Invited paperDevelopment of giant low-field magnetostriction in a-TerfecoHan-based single layer, multilayer and sandwich films N.H.. Duc Faculty of Physics, Cryogenic Laboratory, Vietnam N
Trang 1Invited paper
Development of giant low-field magnetostriction in
a-TerfecoHan-based single layer, multilayer and sandwich
films N.H Duc
Faculty of Physics, Cryogenic Laboratory, Vietnam National University, Hanoi, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam
Abstract
Giant magnetostriction in low magnetic field has been achieved with different approaches, among which single, multilayer and sandwich films with rare-earth base are considered as most promising ones Enhancement of the 3d(Fe,Co)-magnetic moment with respect to those of 3d(Fe) and 3d(Co) strengthens the Tb–FeCo exchange energies in amorphous Tb(Fe0.55Co0.45)1.5 (named as a-TerfecoHan) films This is thought to cause diminishing of the Tb-sperimagnetic cone-angle and, thus, leading to enhancement of the magnetostriction Indeed, a saturation magnetostriction of lg;2B103 has been obtained After annealing, a parallel magnetostrictive susceptibility of
wl8¼ 1:8 102T1has been achieved at moH ¼ 10 mT TbFeCo/YFe multilayers combining exchange-coupled giant magnetostrictive TerfecoHan layers and large-magnetisation Y0.2Fe0.8 ones exhibit an excellent magnetic and magnetostrictive softness Subsequent annealing at 3501C leads to the relaxation of the a-TbFeCo layers and to a nanocrystallisation of the YFe layers In this state, a magnetic coercive field moHC¼ 0:3 mT and a huge magnetostrictive susceptibility, wl8¼ 13 102T1have been obtained M.ossbauer studies indicate that the magnetic softness as observed is associated to the evolution of the nanocrystalline Fe-particles Magnetostriction is also presented for Fe/TerfecoHan/Fe sandwiches In this case, giant magnetostriction is already developed at low fields in as-deposited films Technologically, this seems to be one of the simplest ways to prepare magnetostrictive films for microsystem applications r 2002 Elsevier Science B.V All rights reserved
Keywords: Thin films; Multilayers; Sandwiches; Magnetostriction; M ossbauer spectra
1 Introduction
Magnetostrictive materials are transducer materials
(as well as piezoelectric and shape memory ones), which
directly convert electrical energy into mechanical energy
They are useful in the manufacture of sensors, actuators,
controllers, force and displacement as well as other
electro-acoustic devices For these applications,
trans-ducer materials in the form of thin films are of special
interest because cost-effective mass production is
possi-ble, compatible to microsystem process technologies In
addition, magnetostrictive thin films are particularly
promising as microactuator elements like cantilevers or
membranes, since they combine high-energy output,
high-frequency and remote-control operations [1–5] Due to this potential, interest in such giant magnetos-trictive thin films has rapidly grown over the past few years Owing to the specifications related with applica-tions in microelectromechanical systems (MEMS), materials research has been focused on thin-film materials showing giant magnetostriction (GMS) in combination with soft magnetic properties
Most papers concerning GMS published in the last decade have been devoted to rare-earth based films and multilayers Various attempts have been focused mainly
on amorphous Terfenol and Terfenol-D (a-TbDyFe2) alloys Although the magnetostriction of these amor-phous alloys has been found to be one order of magnitude lower than that of its crystalline counterpart, reliable magnetostrictive devices in MEMS have been
E-mail address: duc@cryolab-hu.edu.vn (N.H Duc).
