Journal of Magnetism and Magnetic Materials 242–245 (2002) 1411–1417 Invited paper Development of giant low-field magnetostriction in a-TerfecoHan-based single layer, multilayer and sandwich films N.H Duc Faculty of Physics, Cryogenic Laboratory, Vietnam National University, Hanoi, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam Abstract Giant magnetostriction in low magnetic field has been achieved with different approaches, among which single, multilayer and sandwich films with rare-earth base are considered as most promising ones Enhancement of the 3d(Fe,Co)-magnetic moment with respect to those of 3d(Fe) and 3d(Co) strengthens the Tb–FeCo exchange energies in amorphous Tb(Fe0.55Co0.45)1.5 (named as a-TerfecoHan) films This is thought to cause diminishing of the Tbsperimagnetic cone-angle and, thus, leading to enhancement of the magnetostriction Indeed, a saturation magnetostriction of lg;2 B10À3 has been obtained After annealing, a parallel magnetostrictive susceptibility of wl8 ¼ 1:8 Â 10À2 TÀ1 has been achieved at mo H ¼ 10 mT TbFeCo/YFe multilayers combining exchange-coupled giant magnetostrictive TerfecoHan layers and large-magnetisation Y0.2Fe0.8 ones exhibit an excellent magnetic and magnetostrictive softness Subsequent annealing at 3501C leads to the relaxation of the a-TbFeCo layers and to a nanocrystallisation of the YFe layers In this state, a magnetic coercive field mo HC ¼ 0:3 mT and a huge magnetostrictive susceptibility, wl8 ¼ 13 Â 10À2 TÀ1 have been obtained Mossbauer studies indicate that the magnetic softness as observed is associated to the evolution of the nanocrystalline Fe-particles Magnetostriction is also presented for Fe/TerfecoHan/Fe sandwiches In this case, giant magnetostriction is already developed at low fields in as-deposited films Technologically, this seems to be one of the simplest ways to prepare magnetostrictive films for microsystem applications r 2002 Elsevier Science B.V All rights reserved Keywords: Thin films; Multilayers; Sandwiches; Magnetostriction; Mossbauer spectra Introduction Magnetostrictive materials are transducer materials (as well as piezoelectric and shape memory ones), which directly convert electrical energy into mechanical energy They are useful in the manufacture of sensors, actuators, controllers, force and displacement as well as other electro-acoustic devices For these applications, transducer materials in the form of thin films are of special interest because cost-effective mass production is possible, compatible to microsystem process technologies In addition, magnetostrictive thin films are particularly promising as microactuator elements like cantilevers or membranes, since they combine high-energy output, E-mail address: duc@cryolab-hu.edu.vn (N.H Duc) high-frequency and remote-control operations [1–5] Due to this potential, interest in such giant magnetostrictive thin films has rapidly grown over the past few years Owing to the specifications related with applications in microelectromechanical systems (MEMS), materials research has been focused on thin-film materials showing giant magnetostriction (GMS) in combination with soft magnetic properties Most papers concerning GMS published in the last decade have been devoted to rare-earth based films and multilayers Various attempts have been focused mainly on amorphous Terfenol and Terfenol-D (a-TbDyFe2) alloys Although the magnetostriction of these amorphous alloys has been found to be one order of magnitude lower than that of its crystalline counterpart, reliable magnetostrictive devices in MEMS have been 0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V All rights reserved PII: S - 8 ( ) 0 - 1412 N.H Duc / Journal of Magnetism and Magnetic Materials 242–245 (2002) 1411–1417 designed on the basis of these materials Magnetic investigations have shown that such amorphous R-Fe alloys could never offer optimal magnetostrictive performances, due to the sperimagnetic character of the Fe and R moments [6] For these traditional magnetostrictive materials, fortunately, an alternative has