DSpace at VNU: Magnetisation process and magneto striction in Fe TerfecoHan Fe sandwich films with perpendicular magnetic anisotropy

Physica B 327 2003 328–333Magnetisation process and magnetostriction in Fe/TerfecoHan/Fe sandwich films with perpendicular magnetic anisotropy N.H.. 334 Nguyen Trai Road, Thanh Xuan, Hano

Physica B 327 (2003) 328–333

Magnetisation process and magnetostriction in

Fe/TerfecoHan/Fe sandwich films with perpendicular

magnetic anisotropy N.H Duca,*, D.T Huong Gianga, V.N Thuca, N.T Minh Honga, N Chaub

a

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

b

Center for Materials Science, Faculty of Physics, Vietnam National University, Hanoi 334 Nguyen Trai Road, Thanh Xuan,

Hanoi, Viet Nam

Abstract

A new magnetostrictive film with out-of-plane magnetic anisotropy is realised in sandwich type of Fe/ Tb(Fe0.55Co0.45)1.5/Fe (denoted as Fe/TerfecoHan/Fe) films with a fixed individual TerfecoHan-layer thickness

tTbFeCo¼ 600 nm and variable Fe layer thickness tFe¼ 0; 10, 15 and 60 nm As-deposited Fe/TerfecoHan/Fe films exhibit a magnetostriction as large as 103 in an applied field of m0H ¼ 0:6 T This magnetostriction is exclusively observed in the field direction parallel to the sample length The development of giant low-field magnetostriction has been performed by heat treatment A maximal magnetostrictive susceptibility wl8¼ 2:3  102T1 is reached in

m0H ¼ 6 mT for the TerfecoHan film annealed at TA¼ 4501C: For Fe/TerfecoHan/Fe sandwich films, however, the magnetic softness is just slightly improved with TA¼ 2501C and 3501C At TA¼ 4501C; both the perpendicular magnetic anisotropy and the magnetic coercivity are reinforced These magnetic behaviours are associated to the fact that the additional Fe-layers accelerated the crystallisation process in the TerfecoHan layer The sandwich films are favourable for making the Fe/TerfecoHan/Fe a low-field magnetostrictive material for microelectromechanical systems

as well as a perpendicular recording medium Mechanisms of the magnetisation process and huge parallel magnetostriction are discussed for the films under investigation

r2002 Elsevier Science B.V All rights reserved

Keywords: Thin films; Microstructure; Magnetisation process; Magnetostriction

1 Introduction

Magnetostriction of thin films has been studied

intensively in the last decade First

microelectro-mechanical devices using magnetostrictive films

have been realised For a more detailed reviewof

the present state of theory and experiments of the

magnetostriction of rare earth–transition metal thin films, we refer the reader to the chapters written by Duc[1]and more recently by Duc and Brommer [2] As a tradition, research of giant magnetostrictive thin films has also been based on amorphous (a) rare earth–iron alloys, in particular a-Tb0.27Dy0.73Fe2(known as a-Terfenol-D) Prac-tically, a record magnetostriction of 1020  106 has been achieved on the a-Tb(Fe0.55Co0.45)2.1

thin film [3] Enhancement of the low-field

*Corresponding author.

E-mail address: duc@netnam.org.vn (N.H Duc).

0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V All rights reserved.

PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 7 8 0 - 5

magnetostrictive susceptibility, however, has been

found in TbCo/FeCo multilayers[4,5] Moreover,

an excellent magnetostrictive softness was reported

for a-Tb(Fe0.55Co0.45)1.5 (denoted as

a-Terfeco-Han) single layer, Fe/a-TerfecoHan/Fe sandwich

and a-TerfecoHan/n-YFeCo multilayer films [6]

