For the sample annealed at 350C, the amorphous state still remains in the TbFeCo layers, whereas a fine grain 7–10 nm size structure was formed in the YFeCo ones.. Magnetisation analysis
Trang 1Microstructure and magnetic studies of magnetostrictive
Terfecohan/YFeCo multilayers D.T Huong Gianga,*, N.H Duca, F Richommeb, S Schulzec
a
Faculty of Physics, Cryogenic Laboratory, Vietnam National University, 334 Nguyen Trai Road, Thanh Xuan, Hanoi, Viet Nam
b
GPM-UMR CNRS 6634, Universit !e de Rouen, Site Universitaire du Madrillet, B.P 12,
76801 Saint-Etienne-Du-Rouvray Cedex, France
c
Institute of Physics, Chemnitz University of Technology, D-09107, Chemnitz, Germany
Abstract
Sputtered [Tb0.4(Fe0.55Co0.45)0.6/(Y0.2Fe0.63Co0.17)]40 multilayers were investigated by means of energy-dispersive X-ray spectroscopy, high-resolution transmission electron microscope and SQUID magnetometer measurements The results show that the amorphous state exists in the whole as-deposited sample For the sample annealed at 350C, the amorphous state still remains in the TbFeCo layers, whereas a fine grain (7–10 nm size) structure was formed in the YFeCo ones Magnetisation analysis indicates the existence of a non-collinear magnetic structure and a field-induced magnetic phase transition, in which the TbFeCo magnetisation tends to rotate along the YFeCo magnetisation direction The magnetic coercivity is discussed in terms of the magnetoelastic interactions
r2003 Elsevier Science B.V All rights reserved
PACS: 75.60.Jk; 75.70.Cn; 81.07.Bc
Keywords: Microstructure; Field-induced magnetic phase transition; Non-collinear magnetic structure; Magnetic coercivity
1 Introduction
During the last few years, there has been a great
interest in magnetostrictive thin films for various
microelectromechanical systems (MEMS)
High-performance magnetostrictive materials have been
realised in the form of amorphous a-TbFeCo
single layer films and/or of magnetostrictive
spring-magnet type multilayers (MSMMs) of
a-TbFeCo/FeCo For more insight on the
magne-toelastic effect and its applications, we refer to Refs.[1–3]and references therein
Applications require not only a large magnetos-triction, but also a large magnetostrictive suscept-ibility at low magnetic fields The idea of preparing MSMMs is to combine large room-temperature magnetostriction layers (e.g TbFeCo) with soft-magnetic layers with a high magnetisation (e.g Fe, Co), in order to enhance the average saturation magnetisation (MS) This leads to the reduction of the saturation field m0HSð¼ 2K=MSÞ instead of decreasing the anisotropy constant (K)[4] In such MSMMs, the FeCo individual layers are usually formed in the crystalline state They exhibit a magnetic coercivity as small as 5 mT [5] An
*Corresponding author.
E-mail address: giangdth79@yahoo.com
(D.T Huong Giang).
0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V All rights reserved.