been found in nanocrystalline R-Fe alloys [7] In these alloys, the crystallites are sufficiently large for the exchange coupling to be effective, but small enough to prevent macroscopic magnetic anisotropy Thus, one may consider the material as an isotropic ferromagnet, in which the magnetostriction is expected to be not reduced while the magnetocrystalline anisotropy is diminished By optimising the annealing temperature and time, the magnetostriction value can be doubled [7], although coercivity values below 100 mT, cannot be reached in these nanocrystalline single layer films [8] In the amorphous state, however, it is strongly preferable to replace the iron by cobalt, because the amorphous alloys near to the composition a-TbCo2 present higher ordering temperatures and higher magnetostriction than the equivalent iron-based alloy [3] As a tradition, we prefer to give this film a name as a-TercoN!eel, indicating the constituents: ter for Tb, co for cobalt and N!eel for the name of the laboratory where this alloy had been discovered In fact, the magnetostriction has been optimised in a series of thin films of the type a(Tb,Dy)(Fe,Co)2 (a-TerfecoN!eel-D) [9,10] Still better performances were obtained on R/T magnetostrictive spring-magnet multilayers, where the saturation field of the magnetostrictive a-TbDyFeCo is lowered by increasing the average magnetisation through exchange coupling with the soft-magnetic FeCo layers [4,5 and refs therein] For comparison, the interesting values of the magnetostriction and the magnetostrictive susceptibility are summarised in Table GMS obtained in a-TerfecoN!eel-D alloys has been explained in terms of an increase in the ferromagnetic coupling strength within the (Fe,Co) subsystem, and the effect of field annealing in inducing a well-defined uniaxial anisotropy (see below) It is well known that the substitution of Dy for Tb gives rise to an increase of the low-field magnetostriction through the reduction of the saturation field However, it is also accompanied by Table Comparison of the magnetoelastic data for magnetostrictive bulk and thin-film materials Materials Bulk crystalline c-Tb0.27Dy0.73Fe2 Single layer films a-TbFe2 a-Tb0.27Dy0.73Fe2 a-TbCo2 a-Tb0.27Dy0.73Co2 a-Tb(Fe0.45Co0.55)2 a-TerfecoHan a-Tb0.27Dy0.73(Fe0.45Co0.55)2 a-SmFe1.6 a-(SmFe2)99.26 B0.74 a-SmCo2 bg;2 (MPa) lg;2 (10À6) À101 2400 568 0.93 [12] 321 300 400 260 1040 1080 330 À380 À670 À161 20 50 155 190 300 1100 430 0.03 0.08 0.25 0.31 0.49 1.80 0.71 40 0.07 [13] [10] [3] [10] [9] Present [9] [1] [14] [5] 1.06 0.49 4.97 1.64 6.55 7.86 13.0 [15] [15] Present [16] [16] [16] Present 2.6 [5,17] [5,17] Present À19.4 À17.2 À24.5 À15.1 À63.5 À65.9 À20.2 25.9 45.6 11.0 qb8 =qB (MPa/T) Multilayers Tb0.27Dy0.73Fe2/Fe Tb0.27Dy0.73Fe2/Finemet TerfecoHan/Fe Tb0.4Fe0.6/Fe0.5Co0.5 Tb0.37Fe0.63/Fe0.65Co0.35 Tb0.27Fe0.73/Fe0.75Co0.25 TerfecoHan/Y0.2Fe0.8 À39 À28 À31.1 À27 À32 410 600 348 530 650 300 3040 1000 4000 4800 7850 Sandwich films Nd0.25Co0.75/Tb0.28Co0.72/Nd0.25Co0.75 Tb0.28Co0.72/Nd0.25Co0.75/Tb0.28Co0.72 Fe/TerfecoHan/Fe À15.2 À16.5 À24.8 248 270 320 560 117 1350 À12 ql8= =qB (10À2 TÀ1) Refs N.H Duc / Journal of Magnetism and Magnetic Materials 242–245 (2002) 1411–1417 - b (MPa) 40 30 20 10 B /at) 5.0 ( Although crystalline TbCo2 compound orders below room temperature (TC ¼ 230 K) as Co is merely metamagnetic [18], the amorphous state stabilises a moment on the Co sublattice due to band narrowing These Co moments are strongly ferromagnetically coupled A Tb-sperimagnetic structure still occurs as in a-R-Fe alloys, but the ordering temperature is now raised up to approximately 500 K, which is already higher than that of a-TbFe2 The magnetostriction of aTb1ÀxCox thin films was studied intensively by Betz et al [3] For a-TbCo2, the room temperature magnetostriction reaches a value of bg;2 ¼ À24 MPa under m0 H ¼ 1:9 T, which corresponds to lg;2 ¼ 400 Â 10À6 : As mentioned above, Duc et al [9,10] have succeeded in enhancing the magnetostrictive properties of amorphous Terfenol-D like alloys by substituting Co for Fe in