The giant magnetostriction observed in these

materials was explained by the enhancement of

the 4f–3d exchange, leading to the diminishing of

the Tb-sperimagnetic cone-angle, whereas their

huge magnetostrictive susceptibility is connected

to the nanostructure of the n-YFeCo layer

Giant magnetostriction originates from the

rotation of the magnetic moments It is usually

observed in uniaxially anisotropic magnetic

sys-tems, one of which are thin films with out-of-plane

magnetic anisotropy Such a giant

magnetostric-tion, however, requires a high magnetic field In

Ref [6], it was reported that the Fe layers played

an important role in improving the magnetic

softness of the Fe/TerfecoHan/Fe sandwiches with

parallel magnetic anisotropy

In this paper, a newmagnetostrictive material

with perpendicular magnetic anisotropy is

realised in sandwich type Fe/TerfecoHan/Fe

films Their microstructure, magnetisation process

and magnetostriction will be described in

compar-ison with those of the TerfecoHan-single layer

films

2 Experimental

The Fe/TerfecoHan/Fe sandwiches with a

fixed individual TerfecoHan-layer thickness

tTbFeCo¼ 600 nm and a variable Fe layer thickness

tFe¼ 0; 10, 15 and 60 nm (denoted as samples S0,

S10, S15 and S60, respectively) were prepared by

RF-magnetron sputtering The TerfecoHan layer

was sputtered under a power of 200 W, while the

sputtering power of the Fe-layers was 100 W A

composite target has been used for the TerfecoHan

layer, a high-purity metal plate for the Fe layers

The substrates were glass microscope cover slips

Both target and sample holder were water-cooled

Samples were annealed in the temperature range

from TA¼ 2501C to 4501C for 1 h in a vacuum of

5  105mbar

Film structure was investigated with X-ray diffraction (XRD) and high-resolution transmis-sion electron microscope (HRTEM) The magne-tisation was measured using a vibrating sample magnetometer (VSM) in magnetic fields up to 1.4 T The magnetostriction was measured using

an optical deflectometer (with a resolution of

2  106rad.)

3 Experimental results and discussions 3.1 Microstructure

Fig 1 presents the XRD spectra for the as-deposited Fe/TerfecoHan/Fe samples S0, S10 and S60 No diffraction peaks indicating crystalline TbFeCo phases are observed Moreover, the existence of the BCC-Fe layers is weekly evi-denced It may relate to the fact that the Fe-layer is too thin (for sample S10) and/or the grain size of the crystalline phase is too small (for sample S60)

to be detected by the XRD However, the BCC-Fe crystalline phase was well determined by cross-section TEM micrographs (Figs 2a,b) An average BCC Fe grain size of about 2 nm can be deduced for sample S10 (Fig 2a) The Fe grain size increases with increasing tFe, e.g it equals 8 nm

in sample S60 (figure not shown) After annealing

at TA¼ 4501C, fine grain (5–7 nm size) structure is observed in the TerfecoHan film S0 In the sample S10, the fine-grain (3–4 nm size) structure still remains in the Fe-layers, while a grain size of the crystalline phase of about 25 nm was formed in the TerfecoHan layer (Fig 2b) The transmission

Intensity (arb unit)

2θ (degree)

S60

S10

S0

bcc-Fe

Fig 1 XRD spectra of as-deposited S0, S1, S60 films.

