VNU jo u r n a l o f s c i e n c e , M a th e m a tic s - P h y s ic s , T.xx, Ny2, 2004 N a n o s tr u c tu r e a n d m a g n e to s tr ic tio n in n o v e l D I S C O N T I N U O U S T e r f e c o h a n / Y Fe E X C H A N G E - S P R I N G T Y P E M U L T IL A Y E R S Do T h i H u o n g G ia n g , N g u y e n H u u D u e 1, P h a m T h i T h u o n g Cryogenic Laboratory, Department of Physics, College o f Sciences - V N U Abstract Sputtered Tb(Fen Co,) !-), -/YxFel x multilayers (0 < X < 0.2) with a T)FeCo layer thickness tn.KpCo - nm and YFeCo layer thickness iYi-vr» - 10 nm hive been studied bv means of the X-ray diffraction (XRD), high-resolution tnnsmission electron microscopy (HR-TEM), conversion electron Mossbauer Sfectrometrv (CEMS) and magnetostriction investigations The results show tlat nanocrystals are naturally formed and coexist within an amorphous matrix 11 Y, layers For this discontinuous exchange-spring multilayers, a parallel magnetostrictive susceptibility ỵ,j: as large as 29.4x10 “ T 1, which is almost half of that (79.6x10 T ') of the Metglas 2605SC, was achieved I n t r o d u c t i o n M agnetostrictive m a te ria ls a re tran sd u ce r m ateria ls (as well as piezoelectric and shape memory ones), which directly convert electrical energy into m echanical energy They are useful in the m anufacture of m icroactu ato rs as well as microsensors [1-3] The perform ance of m icroactuators is p rim arily determ in ed by the value of the m agn etostriction (A), which is the dim ensional chang e resulting from the orientation of m ag n etizatio n from one direction to an o th er The performance of m icrosensors, however, depends r a th e r on the value of th e (parallel) m agnetostrictive susceptibility, X.Ằ// - cỈẢị/ldB, which re p re se n ts th e m ag neto strictiv e response to an applied field For these applications, tra n s d u c e r m a te ria ls in the form of thin films are of special in te re s t because cost-effective m ass production is possible, compatible to m icrosystem process technologies Most papers concerning g ia n t m agnetostriction published in the la st decade have been devoted to r a r e - e a r th based films and m ultilayers As a tra d itio n , various attem p ts have been m ainly focused on am orphous Terfertol (a-T bFe2) and Terfenol-D (ơ-TbDyKe,) alloys (Ter for Tb, D for Dy, fe for Fe and nol for Naval O rdnance Laboratory, w here th ese alloys were discovered) [4] In the a m o rp h o u s state, however, it is strongly p referable to su b stitu te the iron by cobalt, because the amorphous alloys are n e a r th e composition a-TbCo2, t h a t p re s e n ts h igh er ordering tem p era tu re s and h ig h e r m agneto striction th a n the equivalent Fe-based alloy [5] Permanent address: Academic Affairs Department, VNƯ, 144 Xuan Thuy Road, Cau Giay, Hanoi E-mail: ducnli@vnu.edu.un , , Do Thi H u o n g G ia n g Nguye n H uu D u e P h a m Thi Thu ong Because of the im p o rta n t role of th e “L aboratoire Louis N éel” (Grenoble, France) in th eir development, we have proposed to refer to the a-TbCo.), as “a - T e r c o n é e l by an obvious analogy to Terfenol In fact, the m agnetostriction has been optimized in a series of th in films of the type a-(Tb,Dy)(Fe,Co)2 (a-Terfeconéel-D) [6,7] Ir Hanoi, we have developed the am orphous Tb(Fe0.5f,Co0 15) 1.5 film (nam ed a-Tcrfecohan, h e re han m eans Hanoi, i.e th e capita] where studies of this composition have been carried out [6,7]) Still b e tte r perform ances were obtained on m agnetostrictive spring-m agnet m ultilayers, where the sa tu ratio n field of the m agnetostrictive aTbDyFeCo phase is lowered by increasing the average m ag netisatio n through exchange