Nghiên cứu các màng mỏng tử giảo đơn lớp đa lớp chứa đất hiếm và các khả năng ứng dụng của chúng trong các kỹ nghệ kích thước bé

T ín h chfTt vi mô của các màng mỏng cíĩng đã được nghiên cứu bằng phổ Mossabuer.. Các kết quả nghiên cứu tìm hiểu các cơ chế vật lý của loại vật liệu mới này trong từ trưòng cao và ở nh

Đại học Quốc gia Hà nội

4-2001

Báo cáo kết quả thực hiện đề tài khoa học ĐHQG đặc biệt

QG 99 08

T ê n đề tà i: N g h iên cứu c á c m àn g m ỏng từ g iở o đơn IỚỊ 1 , da lớp chứa ổ ất hiếm và cá c

khả n ân g ứng d ụ n g c ủ a chúng tro n g c á c k ỹ nghệ kích thư ớc bé

Chủ trì đề tài: TS Nguyễn Hữu Đức

Bộ mồn V ậ t lý N h iệt độ thấp, K h o a v ạ t lý, Trưòng Đ ại học K hoa học Tự nhiên

Đ H Q G H à nội.

Các cán bộ thực hiện chính:

Trần M âu Danh, NCS, trung tâm IT ĨM S và Bộ môn v ạ t lý N hiệt độ thấp Nguyễn Pliíí T h u ỳ C B G D , Bộ môn V ật lý N h iệt độ (hấp

Hoàng Ngọc Thành C B G D , Bộ môn V ậ t ]ý N hiệt độ thấp

Trẩu Đ ìn h Thọ K ỹ thuật viên, Bộ môn V ật lý N hiệt độ thấp

Nguyễn A nh Tuấn CB hợp đồng, Bộ môn V ậ í lý N h iệt độ thấp

Nguyễn Thành N am Học viên cao học, Bộ môn V ật lý Nhiệt độ thốp

Nguyễn T h ị Ngọc A nh H ọc viên cao học, Trung trim IT IM S

Đ ỗ thị Hương G iang Sinh viên khoa V ậ t lý

V ũ N guyên Thức Sinh viên khoa V ậ t lý

M ộ t số cán bộ khác của Bộ môn V ậ t lý N h iệt độ thấp dã tham gia thảo luận xây dựng Hệ do từ giảo, Bộ môn Vật lý chất rắn và Trung tâm khoa học vật liệu dã giúp đỡ

đo nhiễu xạ Rơnghen, T rung tâm IT IM S đã cho phép sử dụng m áy sputtering để chế tạo mẫu.

Các hợp tác khoa học với Viện Từ học Louis Néel, Grenoble và PTN từ hoc,

Đ H T H Rouen, Pháp đã giiìp các phép đo độ dày màng mỏng, và phổ Mossbauer.

9

Thời gian thực hiện: 2 năm (từ tháng 4/1999)

1 Tóm tỉít các kết quả chính đà đạt được

Đ ã chế tạo thành công ờ V iệ t nam và nghiên cứu có kết quả các m àng m ỏng từ giảo đơn lớp T b (F e ,C o )2 và T b (F e ,C o ), V Đ ã tìm hiểu các cơ chế vật lý của hiện tượng từ

giảo trong các màng mỏng đất hiếm và tìm được các thông số tối ưu cho vật liệu Từ

giảo đã đạt được X ~ 10'? ở H = 0.7 T Đặc biệt, ở tìr trường thấp 10 m T , độ cảm tír giảo

«Xx/' - 1 -8x I0 '2 T 1 T ín h chfTt vi mô của các màng mỏng cíĩng đã được nghiên cứu bằng phổ Mossabuer ĐAy là các vậl liệu có các lliông số so sánh được với các vật liệu cíing loại đang được nghiên cứu ở các phòng thí nghiệm trên thế giới Các kết quà nghiên cứu thực hiện t r o n g năm 1999 vh đà có 3 bài háo đãng ở lên các tạp chí J Appl Pliys

và J o f Phys.: Contiens M atter, (xem các bài bán kèm tlieo trong phần phụ lọc)

l.b Các màng mỏng từ giảo đa lốp

Chế tạo các màng mỏng từ giảo đa lớp dã được thực hiện với kỹ thuật phức lạp hơn H ai loại m àng mỏng đa lớp đã được chế tạo và nghiên cứu (i) T b (F e ,C o ), ,/F e và (ii) T b (F e ,C o ), 5/(Y F e ) Ư u điểm cluing của cả hai loại vật liệu này là tính từ mềm tốt nhưng vẫn giữ được độ cảm từ giảo cao nhờ cấu trúc composit của chííug Loại đa lớp thứ nhất mà đề tài đã chế tạo được có tính chất lương lự như các vạt liệu cùng loại mà các phòng thí nghiệm ở trên thế giới dang đạt dược T h ế mạnh của chúng tôi là khai thác một số ý tưởng vật lý liên quan đến cấu trúc interface của vật liệu Bằng việc xử lý nhiệt, ngoài việc cải thiện tính chất từ giảo của mẫu, vùng interface đã đựơc mờ rộng l a

vh dị hướng từ vuông góc đã hình thành Với tính chất này, vật liệu CÒ 11 có khả năng ứng dụng trong kỹ thuật ghi và lưu trữ thông tin Các kếl quả đã dược xử lý, báo cáo và đăng trong tuyển tập H ộ i nghị v ạ t lý Châu Á - Thái bình dương, 8 /2 0 00 Đ ặc biệt, các kết quà Iihy cĩiiig đã được xuất bả» ở tạp chí Appl Physics Letter., (2 0 0 1 ), xem phụ lục T r o n g bài b á o n à y , n g o à i v i ệ c (h ô n g b á o g iá trị 1ỚI1 củ a d ộ c á m từ g i á o , c h ú n g tỏi CÒI1 đề xuất m ộ t giải pháp c h ọ n đ iể m là m v i ệ c c h o c á c vật liệu từ g iả o

Loại vật liệu đa lớp thứ hai T b (F e ,C o ), ,/(Y F e ) là một (hành công hất ngờ Thông (hường, các lớp FeCo trong các màng mỏng đa lớp đều có cấu trúc tinh thể Đ ó cũng vừa là tru điểm (cho độ từ hoá cao), nhưng cũng là một hạn chế vì nó khống chế luôn cả giới hạn dưới cùn lực kháng từ Trong công trình của cluing tôi, trạng thái vô định hình trong lớp Y F e đã hình thành Xuất phái từ (rạng thái này, nhờ quá trình II nhiệt, cấu trúc vi liạt đã đạt được và kéo tlieo là những lính chất từ và từ giảo lý tưởng:

H t = 0.3 in T và độ cảm từ giảo kỷ lục Xx = 1 3 1 0 2 T 1 Các kết quả đang được gửi

dăng ở lạp chí J Appl Phys (USA) Các kếl quả nhân clirợc của đề thi đang m ờ ra một

ImỚMg nghiên cứu lý thú và có nhiều ý nghĩa thực liễn ờ cả (rong nước và tiên lliế giới Với thành tựu này, cluing tôi đã đirợc ban tổ chức hội Iiglv I '-'-'-' w quốc tế IE M S '0 1

(28/8-1/9/200Í, Grenoble, Pháp) mời tham dự để đọc báo cáo mời tại hội thảo Các kết quả nghiên cứu tìm hiểu các cơ chế vật lý của loại vật liệu mới này trong từ trưòng cao

và ở nhiệt độ thấp cũng đang được xử lý để xuất bản.