doi:10.1016/S0304-8853(03)00063-5
Trang 2improvement of the soft-magnetic properties of
MSMMs was achieved on the TbFe/FeCoBSi and
TbDyFe/FeSiBNbCu multilayers, in which both
magnetostrictive and soft magnetic layers are
amorphous [6,7] Recently, we have reported an
excellent magnetic softness in magnetostrictive
TbFeCo/YFeCo multilayers [3,8] These
multi-layers show a record magnetostrictive
susceptibil-ity wlJð¼ dlJ=dHÞ ¼ 13 102/T1 and a rather
small magnetic coercivity m0HC¼ 0:6 mT The
observed novel magnetic behaviour has been
attributed to the nanostructure of the YFeCo
layers In this paper, such a microstructure is
confirmed by high-resolution transmission
elec-tron microscopy In addition, the magnetic
struc-ture and a field-induced collinear–non-collinear
structure phase transition will be reported The
low magnetic coercivity is discussed in terms of the
magnetoelastic interactions
2 Experimental
[TbFeCo/(YFeCo)]nmultilayers with individual
layer thicknesses tTbFeCo¼ tYFeCo¼ 12:5 nm and
with number of periods n ¼ 40 were prepared by
RF-magnetron sputtering The typical power
during sputtering was 400 W and the Ar pressure
was 102mbar A composite target, consisting of
18 segments of about 20, of different elements,
here Tb, Fe, Co and Y, has been used 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 up to TA¼ 350C for
1 h in a vacuum of 5 105mbar Film composi-tion is determined by energy-dispersive X-ray (EDX) spectroscopy The obtained composition
is Tb0.4(Fe0.55Co0.45)0.6/Y0.2Fe0.63Co0.17 (denoted
as Terfecohan/Y0.2Fe0.63Co0.17), instead of Terfe-cohan/Y0.2Fe0.8as reported previously[3,8] The microstructure was investigated using a high-resolution transmission electron microscope (HRTEM) The magnetisation was measured by means of a SQUID magnetometer in magnetic fields up to 5 T and at temperatures ranging from 5
to 298 K
3 Experimental results and discussion 3.1 Microstructure
Figs 1(a) and (b) show the cross section TEM micrographs of the as-deposited and 350 C-annealed Terfecohan/(Y0.2Fe0.63Co0.17) multi-layers, named M0 and M350, respectively It can
be clearly seen that, for sample M0, the amor-phous state exists in the whole sample, i.e in both the magnetostrictive and the soft-magnetic layers For sample M350, the amorphous state still remains in the Terfecohan layers, whereas a fine grain (7–10 nm size) structure was formed in the YFeCo ones The corresponding TEM electron diffraction patterns are presented inFigs 2(a) and (b) This finding supports the X-ray diffraction
Fig 1 Cross-section HRTEM micrographs of multilayers M0 (a) and M350 (b).
Trang 3results obtained earlier [8] and corroborates the
nanostructure nature of the excellent magnetic
softness of the films under consideration
3.2 Magnetisation and magnetic structure
The in-plane magnetisation curves measured in
decreasing field at 5, 77 and 298 K are presented in
Figs 3(a) and (b) for the samples M0 and M350
Their magnetisation data at different temperatures
and applied magnetic fields are listed in Table 1
All magnetisation curves show a large remanence
The zero-field magnetisation Mt(0 T) listed in
Table 1, however, is the value obtained by
extrapolating to m0H ¼ 0: For the as-deposited
sample, at room temperature Mtð0 TÞ ¼ 480 kA/m
only After annealing at TA¼ 350C, Mt(0 T)
increases to 630 kA/m In accordance with the
M.ossbauer studies[3], this magnetisation
enhance-ment is related to the magnetic evolution in
nanostructured YFeCo layers As the temperature
is decreased, Mt(0 T) slightly increases in both samples M0 and M350 The (FeCo)-rich YFeCo alloys usually exhibit a weakly temperature dependent magnetisation However, the magneti-sation of the Terfecohan film shows a rather strong temperature dependence (see Fig 4) Hence, the observed variation of Mt(0 T) can not
be described by combining the two magnetisation contributions of the individual Terfecohan and YFeCo layers In addition, it is interesting to note that the magnetisation curves show a rather large high-field magnetic susceptibility (whf) For in-stance, for the film M0, whf takes a value of 22.5 (kA/m)/T at T ¼ 298 K and reaches to 25.2 and 52.7 (kA/m)/T at T ¼ 77 and 5 K, respec-tively A similar result is obtained for the sample annealed at 350C These findings may imply a field-induced magnetic phase transition in the investigated samples
Fig 2 TEM electron diffraction patterns of multilayers M0 (a) and M350 (b).
500 600 700 800 900 1000
5 K
77 K
298 K
200
300
400
500
600
700
800
5 K
77 K
298 K
Fig 3 In-plane field-down magnetisation curves of multilayers M0 (a) and M350 (b).
