V N U . j o 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 t r u c t u r e a n d m a g n e t o s t r i c t i o n i n 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 / YFe E X C H A N G E - S P R I N G T Y P E
M U L T I L A Y E R S
Do T h i H u o n g G i a 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, De partment o f 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 - 1 2 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 tla t 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.
1. I n t r o d u c t i o n
M agnetostrictive m a te r ia ls a re tra n s d u c e r m a te ria ls (as well as piezoelectric and shape m emory ones), which directly convert electrical energy into m echanical energy They are useful in th e m a n u fa ctu re of m ic ro a c tu a to rs a s well as microsensors [1-3]. T he p erfo rm an ce of m icro actu ato rs is p rim a rily d e te rm in e d by the value of th e m a g n e to stric tio n (A), which is the d im ensional c h an g e resu ltin g from the o rien tatio n of m ag n e tiz a tio n from one direction to a n o th e r. The performance of m icrosensors, however, depends r a t h e r on the v alu e of th e (parallel) m agnetostrictive su scep tib ility , X.Ằ// - cỈẢị/ldB, which r e p r e s e n ts th e m a g n e to s tric tiv e response to an applied field. For th ese applications, t r a n s d u c e r m a t e r i a ls in th e form of thin films are of special in te r e s t because cost-effective m ass production is possible, compatible to m icro sy stem process technologies.
Most p ap ers co ncerning g ia n t m ag n eto strictio n p u b lish ed in th e la s t decade have been devoted to r a r e - e a r t h b ased films and m u ltilay ers. As a tra d itio n , various a tte m p ts have been m ain ly focused on am orphous Terfertol (a-T b F e2) a n d Terfenol-D (ơ-TbDyKe,) alloys (Ter for Tb, D for Dy, fe for Fe and nol for N aval O rd n a n ce Laboratory, w here th e s e alloys were discovered) [4]. In th e a m o rp h o u s sta te , however, it is strongly p refe rab le to s u b s titu te th e iron by cobalt, b ecau se the am orphous alloys are n e a r th e composition a-TbCo2, t h a t p r e s e n ts h ig h e r o rd erin g te m p e ra tu re s and h ig h e r m ag n e to stric tio n th a n the e q u iv a le n t F e-b ased alloy [5].
1 Permanent address: Academic Affairs Department, VNƯ, 144 Xuan Thuy Road, Cau Giay, Hanoi E-mail: ducnli@vnu.edu.un
1
9 Do Thi H u o n g G i a n g
,
N g u ye n H uu D u e,
P h a m Thi T hu o ng Because of the im p o r ta n t role of th e “L aboratoire Louis N éel” (Grenoble, France) in th e ir developm ent, we have proposed to refer to th e a-TbCo.), as “a - T e r c o n é e lby an obvious analogy to Terfenol. In fact, the m agnetostriction h a s been optimized in a series of th in films of th e type a-(Tb,Dy)(Fe,Co)2 (a-Terfeconéel-D) [6,7]. Ir. Hanoi, we have developed th e am o rp h o u s Tb(Fe0.5f,Co0.15)1.5 film (nam ed a-Tcrfecohan, h e re han m eans Hanoi, i.e. th e capita] w here stu d ies of th is composition have been carried out [6,7]). Still b e tte r perform ances were o b tain ed on m agnetostrictive sp rin g -m ag n et m u ltilay ers, w here the s a tu ra tio n field of th e m agnetostrictive a- TbDyFeCo phase is lowered by increasing th e average m a g n e tisa tio n th ro u g h exchange coupling with the soft-m agnetic FeCo layers [2,3 a n d refs, therein].The conventional m u ltilay ered concept is u sually asso ciatin g m agnetic h a rd with soft layers, which are s tru c tu ra lly hom ogeneous in e ith e r crystalline or am orphous sta te - n am ed as continuous exchange-spring configuration. In th is case, m ag n etizatio n rev e rsa l is th o u g h t to be nucleated w ith in th e soft layer at low applied field and p ro p ag a tes from the soft layers into th e m ag n eto strictiv e layers [8,9]. The n u cleatio n of rev e rsa l usually occurs a t defect points on the sam ple surface and a t interfaces. In th is context, one expects t h a t the reversal can be easier nu cleated in d iscontinuous soft phase, i.e. in la y e rs in which the FeCo n a n o g rain s are em bedded w ithin a non-m agnetic m atrix. The idea to prepare th is novel discontinuous ex ch ange-spring type m u ltila y e r was applied for {Terfecohan/YFeCo} m u ltila y e rs by using the bottom -up ap p ro ach [6]. In th is paper, we rep o rt a direct ap p ro ach to obtain the n a tu ra lly formed n a n o stru c tu re by controlling the Y -concentration in {TerfecohanlYxFej.J m u ltila y ers, t h a t shows a g reat p otential to optim ize both large m agnetostriction and large m agnetostrictive susceptibility.
