Teilletb, a Laboratory for Nano Magnetic Materials and Devices, Faculty of Engineering Physics and Nanotechnology, College of Technology, Vietnam National University, Building E3, 144 Xu
Trang 1Journal of Magnetism and Magnetic Materials 316 (2007) 379–382
Magnetization and magnetostriction process in spring-magnet TbFeCo/Fe multilayers with variable TbFeCo thickness
D.T Huong Gianga, N.H Duca, J Juraszekb, J Teilletb,
a Laboratory for Nano Magnetic Materials and Devices, Faculty of Engineering Physics and Nanotechnology, College of Technology, Vietnam National
University, Building E3, 144 Xuan Thuy Road, Cau Giay, Hanoi, Viet Nam b
Groupe de Physique des Mate´riaux, UMR CNRS 6634, Universite´ de Rouen, 76801 St Etienne du Rouvray, France
Available online 7 March 2007
Abstract
Different natures of the magnetization reversal are studied by means of magnetization and magnetostriction measurements for magnetostrictive ‘‘spring-magnet’’ multilayers of TbFeCo/Fe with a fixed Fe layer thickness of 10 nm and variable TbFeCo layer thickness of 12, 16 and 20 nm In such multilayered systems, magnetization reversal occurs at different coercive fields for each layer related to formation of the domain wall at interfaces The results show that at low temperatures, transition from the ferromagnetic saturation state (with domain walls) to the ferrimagnetic transient saturation state (without domain walls) takes place by the reversal of the magnetic moments in the high-magnetization Fe layers This process is governed by large magnetic anisotropy of the TbFeCo layer Increasing temperature, this anisotropy decreases and transition results from the reversal of magnetic moments in the small magnetization TbFeCo layers Formation of the domain wall at interfaces is clearly evidenced by the negative contribution to the parallel magnetostriction
r2007 Elsevier B.V All rights reserved
PACS: 75.60.Ej; 75.60.Ch; 75.70.i; 75.80.+q
Keywords: Spring-magnet multilayer; Magnetostriction; Magnetization process; Domain wall
1 Introduction
The combination of rare earth–transition metal
(RE–TM) alloys and transition metals in spring-magnet
RE–TM/TM multilayers opened a new approach for
developing low-field giant magnetostriction [1,2] These
conventional magnetostrictive spring-magnet-type
multi-layers (CMSMM) were recently developed in the novel
magnetostrictive multilayers—named discontinuous
mag-netostrictive spring-magnet type multilayers (DMSMM),
in which the soft FeCo-layer is structurally heterogeneous
in nanostructured state Indeed, an excellent magnetic
softness with magnetostriction (l) of the order of 103and
magnetostrictive susceptibility (wl) of about 101T1was
reported for Tb(Fe0.55Co0.45)1.5/(YFeCo) (named as
Terfe-cohan/YFeCo) DMSMM [3,4] In such ferrimagnetic
multilayered systems, magnetization and magnetic aniso-tropy differ from one layer to the next, so 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, the so-called extended domain wall (EDW) formes at the interfaces and results in a negative contribu-tion to the parallel magnetostriccontribu-tion [5] In general, the high-field saturation state (HFS state) is usually related
to the existence of the interfacial EDW This EDW is destroyed in middle field Finally, a temporary saturation state (TS state) without DW exists at low fields Details of these phenomena, however, depend not only on the intrinsic properties of the soft layers but also on the net magnetic moment of the system In a special condition, one observes also the so-called negative coercivity, at which the reversal causes a negative magnetization when the applied field is still positive [6,7]
This paper deals with the influence of individual TbFeCo-layer thickness on magnetic and magnetostrictive
www.elsevier.com/locate/jmmm
0304-8853/$ - see front matter r 2007 Elsevier B.V All rights reserved.
doi: 10.1016/j.jmmm.2007.03.028
Corresponding author Tel.: +33 2 35 14 63 11; fax: +33 2 35 14 69 09.
E-mail address: jacques.teillet@univ-rouen.fr (J Teillet).
