DSpace at VNU: Novel exchange-spring configuration for excellent magnetic and magneto strictive softness

Journal of Magnetism and Magnetic Materials 290–291 2005 800–803Novel exchange-spring configuration for excellent magnetic and magnetostrictive softness N.H.. Chaub a Academic Affair Depa

Journal of Magnetism and Magnetic Materials 290–291 (2005) 800–803

Novel exchange-spring configuration for excellent magnetic

and magnetostrictive softness

N.H Duca, , D.T Huong Giangb, N Chaub

a Academic Affair Department, College of Applied Sciences and Technologies, Vietnam National University, Hanoi, E3 Building,

144 Xuan Thuy Road, Cau giay, Hanoi, Vietnam

b Faculty of Physics, Vietnam National University, Hanoi, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam

Available online 14 December 2004

Abstract

Magnetization and magnetostriction data are reported for discontinuous type exchange-spring Tb(Fe0.55Co0.45)1.5/ (Y0.1Fe0.9) multilayers, in which nanograins coexist with amorphous phase in soft YFe layers This novel exchange-spring configuration exhibits an excellent magnetic and magnetostrictive softness: low magnetic coercivity ðm0HC¼

1 mTÞ; large magnetostriction ðlg;2¼720  106Þand large parallel magnetostrictive susceptibility ðwljj¼dljj=dm0H ¼ 29:7  102T1Þ: In addition, the observed phenomena of the negative contribution to magnetostriction, the formation

of the extended domain wall at the interfaces and the exchange-bias are discussed

r2004 Elsevier B.V All rights reserved

PACS: 75.60.Jk; 75.70.Ak; 75.80.+q

Keywords: Exchange-spring multilayers; Exchange-bias; Magnetization; Domain wall

The conventional exchange-spring concept has

suc-cessfully been applied to the low-field giant

magnetos-triction development in the so-called magnetostrictive

spring-exchange multilayer, in which high

magnetostric-tive (e.g TbFeCo) and soft magnetic layers (e.g FeCo)

alternate[1] In these multilayers, magnetization reversal

is nucleated within the soft layers in low applied fields

and propagates from the soft layers into the

magnetos-trictive layers When the soft FeCo-layer is structurally

homogeneous in either crystalline or amorphous state—

named as continuous structure, the nucleation of

reversal occurs at some defect points on the sample

surface and interfaces The reversal is expected to be

easily nucleated in discontinuous soft phase, in which

nanograins coexist with amorphous phase in soft layers

Such a novel exchange-spring configuration is realized

by controlling the Y-concentration in soft magnetic layers of sputtered TbFeCo/YFe and TbFeCo/YFeCo multilayers [2,3] This approach opens an alternative route towards new magnetostrictive materials and new generation of soft magnetic materials Furthermore, it provides a configuration for studying fundamental reversal mechanism Here, these aspects are presented

in more details for the discontinuous type exchange-spring multilayer of {Tb(Fe0.55Co0.45)1.5/(Y0.1Fe0.9)}, denoted as {Terfecohan/(Y0.1Fe0.9)} with the individual layer thicknesses tTbFeCo¼12 nm and tYFe¼10 nm: The samples were fabricated by RF-magnetron sputtering The sample nanostructure was investigated using high-resolution transmission electron microscopy (HRTEM) The magnetization was measured with a VSM in magnetic fields up to 5 T and in the temperature range from 4.2 to 300 K The magnetostriction was

www.elsevier.com/locate/jmmm

0304-8853/$ - see front matter r 2004 Elsevier B.V All rights reserved.

doi:10.1016/j.jmmm.2004.11.368

Corresponding author Tel.: 84 4 7680461; fax: 84 4 7680460.

E-mail address: ducnh@vnu.edu.vn (N.H Duc).

measured using an optical deflectometer (the

re-solution of 5  106rad), in which the bending of the

substrate due to the magnetostriction in the film was

measured

For the as-deposited sample, a periodic stripe

structure of smooth and unsmooth layers viewed in

HRTEM-cross-sectional micrograph inFig 1ais a good

evidence for the multilayered structure of continuous

(amorphous) Terfecohan layers and discontinuous

(nanocrystalline) Y0.1Fe0.9 layers, respectively Dark

spots observed in unsmooth stripes are noticeable with

an average size of the stripe thickness They are

attributed to BCC-Fe nanograins with an average

diameter of about 10 nm embedded within an

amor-phous matrix The electron diffraction patterns

per-formed in this sample is further evidenced for observed

microstructure behavior (Fig 1b) After annealing at

Ta¼350 1C; the microstructure almost remains

un-changed, whereas the room temperature magnetization

is strongly enhanced (seeFig 3below) This reflects the

fact that the role of annealing is not to lead to the

evolution in the grain size, but to enrich the Fe

concentration in the grains

Shown inFig 2is magnetostriction data Clearly, the

magnetostriction develops rapidly at the magnetic fields

of a few militestla (Fig 2a) Optimization of the large

magnetostriction ðlg;2¼720  106Þ as well as large

parallel magnetostrictive susceptibility ðwljj¼0:3 T1Þ

was obtained for the 350 1C-annealed film The obtained

wljjvalue is almost 30 times higher than that obtained in

the well-known Terfenol-D alloy and comparable with

that of the Metglas 2605SC In higher fields, however,

the magnetostriction exhibits a negative slope (Fig 2a)

