Journal of Magnetism and Magnetic Materials 310 2007 2459–2465Discontinuous spring magnet-type magnetostrictive Terfecohan/YFeCo multilayers: A novel nanostructured material principle fo
Trang 1Journal of Magnetism and Magnetic Materials 310 (2007) 2459–2465
Discontinuous spring magnet-type magnetostrictive Terfecohan/YFeCo multilayers: A novel nanostructured material principle for excellent
magnetic softness
Laboratory for Nano Magnetic Materials and Devices, Faculty of Engineering Physics and Nano Technology, College of Technology, Vietnam National
University, Hanoi, Building E3, 144 Xuan Thuy Road, Cau Giay, Hanoi, Vietnam
Available online 27 November 2006
Abstract
Novel physics and reversal mechanisms of the whole system switching (WS) and individual switching (IS) type are reported for hard/ soft TbFeCo/YFeCo exchange-spring multilayers The WS type usually occurs in multilayered systems, in which the magnetic anisotropy
of hard TbFeCo layers is neglectable For such a system, the ferrimagnetically coupled hard/soft multilayered state is recovered after removing applied fields from the magnetized state At low negative fields, the magnetization switching occurs collectively for all magnetic moments in the whole system In this case, the low-coercivity mechanism is discussed on the basis of a hard/soft interfacial point contact This configuration is realized for TbFeCo/YFeCo discontinuous exchange-spring multilayers, in which the magnetic (Fe,Co) nanograins coexist with non-magnetic amorphous phase in the soft layers In this state, a magnetic coercivity as small as 0.4 mT is achieved It is considered as an excellent magnetic softness of rare-earth-based systems Enhancing the magnetic anisotropy in the hard TbFeCo layers, the magnetization switching follows the IS type at low temperatures Starting to decrease the applied magnetic field from the high-field state, one observes the first reversal of the magnetic moments in the soft high-magnetization YFeCo-layers in positive magnetic fields This is the reason for the observation of the negative coercivity as well as negative-biasing phenomena
r2006 Elsevier B.V All rights reserved
PACS: 75.70.i; 75.30.Et; 75.60.Ej
Keywords: Multilayer; Coercivity; Magnetic softness; Spring-exchange bias; Magnetization reversal
1 Introduction: discontinuous spring magnet-type
magnetostrictive Terfecohan/YFeCo multilayers
Hard/soft exchange-coupled magnetic nanocomposites,
which refer to the ‘‘exchange-spring’’ magnets, have
provided a pathway to increased energy product (Fig 1a)
[1] The fundamental understanding of the exchange-spring
mechanism has also been improved in multilayered form
(Fig 1b and c) Such materials have attracted much
attention in the last two decades Regarding the
high-performance permanent magnets, the experimental
achievements are still far from the theoretical predictions,
while the exchange-spring term has been successfully
applied to the so-called low-field giant magnetostrictive exchange-spring multilayers, in which high magnetostric-tive (e.g TbFeCo) and soft magnetic (e.g FeCo) layers alternate[2–4]
Magnetostrictive materials have particular interest in actuators as well as in sensors Crystalline and amorphous rare earth (R)–transition metal (T) alloys, e.g TbFeCo, usually exhibit giant magnetostriction The combination of
RT alloys and T metals in the exchange-spring multilayers opens a new approach for combining both high magneto-striction (ls) and magnetostrictive susceptibility (wlJ) Indeed, the magnetostriction of the order of 103 and a parallel magnetostrictive susceptibility of about 101T1 were reported for TbFeCo/FeCo multilayers ([4] and references therein) The magnetostrictive softness obtained
in these multilayers is attributed to the magnetization
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0304-8853/$ - see front matter r 2006 Elsevier B.V All rights reserved.
doi: 10.1016/j.jmmm.2006.11.011
Corresponding author Tel.: +84 4 7547203; fax: +84 4 7547460.
E-mail address: ducnh@vnu.edu.vn (N.H Duc).