Trang 2properties in sputtered Tb(Fe0.55Co0.45)1.5/Fe multilayers
with a fixed Fe layer thickness of 10 nm and TbFeCo layer
thickness varying between tTFC¼12, 16 and 20 nm (named
as M12, M16 and M20, respectively)
2 Results and discussion
The magnetization loops were measured using a SQUID
from the helium liquid temperature to room temperature in
fields up to 5 T Shown in Fig 1 are the parallel
magnetization curves measured at 5, 100 and 300 K for
the as-deposited samples For single-layer films,
investiga-tions reveal that the Fe magnetization can take the almost
temperature-independent value of (MFe) ¼ 1700 kA/m,
whereas the Terfecohan magnetization (MTFC) decreases
from 1100 to 250 kA/m when going from 5 to 300 K [8]
This can be confirmed by analyzing the magnetization data
in the TS state (MTS):
MTS¼jtFeMFetTFCMTFCj
tFeþtTFC
where t indicates thickness For the HFS state, however,
net magnetization (MHFS) depends also on dimension of
the EDW through
MHFS¼t0
FeMFeþt0
TFCMTFCþdDWMDW
tFeþtTFC
where t0is the remainder of thickness in the ferromagnetic
Fe and Terfecohan core and d is thickness of the DW By
analyzing HFS magnetization data, one can obtain information for d This problem, however, will be treated elsewhere Presently, we are interested in the nature of the magnetization reversal only
Let us start by considering the 5 K magnetization loops (Fig 1(a)) Note that M20 is an almost magnetically compensated system, but the M12 and M16 are the Fe magnetically dominating samples Thus, Fe magnetization
is strongly sensitive to applied field direction In these experiments, however, starting to decrease the applied magnetic field from the HFS state, one observes first the reversal of the magnetic moment in the Fe layers This is due to the fact that although the Terfecohan layers have smaller net magnetization (with respect to that of Fe layers), their strong magnetic anisotropy continues to pin their magnetization against the magnetic field direction At
5 K, for all investigated systems, the rotation of the Fe magnetization starts in positive fields, almost compensated with TbFeCo magnetization in zero fields and the Fe magnetization rotation process is completed in negative fields (in particular for M12 and M16 samples) In addition, this rotation process (i.e destroying the EDW) starts in the higher field for the sample with thinner TbFeCo thickness
At 100 K, the reversal of the Fe magnetization in positive fields remains for the M16 and M20 samples only (Fig 1(b)) For the M12, due to reduction of TbFeCo magnetic anisotropy, TbFeCo magnetization cannot pin against the field direction anymore but it reverses in
Fig 1 Parallel magnetization loops of TbFeCo (tTFC)/Fe (10 nm) multilayers at different temperatures.
Trang 3positive fields (Fig 1(b) at the top) Finally, at room
temperature, magnetic anisotropy of amorphous TbFeCo
phase can be neglected, the Zeeman energy is dominating
and magnetization reversal takes place firstly in the smaller
magnetization TbFeCo layers as expected (Fig 1(c)) The
magnetization anomalies evidenced for this transition are,
however, rather weak It will be discussed below
Magnetostriction was measured using an optical
deflec-tion method[8].Fig 2presents room-temperature parallel
and perpendicular magnetostriction for the investigated
samples Note that, while the parallel magnetostriction
curves show big anomalies, e.g negative contributions, the
perpendicular magnetostriction exhibits almost ordinary
behavior with a rather weak positive curvature in high
fields This is a good evidence for the formation of the
EDW, which is weakly indicated from the
room-tempera-ture magnetization curves Indeed, the formation of the
Bloch-type DW results in a disorder of the magnetic
moments at the interfaces along which is measured the
parallel magnetostriction This causes a decrease of the
parallel magnetostriction, but preserves the perpendicular
magnetostriction [5] The magnetostriction coefficient
lg,2¼lJl? is presented in Fig 3 This presentation allows a good comparison not only for the magnitude of the domain wall magnetostriction, but also for its contribution with respect to the total magnetostriction Finally, considering the maximum in the parallel magne-tostriction curves as the starting point of the formation of EDW, one can say that the critical field for this transition decreases as TbFeCo thickness increases It is in good agreement with the observation mentioned for the 5 K magnetization data
In summary, different natures of the magnetization reversal are clarified for the magnetostrictive ‘‘spring-magnet’’ multilayers of TbFeCo/Fe with a variable TbFeCo layer thickness At low temperatures, transition from ferromagnetic HFS state (with the existence of the EDW) to the ferrimagnetic TS state (without the existence
of EDW) takes place by the reversal of magnetic moments
in high-magnetization Fe layers This is due to the large magnetic anisotropy of the TbFeCo With the increase of temperature, TbFeCo anisotropy decreases and transition
Fig 2 Room-temperature parallel (lJ) and perpendicular (l?) magnetostriction of TbFeCo (tTFC)/Fe (10 nm) multilayers.
Fig 3 Room-temperature magnetostriction coefficient (l g,2 ) of TbFeCo (tTFC)/Fe (10 nm) multilayers.
Trang 4takes place by the reversal of magnetic moments in the
TbFeCo layers with smaller magnetization The formation
of the domain wall at the interfaces is evidenced by the
negative contribution to the parallel magnetostriction
Acknowledgments
This work was supported by the College of Technology,
Vietnam National University, under the project QG.06.22,
the Programme International de Cooperation Scientifique
(PICS) of France and the INTEREG IIIa European
Research Program no 198
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