2 Danh mục các công trình khoa học đã hoàn thành

2.a Các bài báo đã đãng trên các tạp chí quốc tế:

T?) N.H Due, K Mackay, J Betz and D Givorđ,

M agnetic a n d m agnetostrictive p ro p e rties in am orphous (Tb, D y) (Fe i-xCo^ỉ film s

J Appl Phys., 87 (2000) 834 (USA).

ĩ ) T M Danh, N H Due, H N Thanh and ì. Teillet,

M agnetic, M ỏssbauer a n d m agneíostrícíive stu dies o f am orphous

(Tb,Dy) (Fei.xCoJi.i films

J Appl Phys., 87 (2000) 7208 (U S A )

/3 n) N H Due, T M Danli, H N Thanh, J Teillet and A Lienard,

M a g n e lic , M o s s b a u e r a n d m a g n e to ,s fr ic tiv e s tu d ie s o f a m o r p h o u s T b fF e o is C o o js ) I.s

films

I o f Phys : Condens Matter., 12 (2000) 8283 (U K )

0 N H Due, N A Tuan, D T.N Anh, T M Danh, N p Thuy

G iant m ơgtỉeíostriction o f TbFeCo sin gle-layer a n d TbFeCo/Fe m ultilayer pirns

Proceedings o f the 811' Asia Pacific Physics Conference, Taipei, August 7-10, 2000

Large m agnetostrictive susceptibility in Tb-FeCo/FeCo m ultilayers

Appl Phys Letter (USA), 78,11 (2001)

2.b C ác bài báo đã gửi đ ăn g ử các tạp chí quốc tế:

( T ) N H Duc,

D evelopm ent o f giant low-field magnetostriction in rare-earth based single, multilayer and sandwich films

Invited talk to be presented at JEM S’01, Grenoble, 8-2001

2 N H Duc, T M Danh, N A Tuan and J Teillet

E x c e lle n t m a g n e to s tr ic tio n s o ftn e s s in a m o r p h o u s - T b - F e C o /p o ly m o tp h o tts - Y F e

multilayers

Submitted to J Appl Phys.

2.C Sách chuyên khảo bànjî tiếng Anh

( T ) N H Duc,

G iant m agnetostriction in lanthanide - transition m etal thin film s

Handbook on the Physics and Chemistry o f Rare Earths, Vol 32,

eds K.A Gschneider, Jr and L Eyring, Elsevier Science, (Amterdam), 2001 in press.

N H Duc, T M Danh, N A Tuan and J Teillet

2.d Các bàí báo đã đăng ở các tuyển tập hội nghị khoa học quốc tê tổ chức ỏ Việt nam:

1 N H Duc, T M Danh, N T Nam, H N Thanh, N P Thuy, J Teillet and D Givord

M a g n e tic a n d m a g n e fo s tr ic tiv e p r o p e r tie s in a m o r p h o u s T b ( F e t) Ị iC o o 4 í ) ĩ f i l m s w ith

p e r p e tid ìc ttla r a n is o tr o p y

Proceedings o f the International Workshop on Materials Science, Hanoi, 11/99, p 230

2 N H Duc, T M Danh, H N Thanh, N p Thuy

G ia n t m a g n e to s tr ic tiv e T b -F e C o th in f i l m s a n d a p p lic a tio n s

Proceedings o f the Third Vietnamese-German Workshop on Physics and Engineering,

Ho Chi Minh City, 3-8 April 2000.

3 N.H Duc, F Richomme, N A Tuan, D T líuong Giang and J Teillet

M a g n e tic s o ftn e s s in m a g n e to s tr ic tiv e T b F e C o /Y F e m illlila y e r s

To be presented at JEMS 01, Grenoble, 8-2001

2.C Các bài báo đã báo cáo tại các hội nghị khoa học trong nước

1 N g u y ễn A nli T uấn, Trần MẠu D anh, N gu yỗn Hữu Đ ứ c, N puyỏn Thị N g ọ c A n h ,

Đỗ Thị Hương Giang, Vũ NguyCn Thírc

Đ ộ cảm tứ giào của các màng mỏng TbFeCo và TbF eC o/F e

Hội nghị Khoa học Trường Đ H K H Tự nhiên, 11/2000

2. Nguy ễ n Hữu Đức, Trần Müll D anh và N g u y ễ n Anh Tuấn

Tính từ mềm dặc biệt ỉrong các' inàiĩg mòng từ giảo dơ lớp vô địnlt hình và vi hạt

Hội nghị vật lý loàn quốc, 3/2001.

3 Báo cáo tài chính:

N ăm 1999: Đ ã được cấp 45 triệu đổng Đ ã quyết toán xong.

N ãm 2000: K inh phí đựơc cấp: 15 triệu đồng Đ ã quyết toán xong.

dụng N hóm nghiên cứu của đề tài đã trở nên có uy tín (rên thế giới Cán bộ của đề thi

đã được mời tiếp tục viết sách chuyên khảo về chủ đề M a g n e t o e i a s t i c i t y in A r t i f i c i a l l y

S tru c tu re d M a te r ia ls cho Handbook o f M agnetic M aterials do K H J Buschow biên tập và sẽ xuất bản ờ Am sterdam

Đ é nghị được nghiệm thu đúng thòi Itọn (4/2001) và tạo điểu kiện d ể dề tài được tiếp tục đăng ký trong hai năm tiếp then (2001-2003).

v/i * likij rn i ait.) VOI.UMi; S7 Nt'MHIR 2 15 JANUARY

Magnetic and magnetostrictive properties in amorphous

(Tb0. 2 7Dy()7 3 )(F e 1_ xCox) 2 films

N H Duca)

Cryogenic Laboratory Faculty o f Physics, Notional University o f Hanoi .I.U -N yiiycn In n Thanh Xuan

Hanoi Vietnam

K Mackay, J Betz, and D Givord

Laboratoire rie Magnetisme Louis Neel, C N R S , 3X042 Grenoble Cerle.x 9 Francr

(Received 5 May 1999: accepted for publication 16 September 1999)

Magnetic and mngnetostrictive properties have been investigated for amorphous (Tb0 37Dyo 71)

(F e |_ ,C o ,)j thin films An increase in the 3rf magnetic moment due to the enhancement of T - T

interactions in substituted (Fe Co) alloys was found This leads to stronger R-(Fe.Co) exchange

cnergifcs and then to enhancements of R —sublattice magnetization as well as magnetostriction in

these amorphous R(Fe.Co) thin films In addition, a well-defined in-plane anisotropy is created by

magnetic-ficld annealing for the Co-rich films A large magnetostriction of 4R0X 1 0 '* developed in

low fields of 0.3 T was observed for films with v = 0.47 after magnetic-field annealing The differing

roles of Fe and Co atoms on the magnetization process have also been discussed © 2000

A m e ric a n Institute o f Physics, [S0021-8979(99)06624-4]

I INTRODUCTION

Over the past few years there has been a growing interest

in magnetic thin films with large magnetostriction 1 ' This

interest is motivated by the potential such films show for use

in microsystems actuators.