coupling with the soft-magnetic FeCo layers [2,3 a n d refs, therein] The conventional m ultilayered concept is usually associating magnetic h a rd with soft layers, which are stru c tu lly homogeneous in e ith er crystalline or am orphous sta te - nam ed as continuous exchange-spring configuration In th is case, m agnetization reversal is th ou ght to be nucleated w ithin the soft layer at low applied field and pro pagates from the soft layers into the m agnetostrictive layers [8,9] The nucleation of reversal usually occurs at defect points on the sample surface and at interfaces In this context, one expects t h a t the reversal can be easier nucleated in discontinuous soft phase, i.e in lay ers in which the FeCo nanograins are em bedded w ithin a non-m agnetic m atrix The idea to prepare this novel discontinuous exchange-spring type m ultilay er was applied for {Terfecohan/YFeCo} m ultilayers by using the bottom-up approach [6] In this paper, we report a direct approach to obtain the n a tu lly formed n an o stru ctu re by controlling the Y-concentration in {TerfecohanlYxFej.J m ultilayers, th a t shows a great potential to optim ize both large m agnetostriction and large m agnetostrictive susceptibility E x p e r i m e n t a l p {TerfecoHanlYxF e l x}n m ultilayers with X = 0, 0.1, 0.2, n - 50 and the individual layer thicknesses t Th\?vC0 - 12 nm and t |.VC0 - 10 nm were fabricated by rf-m agnetron sp u tterin g at the C en ter for M aterials Science (College of N a tu l Science, VNU) Composite ta rg e ts were consisted of segm ents of different elem ents (here Tb, Y, Fe, Co) (fig la) The typical plasm a image for sp u tte rin g power of 200 w a nd the Ar pressure of 10 m bar is showed in fig lb The su b s tra te s were glass microscope cover-slips with a nom inal th ickn ess of 150 //m Both ta rg e t and sam ple holders were water-cooled The crystal s tru c tu re of sam ples were studied by X-ray diffraction using the D5005 Siem ens with a cooper anticathode The sam ple n a n o stru c tu re was investigated using high-resolution transm ission electron microscopy (HRTEM) at the In stitu te of Physics, C hem nitz U niversity of Technology (Germany) The conversion electron M ossbauer spectrom etry (CEMS) was recorded using a NanostiUCture and magnetostriction in novel conventional spectrom eter equipped w ith a hom em ade proportional counter The source was a "C o in rhodium m atrix helium -m ethane 39*» (a) Fig / Composite Terfecohan ta g e t (a) and its p lasm a in RF sp u tte rin g (b) The m agnetostriction was m easured using an optical deflectom eter (resolution of x l (i rad), in which the bending of the s u b s tra te due to the m agnetostriction in the films was determ ined E x p e r i m e n t a l r e s u l t s a n d d i s c u s s i o n s 3.1 Nanostructure The X-ray 0-20 diffraction resu lts of the in vestigated TerfecohanlYxFe!.x m ultilayers are shown in fig One observes a narrow and large intensity diffraction pick at 20 = 45° in the X = sample, ch arac teristics of the (110) reflections of bcc-Fe No other diffraction peaks are observed indicating th a t the Terfecohan layers are am orphous The intensity of the (110) reflection pick is strongly reduced for X = 0.1 This is a ttrib u te d to the form ation of bcc-Fe nanograins Finally, the (110) reflection alm ost d isap pears a t X —0.2 reflecting the fact th a t the whole m ultilayer is now amorphous The corresponding electron diffraction p a tte rn s (fig 3a-c) reinforce fu rth e r the conclusions of the X-ray analysis The am orphous sta te existing in Terfecohan layers is characterized by the (typical) first brigh t spread ring from the inside diffraction spot, w hereas the other rings which