Trang 2reversal, which is initially nucleated within the soft layers at
low fields and propagates into the magnetostrictive layers
[5,6] In these conventional spring magnet-type multilayers
(CSMs) (Fig 1b), the individual layers are structurally
homogenous either in the amorphous (TbFeCo) or
crystal-line (FeCo) state, thus the reversal nucleation occurs at
some defect points on the sample surface and interfaces In
this context, the magnetization reversal is expected to be
nucleated more easily in the heterogeneous soft layers
consisting of Fe(Co) nanograins embedded in an
amor-phous matrix (Fig 1c) This novel exchange-spring
configuration is named as discontinuous spring
magnet-type multilayer (DSM) Such DSMs have been realized
for sputtered {Tb0.4(Fe0.55Co0.45)0.6/(YxFe1x)}n and
{Tb0.4(Fe0.55Co0.45)0.6/(Yx(Fe0.7Co0.3)1x)}n (denoted as
{Terfecohan/(YxFe1x)}nand {Terfecohan/Yx(Fe,Co)1x}n,
respectively [5,6]) multilayers with a variable Y content
x ¼ 0, 0.1, 0.2 and the period number n ¼ 50 The
thicknesses of the individual layers are tTbFeCo¼12 nm
and tYFeCo¼10 nm As regards the R (and Y)
concentra-tion dependence of the microstructure, in these studies, a
rather high Tb content of 40 at% is fixed in order to
maintain the amorphous structure in the magnetostrictive
Terfecohan layers The soft magnetic YFeCo layers,
however, can be formed either in homogeneous crystalline
(c), amorphous (a) or in heterogeneous nanostructure (n)
state depending on the Y content and/or additional heat
treatments The structural, magnetic and magnetostrictive
investigations have been performed for various
configura-tions of the exchange-spring multilayers[5–8]
In this paper, we will highlight our recent work on Terfecohan/YFe and Terfecohan/YFeCo DSMs that dis-play several novel physics and reversal mechanisms of the exchange-spring magnets The paper is organized as follows After the introduction, Section 2 presents a description of hard/soft interfacial point contact model for the discontinuous exchange-spring multilayer—a novel nanostructured material principle for excellent magnetic softness Section 3 deals with magnetization reversal types Among them, the so-called negative coercivity is shown Section 4 describes the negative exchange-biasing phenom-enon Finally, conclusion remarks are given in Section 5
2 Hard/soft interfacial point contact model—a novel nanostructured material principle for excellent magnetic softness
According to Kneller and Hawig [1], in exchange-spring magnets the magnetization reversal is originally attributed
to domain wall motion Recently, Liu[9]proposed a new coercivity mechanism, in which the interfacial exchange coupling restricts the magnetization rotation Such magne-tization rotation is incoherent, leading to a useful magnetic coercivity This new coercivity concept was supported by experimental results obtained in rare-earth nanocomposites YCo5/a-Fe, Sm2Co17/Co, Nd2Fe14B/a-Fe, etc
Contrary to high-performance magnets, in several application aspects in microsystems, e.g magnetostriction, the rotation of the rare earth magnetization must be developed at low fields In this case, spring magnet-type magnetostrictive multilayers have been prepared as already mentioned above With regards to the magnetic (and/or magnetostrictive) softness, one can apply the traditional way to increase the thickness of the soft layer (roughly twice the width of a domain wall dhin the hard phase[10]) Presently, we propose another way by minimizing the above-mentioned incoherent rotation in spring magnet-type multilayers This can be realized in a configuration where soft/hard interfaces become magnetically soft/hard interfacial point coupling The idea is illustrated inFig 2
In a two-dimensional description, the orientation of magnetic moments in hard and soft layers of the CSM in the zero demagnetizing field is shown in Fig 2a In this case, the soft/hard interfacial exchange coupling is homo-genous and the incoherent rotation of the magnetic moments in the soft layers is reinforced The interfacial exchange coupling is a short-range interaction that is effective only for hard/soft atomic neighbors In order to lift the restriction of the rotation of the magnetic moments
in the soft layers, the magnetic moments of atoms near the interface should be reduced or annulled as illustrated in
Fig 2b and c, respectively This corresponds to the two typical configurations of soft layers in DSMs: the weakly magnetic amorphous matrix (wmm) (Fig 2b) and the non-magnetic amorphous matrix (nmm) (Fig 2c)
TEM electron diffraction patterns of the Terfecohan/ YFe multilayers are given on the left in Fig 3 The
Fig 1 Illustration of the exchange-spring materials: nanocomposite (a),
conventional multilayered (b) and novel discontinuous multilayered (c)
types.