a-(Tb,Dy)(Fe,Co)2 The simple arguments were that, in general, R-Fe exchange energies are larger than the equivalent R-Co interaction energies [19] This arises from the fact that the Fe moment is significantly larger than the Co one, while the R–T intersublattice exchange constant is approximately the same for T=Fe and Co Fortunately, the T–T interactions tend to be stronger in (Fe,Co)Fthan in either Fe- or Co-based alloys [20] This results in an increase of TC for a given R:T ratio The stronger R-FeCo exchange energies should then lead to a closing of the sperimagnetic cone angle and thus to an enhancement of the magnetostriction The main results obtained for the (Tb0.27Dy0.73)(Fe1ÀxCox)2 system [10] are summarised in Fig 1(a–c) At room temperature, the films become magnetically rather soft The strongest coercive field (mo HC ¼ 15 mT only) is found at x ¼ 0:63: The largest magnetostriction of bg;2 ¼ À28:8 MPa (lg;2 ¼ 480 Â 10À6 ) was found in the middle of the composition range (at x ¼ 0:47) and it can be obtained in the rather low applied magnetic field of 60 mT A detailed analysis yields values between 481 and 4.8 4.6 4.4 4.2 1.8 B /at) Magnetostriction in a-TerfecoN!eel-D and aTerfecoHan thin films 531 for the R-sperimagnetic cone-angle y [9,10], in accordance with some literature values [6] The variation in y also implies a variation in the average (Tb,Dy) moment as a function of x (see Fig 1b) From the measured magnetisation data, the 3d-moment M3d can be determined as a function of x (see also Fig 1c) Clearly, a similar composition dependence of M3d as observed in the crystalline R-(Fe,Co) alloys is found and a maximum is reached for x ¼ 0:47 where there is sufficient Co to ensure good ferromagnetic T–T coupling as well as sufficient Fe giving the larger magnetic moment This 3d magnetic moment enhancement caused by Co substitution was confirmed by Mossbauer studies [11] in a-TerfecoHan films In both configurations of parallel and perpendicular magnetic anisotropy, a hyperfine-field value Bhf of 23.5 T was reported, whereas it equals only 21 T for a-TbFe2 This shows a possibility to enhance the magnetostriction in this type of alloys by increasing the Tb concentration Indeed, we have found a record giant magnetostriction of lg;2 B10À3 for a-TerfecoHan (see Fig there only l8 is presented) [11] For this alloy, an interesting value of the magnetoelastic susceptibility wl8 ẳ ql8 =qmo Hịị of 1.8 102 TÀ1 was achieved at mo HC ¼ 15 mT Although this result is not so good as for the multilayers to be discussed in the next section (see also Table 1), it does suffice for microactuator applications ( a reduction in the saturation magnetostriction Co substitution results in an enhancement of both low-field and saturation magnetostriction Thus, we can expect a further enhancement of the magnetostriction in these alloys by increasing the Tb concentration at the expense of both Dy as well as (Fe, Co) An optimum was found for a-Tb(Fe0.55Co0.45)1.5 (denoted as a-TerfecoHan, where Han means Hanoi, i.e the capital where studies of this composition have been carried out) In this paper, we present results of our studies of GMS in the a-TerfecoHan based single layer, multilayer and sandwich films Microscopic properties of these magnetostrictive materials are discussed in connection with the Mossbauer studies 1413 1.6 1.4 1.2 1.0 0.0 0.2 0.4 0.6 0.8 1.0 x Fig Concentration dependence of magnetoelastic coupling constant (b), rare-earth magnetic moment (MR ) and 3dmagnetic moment (Md ) in (Tb0.27Dy0.73)(Fe1ÀxCox)2 N.H Duc / Journal of Magnetism and Magnetic Materials 242–245 (2002) 1411–1417 1414 400 350 -6 λ // (10 ) 300 250 200 150 100 50 -1 -0.5 µ oH (T) 0.5 Fig Magnetostriction of as-deposited (1), 2501C- (2) and 3501C- (3) annealed TerfecoHan films 300 λ// (10-6) 250 200 150 100 50 -0.8 -0.4 0.4 0.8 µoH (T) Fig Magnetostriction of as-deposited (closed circle) and 3501C-annealed (open circle) {TerfecoHan/Fe} multilayers Magnetostriction in a-Terfecohan based multilayers The starting idea of preparing the spring-magnet type