2. E x p e r i m e n t a l
p
{TerfecoHanlYxF e l x}n m u ltila y ers with X = 0, 0.1, 0.2, n - 50 and the individual layer th ick n esses tTh\?vC0 - 12 nm and t|.VC0 - 10 nm were fab ricated by rf-m agnetron s p u tte rin g a t th e C e n te r for M a terials Science (College of N a tu ra l Science, VNU).
Composite ta r g e ts were consisted of seg m en ts of d ifferent e le m e n ts (here Tb, Y, Fe, Co) (fig. la). The typical p lasm a image for sp u tte r in g power of 200 w a n d the Ar p ressu re of 10 2 m b ar is showed in fig. lb. The s u b s tr a te s were glass microscope cover-slips w ith a n o m in al th ic k n ess of 150 //m. Both t a r g e t a n d sam ple holders were water-cooled.
The crystal s tr u c tu r e of sam ples were studied by X-ray diffraction using the D5005 Siem ens with a cooper anticathode. The sam p le n a n o s tr u c tu r e was investigated usin g high-resolution tran sm issio n electron microscopy (HRTEM) a t the I n s titu te of Physics, C hem nitz U niversity of Technology (Germany). The conversion electron M o ssb a u er spectrom etry (CEMS) w as recorded using a
NanostiUCture a n d m ag ne tos tri ctio n in novel. 3 conventional sp e ctro m eter equipped w ith a h o m em ad e h e liu m -m eth an e proportional counter. The source was a " C o in rhodium m atrix .
39*»
(a)
Fig /. C om posite T erfecohan ta g e t (a) and its p la s m a in R F s p u tte r in g (b) The m ag n eto strictio n w as m e a su re d u sing an optical d eflectom eter (resolution of 5 x l 0 (i rad), in which th e ben d in g of the s u b s tr a te due to th e m agnetostriction in th e films was determ ined.
3. 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 re s u lts of th e in v estig ate d TerfecohanlYxFe!.x m ultilayers are shown in fig. 2. One observes a narrow and large intensity diffraction pick at 20 = 45° in the X = 0 sam ple, c h a ra c te ristic s of the (110) reflections of bcc-Fe. No o th er diffraction p eaks are observed in d icatin g t h a t the Terfecohan layers are am orphous. The in te n sity of th e (110) reflection pick is strongly reduced for X = 0.1. This is a ttr ib u te d to th e form ation of bcc-Fe nan o g rain s Finally, th e (110) reflection alm ost d isa p p e a rs a t X — 0.2 reflecting the fact t h a t th e whole m u ltila y er is now am orphous. The corresponding electron diffraction p a tte r n s (fig. 3a-c) reinforce f u r th e r the conclusions of the X-ray analysis. The am o rphous s ta te existing in Terfecohan layers is c h arac terize d by the (typical) first b rig h t sp re ad rin g from the inside diffraction spot, w h e rea s the other rings which are c h a ra c te ristic s of the YsFe,.s layers, ex h ib it d rastically different behaviours with the variab le Y-concentration. They are a lm o st complete sh a rp rings for .V = 0 3a) and spotty rings for X = 0.1 (fig. 3b) in d ic atin g th e crystalline sta te of Fe layers and the n a n o cry stallin e s ta te of th e Y ,„F eIU) layers, respectively. For X
= 0.2, these rings become sp read (fig. 3c) t h a t evidence for th e am o rp h o u s sta te of Y(l .2Fen K layers.