electron diffraction patterns of corresponding

TerfecoHan nanocrystalline phases are also

illu-strated in images inset in Fig 2 The observed

different annealing effects could be attributed to

the nucleus formed in TerfecoHan/Fe interfaces

3.2 Magnetic hysteresis and magnetisation

orientation

Fig 3 shows the magnetic hysteresis loops

measured in magnetic fields applied parallel and

perpendicular to the film plane at room

tempera-ture for several as-deposited films Note that, the

in-plane magnetisation requires a magnetic field

higher than 0.35 T to saturate and that the

remanence is almost zero (e.g in samples S0, S10

and S15) These features suggest the existence of a

perpendicular anisotropy, in addition to the usual

shape anisotropy For sample S60, the

magnetisa-tion seems to consist of both perpendicular and

parallel magnetic components Indeed, it is found

from M.ossbauer studies [7] that, in this sample,

the Fe-magnetic moments in TerfecoHan layer

orient along the out-of-plane direction, whereas

those in the Fe-layers lie in the film plane In this

context, it is possible that a 901-domain wall may

be created in the interfaces

By annealing, the in-plane (soft) magnetic state

is quickly established and improved in the sample

S0 (see Fig 4a) For the samples S10 and S15,

however, the in-plane magnetic anisotropy can be

formed with annealing at TAp2501C only At

higher-temperature annealing, one observes not only the re-establishment of the out-of-plane magnetic anisotropy, but also a strong enhance-ment of the magnetic coercivity (see e.g.Fig 4bfor S15) Finally, besides the establishment of the in-plane magnetic anisotropy, the sample S60 annealed at 4501C exhibits also a field-induced magnetic transition at m HE200 mT (Fig 4c)

Fig 2 TEM bright field images of the as-deposited (a) and 4501C-annealed (b) sample S10 TEM diffraction patterns of the corresponding TerfecoHan phases are shown in the insets.

-1 0 1

µo H ( T )

M 1T

-1 0 1

-1 0 1

M 1T

//

//

//

S15 S0

S60

Fig 3 Magnetic hysteresis loops of the as-deposited samples S0, S15 and S60.

N.H Duc et al / Physica B 327 (2003) 328–333 330

Such a magnetic behaviour is usually observed in

sandwich films made by stacking coupled layers

with typical thicknesses around 100 nm [8] It w as

associated with the different magnetisation

rever-sal of individual layers, leading to the formation of

the so-called extended domain wall at the

inter-faces The magnetisation process, thus, depends

not only on the crystallisation of the TerfecoHan

phase, but also on the film structure (e.g thickness

as well as the microstructure of the Fe-layers) and

magnetisation configuration

3.3 Magnetostriction

We measured two coefficients, l8and l>, which

correspond to the applied field, in the film plane,

being, respectively, parallel and perpendicular to

the measuring direction, which is along the sample

length l>is small, whereas a l8value of 103was achieved for the films S0, S10 and S15 As far as our knowledge, such behaviour is usually observed

in the in-plane induced uniaxial magnetic aniso-tropic films[3] The observed phenomenon will be tackled below The l8 data, measured in an applied magnetic field m0H = 0.6 T are shown in Fig 5 and listed in Table 1 for the as-deposited sandwiches It is clearly seen that the parallel magnetostriction initially increases almost linearly

A saturation tendency can be evidenced in magnetic fields higher than 0.3 T This implies that it is rather difficult to rotate spins into the film-plane An almost constant low-field magne-tostrictive susceptibility wl8¼ ql8=qðm0HÞE0:25

102T1 can be deduced for the samples S0, S10 and S15 For the sample S60, wl8¼ 0:06

102T1only (seeTable 1for more details) Annealing at temperatures TAp4501C reduces the saturation magnetostriction but enhances the low-field magnetostriction in the single layer TerfecoHan films This is due to the disappearance

-1

0

1

-1

0

1

-1

0

1

S60

T A = 450 o C

S0

T A = 450 o C

S15

T A = 450 o C

//

//

//

Fig 4 Magnetic hysteresis loops of the 4501C-annealed

samples S0, S15 and S60.

Fig 5 Parallel magnetostriction of as-deposited films.

Table 1 Magnetostriction data (see text) for as-deposited Fe/Terfeco-Han/Fe films

Samples l 8 (106) wl8(102T1)

of the perpendicular magnetic anisotropy

lead-ing to, on the one hand, an isotropic in-plane

distribution of the magnetostriction (l8=l>E  1)

and, on the other hand, to the reinforcement of the

domain wall motion contributions to the

magne-tisation process For this film, the optimum

annealing temperature is 4501C In this case, the

magnetostriction l8¼ 290  106is already

devel-oped in rather lowapplied magnetic fields of about

20 mT In addition, its coercive field is less than

5 mT It is worthwhile to mention that in an

applied field less than 10 mT, the (parallel)

magnetostrictive susceptibility has reached its

maximal value, wl8¼ 2:3  102T1 These results

reproduce well those reported previously [9]