R-Fe ( R = rare earth) based alloys offer the possibility to

develop very large magnetostriction al room temperature

This is due to the highly aspherical 4 / orbitals remaining

oriented by the strong coupling between R and Fc moments

In order to exploit (his property at reasonably low fields, it is

essential to have low macroscopic anisotropy A first route to

low anisotropy is by using cubic compounds 111 which the

second-order anisotropy constants vanish This is the case for

the RFei laves phase compounds of which TbFe2 (tcrlcnol),

a fcrrimagnet with 7'C = 710K is probably the best known 1

having 1753X 10“ f’ The anisotropy can be further de­

creased by substitution of Th and Dy in these compounds

This is due to Dy and Tb having opposite signs of the

Sloven's (3, coefficient and thus their contribution to the

fourth-order anisotropy being of opposite sign This leads to

the magnetostriction, albeit less than in pure TbFe, being

saturated in much lower fields This is the case for the

terfenol-D material, the crystalline (Tbf|27Dy07,)Fci com­

pound which has found many applications as high-power

actuators.

An alternative route to low macroscopic anisotropy is by

using amorphous materials In Fe-based amorphous alloys,

both positive and negative exchange interactions exisl' lead­

ing to magnetic frustration in the Fe sublallicc In amorphous

o-YFc alloys, this results in a concentrated spin-glass behav­

ior below room temperature In a-RFc alloys, where R is a

magnetic rare earth, the additional contributions of R-Fe

ex-‘’Aultior to whnm cpmrspundence should be addressed; electronic mail:

ducfo'cryolnh cdu.vn

0021 ’8979/2OOO/87(2)/034/6/$ 17,00

change and local crystalline electric-field interactions lead to the formation of sperimagnelic structures ’ The ordering temperatures are above room temperature [ r r = 410K for fl-Tbn }iFe0fi£ (Refs, 6 and 7)] It is however, still rather low and is thus detrimental lo large magnetostrictions being ob­ tained in such materials at room temperature.

Actually, with a view lo obtaining large magnetostric­ tions in the amorphous stale, it is interesting to consider the equivalent n-RCo-based alloys Although crystalline RCo- compounds order below 300 K as Co is merely paramagnetic.* the amorphous state stabilizes a moment on the Co sublatlice due to band narrowing These Co moments are strongly lerromagneticalh coupled A sperimagnctic structure occurs as in a-RFe alloys but the ordering tempera­ ture is now raised up to 600 K (Ref, 7) for Tb„i,Cow Re­ cently, we have studied rt-Tb.Co, _ , and shown that large magnetostrictions of />V’ = 300X 10 at 300 K are obtained ror.T~0.33.9

In general, however R- Fe exchange energies are larger than the equivalent R-C o interaction energies 1,1 This arises from the fact the Fe moment is significantly larger than the

Co one while the R -T intersublattice exchange constant (T=!ransition metal) is approximately the same for T = F e and Co In addition, the T - T interactions tend to be stronger

in (FeCo)- than in either Fe- or Co-based alloys.1* This re­ sults in an increase of T r for a given R:T ratio The stronger R-FeCo exchange energies should then lead to an enhance­ ment of the R moment at room temperature and thus the magnetostriction in these amorphous alloys Rcccntly,

we have studied the magnetostriction in amorphous (Tb| - vDy, )(Fe0.i5CO|i5 5 ) 2 1 thin films A magnetostriction of 1020X 10 was obtained for amorphous Tb(Fe0jjCoo55)3 1 Indeed, this is much larger than that seen in other amorphous films of either TbFe or TbCo.

© 2000 American Institute of Physics B34

I

dI Due el ai 835

n ihe present article, we have studied the influence of

e:Co ratio on the magnetization and magnetostriction of

:7Dyo7i)(F e ,_ ,C o v) 2- We will show that the Fe:Co ra-

f 50:50 responds approximately to the optimum compo-

i for the giant magnetostriction.

<PERIMENT

The films were prepared by rf magnetron sputtering The

al power during sputtering was 300 W and the Ar pres-

was 10~ 2 mbnr A composite target was used allowing a

range of alloys to be made in a controllable way with-

large cost of materials The target consisted of 18 seg-

s of about 20°, of different elements (here, Tb, Dy, Fe,

These were made by spark cutting pure element disks,

were then assembled and stuck to a Cu sample holder

! silver paint It was verified by Rutherford backscatter-

;pcctroscopy (RBS) and X-ray energy-dispersive spec-

opy (XEDS) measurements that no Cu and Ag contami-

n has occurred, The target-subslrnte distance was 8 cm

substrates were glass microscope cover slips with a

nal thickness of 150 /uni, Roth target and sample holder

water cooled.

The ratio of the deposition rates of R = T b Dy lo T = F e ,

s 0.85 Thus, for the (T b0 2 7Dyi)7 1)(F e1 _,C o T) , films

: here, the Tb(Dy) and Fc(Co) concentrations could, in

iple, be varied in steps of about 14% and 9% respec-

y The resulting composition, contamination, and the

losition homogeneity were measured using XEDS and

analyses The thicknesses were measured mechanically

> an rr-stcp and the sample mass was determined from

nass difference of the substrates before and after sput-

g The typical film thickness was 1.2 /im X-ray 0 - 1 0

action showed the as-deposited samples to be amor­

s.

Samples were annealed al 150° and 250 °C for I h under

ignetic field of 2.2 T in order to relieve any stress in­

d during the sputtering process and to induce a well­

ed uniaxial in-plane anisotropy, Subsequent x-ray 0 - 2 0

iction showed no evidence of recrystallization after an-

ng.

The magnetization measurements were carried out using

•rating sample magnetometer in a field of tip 8 T from

a 800 K.

The magnetostriction was measured using an optical de-

imeter (resolution of 5 X I0 ~8rad), in which the bend-

>f the substrate due to the magnetostriction in the film

measured This allows the magnetoelastic coupling co­

g e n t of film (/j) to be directly determined1 ' 14 using

e « is the deflection angle of the sample as a function of

ed field, I is the sample length, and £ , and v , are the

ig's modulus and Poission's ratio for the substrate

li are taken to be 72 GPa and 0.21, respectively hs and

e the thicknesses of the substrate and film, respectively,

s typically of the order of 13 mm.

FIG I Hyntcrcsis liKips m 4.2 K for scvcr;il I Ttin -jDy,,71>t FC| ,Co,); lliin films: (I) -,< =n (3) *=0.31 and (.1) —.r = 1.0 •

b is proportional to the magnetostriction via the Young's

modulus ( Ef) and Poisson's ratio (v, ) of the film These cannot be reliably measured for thin films However, for comparison, we also give values of \ calculated using

We measured two coefficients at saturation, hv and b ,

which correspond to the applied field, always in the film plane, being, respectively, parallel anti perpendicular to the sample length (i.e the measurement direction) In addition, the perpendicular direction corresponds to the easy axis in­ duced after field annealing The intrinsic material-dependent parameter h Y2 (or X ’ -1) is just the difference bf — b 1 (or

The coercive fields (¿tn/Vr ) reach their highest value of 3.4 T for.r = 0 With increasing Co concentration, coercivity decreases rapidly down to about 0.5 T for 0 6 7 '= t« 1.0 [sec Fig 2(a)] The * 1l( also decreases with increasing Co concen­ tration, to a minimum at ,v = 0.47 and then slightly increases with further increasing ,v.

In all cases, fxnHr also decreases with increasing tem­

perature [sec the inset in Fig 2(a)], while the * hf is strongly enhanced This is due to the rapid decrease local anisotropy

of the R atoms as the temperature is increased compared lo

Due el at.