Trang 4In the multilayer under consideration, the individual layers are thick enough for magnetic coupling but they are thinner than the magnetic exchange length, for which domain walls cannot
be formed at the interface In this state, the 3d–3d exchange interactions ensure that parallel coupling
of the (Fe,Co)-magnetic moments persists throughout the entire multilayer Without creating domain walls at the interfaces, the multilayer behaves as one piece of material Then, magnetisa-tion processes result from the average of the magnetic characteristics of each individual layer Assuming a collinear magnetic structure, the (experimental) total magnetisation Mt of the multilayer, which is dominated by the YFeCo contribution, can be described as a function of thickness tiand magnetisation Miof the individual layers as follows:
Mt¼tYFeCoMYFeCo tTbFeCoMTbFeCo
tYFeCoþ tTbFeCo : ð1Þ The total magnetisation Mt; measured at different temperatures and in different applied magnetic fields, is listed inTable 1 In Ref [3], a hyperfine field of Bhf ¼ 27 and 34 T was reported for the as-deposited and 350C-annealed films, respectively By scaling the BCC-Fe hyperfine field (33 T) with its magnetisation (1740 kA/m), the FeCo-magnetisation and then the magnetisation
of the YFeCo layer (MYFeCo) can be deduced from the strength of the corresponding hyperfine field It turns out that MYFeCoequals 1140 and 1450 kA/m
in the as-deposited and 350C-annealed films,
0 400 800 1200
0 50 100 150 200 250 300 350
T (K)
Terfecohan
µo H =1 T
Fig 4 Temperature dependence of magnetisation of Terfeco-han film in moH ¼ 1 T.
(mo
mo
mo
mo
mo
mo
mo
mo
mo
mo
mo
mo
mo
mo
Trang 5respectively Below, this magnetisation value is
assumed to be temperature independent
Inserting the experimental value of the
structur-al and magnetic parameters, e.g tTbFeCo; tFe;
MYFeCo and Mt into Eq (1), the magnetisation of
the individual TbFeCo layer (MTbFeCo) can be
derived on the basis of the collinear situation The
obtained results are also listed inTable 1, and are
plotted as a function of applied field in Figs 5(a)
and (b) The results show that at room
tempera-ture MTbFeCo(0 T) is 236 and 209 kA/m for the
films M0 and M350, respectively At low
tempera-tures, MTbFeCo(0 T) is slightly enhanced in M0,
however, it is weakly decreased in M350 The
positive sign of MTbFeCo(0 T) confirms the
anti-parallel orientation of TbFeCo and YFeCo
magnetisation In addition, the obtained
MTbFeCo(0 T) value of the as-deposited and
350C-annealed samples is comparable to the
room temperature value of 259 kA/m of the
Terfecohan film (see Fig 4) The decrease of
MTbFeCo(0 T) with decreasing temperature does
not mean a reduction of the magnitude of the
TbFeCo magnetisation, but a reduction of its
contribution along the applied-field direction It
may be associated to the existence of a zero-field
non-collinear magnetic structure of the TbFeCo
and YCoFe magnetisation In this context, it is
interesting to mention that, while Mt increases
strongly with increasing applied fields,
MTbFeCoðm0HÞ is decreased (Fig 5) Specially, in
the investigated magnetic fields we observe also a
change in sign of MTbFeCoðm HÞ at T ¼ 5 K for the
sample M0 (Fig 5a) and at T ¼ 5 and 77 K for the sample M350 (Fig 5b) This finding supports the above mentioned picture of a non-collinear magnetic structure This argument seems to be strengthened at low temperatures At T ¼ 5 K, however, jMTbFeCoð5 TÞj reaches 406 kA/m only, which is still lower than the value of 1000 kA/m observed for Terfecohan film at low temperature (see Fig 4) This implies that the rotation is not completed in the investigated magnetic fields (moHp5 T)
3.3 Magnetic coercivity The in-plane magnetic hysteresis loops mea-sured at 298, 77 and 5 K are presented in
Figs 6(a)–(c) for the samples M0 and M350 All samples show a rather low coercive field, e.g at room temperature, moHCequals to 3.5 and 0.6 mT for samples M0 and M350, respectively As the temperature is decreased, moHC is enhanced However, it is worthwhile to mention that, at a certain temperature, moHC of M350 is always smaller than that of M0 This behaviour may also
be described in the relation with the magnetic enhancement in the nanocrystalline YFeCo layers Indeed, the role of magnetoelastic interactions in controlling the magnitude of switching field and coercivity was investigated for TbFe/FeCo multi-layers [9] Results showed that magnetoelastic constraints at the TbFe/FeCo interfaces, arising from different magnetostriction values (li) in adjacent layers, lead to biaxial stress and, as a
-300 -200 -100 0 100 200 300
5 K
77 K
298 K
-500 -400 -300 -200 -100 0 100 200 300
MTbFeCo
5 K
77 K
298 K
µoH (T) (b) µoH (T) (a)
Fig 5 Plot of MTbFeCoas a function of applied magnetic field.