Trang 3amorphous state existing in the Terfecohan layers of all
three samples is characterized by the (typical) first bright
spread ring from the inside diffraction spot, whereas the
other rings, which are characteristics of the YxFe1xlayers,
exhibit drastically different behaviors with the variable Y
concentration They are almost complete sharp rings for
x ¼ 0 (Fig 3a, left) and spotty rings for x ¼ 0.1 (Fig 3b,
left) indicating the crystalline state of Fe layers and the
coexistence of Fe nanocrystallites in an amorphous matrix
of the Y0.1Fe0.9 layers, respectively Finally, for x ¼ 0.2,
these rings become spread (Fig 3c, left) that evidence for
amorphous state of Y0.2Fe0.8 layers Periodic stripe
structures of layers are viewed in HR-TEM cross-sectional
micrographs on the right of Fig 3 for the as-deposited
samples These images are characterized by typical (dark)
smooth trips of amorphous Terfecohan layers and different
microstructure of YxFe1xlayers A percolation of
BCC-Fe grains is observed in the x ¼ 0 sample The Terfecohan/
Fe multilayer is considered as the CSM with an almost
homogenous interfacial hard (amorphous)/soft (crystalline)
exchange coupling Well-separated dark spots observed in
unsmooth Y Fe 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 amorphous matrix This is a typical observation of the DSM, in which hard/soft interfacial point contacts are formed As usual, the stripes are (light) smooth for the amorphous Y0.2Fe0.8 layers The Terfeco-han/Y0.2Fe0.8multilayer, thus, is the CSM with the almost homogenous hard (amorphous)/soft (amorphous) interfa-cial exchange coupling The observed transformation to the nanostructure was associated with the reduction of the thermodynamic driving force for the crystallization caused
by the Y substitution[11] This is the direct approach to the nanostructure based on a critical concentration of the yttrium element Similar behaviors are found for Terfeco-han/Yx(Fe,Co)1xsystem In addition, the DSM can also
be obtained from the CSM by the conventional bottom-up
Fig 2 Magnetic configurations in CSM (a), DSM with wmm (b) and
DSM with nmm (c).
Fig 3 TEM electron diffraction patterns (left) and HR-TEM micro-graphs (right) of TbFeCo/Y x Fe 1x multilayers: (a) x ¼ 0; (b) x ¼ 0.1 and (c) x ¼ 0.2.
Trang 4approach This is the case of the 350 and 450 1C annealed
Terfecohan/Y0.2Fe0.8multilayers
Magnetic coercivity of investigated samples is
deter-mined from the magnetic and magnetostrictive hysteresis
loops The results are collected in Table 1 The hard/soft
interfacial point contact model seems to be valid for the
as-deposited samples: the observed magnetic coercivity is
higher in CSM (moHC45 mT) with respect to that in DSM
(moHC¼3 mT) The low-coercive force of the as-deposited
Terfecohan/Y0.1Fe0.9 multilayer may be attributed to the
specific nanostructure, in which each Fe nanograin is
largely decoupled from the other ones via the non-magnetic
matrix forming hard/soft interfacial point contacts
(de-noted as DSM with nmm) (Fig 2c) After releasing the
stress induced during the deposition, the coercivity of
1 mT is reached in the 350 1C-annealed Terfecohan/
Y Fe and Terfecohan/Y Fe DSMs with nmm (see
inTable 1) The coercivity reduction, in particular, is much effective after annealing at 450 1C: the difference in moHC
between CSM and DSM with nmm is four times However, almost no difference in the coercivity is found between
350 1C-annealed Terfecohan/FeCo CSMs and Terfecohan/ Y(Fe,Co) DSMs At present, it is possible to assume that although the nanograins were formed in the YFeCo layers, their matrix is still magnetic (wmm) This reflects the fact that the effect on the coercivity between the CSM and DSM with wmm described inFig 2a and b, respectively, is not much different Further increasing the annealing temperature, the evolution of nanograins and the phase segregation lower the FeCo concentration in the matrix As
a consequence, the DSM with nmm is obtained for Terfecohan/Y0.2(Fe,Co)0.8 In this case, a magnetic coer-civity as small as 0.4 mT is achieved This could be an excellent magnetic softness being able to obtain for
Table 1
The values of coercivity m o H C (mT) of TbFeCo/Y x Fe 1x and TbFeCo/Y x (Fe,Co) 1x CSMs and DSMs with non-magnetic amorphous matrix (nmm) and weakly magnetic amorphous matrix (wmm) in soft layers
Fig 4 Magnetization data measured at 5 K (top) and 100 K (bottom) for Terfecohan/Y 0.1 (Fe,Co) 0.9 (a) and Terfecohan/Y 0.2 (Fe,Co) 0.8 (b).