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 6 1 - 1
Trang 2designed on the basis of these materials Magnetic
investigations have shown that such amorphous R-Fe
alloys could never offer optimal magnetostrictive
performances, due to the sperimagnetic character of
the Fe and R moments [6] For these traditional
magnetostrictive materials, fortunately, an alternative
has been found in nanocrystalline R-Fe alloys [7] In
these alloys, the crystallites are sufficiently large for the
exchange coupling to be effective, but small enough to
prevent macroscopic magnetic anisotropy Thus, one
may consider the material as an isotropic ferromagnet,
in which the magnetostriction is expected to be not
reduced while the magnetocrystalline anisotropy is
diminished By optimising the annealing temperature
and time, the magnetostriction value can be doubled [7],
although coercivity values below 100 mT, cannot be
reached in these nanocrystalline single layer films [8] In
the amorphous state, however, it is strongly preferable
to replace the iron by cobalt, because the amorphous
alloys near to the composition a-TbCo2present higher
ordering temperatures and higher magnetostriction than
the equivalent iron-based alloy [3] As a tradition, we
prefer to give this film a name as a-TercoN!eel, indicating the constituents: ter for Tb, co for cobalt and N!eel for the name of the laboratory where this alloy had been discovered In fact, the magnetostriction has been optimised in a series of thin films of the type a-(Tb,Dy)(Fe,Co)2 (a-TerfecoN!eel-D) [9,10] Still better performances were obtained on R/T magnetostrictive spring-magnet multilayers, where the saturation field of the magnetostrictive a-TbDyFeCo is lowered by in-creasing the average magnetisation through exchange coupling with the soft-magnetic FeCo layers [4,5 and refs therein] For comparison, the interesting values of the magnetostriction and the magnetostrictive suscept-ibility are summarised in Table 1
GMS obtained in a-TerfecoN!eel-D alloys has been explained in terms of an increase in the ferromagnetic coupling strength within the (Fe,Co) subsystem, and the effect of field annealing in inducing a well-defined uniaxial anisotropy (see below) It is well known that the substitution of Dy for Tb gives rise to an increase of the low-field magnetostriction through the reduction of the saturation field However, it is also accompanied by
Table 1
Comparison of the magnetoelastic data for magnetostrictive bulk and thin-film materials
(MPa) (10 6 ) (MPa/T) (10 2 T 1 ) Bulk crystalline
Single layer films
Multilayers
Sandwich films
Trang 3a reduction in the saturation magnetostriction Co
substitution results in an enhancement of both low-field
and saturation magnetostriction Thus, we can expect a
further enhancement of the magnetostriction in these
alloys by increasing the Tb concentration at the expense
of both Dy as well as (Fe, Co) An optimum was found
for a-Tb(Fe0.55Co0.45)1.5 (denoted as a-TerfecoHan,
where Han means Hanoi, i.e the capital where studies
of this composition have been carried out)
In this paper, we present results of our studies of
GMS in the a-TerfecoHan based single layer, multilayer
and sandwich films Microscopic properties of these
magnetostrictive materials are discussed in connection
with the M.ossbauer studies
2 Magnetostriction in TerfecoN!eel-D and
a-TerfecoHan thin films
Although crystalline TbCo2compound orders below
room temperature (TC¼ 230 K) as Co is merely
metamagnetic [18], the amorphous state stabilises a
moment on the Co sublattice due to band narrowing
These Co moments are strongly ferromagnetically
coupled A Tb-sperimagnetic structure still occurs as in
a-R-Fe alloys, but the ordering temperature is now
raised up to approximately 500 K, which is already
higher than that of a-TbFe2 The magnetostriction of
a-Tb1xCoxthin films was studied intensively by Betz et al
[3] For a-TbCo2, the room temperature
magnetostric-tion reaches a value of bg;2¼ 24 MPa under
m0H ¼ 1:9 T, which corresponds to lg;2¼ 400 106:
As mentioned above, Duc et al [9,10] have succeeded
in enhancing the magnetostrictive properties of
amor-phous Terfenol-D like alloys by substituting Co for Fe
in a-(Tb,Dy)(Fe,Co)2 The simple arguments were that,
in general, R-Fe exchange energies are larger than the
equivalent R-Co interaction energies [19] This arises