2 (a) Coercive field ixnH r as a function or Co concentration at 4.2 K

shows the temperature dependence of f i nI I r for jr = 0 83 (b) Coer-

field as a function of Co concentration at 300 K: (I) the

as-ited films (2) after annealing at liO"C, anti (3) after annealing

re.

ixchange field In Fig 2(b), we prescnl f i0Hc at 300 K

motion of x All tbe films are magnetically rather soft at

n temperature and there is a maximum in at x

The spontaneous magnetization values at 4.2 and 300 K

the as-deposited (T b 02 7Dyo71)(Fe| _ ,C ot ) 2 films ex-

olatcd to zero field are shown in Fig 3 At 4.2 K there is

nximum at r = 0.47 while at 300 K, within experimental

rs the magnetization is independent of the Co conccn-

on This is in contrast with (he behavior observed for the

esponding crystalline alloys where M s always shows a

inium in the middle of the composition range due to the

incemcnt of the 3d magnetic moment ( M w ) In the

irphous case, however, an increase in will close the

x

3 Variation of spontaneous magnetization as a function of x at 4.2 anil

K for (Tbo?1Dyn70(Fe,_tCo, ) 2 thin films.

(T) FIG 4 Hysteresis loops for the (Tbq:,Dy„,j)Co2 (1) as-deposited film and (2) after annealing along induced easy axis and (3) hard axis.

R-sperimagnetic conc The maximum in M , at r = 0.47 re­

flects that, at low temperature, the enhancement of M is smaller than the associated increase in the magnetization of the R sublattice ({ A /R)).

Samples were annealed at temperatures between 150 and

250 0C in an applied magnetic field of 2.2 T The field de­ pendences of the magnetization before and after annealing are shown in Fig 4 for x ~ 1 For the as-dcposited samples,

the magnetization reversal process is progressive and isotro­ pic with a rather large coercive field This property is often observed in sperimagnetic systems where domains of corre­ lated moments are formed due to the competition between exchange interactions and random local anisotropy These domains, termed Imry and Ma domains,|, ,r' are oriented more or less at random in zero field but can be reoriented relatively easily under applied field.

After annealing, there are a number of clear differences

in the magnetization proccss First, (he coercive field is strongly rcduccd Figure 2(b) shows the coercive field as a function of composition before and after annealing After annealing at 250 BC, Unlfc is less than 0.002 T for samples

with r = 0.0 and 1.0, A slight maximum of fxnH c around the

middle of the composition range is siiII observed, however, with /xo/Yc ~ 0 0 0 6 T only Second, for this sample, there is now a well-defined easy axis with an increased low-ficld susceptibility These properties are characteristic of systems which show uniaxial anisotropy This field-annealing in­ duced anisotropy suggests that a process of single-ion direc­ tional ordering1 7 has occurred, in which there is a local re­ orientation of the Tb easy axes along the field direction The composition dependence of this uniaxial anisotropy is, how­ ever, more complex and will be discussed further in connec­ tion with the magnetostriction data The field annealing also causes a reduction in ^ (lf, indicating that the cone distribu­ tion of the Tb moments is somewhat closed,

B M a g n e to s tric tio n

In general, the comparison of f>, and hL indicates clearly

the anisotropy state of the sample If the zero-field state is fully isotropic, then b i = —2 b 1 , and if it is isotropic in the

plane, then b, = — hL IS For a well-defined in-plane, uniaxial

system, magnelizaiion reversal under a field applied along the easy axis, occurs by 180° domain-watl displacement Ne-

t

Appl Phys., Vol 87, No, 2, 15 January 2000 Due el al. 837

M, " O)

710 5 (o) Magnotoslriction for t = 0.83: (I) as-deposited film, (2) anneal

ng al I50°C and (3) 250'C (b) Magnetostriction for jr = 0: (I) as-

lepositcd film, and (3) 250 °C.

¡lecting domain-wall contributions, no magnetostriction is

associated with this process Thus, hi should be zero and

Figure 5 shows the effect of annealing on the magneto­

striction for two alloys with * = 0.83 and * = 0 For jr = 0.83

[see Fig 5(a)], we see that annealing increases the ratio of i>n

lo b± while b y 7 rests roughly constant This is due to the

creation of an in-plane uniaxial anisotropy as seen from mag­

netization measurements In addition, we see that this anisot­

ropy is completely induced after annealing at 150°C and is

accompanied by a reduction in the saturation field Subse­

quent annealing at 250 °C simply further reduces the satura­

tion field For the * = 0 sample [see Fig 5(b)], we see a

different behavior Before annealing, the approach to satura­

tion is rather slow and the ratio of b to b± indicates an

initial anisotropy After annealing, the saturation field is re­

duced and this initial anisotropy is destroyed, leaving the

sample almost isotropic However, br 2 (measured at 1.8 T)

actually increases after annealing probably due lo the reduc­

t i o n in the saturation field.

These differences are reflected across the whole compo­

sition range and the results obtained are summarized in Fig

6(a) As outlined above, it is clear that the annealing affects

very differently the Fe-rich alloys compared to the Co-rich

ones For the Co-rich alloys, b t increases significantly after

annealing while b r2 rests virtually unchanged For the Fe-

rich alloys, we see the opposite effect in that b y 2 increases

significantly after annealing while b n rests virtually un­

changed The annealing seems to destroy the initial as-

deposited anisotropy and does not induce an in-plane

uniaxial anisotropy These differences in anisotropy are also

X

X FIG 6 (a) Magnetostriction Xy2(1.8T) and X,(0.06T) for the (Tbo.27Dyo.73HFei -.,0 ^ ) 2 as-deposited thin films (I) and (I), films nn nealed at I50°C (2 and 2 ') and at 250*0 (3 and ,V) (b) Ratio b %lb ± as a function of x before and after annealing

reflected in Fig 6(b), which shows the ratio of b„ to />,

before and after annealing This will be discussed later The largest magnetostriction of \ r'2= 480X 10~s and Xn = 250

X1 0 is found in the middle of the composition range at

x = 0.47 and can be obtained in very low applied magnetic

fields of 0.06 T.

IV DISCUSSION

The magnetic properties of these alloys are rather com­ plex but it is important to attempt to understand them in order to better optimize the magnetostrictive properties of such alloys with respect to potential applications One of the main differences between the magnetic properties of amor­ phous R T2 alloys and their crystalline counterparts is the sperimagnetic distribution of R and Fe moments in the amor­ phous case, ' 2 This sperimagnetic structure arises from the competition between exchange interactions and random local anisotropy and leads to the formation of domains of corre­ lated moments These domains are oriented more or less at random in zero field and ihc macroscopic anisotropy energy, which determines the coercive field, is an average of the random local anisotropy over the volume of each domain 10

At low temperature, these domains are small and this ex­ plains the large coercivc fields found in these alloys The sperimagnetic cone, within which the Tb and Dy moments lie, can be somewhat closed due to an increase in the mo­ lecular field of the T sublaltice acting on them and this could account for the maximum seen in M s and the minimum in

Afhf f ° r * = 0.47 At room temperature, however, this en­ hancement of the 1 sublattice moment is less clear The mag-

w. Due et si.

FIG 7 Calculated variation of (M u ) and M from magnetostriction data

as a function of jr.

netostriction is, on the other hand, much more sensitive to

changes in the R-sublattice magnetization and we will now

discuss this effect.