Trang 5magnetic rare-earth-based materials For the 450 1C
annealed Terfecohan/Y0.1(Fe,Co)0.9, the ferromagnetic
behavior seems to be maintained in the YFeCo matrix,
so that its coercivity is comparable with that in CSMs
As a remark, the presented experimental results seem to
phenomenologically support the hard/soft interfacial point
contact model for the low-coercivity mechanism This
model is, however, rather ideal Practically, the HR-TEM
micrographs give a good impression that the Terfecohan
layers block the grain growth, so there must be a rather
large contact area One could perhaps surmise that some
nucleation centers for re-crystallization are quite near the
border, and that such a grain could be a handle to rotate
the moments in the Terfecohan layer
3 Magnetization reversal types and the observation of the
so-called negative coercivity
In artificial ferrimagnetic multilayered systems,
magne-tization and magnetic anisotropy differ from one layer to
the next, so the magnetization reversal occurs at different
coercive fields for each layer For TbFeCo/YFeCo DSMs
under consideration, in the low-field saturation state
(LFS-state), magnetization is dominated by the soft YFeCo
layers and the ferrimagnetic multilayered state is
char-acterized by the parallel orientation of 3d magnetic
moments in the whole sample In external magnetic fields,
the ferromagnetic multilayered state is established by the
rotation of the hard TbFeCo-layer magnetization along the
applied field direction In this case, the 3d magnetic
moments in soft and hard layers are antiparallelly oriented,
leading to the formation of a so-called extended domain
wall (EDW) at the interfaces Thus, the high-field
satura-tion state (HFS-state) always accompanies with the
existence of the interfacial EDW As the external field is
decreased, this EDW is destroyed in a middle field and
finally, the ferrimagnetic multilayered state (without EDW)
returns at low fields The details of these (reversal)
phenomena, however, depend on not only the net magnetic
moment of the system, but also the magnetic anisotropy of
the hard layers In special conditions, one also observes the
so-called negative coercivity, at which the reversal causes
a negative magnetization when the applied field is still
positive[12]
The magnetization loops measured using a SQUID in
the fields up to 5 T are shown in Figs 4 and 5 Let us
consider the 100 K magnetization loops shown at the
bottom of Fig 4 At this temperature, the magnetic
anisotropy of the TbFeCo layers is still negligible and the
Zeeman energy is dominated As the magnetic field is
decreased from the HFS-state, the magnetization reversal
takes place firstly in the smaller magnetization TbFeCo
layers as expected (individual switching (IS) type) In this
case, the LFS-state corresponds to the ferrimagnetic
multilayered one and in low negative fields the reversal
occurs for whole systems (whole system switching (WS)
type) Similar phenomenon can be described for reversal
processes at room temperature At 5 K, all the samples are still the Fe magnetically dominated systems Thus, the Fe magnetization is, in principle, in priority to favor the applied field direction At present, however, on decreasing the applied magnetic field from the HFS-state, the first reversal of IS type occurs with 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 aniso-tropy continues to pin their magnetization against the magnetic field direction For Terfecohan/Y0.1(Fe,Co)0.9, the rotation of Fe magnetization starts in positive fields, almost compensates with TbFeCo magnetization in zero fields and the Fe magnetization reversal process is completed in negative fields (Fig 4a, top) For Terfeco-han/Y0.2(Fe,Co)0.8, the IS-type reversal of the Fe magne-tization is completed and causes a negative magnemagne-tization when the applied field is still positive (Fig 4b, top) This is the observation of the so-called negative coercivity The nature of the Fe magnetization reversal in the positive
Fig 5 Magnetization data measured at 5 K for Terfecohan (t TFC )/Fe (10 nm): t ¼ 20 nm (a), 16 nm (b) and 12 nm (c).