Trang 6consequence, to a corresponding stress-induced
anisotropy Applying this to the present case, we
write the coercivity as
moHC¼3ðlTbFeCo lYFeCoÞ
2
E
MYFeCo MTbFeCo
here, E is the Young modulus of TbFeCo
This model was postulated on the basis of a
magnetic collinear structure At present, however,
it can be applied to discuss the observed magnetic
softness improvement Indeed, the observed
de-crease of moHCwith increasing annealing
tempera-ture can be explained by the enhancement of
the average magnetisation /MSð¼ MYFeCoF
MTbFeCoÞ: The almost zero-coercivity observed in
the 350C-annealed film, on the other hand, may
also be associated to the development of the
magnetostriction in the nanostructured YFeCo
layers with respect to its amorphous state and thus
to the compensation of the (lTbFeCo lYFeCo) factor in Eq (2)
4 Concluding remarks Besides the excellent magnetic softness and giant low-field magnetostriction, multilayers consisting
of a periodic sequence of amorphous TbFeCo and fine grain structure YFeCo as soft-magnetic interlayers exhibit also the existence of a non-collinear magnetic structure and a field induced magnetic phase transition Their low magnetic coercivity can be described on the basis of the magnetoelastic interactions
Acknowledgements This work was supported granted by the Vietnam National University, Hanoi within the project QG.02.06 and by KC.02.13
M0
5 K
-900 -600 -300 0 300 600 900
5 K
M350
M0
77 K
-600 -300 0 300 600
-600 -300 0 300 600
-600 -300 0 300 600
M0
298 K
-800 -400 0 400 800
-800 -400 0 400 800
M350
298 K
M350
77 K
Fig 6 Magnetic hysteresis loops for the multilayers M0 and M350 at T ¼ 298; 77 and 5 K.
Trang 7[1] N.H Duc, in: K.A Gschneirdner Jr., L Eyring, G.H.
Lander (Eds.), Handbook on the Physics and Chemistry of
Rare Earths, Vol 32, Elsevier, Amsterdam, 2001, p 1
(Chapter 205).
[2] N.H Duc, P.E Brommer, in: K.H.J Buschow (Ed.),
Handbook on Magnetic Materials, Vol 14, Elsevier,
Amsterdam, 2002, p 89.
[3] N.H Duc, J Magn Magn Mater 242–245 (2002)
1411.
[4] N.H Duc, T.M Danh, N.A Tuan, J Teillet, Appl Phys Lett 78 (2001) 3648.
[5] E Quandt, A Ludwig, J Betz, K Mackay, D Givord, J App Phys 81 (1997) 5420.
[6] P Farber, H Kronm uller, J Appl Phys 88 (2000) 2781 [7] A Ludwig, E Quandt, J Appl Phys 87 (2000) 4691 [8] N.H Duc, F Richomme, N.A Tuan, D.T Huong Giang,
T Verdier, J Teillet, J Mag Magn Mater 242–245 (2002) 1425.
[9] H.D Chopra, D.X Yang, P Wilson, J Appl Phys 87 (2000) 5780.