Assuming that the R moments have the same value as in

the crystalline laves phase, we can estimate the magnetostric­

tion of a sperimagnelic system with respect to a collinear

femmagnctic one using

^■2= ^ ( < « Z) M )

where is the direction cosine for each rare-earth moment

with respect to the field direction and / ;£ ,2 is the intrinsic

magnetoelastic coupling coefficient (i.e., that of the collinear

ferrimagnct) Here, we take bjn7 = 127 MPa, the room-

temperaturc value of b y'7 in isotropic polycrystallinc crystal­

line (Th() 2 7Dy0 7 1)Pe2.70 Assuming a uniform probability dis­

tribution of easy axes within a cone, we can deduce the

characteristic sperimagnelic cone angle (6) For the films un­

der consideration, this gives values of between 48° and 53°,

which are typical of those reported in the literature 5,21 This

variation in 0 implies that there is a variation in the average

(Tb Dy) moment as a function of x Using M (Tb.Dy)

= 7 2 7 /ifl, the room-temperature value in

(Tb0,7Dy0 7,)Fej 4 we can deduce (M n ny) = W (TfcDy){n-,), as

a function of r and this is plotted in Fig 7 From the mea­

sured magnetization data, we can now deduce as a func­

tion of x (Fig 7), The values thus determined are in good

agreement with those found for AiJ<f in “ pure" o-TbCo2 and

d-TbFe2 alloys6 at room temperature This clearly indicates

that there is an enhancement in M for the substituted

rr-R(Fe,Co) 2 alloys and a maximum is reached Tor jt = 0.47

where there is sufficient Co to ensure good ferromagnetic

T - T coupling as well as sufficient Fe giving the larger mag­

netic moment We have, of course, neglected the variation in

ordering temperature, and hence, the intrinsic R-moment

value at room temperature associated with such an enhance­

ment of the T - T interactions However, this simple analysis

illustrates the importance of considering the influence of the

spcrimagnetic structure on the magnetostriction and the mag­

netic properties of such alloys.

An intriguing aspect in this study is the variation of the

anisotropy state as a function of T composition, before and

after annealing The comparison of bt to bL is a useful tool

for understanding the role of Co in these alloys [Fig 6(b)],

For the Fe-rich alloys before annealing h ,/h L is large indi­

cating a well-defined initial anisotropy After annealing,

btlb L^ - 2 suggests that the zero-field magnetization State

is isotropic The as-deposited material is not completely satu­ rated at 1.8 T, while after annealing saturation is achieved at around I T This leads to the measured increase in (¿>„

— foj ) at 1.8 T after annealing For the as-deposited Co-rich alloys, bnlb L * * - I indicates that (he film is isotropic in the

plane After annealing at 250 °C, this ratio is significantly increased showing that a well-defined in-plane anisotropy direction has been induced Figure 6(b) shows the variation

of bl>fb1 as a function of Co concentration It clearly indi­

cates that after annealing the easy axis becomes better de­ fined with increasing Co content This may be accounted for

as follows During the annealing process, it is the local in­ ternal molecular field that is responsible for the reorientation

of the R moments The external field merely saturates the material in a given direction For the Fe-rich alloys, the sperimagnelic nature of the Fe-sublattice distribution is con­ veyed to the R sublattice and gives no net anisotropy How­ ever the strongly ferromagnetically coupled Co sublatticc is well ordered and its molecular field acts to orient the R sub­ lattice in one direction, giving rise to the observed uniaxial anisotropy The differing anisotropies seen in the as- deposited stale are more difficult to account for precisely, but

it has often been noted that Fe-based RT compounds have a different anisotropy state compared to their Co-based coun­ terpart.

We can further illustrate this variation in anisotropy by associating the field dependence of the magnetostriction with different types of magnetization processes, For a system of randomly oriented spin and random distribution of domain walls, the magnetization process takes place in two steps.2’ First, the motion of 180° domain walls leads to a magnetiza­ tion of Mq without any contribution to magnetostriction In the second step, the spins rotate into the direction of the applied magnetic licld leading to the change of both magne­ tization and magnetostriction For the case M 0 — the relation between magnetostriction and magnetization is given asis

(3) For the rotation of magnetization out of the easy axis, the magnetostriction is related to magnetization as follows:22

X ( //) /X nu, = [ W ( f f ) /W in„ ] 2 (4) The results of this analysis are presented in Fig 8 The experimental data for the (Tb Dy)Fe, film are rather well described by Fq (3) With increasing Co concentration, the

vs curves shift towards the line described by

Eq (4) This further confirms that Co substitution is advan­ tageous to the creation of a well-defined easy axis in this system.

Finally, the room-temperature magnetostriction is strongly influenced by the Curie temperature of the investi­ gated alloys It is worth reporting here that one has found the

Tc value of 440 K for the n-(Tbn 27Dy0 7j)(F e , - rCo, ) 2 film with * = 0.63 Indeed, this Tc value is much higher than that

reported for ii-(T b r)27Dyn73)Fe2(7'f:= 3 7 0 K see also, e.g Ref 23) The larger 7> is associated also to the stronger

Duc et al. 839

M/M max } 8 Experimental and theoretical relations between normalized magne-

triction and magnetization for amorphous (Tbn jjDyn 71)(Fe, _ Co,), thin

is.

-FeCo exchange energies This is one of the reasons why

i room-temperature magnetostriction was enhanced in

lorphous (Tb, Dy)(Fe, Co) films.

CONCLUDING REMARKS

In conclusion, we would like to point out that

ger magnetostrictions are obtained in amorphous

b, Dy)(Fe,C o) films as compared to their parent amor­

ous films of either (Tb, Dy)Fe or (Tb, Dy)Co This has

en explained in terms of an increase in the ferromagnetic

upling strength wiihin the (Fe, Co) sublattice In addition,

well-defined uniaxial anisotropy can be induced by

ignetic-ficld annealing for Co-rich films.

It is well known that the substitution of Dy for Tb gives

e to the increase of the magnetostriction at low magnetic

Ids, through the reduction of the saturation field However,

is also accompanied by a reduction in the saturation mag-

tostriction In this study, we have shown that Co substitu-

in, coupled with the effects of annealing, results in an en-

ncement of both the low-field and saturation

agnctostriction Thus, we can expect a further enhancement

the magnetostriction in these alloys by increasing the Tb

ncentration Indeed, we have obtained a giant magneto-

■iction or \ y2= 1020X l ( T6 at 1.8 T with \„ = 585X 10~ 6

0.1 T in amorphous Tb(Feg5;Cbo4 5 ) 2 12

ACKNOW LEDGMENTS

The authors thank Dr E du Tremolet de Lncheisscrric for helpful discussions This work was carried out as part of the E C funded " M A G N IF 1 T " project (Contract No BRE2-0536) The work of one of the authors (N H D.) is partly supported by the National University of Hanoi within Project No QG.99.08.

1 E Quandt J Alloys Compd 258 126 (1997).

3E Tremolet de Lachcisserise, K Mackey, 1. Betz, and J C Peuzin, J AppI Phys 275-277, 685 (1998).

1N H Due in Handbook on the Physics and Chemistry o f Rare F.anhs,

edited by K A Gschneidner, Jr and L Eyring (Nortb-Holland, Amster­ dam), Vol 28 (lo be published).

J A E Clark, in Ferromagnetic Materials, edited by E P Wohlfarth, Vrl

I (North-Ilolland, Amsterdam, 1980), Vol I p 531.