from the fact that the Fe moment is significantly larger
than the Co one, while the R–T intersublattice exchange
constant is approximately the same for T=Fe and Co
Fortunately, the T–T interactions tend to be stronger in
(Fe,Co)Fthan in either Fe-or Co-based alloys [20]
This results in an increase of TC for a given R:T ratio
The stronger R-FeCo exchange energies should then
lead to a closing of the sperimagnetic cone angle and
thus to an enhancement of the magnetostriction The
main results obtained for the (Tb0.27Dy0.73)(Fe1xCox)2
system [10] are summarised in Fig 1(a–c) At room
temperature, the films become magnetically rather soft
The strongest coercive field (moHC¼ 15 mT only) is
found at x ¼ 0:63: The largest magnetostriction of
bg;2¼ 28:8 MPa (lg;2¼ 480 106) was found in the
middle of the composition range (at x ¼ 0:47) and it can
be obtained in the rather low applied magnetic field of
60 mT A detailed analysis yields values between 481 and
531 for the R-sperimagnetic cone-angle y [9,10], in accordance with some literature values [6] The variation
in y also implies a variation in the average (Tb,Dy) moment as a function of x (see Fig 1b) From the measured magnetisation data, the 3d-moment M3d can
be determined as a function of x (see also Fig 1c) Clearly, a similar composition dependence of M3d as observed in the crystalline R-(Fe,Co) alloys is found and
a maximum is reached for x ¼ 0:47 where there is sufficient Co to ensure good ferromagnetic T–T coupling as well as sufficient Fe giving the larger magnetic moment This 3d magnetic moment enhance-ment caused by Co substitution was confirmed by
M.ossbauer studies [11] in a-TerfecoHan films In both configurations of parallel and perpendicular magnetic anisotropy, a hyperfine-field value Bhf of 23.5 T was reported, whereas it equals only 21 T for a-TbFe2 This shows a possibility to enhance the magnetostriction in this type of alloys by increasing the Tb concentration Indeed, we have found a record giant magnetostriction
of lg;2B103for a-TerfecoHan (see Fig 2 there only l8
is presented) [11] For this alloy, an interesting value of the magnetoelastic susceptibility ðwl8¼ ql8=qðmoHÞÞ of 1.8 102T1 was achieved at moHC¼ 15 mT Although this result is not so good as for the multilayers
to be discussed in the next section (see also Table 1), it does suffice for microactuator applications
4.2 4.4 4.6 4.8 5.0
B
0 10 20 30 40
1.0 1.2 1.4 1.6 1.8
x
B
Fig 1 Concentration dependence of magnetoelastic coupling constant (b), rare-earth magnetic moment (MR) and 3d-magnetic moment (M ) in (Tb Dy )(Fe Co )
Trang 43 Magnetostriction in a-Terfecohan based multilayers
The starting idea of preparing the spring-magnet type
magnetostrictive multilayer (MSMM) is to decrease the
saturation field moHs (¼ K=2Ms) by enhancing the
average saturation magnetisation (Ms) instead of
decreasing the anisotropy constant K: The
magnetos-triction of the as-deposited TerfecoHan/Fe multilayer,
however is half of that of the corresponding single layer
film This is consistent with the fact that the
magnetos-triction of Fe is negligible Annealing effects cause the
saturation magnetostriction to decrease, but to be
developed in lower magnetic fields (Fig 3) The
reduc-tion of the magnetostricreduc-tion may be due to the
interdiffusion of atoms between the layers, which leads
to a decrease of the rare-earth concentration and, then,
of the magnetostriction in the interface phases Indeed, analysis of magnetisation data showed that annealing at
TA¼ 3501C made the interface spacer to extend about
1 nm more This change, however, was not detected by the M.ossbauer study As presented in Fig 4, the
M.ossbauer spectrum of the as-deposited and 3501C-annealed films and their hyperfine field distributions are almost similar They can be characterised by two separate contributions from ferromagnetic a-Terfeco-Han and BCC-Fe components
In the absence of long-range anisotropy in amorphous TbFeCo layers, along with negligible magneto-crystal-line anisotropy in (FeCo) layers, the coercivity of MSMM is ranging between 4 and 10 mT In attempts
to improve the soft-magnetic properties of the highly magnetostrictive nanocrystalline layers by preparing MSMM’s with soft magnetic interlayers, Quandt and Ludwig [4], and Farber and Kronm.uller [15] have studied TbFe/FeCoBSi and TbDyFe/FeSiBNbCu multi-layers, respectively An almost vanishing hysteresis was obtained, at the expense, however, of the magnetostric-tion [4] We prepared a {12 nm TerfecoHan/13 nm