are ch aracteristics of the YsFe,.s layers, exhibit d rastically different behaviours with the variable Y-concentration They are alm o st complete sh a rp rings for V = 3a) and spotty rings for X = 0.1 (fig 3b) indicating the crystalline state of Fe layers and the nanocrystalline sta te of the Y ,„FeIU) layers, respectively For X = 0.2, these rings become spread (fig 3c) th a t evidence for the am orphous state of Y(l 2Fen Klayers A periodic stripe stru c tu re of smooth and unsm ooth layers in HRTEM-crosssectional m icrograph viewed in Fig 4a is a good evidence for the m ultilayered stru ctu re of continuous (amorphous) Terfecohan layers and discontinuous (nanocrystalline) Y0.1F e 0.9 layers Dark spots observed in unsm ooth strip es are Do Thi H u o n g Giang, Nguyen Huu Due, P h a m Thi Thuong noticeable with an average size of the stripe thickness T hey are a ttrib u te d to bccp e nanograins with an average diam eter of about 10 11111 These nanog rains are considered as the origin for the weak X-ray diffraction peak and b right spots in the electron diffraction p a tte rn s already m entioned above in fig and 3b Sim ilar behavior was observed for {TerfecohanlYn ,(Fe,Co)o9} m u ltilay ers [8], For X = 0.2 however, the am orphous s ta te resu lts in a periodic, smooth and homogenous stripe structu re, see fiIg 4b b c c -F e ị 'in c hTO -0 ■ - \ I 0,2 20 b c c -F e i 30 I Angle1(22Ifc ineia) eta) Angle Fig X-ray sp e ctra :tra of as-deposited Terfecohan/Y^Fei TerfecohanlY T s m u ltila y e rs Fig Electron diffraction spectra of as-deposited Terfecohan/YsF e ] s m ultilayers Fig The bright-field high resolution TEM -cross-sectional m ic ro g p h s of TerfeeohanlYxF e l x m ultilayers: (a) X = 0.1 and (b) X = 0.2 Nanostructure and magnetostriction in novel T ie tran sfo rm a tio n of the am orphous sta te can be associated to the reduction of the therm odynam ic driving force for crystallisation caused by the Y su bstitution in the YxFe,.x layers Sim ilar behaviour was previously rep orted for evaporated ZxF e lx films [10] In ref [10], it was found th a t the am orphous Fe phase is only stable It small th ickn ess and the crystallisation sets in if the thickness exceeds a critical value of about nm The critical thickness can, however, reach a value of 30 nm in the Ftìí);tZr- film ev ap orated on Zr base layers In general, it is possible to note t h a t the tran sfo rm a tio n of' the am orphous sta te of Fe is shown to be depended on the rare-earth (R and/or Y) concentration The n a n o stru c tu re can be n a turally formed in as-deposited (R.Y)Fe layers at a critical (R.Y)-concentration Or,.) only H ere V, ~ 0.1 F u r th e r increasing re -e a rth content stabilizes the am orphous state This is the reason t h a t the Terfecohan phase w ith high Tb-concentration (xTb = 0.4) always exists in the am orphous sta te in all investigated samples 3.2 Mõssbauer spectra Fig p resen ts the CEM spectra for the as-deposited TerfecohanlYxF e l x multilayers For X = 0, the m agnetic sextet of bcc-Fe is p ro m in e n t in the M ossbauer spectra (fig 5a) The lines of the sextet are broadening and a param agnetic contribution occurs in the X = 0.1 sam ple (fig 5b) and finally, the param agnetic contribution becomes pro m inent for X = 0.2 (fig 5c) The spectra have been fitted with a wide contribution of hyperfine field to ta k e n into account all the environm ents experienced by Fe57 nuclei The obtained hyperfine field distributions P(Bm) are included in figure For X = 0, the P(Bht) can be d istinguished with two almost se p ara ted components: (i) the low hyperfine-field com ponent with an average value of = 22 T and (ii) the high hyperfine-field with = 32.5 T Taking