5J M D Coey, D Givord, A Lienard, and J P Rebouiltal, J Phys F II,

2707 (1981).

ftP Hansen G Much, M Rosenkranz, and K Witter, J Phys 66, 756 (1989).

7K Lee and N, Heimon, AIP Conf Proc 18 108 (1973).

*R Lemairc R., Cobalt (Engl Ed.) 1968 33.

?j Betz, Thesis, University Joseph Fourier of Grenoble (1997).

,0J P Liu, F R de Doer, P F de ChStel R Coehoom, and K H J Buschow, J Magn Magn Mater 134 159 (1994).

" j P Gavigan, IX Givord, II S, Li and J Voiron, Physic» B 149, 345 (1988).

, ! N H Duc, K Mackay J Betz, and D Givord, J AppI, Phys 79, 973 (1996)

E Tremolet de Lacheisserise and J C Peuzin, J Magn Magn Mater

136, 189 (1994).

1dJ Betz, E du Tremolet de Lachcisserise and L T, Baczewski, AppI Phys Lett 68 132 (1996).

15 Y Imry and S Ma Phys Rev Lett .15, 1399 (1975).

1f B Boucber, A Lienard I, P Rebouillat, and J Schweizer, J Phys F 9,

1421 (1979) ,7L Neel, Compte Rendu 273, 1468 (1953): 1 Phys Radium 15, 225 (1954).

"F Schatz, M Hirscher M Schnell, G Flik, and Ft Kromuller J AppI Phys 76 5380 (1994).

19R Alben, J I Bundrik and G S Cargill, Metallic Glasses (American Society for Metals, Metals Park, OH, 1978), Chap 12

jnThe values given in Ref 4 are for somewhat lex lured samples Here, we calculate b, ! for an isotropic polycryslalline sample of Tb027Dyn jjFe, using single-crystal data frT,I=3GXs with l/(2G) = 2/5Jr+ 3/5j, and

= 0.6\ i,i , E du Tremolet de Lacheisserise (private communication).

31 P Hansen, in Ferromagnetic Materials, edited by K H, J Buschow (North-Holland, Amsterdam, 1991) Vol 6, p 289

22S Chikazumi, Physics o f Magnetism (Willey, New York, 1964).

” K Ried, M Schnell, F Schatz, M Hirscher, B Ludescher, W Sigle, and

H, Kromuller, Phys Status Solidi A 167, 195 (1998).

JOURNAL OF APPLIED PHYSICS VOLUME H7 NUMHF.R M ) 15 MAY ;<XX)

M a g n e tic , M o s s b a u e r and m a g n eto strictive studies of a m o rp h o u s

T b (F e 0 55COo.45)i.5 film s

T M D anh, N H D uo,3’ a n d H N T hanh

C rvo y e n ic La b o ra to ry , F a c u lty o f Physics, N a tio n a l U niversity o f H an o i 334 N q u v e n T rat Than h X u a n

H a n o i, Vietnam

J Teillet

La b o ra to ire tie M aynetism e et Ap plication s G M P - U M R 6634 Université de Rouen,

76 8 2 1 M fi’ il-S a in l-A ig n a n Fra n c e

(Received 13 January 2000: accepted tor publication 19 February 2000)

The Tb(Fe,, jjCoo^sJi 5 films were fabricated by rf magnetron sputtering from a composite target

Samples were investigated by means of x-ray diffraction, vibrating sample magnetometer,

conversion electron Mossbauer spectra, and magnetostriction measurements The as-deposited film

is an amorphous alloy with a perpendicular magnetic anisotropy and an intrinsic magnetostriction

\ = I080X IO-,> in an applied field of 0.7 T In this state, it was determined that the hvperfine field

/?t,r= 23.5 T and the cone-angle between the Fe moment direction and the film-normal direction

/3= 12° Alter annealing in the temperature range of T A = 250-450 "C the amorphous structure still

remained, however the anisotropy was changed fo a parallel one The soft magnetostrictive behavior

has also been improved by these heat treatments: the parallel magnetostriction \ B = 465X 1 0 '* was

almost developed in low applied fields of less than 0.1 T and, especially, a huge magnetostrictive

susceptibility = c/\,|/î/(/(a0// ) = I.8 X 10~: T _I was obtained at /¿n/ / = ! 5 m T <D 2000

American Institute o f Physics [S0021 -8979(00)06210-1 ]

I INTRODUCTION

It has been known for a few years that there has been a

growing interest in magnetic thin films with large

m agnetostriction.I"* This interest is motivated bv the poten­

tial such films show for use in microsystems actuators For

these applications, large low-field magnetostrictive suscepti­

bilities = d \ / c l ( f i nH ) > 2 X 1 0 "2 T _ i and low coercive

fields 100mT, are required R -F e (R = ra re earth)

based thin films offer the possibility of developing very large

magnetostriction at room temperature Numerous investiga­

tions on T b -F e and (Tb D y)-F e based thin films have been

carried out In order to get low macroscopic anisotropy, ma­

terials have been used in the amorphous state In Fe-based

amorphous alloys, however, both positive and negative

F e-F e exchange interactions exist.5 leading to magnetic frus­

tration in the Fe sublattice in amorphous R -F e (o-RFe) al­

loys, where R is a magnetic rare-earth, the additional contri­

butions o f R -F e exchange and local crystalline electric field

interactions lead to the formation of sperimagnetic

structures.5 6 The ordering temperatures are above room tem­

perature [7 > = 4 I0 K for fi-TbnjiFcoift, Ref 6 and refer­

ences therein) It is however, still rallier low and is thus

detrimental to large magnetostrictions in such materials at

room temperature f \ = s 3 0 0 x IO- * in — I T) The opti­

mization of magnetostriction and ordering temperature have

been reported for TbDyFe/Nb multilayers by combining the

advantages o f a crystallized film (high T and giant X) with

' \ u I hi >r I o whom torrevpomletice 'iliould he ultlrcssed: clccirnmc mail:

A sperimagnetic structure occurs as in d-RFe alloys but the ordering temperature is now raised up to 600 K (Ref 6) for Tb„jjCo** In practice, Betz7 has investigated rt-TbTC o ,_ r films and shown that large magnetostriction of \ = 350

X l()-fi at 300 K was obtained for t~ 0 3 3 Recently, we have studied the magnetization and m ag­netostriction in the amorphous (T b l _ ,D y r H Fe045Co0 55)2 i (Ref 8) and (Tbo 2?Dyo 7i)(F e | _ cCor) 2 (Ref 9) thin films In these alloys, the R -F eC o exchange energies are stronger than those in the " p u re " a-RFe and <i-RCo alloys:*lt was thought to be ihe reason for the enhancement of the R m o­ment at room temperature and thus the magnetostriction In­deed a magnetostriction of I020X 10"'’ was obtained in the applied field of 2 T for amorphous Tb(Fe,)15Con jj)- In this article, we studied zero-lieid annealing effccts on the magnetization Mossbauer spectra, and magnetostriction of ihe amorphous Tb(Fen,55^ 045)1 j films with a perpendicular anisotropy The obtained magnetic and magnetostrictive characters of these annealed films proved to be rather prom ­ising for application requirements

0021 - 0979/2OOO/87( 101/7208/5/Î17 00 7208 '0 2000 American Institute of Physics

J Appl Phys., Vol 87, No 10 15 May 2000 Danh el al 7209

2- theta (degrees) FIG I X-ray diffraction patterns of the TbfFe035Coo4j)15 films.