In multilayered systems, properties such as

magnetiza-tion or anisotropy differ from one layer to the next, so

that the magnetization reversal occurs at different

coercive fields for each layer When the reversal takes

place in a given layer but not in the adjacent one, a

so-called extended domain wall (EDW) will be formed at

the interfaces and results in a negative contribution to

the parallel magnetostriction [4] As will be indicated below, this is not the case for the films under investigation

Magnetization data are presented inFig 3a,b for the as-deposited and annealed films, respectively Except the difference in the value of the magnetization and coercivity, the magnetic hysteresis loops of both samples exhibits the following common behaviours: (i) a field-induced magnetic transition at m0Ht and (ii) a

Fig 1 Bright field TEM image (a) and electron diffraction pattern (b) of as-deposited {Terfecohan/Y Fe } multilayers.

Fig 2 High-field (a) and low-field (b) magnetostriction data of as-deposited (open circles) and 350 1C-annealed (closed circles) {Terfecohan/(Y 0.1 Fe 0.9 )} multilayers.

phenomenon of exchange-biasing at low temperatures.

The observed phenomena become more pronounced as

the temperature decreases For these films, it is

reason-able to assume that the magnetization in the Tefecohan

layers is dominated by Tb[5] Thus, the corresponding

magnetization configurations of the magnetization

process are illustrated in the inset of Fig 3a The

EDW formation is well established above Ht; which is

much higher than fields where the negative contribution

of magnetostriction starts to occur Moreover, the

room-temperature field-induced transition is similar for

both samples, but their high-field magnetostrictive

susceptibility is quite different In this context, the

observed negative magnetostriction component could be

connected to the nature of the YFe layers, which is

strongly modified by annealing The question, however,

still opens for studies in more details

For the as-deposited Terfecohan/Y0.1Fe0.9multilayer,

the observed room-temperature coercivity value of 3 mT

is still high However, it was about a half of that

obtained in the corresponding conventional Terfecohan/

Fe(Co) systems [1–3] This may be attributed to the

specific discontinuous structure, in which each Fe

nanocrystal is largely decoupled from the other ones The coercivity as small as 1 mT is reached in the film after annealed at Ta¼350 1C: The coercivity reduction

is usually related to the releasing stress introduced during the deposition With regards to the proposed advantage of the discontinuous type exchange-spring configuration for the magnetic softness, this result can also be attributed to the enrichment of Fe in the nanograins, which enhance the decoupling of the Fe nanocrystallites with each other via the real non-magnetic matrix

The exchange-biasing phenomenon is a property of antiferromagnetic (AF)/ferromagnetic (F) bilayer sys-tems Similar behavior is found in exchange-spring magnets, where the hard layer replaces the AF layer as biasing layer [6] At present, the observed phenomenon may relate to the enhancement of the hysteresis of the field-induced transitions below 100 K In this case, the investigated magnetization curves can be considered as minor loops only The recoil curves show the exchange-spring behavior, which resembles the exchange-bias loops

of other systems (seeFig 4) At T ¼ 10 K; the exchange field ðm H Þ equals to 0.17 and 0.09 T for the

Fig 3 Magnetic hysteresis loops measured in the magnetic fields applied in the film-plane for as-deposited (a) and 350 1C-annealed (b) {Terfecohan/(Y 0.1 Fe 0.9 )} multilayers.

as-deposited and annealed films, respectively Scaling the

magnetization of the soft layer Ms to the formula of

m0Hex¼g=Msts; it turns out that the energy of a domain

wall g is the same order of magnitude in the two

samples

This work was supported by the State Program for Nanoscience and Nanotechnology of Vietnam under the Project 811.204 and by the Italian–Vietnamese Program

of Cooperation in S&T—Project 8BS3

References

[1] E Quandt, A Ludwig, J Betz, K Mackay, D Givord, J Appl Phys 81 (1997) 5420.

[2] D.T Huong Giang, N.H Duc, V.N Thuc, L.V Vu, N Chau, Appl Phys Lett 85 (2004) 1565.

[3] N.H Duc, D.T Huong Giang, N Chau, J Magn Magn Mater 282 (2004) 44.

[4] D Givord, J Betz, K Mackay, J.C Toussaint, J Voiron, S.D Wu¨chner, J Magn Magn Mater 159 (2004) 71 [5] N.H Duc, D.T Huong Giang, V.N Thuc, I Davoli, F Richomme, J Magn Magn Mater 272–276 (2004) E1597 [6] E.E Fullerton, J.S Jiang, S.D Bader, J Magn Magn Mater 200 (1999) 392.

Fig 4 Exchange-biasing observed in as-deposited

{Terfeco-han/(Y 0.1 Fe 0.9 )} multilayer.