A periodic strip e s tr u c tu r e of smooth and u n sm o o th lay e rs in HRTEM-cross- sectional m icrograph viewed in Fig. 4a is a good evidence for the m ultilayered s tru c tu re of con tin u o u s (am orphous) Terfecohan lay ers a n d discontinuous (nanocrystalline) Y0.1F e0.9 layers. Dark spots observed in unsm o o th strip es are
4 Do Thi H u o n g Giang, N g u ye n Huu Due, P h a m Thi T h u o n g noticeable with a n av erag e size of the strip e thickness. T hey are a ttr ib u te d to bcc- p e n a n o g ra in s with an av erag e d iam ete r of about 10 11111. These n a n o g ra in s are considered as th e origin for th e weak X-ray diffraction peak a n d b rig h t spots in the electron diffraction p a tte r n s alre ad y m entioned above in fig. 2 and 3b. S im ilar behavior was observed for {TerfecohanlYn ,(Fe,Co)o9} m u ltila y e rs [8], For X = 0.2 however, th e a m o rp h o u s s ta te re s u lts in a periodic, smooth and hom ogenous stripe stru c tu re , see fiIg. 4b.
b c c -F e
ị
'inc h3 -
TO
- ■0 I b c c - F e
1 - i I
\ 0,2 20 30 Angle 12 Ifceta)
:tr a of as-deposited TerfecohanlY. T Angle (2 ineia)
Fig 2. X-ray s p e c tra of as-deposited Terfecoh an/ Y^Fe i s m u ltila y e r s
Fig 3. Electron diffraction sp e ctra of as-deposited T e rf e co ha n /Y sF e ] s m u ltila y ers
Fig 4. The b rig h t-field high resolution T E M -cross-sectional m ic ro g ra p h s of TerfeeohanlYxF e l x m u ltilay ers: (a) X = 0.1 and (b) X = 0.2
Nanostructure a n d m a gn e to str ic tio n in novel. 5 T ie tra n s fo rm a tio n of the am orphous s ta te can be asso ciated to the reduction of th e therm odynam ic driving force for c ry stallisa tio n caused by th e Y su b stitu tio n in the YxFe,.x layers. S im ilar beh av io u r was previously re p o rte d for evaporated ZxF e lx films [10]. In ref. [10], it was found t h a t th e a m o rp h o u s Fe phase is only sta b le It small th ic k n e ss and th e c ry stallisatio n sets in if th e th ic k n ess exceeds a critical value of a b o u t 2 nm. The critical th ick n ess can, however, reach a value of 30 nm in the Ftìí);tZr- film e v ap o rate d on Zr base layers. In g eneral, it is possible to note t h a t the tra n s fo rm a tio n of' the am o rphous s ta te of Fe is show n to be depended on th e rare-earth (R and/or Y) concentration. The n a n o s tr u c tu r e can be n a tu ra lly formed in as-deposited (R.Y)Fe layers at a critical (R.Y)-concentration Or,.) only.
H ere .V, ~ 0.1. F u r t h e r increasing r a r e - e a r th content sta b ilize s th e am orphous sta te . This is th e reaso n t h a t the Terfecohan phase w ith high Tb-concentration (xTb
= 0.4) always exists in the am o rp h o u s sta te in all in v estig ate d sam ples.
3.2. Mõssbauer spectra
Fig. 5 p re s e n ts the CEM sp ectra for th e as-deposited TerfecohanlYxF e l x m ultilayers. For X = 0, th e m agnetic sextet of bcc-Fe is p ro m in e n t in the M ossbauer spectra (fig. 5a). The lines of th e sextet are b ro aden in g a n d a p a ra m ag n e tic contribution occurs in th e X = 0.1 sam ple (fig. 5b) and finally, th e p aram ag n etic contribution becomes p ro m in e n t for X = 0.2 (fig. 5c). The sp e ctra have been fitted with a wide c o n trib u tio n of hyperfine field to ta k e n into account all the en vironm ents experienced by Fe57 nuclei. The o b tained h y p erfin e field d istrib u tio n s P(Bm) are included in figure 5. For X = 0, the P( B ht) can be d istin g u ish ed with two almost se p a ra te d components: (i) the low hyperfine-field com ponent with an average value of <5|,|> = 22 T and (ii) th e high hyperfine-field with <Bhf> = 32.5 T.
Taking in to account the fact t h a t the P(B M) of th e bcc-Fe is c h a ra c te rise d by a peak at Bm = 32.4 T. th e observed low hyperfine-field ferro m ag n etic p h ase can be a ttrib u te d to the a-Terfecohan phase. Indeed, a value of <BM> = 22 T was reported for single Terfecohan lay er film [111- In addition, it is also able to e stim a te the Fe fractions, which a re of 30 % a n d 70 % in th e Terfecohan a n d Fe layers, respectively.