II EXPERIMENT

The films were prepared by rf magnetron sputtering The

typical power during sputtering was 400 W and the Ar pres­

sure was 1 0 '* mbar A composite target consisted of 18 seg­

ments of about 20°, of different elements (here Tb, Fe Co)

The substrates were glass microscope cover slips with a

nominal thickness of 150 ¿im Both target and sample holder

were water cooled The thickness was measured mechani­

cally using an a step and the sample mass was determined

from the mass difference of the substrates before and after

sputtering The typical film thickness was 1.2 ¿un without

any coating The rilm structure was investigated by x-ray

{0 -76 ) diffraction (XR D) with Cu Kn rays The'results

showed the as-deposited samples to be amorphous (see

Fig I).

Samples were annealed at temperatures from 250 to

450°C for 1 h in a vacuum of 5 X ! 0 _ 4 mbar in order to

relieve any stress induced during the sputtering process Sub­

sequent 0 -2 0 XRD showed no evidence of a global crystal­

lization after annealing, but the peaks of Tb oxides and

a-(Fe, Co) (see also Fig 1) appear to be due to the surface

oxidation.

The magnetization measurements were carried out using

a vibrating sample magnetometer in a field of up to 1.3 T at

300 K.

The conversion electron Mossbaucr specira (CEMS) at

room temperature was recorded using a conventional spec­

trometer equipped with a homemade hclium-methane pro­

portional counter The source was a ,7Co in rhodium matrix

The (ilms were set perpendicular to the incident y beam The

spectra were titled with a leasl-squares technique using a

histogram method relative to discrete distributions, con­

straining the linewidths of each elementary spectrum to he

the same Isomer shifts are given relatively to a-Fe at 300 K

The average "cone angle" (3 between the incident y-rnv di­

rection (being, m the film-normal direction I ant) lhat of the

hypertine Held (or the Fe magnetic moment direction) is

estimated from the Iine-intensity ratios 3 : v : l: l: r : 3 o| the

six Mossbaucr lines, where r is related to 0 by sin /J

The magnetostriction was measured using an optical de- (lectometer Iresolution of 5 * 10 *’ rad), in which the bend­ ing of the substrate due to the magnetostriction in the film was measured.'1" 11

III EXPERIMENTAL RESULTS AND DISCUSSION

A M agnetization

Figure 2 presents the magnetic hysteresis loops mea­ sured with applied magnetic field in the film-plane and film­ normal directions for the as-deposited and several annealed Tb(Fen3 3Coo4 5)i 5 films The magnetization curves have been plotted versus the internal field = /xnW.„

- N M ) using a usual demagnetization factor N = N ■, = 0 in

the film-plane direction but an experimentally determined value of N = N l in the film-normal direction The value ol

N l was chosen in a way that the steepest part of the magne­ tization curve is transformed into a vertical line 12 The result­ ing effective values of N , are equal to 0.5 0,9, 0.95 and 1.0

for the as-deposited film, film annealed at r1 = 250°C

3 5 0 'C and 4 5 0 ^ respectively Comparing to /V '*= l as expected for an infinite plate, the obtained value for the as- deposited film is too small This is, however, characteristK

of the nucleation of tripe domains 1 1

For all samples, the in-plane magnetization is almost iso­ tropic The as-deposited sample shows a perpendicular tnac- netic anisotropy [Fig 2(a)] Its coercive Held is rather larse (film-normal coercivity ^ oWC i = 0 1 3 2 T and film-plane co- ercivity Hi\Ht — 0.08 T) and the magnetization docs not

completely saturate even at 1.3 T While, intrinsically related

to the strong local anisotropy of the R atoms and iheir ran dom distribution ot easy axes present in such sperimaunetk systems, the coerciviiv is strongly affected by internal stress microstructure, and homosteneitv 14 The high-tield susccptt-

'210 J Appl Phys Vol 87 No 10 15 May 2000 Danh el ai

TABLE I, Room temperature ma|n<lic and mngnetostriclive chnncteriMiCfl

>f ihe amorphous Tb(Fe^ jjConjjJi j films: T A , M< n „ H a , ( B u ) 0 and

*■ A, — K L) ore Ihe annealing temperature saturation magnetization, film-

ilane coercive Held, avenge hypertine field Fc-spin oriented angle, and

ntrinsic magnetostriction, respectively.

r^tKi Mj (kA/ml ,u„«n (mT) <oM) m 0 tdeg) \ ( n r ‘ )

js-de posed 320 SO 22.5±l),3 I2±5 1080

lility is also typical of sperimagnetic systems and is assor­

ted with the clcsing of the cone distribution of R moments

s the field is increased.*

After annealing, there are a number of clear differences

n the magnetization process First, the magnetic anisotropy

hanges from a perpendicular to a parallel one [see Figs 2(c)

nd 2(d)], Second, the coercive field is strongly reduced

Table I) for instance, with the annealing at 450 °C Mo^ch's

qual to 6 mT Third, the saturation magnetization decreases

see also Table I) but can be easily reached at low magnetic

¡elds In agreement with the XRD results, the reduction of

le magnetization may relate to the process of oxidation dur-

lg vacuum heat treatment (see Sec Ill B) This effect was

reviously reported by Wada et a i.'! Finally, the annealing

Iso causes a reduction in the high-tield susceptibility, indi-

ating that ihe cone distribution of the Tb moments is easy to

lose.

From the magnetization curves measured with applied

lagnetic field in the film-plane and film-normal directions

'C estimated the uniaxial anisotropy constant K„ to be'117

_I/m3 for the as-deposited film This value is comparable

'ith that reported in literature {e.g Refs 12 and 14) Re-

arding the magnetoelastic anisotropy, any magnetostrictive

laterial always tries to compensate the external or internal

ress by appropriate rotation of spins For a film with posi-

ve magnetostriction, tensile stress leads to a spin orientation

i the film plane, whereas for compressive stress the spins

rient along the film normal At present, since the thermal

tpansion coefficients of the Tb-FeCo film and the glass

ibstrate would result in an in-plane anisotropy, it is possible

tat the observed perpendicular anisotropy must be of intrin-

c origin, associating with the structural anisotropy induced

jring Ihe sputtering process This was already confirmed

rectly in the most careful studies of the local structures 1* 17

^ he elimination of the coercivity and anisotropy with anneal-

g reflects that an isotropic amorphous structure has lower

lergy than the as-deposited anisotropy state The relaxation

the anisotropy without crystallization, thus, is a simple

laxation of the amorphous structure resulting in a more

able and homogenous film structure.