For the sandwiches S10 and S15, the

magnetos-triction follows well the observed magnetic

beha-viours The initial magnetostrictive susceptibility

reaches the maximal value wl8¼ 1:3  102T1at

TA¼ 2501C This low-temperature heat treatment

is favourable for making the {Fe/TerfecoHan/Fe}

films a low-field giant-magnetostrictive material

for MEMS At higher-temperature heat

treat-ments, a perpendicular anisotropy type of

magne-tostriction is established again, so that these films

become a perpendicular recording medium

We usually associate the field dependence of the

magnetostriction with different types of

magneti-sation processes[1,9] This approach has also been

applied for the films under investigation It turns

out that the magnetisation process is governed by

the rotation of spins in samples S0, S10 and S15,

whereas it takes place in two-steps in sample S60

First, the motion of domain walls leads to a

magnetisation of M0¼1

2MSwithout any contribu-tion to magnetostriccontribu-tion In the second step, the

spins rotate into the direction of the applied

magnetic field leading to the change of both

magnetisation and magnetostriction

Finally, let us nowtackle the problem,

men-tioned above, to understand the disappearance of

the perpendicular magnetostriction in the

out-of-plane spin systems For simplicity, one may

assume that the 4f-electron density distribution is

a plate form as illustrated in Fig 6 We now

consider the position of the four atoms located in

the film plane and along the x- and y-axis In a

zero applied magnetic field, these four atoms are

equivalent with respect to the distance from the reference atom (Fig 6a) The l8-configuration, as already described above, corresponds to the applied field being parallel to the sample length (i.e along the y-axis, see Fig 6b) Due to the 4f-magnetic moment rotation into the field direction, the interatomic distance along the y-axis increases, whereas the interatomic distance along the x-axis remains the same As a consequence, a huge l8 is detected In contradistinction, in the l> config-uration (Fig 6c), only the interatomic distance change in the x-direction is observed This explains the almost zero perpendicular magnetostriction in the out-of-plane anisotropic films

4 Concluding remarks Magnetic and magnetostrictive properties of the out-of-plane magnetic anisotropic {Fe/Terfeco-Han/Fe} films have been investigated and dis-cussed in connection with their microstructure This newtype of magnetic material appears to be rather promising for MEMS as well as for application as perpendicular recording medium

Fig 6 Magnetostriction mechanism in films with out-of-plane magnetic anisotropy.

N.H Duc et al / Physica B 327 (2003) 328–333 332

The magnetisation process and, in particular, the

magnetostriction mechanism are proposed for

films having perpendicular magnetic anisotropy

Acknowledgements

This work is supported by the Vietnam National

University, Hanoi within project QG 02 06

References

[1] N.H Duc, in: K.A Gschneirdner, L Eyring, G.H Lander

(Eds.), Handbook on the Physics and Chemistry of Rare

Earths, Vol 32, Elsevier Science, Amsterdam, 2001, p 1 Chapter 205.

[2] N.H Duc, P.E Brommer, in: K.H.J Buschow(Eds.), Handbook on Magnetic Materials, Vol 14, Elsevier Science, Amsterdam, 2002, p 89, Chapter 2.

[3] N.H Duc, K Mackay, J Betz, D Givord, J Appl Phys 79 (1996) 973.

[4] E Quandt, J Appl Phys 75 (1994) 5653.

[5] N.H Duc, T.M Danh, N.A Tuan, J Teillet, Appl Phys Lett 78 (2001) 3648.

[6] N.H Duc, J Magn Magn Mater 242–245 (2002) 1411 [7] F Richomme, N.H Duc, D.T Huong Giang, J Teillet, in preparation

[8] D Givord, J Betz, K Mackay, J.C Toussaint, J Voiron, S.

W uchner, J Magn Magn Mater 159 (1996) 71.

[9] T.M Danh, N.H Duc, H.N Thanh, J Teillet, J Appl Phys 87 (2000) 7208.