magnetostrictive multilayer (MSMM) is to decrease the saturation field mo Hs (¼ K=2Ms ) by enhancing the average saturation magnetisation (Ms ) instead of decreasing the anisotropy constant K: The magnetostriction of the as-deposited TerfecoHan/Fe multilayer, however is half of that of the corresponding single layer film This is consistent with the fact that the magnetostriction of Fe is negligible Annealing effects cause the saturation magnetostriction to decrease, but to be developed in lower magnetic fields (Fig 3) The reduction of the magnetostriction may be due to the interdiffusion of atoms between the layers, which leads to a decrease of the rare-earth concentration and, then, of the magnetostriction in the interface phases Indeed, analysis of magnetisation data showed that annealing at TA ¼ 3501C made the interface spacer to extend about nm more This change, however, was not detected by the Mossbauer study As presented in Fig 4, the Mossbauer spectrum of the as-deposited and 3501Cannealed films and their hyperfine field distributions are almost similar They can be characterised by two separate contributions from ferromagnetic a-TerfecoHan and BCC-Fe components In the absence of long-range anisotropy in amorphous TbFeCo layers, along with negligible magneto-crystalline anisotropy in (FeCo) layers, the coercivity of MSMM is ranging between and 10 mT In attempts to improve the soft-magnetic properties of the highly magnetostrictive nanocrystalline layers by preparing MSMM’s with soft magnetic interlayers, Quandt and Ludwig [4], and Farber and Kronmuller [15] have studied TbFe/FeCoBSi and TbDyFe/FeSiBNbCu multilayers, respectively An almost vanishing hysteresis was obtained, at the expense, however, of the magnetostriction [4] We prepared a {12 nm TerfecoHan/13 nm Y0.2Fe0.8} multilayer After production, both TbFeCo and YFe are amorphous In this state, the multilayer exhibits already a soft magnetic and magnetostrictive character with a coercivity mo HC ¼ 3:5 mT and a parallel magnetostrictive susceptibility wl8;max ¼ 3:8 Â 10À2 TÀ1 (Fig 5a and b) This magnetostrictive softness has been strongly improved by heat treatments: mo HC ¼ 0:3 mT and wl8 ¼ 13 Â 10À2 TÀ1 in a field of À1.8 mT (Fig 5b) The Mossbauer study shows that these novel properties are associated with the development of nanostructure in the FeCo layers (see Fig 6) Initially, the Mossbauer spectrum consists of a magnetic sextet with broad lines superimposed on a paramagnetic contribution With increasing the annealing temperature, this paramagnetic contribution decreases and the lines of the sextet are sharpening Finally, at TA ¼ 3501C the magnetic sextet becomes prominent The obtained hyperfine field distribution PðBhf Þ of the as-deposited multilayer shows a minor paramagnetic component with /Bhf S ¼ T and a fraction Apar ¼ 15%: The major ferromagnetic contribution distributes in a broad hyperfine-field range with a maximum at Bhf ¼ 27 T and a fraction Aamor ¼ 85%: Taking into account the fact that the PðBhf Þ of the amorphous TerfecoHan phase is characterised by a peak at Bhf ¼ 23:5 T (see Fig 4), the observed paramagnetic and the high hyperfine-field ferromagnetic phase can be attributed to the not-well crystallised state in the YFe spacers For the 2501C-annealed film, in the hyperfine field distribution, PðBhf Þ; three almost separated components can be distinguished: (i) the paramagnetic component with /Bhf S ¼ T and Apar ¼ 5%; (ii) the low hyperfine-field ferromagnetic component with /Bhf S ¼ 23:5 T and a fraction Aamor ¼ 30% and (iii) N.H Duc / Journal of Magnetism and Magnetic Materials 242–245 (2002) 1411–1417 1415 Velocity (mm/s) -10 +10 P (Bhf) (%) Emission 1.02 (a) 1.00 1.02 30 20 10 0 10 20 30 40 30 40 30 20 10 (b) 0 1.00 10 20 B hf (T) Fig Mossbauer spectra and hyperfine-field distributions of the {TerfecoHan/Fe} multilayers: (a) the as-deposited film, (b) after annealing at 3501C 200 0.15 2 (T-1) 0.10 100 χλ // λ// (10-6) 150 0.05 50 -15 (a) -10 -5 10 0.00 -15 15 µoH (mT) (b) -10 -5 µoH (mT) 10 15 Fig Low field dependence of the