M o s s b a u e r s p e c tr a

The CEMS is suitable to investigate hvperfine param-

srs of the iron nuclei within a depth range of about 2 0 0 nm

:>m the film surface Within this space, however, ihe con-

button of the iron nuclci to the CEM spectrum is not the

((B M)) and the Fe spin reorientation (/? angle) can be ex­

tracted from these spectra The perpendicular anisotropy of the as-deposited film is characterized by the almost disap­ pearing second and fifth Mossbauer lines [Fig 3(a)) For ihis sample, the spectrum has been fitted with a wide contribution

of hyperfine field P ( /?hr) to take into account all the environ­

ments experienced by Fe’ 7 nuclei This provides an average value of (B m} ~ 23.5 T and ( p ) = \2° It is worth mentioning

here that beside the peak at 22.5 T which corresponds to the magnetostrictive Tb(Fe.Co>| 5 alloy, the P {B hf) distribution

extends also to higher hvperfine fields ( “"30 T) The spec­ trum of the 4 5 0 '’C-annealed lilm [Fig 3(b)] is fitted with a wide distribution of hyperfine field P (B h() too For this sample, the peak at 22.5 T still exists in the P (B hf) curve,

however, it is weakened and broadened Moreover, the high hyperfine-field contribution becomes dominant A sharp

P (B i,r) peak is reached at 34.5 T In accordance with the

XRD results, this major ferromagnetic component (82% of the total spectrum area) is associated with the contribution of the crystallized o'-(Fe.Co) phase formed at die film surface due to the oxidation The fraction of the magnetostrictive alloy (18% of the total spectrum area) is small As already mentioned in the beginning of this subsection, this reflects that the thickness of the oxidation layer is sufficiently thick

in annealed films.

The (Bi,,) values obtained for the ri-Tb(Fe0 3 3Co0 45)| 5

phase are comparable with those reported for the Laves phase RFe- compounds Such a value implies a strong

} d - } U exchange coupling The 3</ magnetic moment ,W1(/

is determined by scaling with (B hf) taking (#*M) = 3 3T and

W 1</ = 2.2,uH/atotns for »-Fe It results in Myd- 1

atoms This finding is in good agreement with that deduced from magnetization data lo r« -(T h Dv)(Fe.Co): films ’ This large room-tcmpernutrc 3r/ magnetic moment indicates that

in the composition under consideration there was sufficient

Co to ensure good lermmagnelic T - P coupling as well as sufficient Fe giving the large magnetic moment

*ppl Phys Vol 87 No 10, 15 May 2000 Danh el al. 7211

ti,H (T)

! 4 Parallel magnetostrictive hysteresis loops in (he external fields for

Tb(Fe<)jjCo04j)| j films: (I) as-deposited film and (2) after annealing at

“C and (3) 450 °C.

IW M ,„

FIG 5 Experimental and theoretical relations between normalized muene- tostriction and magnetization for amorphous Tb<Fe«|jjCoo4j)| j films (I) and f2) theoretical curves described for Eqs (I) and (2) respectively (■) as deposed ( • ) r , = 250°C fAl 350 ”C and fO) 450'C.

M a g n e to stric tio n

We measured two coefficients and , which coiTe-

)nd to the applied field, in the film plane being, respec-

ely, parallel and perpendicular to Ihe sample length For

: films under investigation, magnetostriction is almost iso­

pic in the plane V>i / X ^ — 1 The intrinsic

magneiostric-n data \ = measured in the applied magnetic field

fjLnff = 0.7T are listed in Table I It is clearly seen that the

ignetostriclion of a magnitude of 10“ 3 was achieved The

rallel magnetostrictive hysteresis loops are shown in Fig

For the as-deposited sample, the magnetostriction in-

:ases almost linearly in Ihe investigated magnetic fijld

iges This implies that it is rather difficult to rotate spins

o the film plane The largest magnetostriction obtained at

* T is \i| = 550x I0 ~ 6 The annealing at temperatures be-

een 7^ = 250 and 4 5 0 °C reduces the high-field magneto-

iction but enhances the low-field magnetostriction The

timum annealing is at 7’A= 4 5 0 °C In this case, the mag-

tostriction of Xi = 465X 10~s is saturated at ¿¿07/ = 0 I T

d \ , = 340X 10- 6 is already developed in very low applied

ignetic fields of 20 mT In addition, its coercive field is

;s than 5 mT It is worthwhile to mention that in the ap­

ed field of 15 mT ihe magnelostrictive susceptibility has

10” 2 T -1 These magnetic and magnetostrictive character-

ics are very promising for microsystem applications.

^ We can further associate the field dependence of the

ignetostriction with different types of magnetization pro-

sses For a system of randomly oriented spin and random

itribulion of the domain walls, the magnetization process

ces place in two steps, 111 First, the motion of 180” domain

ills leads to a magnetization of M n without any comribu-

m to magnetostriction In the second step, the spins rotate

ihe direction of the applied magnetic field, leading to the

ange of both magnetization and magnetostriction For

lorphous alloys of randomly oriented spins and ol a ran-

m distribution of domain walls, the M „ is expected lo be

M mJ 2 In this case, ihe relation between macnetostric-

>n and magnetization is given as

For the motion of 90° domain walls, i.e the rotation of magnetization oul of the easy axis, the magnetostriction is related to magnetization as follows:IS

Taking the values measured in /_in/ / = 0 7 T as and the relation between the normalized magnetostriction and magnetization is presented in Fig 5 For the as-deposited film with the perpendicular anisotropy, almost no magneto­ striction takes place at M /M mix<0.3 and the experimental data are close to the curve described by Eq 12) II is possible

that, in this film, the magnetization process is governed mainly by the rotation of spins As the annealing temperature increases, the \ / X m„ starts at a higher M IM m, t value, lor

instance /V//Aimlx=0.6 and 0.75 for 7"., = 250 and 350 “C respectively This further confirms the randomly oriented spin structure At 7\t = 450°C , the vs M /M mtx curve

also starts at A //A fm„ = 0 6 , and follows well that of 250°C.

IV CONCLUDING REMARKS

It is well known that, in a traditional route, the substitu­ tion of Dy for Tb has been used to compensate the anisot­ ropy and Ihen to increase the magnetostriction at low mag­ netic fields However, it is also accompanied by a reduction

in the saturation magnetostriction At present, we have shown that Co substitution, coupled with the effecty&f zero- field annealing, results in an enhancement of both the low- field and the saturation magnetostriction Actually, we have obtained a giant magnetostriction of \„ = 3 4 0 X 10~IS in field

fj.nH < 2 0 mT and ' 8x 10" ~ T _ in amorphous Tb(Fe0 5,Co0 jj), s Such films are very promising for appli­ cations.

In this study, the Co substitution led to the enhancement

ol the 3i/(F e.C o )-3r/(F e.C o t as well as 4 /(T b )-3 < /(F e C o i exchange energies and then the intrinsic magnelosiriution The soli magnetic and magnetostrictive behaviors, on one hand, relate to the closing of the Tb-cone anale but on ihe oiher may also be associated with the soft magnetic ar-iFc Co) layer, which was scurecaicd at the surface durinu

7212 J Appl Phys Vo) 87, No 10, 15 May 2000 Oanh et al.

ihe heal treatments The study of the influence of the soft

magnetic layer on the soft magnetostrictive character is in

progress.

ACKNOWLEDGMENTS

This work was granted by the National University of

Hanoi within Project No QG.99.08 The authors express

their thanks to Dr Le Van Vu for the x-ray diffraction mea­

surements The stay of N H Due at the GMP, University

of Rouen is supported by the Ministère Française

de l’Education Nationale, de la Recherche et la Technologie

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J Phys.: Condens Matter 12 (2000) 8283-8293 Printed in the UK PII: S0953-8984(00) 15123-9

Structural, magnetic, Mossbauer and magnetostrictive studies

N H DuctU, T M Danhf, H N Thanhf, J Teilleti and A Liénard§

t Cryogenic Laboratory, Faculty of Physics, National University of Hanoi, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam

t Laboratoire de Magnétisme et Applications, GMP-UMR 6634, Université de Rouen,

II Corresponding author.