This is in good a g re e m e n t with those of 28.6 % a n d 71.4 % deduced for the Fe concentration in th e two corresponding layers. For X = 0.1 (fig. 5b), th e low hyperfine-field ferro m ag n etic com ponent with <Bhi> = 22.5 T alm ost re m a in s with fraction A = 30.5 %. The fraction of th e high hyperfine-field ferrom agnetic component (with <B|,|> = 31.5 T) , however, is reduced (Ar„„. = 54 %) and an additional p a ra m a g n e tic component w ith <Bhi> = 4 T a n d A pill. = 5.5 %, h as occurred.
The m ea su re of th e B hf is in a g re em e n t with the form ation of th e Fe nanograins.
The high hyperfine-field ferrom agnetic fraction alm ost d isa p p e a rs in the X- = 0.2 sam ple (see fig. 5c). For th is sam ple, th e major ferro m ag n etic contribution d istrib u te s in a broad hyperfine-field range with a m ax im u m a t B M = 22 T and a
6 Do Thi H u o n g G i a n g
,
N g u ye n H u u Due, P h a m Thi T h u o n g fraction Afvrv - 65 %. P ( B hĩ) shows a m inor p a ra m a g n e tic c o m p o n e n t w ith <B hf> < 10 T and a fraction A p;u. = 35 %.3.3. Magnetostriction
The m ag n eto strictio n was m easu red in m agnetic fields up-to 0.4 T applied i n plane, parallel and p e rp e n d ic u la r to the long side of th e s a m p le giving xn and Ầly respectively. The re s u lts of Ả (= Ả„ - À j are p re se n te d in fig. 6a for as-deposited films. It is clearly seen th a t, for X = 0 and 0.1, th e m a g n e to s tric tio n is well developed with a r a t h e r large m ag n eto strictiv e su sce p tib ility a t low fields, reaches a m axim um and finally decreases a t high fields. The o b serv ed n e g a tiv e co n trib u tio n to m ag n eto strictio n is rela te d to th e form ation of an e x te n d e d dom ain wall at interfaces, which was alre ad y discussed elsew here [12]. T he m a g n e to s tric tio n of X = 0.2 sam ple is, however, r a t h e r difficult to s a t u r a t e due to its p e rp e n d ic u la r anisotropy n a tu re . Low-field p arallel m ag n e to stric tiv e su s c e p tib ility d a ta are presen ted in fig. 6b. The m agnetostriction as well as low-field parallel m agnetostrictive susceptibility reach m axim um v a lu e s in X = 0.1 sam ple: Ã = 4 2 0 x l0 r> and Xxn = 17.3x10 2 T The m agnetostriction o b ta in e d is co m p a rab le to the value deduced from th e d a ta of the single-layer sa m p le s, e.g. X = 1 0 8 0 x 1 0 6 in Terfecohan [6,7]. For th e X = 0.1 sam ple, the value of X,.// is 4 tim e s la rg e r th a n th a t of X = 0 and 2 o rd ers la rg e r th a n t h a t of X = 0.2. This r e s u l t s directly from the low coercivity m echanism proposed above. The (compressive) s t r e s s e s ex istin g in as- sp u tte re d films are rele ased by low te m p e r a tu re a n n e a lin g (at T A < 350°c for 1 hour). This leads to the change in the orientation of th e m agnetic easy axis and thus enhances noticeably the satu ratio n magnetostriction a n d low-field parallel magnetostrictive susceptibility. This is clearly evidenced in fig. 7. For X = 0.1 sample, a large satu ratio n magnetostriction Ả = 720xl0'6 but a low coercivity of 1.1 mT can be
V e lo c ity (m m /s )
12
20 25 1 02
Fig 5. CEMS and hyperfine-field d istr ib u tio n of a s-d e p o s ite d TerfecohanlYNFe,.N m u ltila y e r s
Na n os tru ct ur e a n d m a g n e t o s t r i c t i o n in novel. 7 reached. C o n se q u e n tly , X>M achieves a m axim al value as large as 29.7x10 ' T ' at u M = 2.1 mT. T he o b ta in e d x>.n value is alm ost 30 tim es h ig h er th a n t h a t obtained in Terfenol-D and 4 tim es h ig h e r th a n th a t obtained in m u ltilay ers by Q u an d t et. al.
