Mater 159 (1996) 71 T.M Danh, N.H Duc, H.N Thanh, J Teillet, J Appl Phys 87 (2000) 7208 ... material with perpendicular magnetic anisotropy is realised in sandwich type Fe/ TerfecoHan /Fe films Their microstructure, magnetisation process and magnetostriction will be described in comparison with. .. reported that the Fe layers played an important role in improving the magnetic softness of the Fe/ TerfecoHan /Fe sandwiches with parallel magnetic anisotropy In this paper, a new magnetostrictive material... The Fe/ TerfecoHan /Fe sandwiches with a fixed individual TerfecoHan- layer thickness tTbFeCo ¼ 600 nm and a variable Fe layer thickness tFe ¼ 0; 10, 15 and 60 nm (denoted as samples S0, S10, S15 and