The results show that in the as-deposited multilayer, the TbFeCo and YFe layers are amorphous.. These novel magnetic properties are associated to the evolution of a nanocrystalline struc
Trang 1Excellent magnetic softness in magnetostrictive TbFeCo/YFe
multilayers N.H Duca, F Richommeb, N.A Tuana, D.T Huong Gianga, T Verdierb,
J Teilletb,*
a Faculty of Physics, Cryogenic Laboratory, Vietnam National University, Hanoi, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam
b Groupe de Physique des Materiaux, Faculte des Sciences de Rouen, Universit !e de Rouen, UMR CNRS 6634, 76821 Mont-Saint-Aignan,
France
Abstract
The {Tb(Fe0.55Co0.45)1.5/Y0.2Fe0.8}40 multilayers are investigated by X-ray diffraction and SQUID measurements The results show that in the as-deposited multilayer, the TbFeCo and YFe layers are amorphous An excellent magnetic softness with a coercivity of 0.3 mT was achieved after annealing at 2501C These novel magnetic properties are associated to the evolution of a nanocrystalline structure in the YFe layers Different temperature dependences of the coercive field are described for the amorphous and nanocrystalline multilayers r 2002 Elsevier Science B.V All rights reserved
Keywords: Multilayers; Magnetostrictive materials; Soft magnetic materials; Coercivity
The need for microactuator applications has
stimu-lated the development of thin films with a large
magnetostriction and a large magnetostrictive
suscept-ibility For this reason, spring-magnet-type
magnetos-trictive multilayers (MSMMs), e.g TbFeCo/FeCo,
which combine layers with a large room-temperature
magnetostriction and soft magnetic layers with a high
magnetisation were investigated [1–2] In the absence of
a long-range anisotropy in amorphous TbFeCo layers,
along with negligible magneto-crystalline anisotropy in
(FeCo) layers, the coercivity of MSMMs ranges between
4 and 10 mT Recently, attempts to improve the soft
magnetic properties of MSMMs were performed by
Quandt and Ludwig [3] and Farber and Kronm.uller [4]
on TbFe/FeCoBSi and TbDyFe/FeSiBNbCu
multi-layers, respectively In these as-deposited multimulti-layers,
both magnetostrictive and soft magnetic layers are in an
amorphous state, from which a nanocrystalline structure
can be controlled Then, a better magnetic softness was
expected to be obtained Practically, an almost negligible
magnetic hysteresis, but at the expense of the magnetos-triction, was observed [4] In this paper, we study the {Tb(Fe0.55Co0.45)1.5/Y0.2Fe0.8} multilayers
The [Tb(Fe0.55Co0.45)1.5/(Y0.2Fe0.8)]n multilayers with individual layer thicknesses tTbFeCo¼ 12 nm,
tYFe¼ 13 nm and with n ¼ 40 were prepared by rf-magnetron sputtering from composite targets The substrates were glass microscope cover slips with a nominal thickness of 150 mm Both target and sample holder were water-cooled Samples were annealed at temperatures from TA¼2501C to 5001C for 1 h in a vacuum of 5 105mbar
The X-ray diffraction (XRD) results are presented in Fig 1 Besides a broad peak corresponding to Tb2O3
oxide, the as-deposited TbFeCo/YFe multilayer does not show any clear Bragg peaks, indicating the existence
of the amorphous state in the TbFeCo as well as in the YFe layers The nanostructure associated with the
BCC-Fe or iron-rich BCC-BCC-Fe(Y) phase in the YBCC-Fe layers seems to occur for the samples annealed at TA¼ 2501C:
It is well evidenced at TA¼ 3501C and is further confirmed by the presence of three expected peaks of BCC-Fe(Y) in the XRD parttens at TA¼ 5001C: This argument is consistent with the results of the M.ossbauer
*Corresponding author Tel.: 2-35-14-66-19; fax:
+33-2-35-14-66-52.
E-mail address: jacques.teillet@univ-rouen.fr (J Teillet).
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V All rights reserved.
PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 9 9 1 - X
Trang 2studies [5] It is worth mentioning here that, at this TA;
no reflection corresponding to the crystallisation of the
TbFeCo layers is observed The crystallisation process
of the BCC-Fe(Y) phase, therefore, occurred at rather
low temperatures The reason may be related to the
influence of the rare earth (or yttrium) concentration
As regards the magnetic evolution during the
crystal-lisation process, we investigated the time dependence of
the magnetisation during a field annealing The result
obtained in the case of TA¼ 400 K and m0H ¼ 1 T is
presented in Fig 2 It is clearly seen that a magnetisation
enhancement followed by an exponential tendency was
found
The magnetic hysteresis loops measured in magnetic
fields applied parallel to the film plane are presented in
Figs 3(a–d) for the as-deposited, 2501C-, 3501C- and
5001C-annealed Tb(Fe0.55Co0.45)1.5/(Y0.2Fe0.8) samples,
respectively For the as-deposited sample, the saturation
magnetisation MS¼ 426 kA/m only After annealing,
MSincreases to 580 kA/m at TA¼ 2501C and reaches a
saturation value of 620 kA/m at T X3501C: The total
(measured) magnetisation MS of the multilayer can be described in relation with thickness tiand magnetisation
Miof individual layers as follows [2]:
MS¼tYFeMYFe tTbFeCoMTbFeCo
tYFeþ tTbFeCo
Introducing the experimental value of the structural and magnetic parameters, e.g tTbFeCo;tYFe; MS and
MTbFeCo¼ 250 kA/m into Eq (1), the magnetisation of the individual YFe layer (MYFe) can be derived The magnetisation value MYFeequals 1050 kA/m for the as-deposited sample At TAX3501C; MYFe reaches
1425 kA/m, which implies a Fe-magnetisation value of
1770 kA/m It is rather close to that of BCC-Fe (1740 kA/m) This result is consistent with that deduced from the M.ossbauer experiments [5]
The multilayers under consideration, in particular, show a rather low coercive field at room temperature:
θ (degree)
TA = 500 o C
TA = 350 o C
TA = 250 o C
As-deposited
Fig 1 X-ray diffraction patterns of TbFeCo/YFe multilayers:
BCC-Fe(Y) (closed triangles), and R 2 O 3 , F e 2 O 3 (open
trian-gles).
1.00
1.04
1.08
t (minutes)
Fig 2 Time dependence of the magnetisation measured at
T ¼ 400 K for the TbFeCo/YFe multilayer (B ¼ 1 T).
-1.0 0.0 1.0
-1.5 0.0 1.0
-1.0 0.0 1.0
-1.0 0.0 1.0
As-deposited
T A = 250 o C
T A = 500 o C
T A = 350 o C
(a)
(b)
(c)
(d)
Fig 3 Magnetic hysteresis loops of TbFeCo/YFe multilayers
Trang 3m0HC equals to 3.5, 0.3, 0.6 and 3.0 mT for the
as-deposited, 2501C-, 3501C- and 5001C-annealed films,
respectively In spring-magnet-type multilayers, the
decrease of the coercivity is usually related to the
enhancement of the (total) magnetisation At present,
moreover, it is also associated to the relaxation of the
amorphous state resulting in more stable and
homo-geneous TbFeCo layers, and to the formation of the
nanostructure in the YFe layers The observed increase
of m0HCin the 5001C-annealed sample can be attributed
to the growth of Fe(Y) grains
As temperature decreases, the coercivity is enhanced (Fig 4a) At T ¼ 5 K, m0HCequals 91.7, 13.4 and 15 mT for the as-deposited, 2501C- and 3501C-annealed sam-ples, respectively The variation of the normalised coercive field (HCðTÞ=HCðT¼ 5 KÞ) as a function of the temperature is plotted in Fig 4b This figure shows different temperature dependences of the coercivity: while m0HC follows a polynomial tendency
m0HCB120:0062T þ 105T2for the as-deposited multi-layer, it follows a power law of m0HCB5=T0:9 for the films annealed at TA¼ 2501C and 3501C The coercivity
in these multilayers, therefore, must be described by two different mechanisms corresponding to the amorphous and nanostructure states This, however, is still open at present
In conclusion, the optimum condition for the mag-netic softness was found for the Tb(Fe0.55Co0.45)1.5/ (Y0.2Fe0.8) multilayers This excellent magnetic property
is associated with the formation of the nanostructure in the YFe layers
This paper is partly supported by the Fundamental Research Program of Vietnam under project 420301
References
[1] E Quandt, A Ludwig, J Betz, K Mackay, D Givord,
J App Phys 81 (1997) 5420.
[2] A Ludwig, E Quandt, J Appl Phys 87 (2000) 4691 [3] E Quandt, A Ludwig, J Appl Phys 85 (1999) 6232 [4] P Farber, H Kronm uller, J Appl Phys 88 (2000) 2781 [5] N.H Duc, Magma, in this proceeding.
0
20
40
60
80
100
µo
H c
25 0˚ C
35 0˚ C
as deposited
H c = 5 T -0 9
H c = 1 - 0 0062 T + 10 -5 T 2
0.5
0.5
1.0
T (K)
H c
25 0˚ C
35 0˚ C
as deposited
(a)
(b)
Fig 4 Temperature dependence of the coercive field in
TbFeCo/YFe multilayers.