[13,14], In com parison w ith th e m agnetostrictive Metglas 2605SC (xxn - 79.6x10'- T '), the o b tain ed X'M v a lu e is still lower [15], b u t the p re s e n t sam p le shows much larg er m ag n eto strictio n . T h is s p e c ta c u la r re s u lt illu s tr a te s th e significance of the approach, w hich we h a v e developed in view of optim izing both m ag n eto strictio n and m ag n eto strictiv e su sce p tib ility .
n H ( m T ) ^ o H ( m T )
Fig 6. M a g n e to s tr ic tio n (a) a n d p ara lle l m ag n e to stric tiv e su sce p tib ility h y ste resis loops (b) of a s-d e p o s ite d TerfecohanlYxFe!.x m u ltila y e rs
Fig 7. M a g n e to s tr ic tio n (a) a n d p a ra lle l m a g n e to stric tiv e su sce p tib ility h y ste resis loops (b) of 3 5 0 °C -a n n ea le d TerfecohanIYxFei.x m u ltila y e rs
4. C o n c l u d i n g r e m a r k s
In conclusion we h a v e described the direct ap p ro ach to discontinuous m ag n e to stric tiv e e x c h a n g e -sp rin g m ultilayers, in which the n a n o stru c tu re is n a tu ra lly form ed in YFe soft lay e rs by controlling th e Y-concentration. This novel
8 Do Thi H u o n g Giang, N g u y e n H uu Due, P h a m Thi T h u o n g ex ch ange-spring config u ratio n opens an a lte rn a tiv e ro u te to w ard s new high- p erform ance m ag n e to stric tiv e m a te ria ls t h a t both larg e m ag n e to s tric tio n a n d large m ag n e to stric tiv e susceptibility c a n be combined. F u rth e rm o re , it p ro v id es a new g e n eratio n of ex ch ange-spring m agnetic configuration for stu d y in g f u 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 . T his work was su p p o rted by th e S ta te P ro g ra m for Nanoscience a n d Nanotechnology of Vietnam u n d e r th e Project 811.204.
R e f e r e n c e s
I. F. Claeyssen, N. L h e rm e t , R. Le Letty and p. Bouchilloux, J. All oys Com pd 258(1997) 61.
■>' N.H. Due, in: H a n d b o o k on Physics a n d Che mistry o f the Rare E a r t h s K.A.
Gs~hneirdner, J r ., L. E yring a n d G.H. L ande (Elsevier Science, A m ste rd am ), Vol. 32(2001) 1.
3. N.H. Due, P.E. B rom m er, in: Han dbook on Magnetic M at erials, K.H.J.
Buschow ed., (E lsevier Science, A m sterdam ), Vol. 14(2002) 89
4. A.E. C lark, in: Ferromagnetic Materials, ed. E.p. W ohlfarth, (N o rth -H o llan d Am sterdam ), Vol. 1(1980) 531.
3. P.E. B rom m er, N.H. Due, in: Han dbook on Ma gnetic Mat er ial s, K.H.J.
Buschow ed., (Elsevier Science, A m sterdam ), Vol. 12(1999) 259.
3. N.HL Due, P.E. B rom m er, in: Encyclopedia o f Materials: Science a nd Technology, K.H.J. Buschow ed., (Elsevier Science, A m ste rd am ), (2004).
7 N.HL Due, J. Magn. Magn. Mater., 242-245(2002) 1411.
3. N.HL Due, D.T. H uong Giang, N. Chau, J. Magn. Magn. Mater., 265(2004).
9. 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 a n d K. Sam wer, Phys. Rev., B 59(1999) 13719.
I I , T.M. D anh, N.H. Due, H.N. T h a n h a n d J . Teillet, J. Appl. Phys., 87(2000) 72)8.
12. N. H. Due, D. T. H uong Giang, V. N. Thuc, I. Davoli, F. Richom m e, J. Magn.
Mcgn. Mater., (2004), in press.
13. E. Q u a n d t, A. Ludwig, J. Betz, K. Mackay, D. Givord, J. App. Phys. 81(1997) 5420.
1 4. Ludwig, E. Q u an d t, J. Appl. Phys. 87(2000) 4691.
1 5. E. T rém olet de L ach eisserise, D. Gignoux, M. Schlenker, M at er ial s a nd Applications, M ag n et i sm , Kluwer Academic Publisher, Vol. 2(2002) 227.