parallel magnetostriction (a) and magnetostrictive susceptibility (b) for the {TerfecoHan/Y0.2Fe0.8} multilayers: curves (1) as-deposited and curves (2) 3501C-annealed multilayers the high hyperfine-field ferromagnetic component with /Bhf S ¼ 31:5 T and a fraction Aferr ¼ 65%: The Bhf results are in agreement with the development of the BCC-Fe phase After annealing at 3501C, the low hyperfine-field peak at Bhf ¼ 23:5 T still remains (with Aamor ¼ 20%), the high hyperfine-field peak (with Aferr ¼ 80%) is shifted up to 34 T, but the paramagnetic phase disappeared in the sample This was also confirmed by X-ray diffraction results [21] Such an excellent magnetostrictive softness is rather promising for MEMS Magnetostriction in TerfecoHan based sandwich films To make an attempt at simplifying the production process of soft magnetostrictive films, we have prepared and investigated three {Fe/TerfecoHan/Fe} sandwich films with the fixed TerfecoHan layer thickness of 800 nm and a variable Fe thickness tFe ¼ 12; 50 and 75 nm, which are named as the sandwich A, B and C, respectively Their coercive field and magnetostriction data are listed in Table It is worthwhile to note that although these sandwiches show a somewhat smaller magnetostriction with respect to corresponding single layer and multilayer films, their magnetic softness is comparable with that of corresponding multilayers In particular, the as-deposited sandwich A (with smallest Fe-thickness) exhibits already a coercivity as small as 4.5 mT The observed behaviour may be related to the magnetisation reversal of the Fe layer at the surface of the TerfecoHan layer Anyway, these films would open advanced possibilities to fabricate magnetostrictive films for MEMS N.H Duc / Journal of Magnetism and Magnetic Materials 242–245 (2002) 1411–1417 1416 Velocity (mm/s) -12 +12 12 10 1.01 (a) (a) 1.00 1.02 10 20 30 40 10 20 30 40 20 30 40 20 P (Bhf) Emission n (b) (b) 15 10 0 1.00 1.02 20 (c) 15 (c) 10 1.00 10 Bhf (T) Fig Mossbauer spectra and hyperfine-field distributions of the {TerfecoHan/Y0.2Fe0.8} multilayers: (a) the as-deposited film, (b) after annealing at 2501C and (c) 3501C Table Coercive field and magnetostriction of several TerfecoHan based sandwich films TA Sandwich A Sandwich B g;2 Sandwich C (1C) mo HC (mT) l (10À6) mo HC (mT) l (10À6) mo HC (mT) lg;2 (10À6) 30 150 250 350 500 4.5 5.0 4.7 3.9 5.0 450 420 400 350 400 15.4 11.2 15.4 4.5 5.0 320 280 330 350 350 13.8 13.7 6.8 5.4 5.8 418 400 460 400 475 Acknowledgements The project QG 99.08 of the Vietnam National University, Hanoi and the project 420.301 of the Fundamental Research Program of Vietnam support this work Samples were prepared at the International Training Institute for Materials Science (ITIMS) The collaboration with Dr T.M Danh, Dr H.N Thanh, N.A Tuan, D.T Huong Giang, V.N Thuc, Prof N.P Thuy at the Cryogenic Laboratory and ITIMS, that with Dr D Givord at the Laboratoire Louis N!eel and that with Prof J Teillet at the University of Rouen has been very useful to complete this work References [1] T Honda, K.I Arai, M Yamaguchi, J Appl Phys 76 (1994) 6994 g;2 [2] F Claeyssen, N Lhermet, R Le Letty, P Bouchilloux, J Alloys Compounds 258 (1997) 61 [3] J Betz, K Mackay, D Givord, J Magn Magn Mater 207 (1999) 180 [4] A Ludwig, E Quandt, J Appl Phys 87 (2000) 4691 [5] N.H Duc, in: K.A Gschneidner Jr., L Eyring (Eds.), Handbook of Physics and Chemistry of the Rare Earths, Vol 32, Elsevier Science, North-Holland, Amsterdam, 2001 [6] P Hansen, in: K.H.J Buschow (Ed.), Handbook of Magnetic Materials, Vol 6, Elsevier Science, NorthHolland, Amsterdam, 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TerfecoHan/Fe multilayer, however is half of that of the corresponding single layer film This is consistent with the fact that the magnetostriction of Fe is negligible Annealing effects cause the saturation... a-TerfecoN!eel-D alloys has been explained in terms of an increase in the ferromagnetic coupling strength within the (Fe,Co) subsystem, and the effect of field annealing in inducing a well-defined uniaxial