Trang 6fields is confirmed in the magnetization loops of the
Terfecohan (tTFC)/Fe (10 nm) multilayers with a fixed Fe
layer thickness of 10 nm and the TbFeCo layer thickness
varying as tTFC¼12, 16 and 20 nm (Fig 5): with
decreasing TbFeCo layer thickness, the Fe magnetization
almost compensates with TbFeCo magnetization for
tTFC¼20 nm (Fig 5a) and causes negative net
magnetiza-tion for tTFC¼12 and 16 nm (Fig 5b and c) Note that
these magnetization reversal types can be described by
Monte Carlo simulations [10–13]
4 Negative exchange biasing
The phenomenon of exchange biasing is a property of
many antiferromagnetic (AF)/ferromagnetic (F) bilayer
systems It is observed by a displacement of hysteresis loop
of the ferromagnetic layer toward negative fields when the
sample is cooled through the Ne´el temperature of the AF
layer This is defined as the positive exchange biasing
Recently, similar physical and reversal mechanisms were
found in exchange-spring magnets, where the hard layer
replaces the AF layer as biasing layer [10] In this case,
a complement understanding of the exchange-bias
pro-blem is involved with the development of domain walls, i.e
with a twisted magnetic structure at the AF/F interfaces
For Terfecohan/Y0.1Fe0.9 multilayer, besides the
field-induced magnetic transition, at which the EDW is
destroyed, the observation of the phenomenon of positive
exchange biasing at low temperatures was reported[8] At
present, as showed inFig 6, a so-called negative exchange
biasing, i.e a displacement of the hysteresis loop of the
ferromagnetic layers toward positive fields, is observed Indeed, the minor hysteresis loops have been recorded in the temperature range from 5 to 100 K for TbFeCo/
Y0.2(Fe,Co)0.8 multilayer The measurement was obtained
by sweeping the field from 5 to 0.4 T In the hysteresis loops that correspond only to the reversal of the soft layers (IS type), the value of the exchange-bias fields is found to decrease from 165 to 115 mT and 36 mT when the temperature increases from 5 to 25 K and 50 K, respec-tively Finally, the positive exchange biasing disappears at
100 K, at which the WS-type reversal is governed (Fig 6d) This observation exhibits the important role of the hard layer anisotropy
5 Concluding remarks This paper deals with two types of magnetization switching, denoted as WS type and IS type For the WS type, the low-coercivity mechanism is discussed on the basis of a hard/soft interfacial point contact This is the configuration realized for TbFeCo/YFeCo DSMs, in which the magnetic (Fe,Co) nanograins coexist with non-mag-netic amorphous phase in soft layers In this state, a magnetic coercivity as small as 0.4 mT is achieved This is considered as the novel nanostructured material principle for excellent magnetic softness Due to the enhancement of the magnetic anisotropy in the hard TbFeCo layers, the magnetization switching follows the IS type firstly in soft layers at low temperatures This is the reason for the observation of negative coercivity as well as negative-biasing phenomena
Fig 6 Minor loops measured by sweeping the field from 5 to 0.4 T (closed circles) and magnetization data (opened circles) at 5 K (a), 25 K (b), 50 K (c) and 100 K (d) for TbFeCo/Y (Fe,Co) films.
Trang 7This work is supported by the State Program for
Fundamental Research in Natural Sciences under Project
410.406 and by the College of Technology, Vietnam
National University
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