in to account the fact th a t the P(BM) of the bcc-Fe is ch arac terise d by a peak at B m = 32.4 T the observed low hyperfine-field ferrom agnetic phase can be a ttrib u te d to the a -Terfecohan phase Indeed, a value of = 22 T was reported for single Terfecohan layer film [111- In addition, it is also able to estim ate the Fe fractions, which a re of 30 % and 70 % in the Terfecohan and Fe layers, respectively This is in good a g re e m e n t with those of 28.6 % and 71.4 % deduced for the Fe concentration in the two corresponding layers For X = 0.1 (fig 5b), the low hyperfine-field ferrom agnetic com ponent with = 22.5 T alm ost rem ains with fraction A = 30.5 % The fraction of the high hyperfine-field ferrom agnetic component (with = 31.5 T) , however, is reduced (Ar„„ = 54 %) and an additional p a m ag n e tic component with = T and A pill = 5.5 %, has occurred The m easure of the B hf is in agreem en t with the form ation of the Fe nanograins The high hyperfine-field ferrom agnetic fraction alm ost d isa p p ea rs in the X- = 0.2 sample (see fig 5c) For this sample, the major ferrom agnetic contribution distrib u tes in a broad hyperfine-field range with a m axim um at B M = 22 T and a , Do Thi H u o n g Gia ng Nguyen H u u Due, P h a m Thi T h u o n g fraction Afvrv - 65 % P (B hĩ) shows a minor param agnetic c o m p onent w ith < 10 T and a fraction A p;u = 35 % V e lo c ity (m m /s) 12 20 25 02 Fig CEMS and hyperfine-field d istrib u tio n of as-d ep o site d TerfecohanlYNFe,.Nm u ltila y e rs 3.3 Magnetostriction The m agnetostriction was m easured in m agnetic fields up-to 0.4 T applied in plane, parallel and perp endicular to the long side of the sa m p le giving xn and Ầly respectively The resu lts of Ả (= Ả„ - À j are presen ted in fig 6a for as-deposited films It is clearly seen that, for X = and 0.1, th e m ag n e to stric tio n is well developed with a r a th e r large m agnetostrictive su sceptibility a t low fields, reaches a m axim um and finally decreases at high fields The observed n eg ativ e contribution to m agnetostriction is related to the formation of an e x te n d ed dom ain wall at interfaces, which was already discussed elsewhere [12] The m a g n e to stric tio n of X = 0.2 sam ple is, however, th e r difficult to s a tu r a te due to its perp en d icu lar anisotropy n a tu re Low-field parallel m agnetostrictive su sce p tib ility d a ta are presented in fig 6b The m agnetostriction as well as low-field parallel m agnetostrictive susceptibility reach m axim um values in X = 0.1 sam ple: Ã = 420xl0 r>and Xxn = 17.3x10 T The m agnetostriction o b tain ed is com parable to the value deduced from the d a ta of the single-layer sam p les, e.g X = 080x106 in Terfecohan [6,7] For the X = 0.1 sample, the value of X,.// is tim e s la rg e r th a n th a t of X = and orders larg e r th a n th a t of X = 0.2 This r e s u lts directly from the low coercivity m echanism proposed above The (compressive) s tre s s e s existing in assp uttered films are released by low te m p e tu re a n n e a lin g (at T A < 350°c for hour) This leads to the change in the orientation of the m agnetic easy axis and thus enhances noticeably the saturation magnetostriction a n d low-field parallel magnetostrictive susceptibility This is clearly evidenced in fig For X = 0.1 sample, a large saturation magnetostriction Ả = 720xl0'6 but a low coercivity of 1.1 mT can be Nanostru cture a n d m a g n e to s tr ictio n in novel reached C onsequ ently, X>M achieves a maximal value as large as 29.7x10 ' T ' at u M = 2.1 mT The o b tain ed x>.n value is alm ost 30 tim es higher th a n th a t obtained in Terfenol-D and tim es h ig h er th an th a t obtained in m ultilayers