Y0.2Fe0.8} multilayer After production, both TbFeCo and YFe are amorphous In this state, the multilayer exhibits already a soft magnetic and magnetostrictive character with a coercivity moHC¼ 3:5 mT and a parallel magnetostrictive susceptibility wl8;max¼ 3:8 102T1 (Fig 5a and b) This magnetostrictive softness has been strongly improved by heat treatments:
moHC¼ 0:3 mT and wl8¼ 13 102T1 in a field of
1.8 mT (Fig 5b) The M.ossbauer study shows that these novel properties are associated with the develop-ment of nanostructure in the FeCo layers (see Fig 6) Initially, the M.ossbauer spectrum consists of a magnetic sextet with broad lines superimposed on a paramagnetic contribution With increasing the annealing tempera-ture, this paramagnetic contribution decreases and the lines of the sextet are sharpening Finally, at
TA¼ 3501C the magnetic sextet becomes prominent The obtained hyperfine field distribution PðBhfÞ of the as-deposited multilayer shows a minor paramagnetic component with /BhfS ¼ 6 T and a fraction Apar¼ 15%: The major ferromagnetic contribution distributes
in a broad hyperfine-field range with a maximum at
Bhf¼ 27 T and a fraction Aamor¼ 85%: Taking into account the fact that the PðBhfÞ of the amorphous TerfecoHan phase is characterised by a peak at
Bhf¼ 23:5 T (see Fig 4), the observed paramagnetic and the high hyperfine-field ferromagnetic phase can be attributed to the not-well crystallised state in the YFe spacers For the 2501C-annealed film, in the hyperfine field distribution, PðBhfÞ; three almost separated com-ponents can be distinguished: (i) the paramagnetic component with /BhfS ¼ 4 T and Apar¼ 5%; (ii) the low hyperfine-field ferromagnetic component with /B S ¼ 23:5 T and a fraction A ¼ 30% and (iii)
0
50
100
150
200
250
300
350
400
-6 )
1 2
3
Fig 2 Magnetostriction of as-deposited (1), 2501C-(2) and
3501C-(3) annealed TerfecoHan films.
0
50
100
150
200
250
300
-0.8 -0.4 0 0.4 0.8
µo H (T)
λ ////
-6 )
Fig 3 Magnetostriction of as-deposited (closed circle) and
3501C-annealed (open circle) {TerfecoHan/Fe} multilayers.
Trang 5the high hyperfine-field ferromagnetic component with
/BhfS ¼ 31:5 T and a fraction Aferr¼ 65%: The Bhf
results are in agreement with the development of the
BCC-Fe phase After annealing at 3501C, the low
hyperfine-field peak at Bhf¼ 23:5 T still remains (with
Aamor¼ 20%), the high hyperfine-field peak (with
Aferr¼ 80%) is shifted up to 34 T, but the paramagnetic
phase disappeared in the sample This was also
confirmed by X-ray diffraction results [21] Such an
excellent magnetostrictive softness is rather promising
for MEMS
4 Magnetostriction in TerfecoHan based sandwich films
To make an attempt at simplifying the production
process of soft magnetostrictive films, we have prepared
and investigated three {Fe/TerfecoHan/Fe} sandwich films with the fixed TerfecoHan layer thickness of
800 nm and a variable Fe thickness tFe¼ 12; 50 and
75 nm, which are named as the sandwich A, B and C, respectively Their coercive field and magnetostriction data are listed in Table 2 It is worthwhile to note that although these sandwiches show a somewhat smaller magnetostriction with respect to corresponding single layer and multilayer films, their magnetic softness is comparable with that of corresponding multilayers In particular, the as-deposited sandwich A (with smallest Fe-thickness) exhibits already a coercivity as small as 4.5 mT The observed behaviour may be related to the magnetisation reversal of the Fe layer at the surface of the TerfecoHan layer Anyway, these films would open advanced possibilities to fabricate magnetostrictive films for MEMS
0
50
100
150
200
(a)
1 2
0.00 0.05 0.10 0.15
λ//
-6 )
// (T
-1 )
(b)
2
1
Fig 5 Low field dependence of the parallel magnetostriction (a) and magnetostrictive susceptibility (b) for the {TerfecoHan/Y 0.2 Fe 0.8 } multilayers: curves (1) as-deposited and curves (2) 3501C-annealed multilayers.
0
1.00
1.02
1.00
1.02
0 10 20 30
40 0
10 20 30
Velocity (mm/s)
(a)
(b)
Fig 4 M ossbauer spectra and hyperfine-field distributions of the {TerfecoHan/Fe} multilayers: (a) the as-deposited film, (b) after annealing at 3501C.