by Q uandt et al [13,14], In com parison w ith the m agnetostrictive Metglas 2605SC (xxn - 79.6x10'- T '), the obtained X'M v a lu e is still lower [15], but the p resent sam ple shows much larger m agnetostriction T his sp e cta cu lar resu lt illu stra te s th e significance of the approach, which we have developed in view of optim izing both m agnetostriction and m agnetostrictive susceptibility n H (mT) ^oH ( m T ) Fig M a g n e to s tric tio n (a) a n d parallel m agneto strictive su sceptibility h ysteresis loops (b) of as-d ep o sited TerfecohanlYxFe!.x m u ltila y ers Fig M a g n e to s tric tio n (a) a n d parallel m agn etostrictive suscep tib ility hysteresis loops (b) of 350 °C -ann ealed TerfecohanIYxFei.x m u ltila y ers C o n c l u d i n g r e m a r k s In conclusion we h ave described the direct approach to discontinuous m agnetostrictive ex ch a n g e-sp rin g m ultilayers, in which the n a n o stru ctu re is natu rally form ed in YFe soft layers by controlling the Y-concentration This novel Do Thi H u o n g Giang, Ngu yen H uu Due, P h a m Thi T h u o n g exchange-spring configuration opens an altern a tiv e ro u te tow ards new highperform ance m agnetostrictive m ate ria ls th a t both large m agneto strictio n a n d large m agnetostrictive susceptibility can be combined F u rth e rm o re , it provides a new generation of exchange-spring m agnetic configuration for studying fu n d a m e n ta l reversal m echanism A c k n o w l e d g e m e n t This work was supported by th e S ta te P ro g m for Nanoscience and Nanotechnology of Vietnam under the Project 811.204 R e fe r e n c e s I F Claeyssen, N L h erm et , R Le Letty and p Bouchilloux, J Alloys Compd 258(1997) 61 ■>' N.H Due, in: Handbook on Physics a n d Chemistry o f the Rare E a r t h s K.A Gs~hneirdner, Jr., L E yring a n d G.H Lande (Elsevier Science, A m sterdam ), Vol 32(2001) N.H Due, P.E Brom m er, in: Handbook on Magnetic Materials, K.H.J Buschow ed., (Elsevier Science, A m sterdam ), Vol 14(2002) 89 A.E Clark, in: Ferromagnetic Materials, ed E.p W ohlfarth, (N orth-H olland Amsterdam), Vol 1(1980) 531 P.E Brom mer, N.H Due, in: Handbook on Magnetic Materials, K.H.J Buschow ed., (Elsevier Science, A m sterdam ), Vol 12(1999) 259 N.HL Due, P.E Brom m er, in: Encyclopedia o f Materials: Science Technology, K.H.J Buschow ed., (Elsevier Science, A m sterdam ), (2004) N.HL Due, J Magn Magn Mater., 242-245(2002) 1411 N.HL Due, D.T Huong Giang, N Chau, J Magn Magn Mater., 265(2004) D.r Huong Giang, N.H Due, V.N Thuc, L v Vu, N Chau, J Appl Phys Le t., (2004), in press 10 Ư H err, H Geisler, H Ippach and K Samwer, Phys Rev., B 59(1999) 13719 II, T.M Danh, N.H Due, H.N T h a n h and J Teillet, J Appl Phys., 87(2000) 72)8 12 N H Due, D T Huong Giang, V N Thuc, I Davoli, F Richomme, J Magn Mcgn Mater., (2004), in press 13 E Q uandt, A Ludwig, J Betz, K Mackay, D Givord, J App Phys 81(1997) 5420 14 Ludwig, E Q uandt, J Appl Phys 87(2000) 4691 15 E Trém olet de L acheisserise, D Gignoux, M Schlenker, Materials and Applications, Mag netism , Kluwer Academic Publisher, Vol 2(2002) 227 and ... nucleated in discontinuous soft phase, i.e in lay ers in which the FeCo nanograins are em bedded w ithin a non-m agnetic m atrix The idea to prepare this novel discontinuous exchange-spring type. .. homogeneous in e ith er crystalline or am orphous sta te - nam ed as continuous exchange-spring configuration In th is case, m agnetization reversal is th ou ght to be nucleated w ithin the soft layer at. .. variable Y-concentration They are alm o st complete sh a rp rings for V = 3a) and spotty rings for X = 0.1 (fig 3b) indicating the crystalline state of Fe layers and the nanocrystalline sta te of