Trang 6The project QG 99.08 of the Vietnam National
University, Hanoi and the project 420.301 of the
Fundamental Research Program of Vietnam support
this work Samples were prepared at the International
Training Institute for Materials Science (ITIMS) The
collaboration with Dr T.M Danh, Dr H.N Thanh,
N.A Tuan, D.T Huong Giang, V.N Thuc, Prof N.P
Thuy at the Cryogenic Laboratory and ITIMS, that
with Dr D Givord at the Laboratoire Louis N!eel and
that with Prof J Teillet at the University of Rouen has
been very useful to complete this work
References
[1] T Honda, K.I Arai, M Yamaguchi, J Appl Phys 76
(1994) 6994.
[2] F Claeyssen, N Lhermet, R Le Letty, P Bouchilloux, J Alloys Compounds 258 (1997) 61.
[3] J Betz, K Mackay, D Givord, J Magn Magn Mater.
207 (1999) 180.
[4] A Ludwig, E Quandt, J Appl Phys 87 (2000) 4691 [5] N.H Duc, in: K.A Gschneidner Jr., L Eyring (Eds.), Handbook of Physics and Chemistry of the Rare Earths, Vol 32, Elsevier Science, North-Holland, Amsterdam, 2001 [6] P Hansen, in: K.H.J Buschow (Ed.), Handbook of Magnetic Materials, Vol 6, Elsevier Science, North-Holland, Amsterdam, 1991, p 289.
[7] B Winzek, M Hirscher, H Kronm uller, J of Alloys, Compounds 283 (1999) 78.
[8] S.F Fischer, M Kelsch, H Kronm uller, J Magn Magn Mater 195 (1999) 545.
[9] N.H Duc, K Mackay, J Betz, D Givord, J Appl Phys.
79 (1996) 973.
[10] N.H Duc, K Mackay, J Betz, D Givord, J Appl Phys.
87 (2000) 834.
[11] N.H Duc, T.M Danh, H.N Thanh, J Teillet, A Lienard,
J Phys.: Condens Matter 12 (2000) 8283.
0 2 4 6 8 10 12
0 5 10 15 20
0
1.00 1.01
1.00 1.02
1.00
1.02
B hf
Velocity (mm/s)
0 5 10 15 20
B hf (T)
(a)
(a)
(b)
(c) (c)
(b)
Fig 6 M ossbauer spectra and hyperfine-field distributions of the {TerfecoHan/Y 0.2 Fe 0.8 } multilayers: (a) the as-deposited film, (b) after annealing at 2501C and (c) 3501C.
Table 2
Coercive field and magnetostriction of several TerfecoHan based sandwich films
Trang 7[12] A.E Clark, in: E.P Wohlfarth (Ed.), Handbook of
Ferromagnetic Materials, Vol 1, Elsevier Science,
North-Holland, Amsterdam, 1980, p 539.
[13] Y Hayashi, T Honda, K.I Arai, K Ishiyama, M.
Yamaguchi, IEEE Trans Magn 29 (1993) 3129.
[14] J.Y Kim, J Appl Phys 74 (1993) 2701.
[15] P Farber, H Kronm uller, J Appl Phys 88 (2000)
2781.
[16] E Quandt, A Ludwig, Mat Res Soc Symp Proc.,
Mater Res Soc 459 (1997) 565.
[17] D Givord, A.D Santos, Y Souche, J Voiron, S.
W uchner, J Magn Magn Mater 121 (1993) 216.
[18] N.H Duc, P.E Brommer, in: K.H.J Buschow (Ed.), Handbook of Ferromagnetic Materials, Vol 11, Elsevier Science, North-Holland, Amsterdam, 1999, p 259 [19] N.H Duc, in: K.A Gschneidner Jr., L Eyring (Eds.), Handbook of Physics and Chemistry of the Rare Earths, Vol 24, Elsevier Science, North-Holland, Amsterdam,
1977, p 339.
[20] J.P Gaviagan, D Givord, H.S Li, J Voiron, Physica B
149 (1988) 345.
[21] N.H Duc, F Richomme, N.A Tuan, D.T Huong Giang,
T Verdier, J Teilet, J Magn Magn Mater., in these proceedings.