Spacer layer effect and microstructure on multi-layer [NdFeB/Nb]n films

Article abstract of DOI:10.1016/S0304-8853(01)00624-2

Spacer layer effect on multi-layer [NdFeB/Nb]_n films has been investigated from the variation of magnetic properties and microstructure of the films. From a HRTEM cross-section view observation, the average grain size of [NdFeB/Nb]_n multi-layers was controlled by both annealing temperature and thickness of NdFeB layer. Selected area diffraction pattern indicated that the structure of Nb spacer layer was amorphous. The grain size and coercivity of [NdFeB_x/Nb]_n films change from 50 nm and 16.7 kOe to 167 nm and 9 kOe for films with x = 40 nm, n = 10 and x = 200 nm, n = 2, respectively.

Full text of DOI:10.1016/S0304-8853(01)00624-2

Journal of Magnetism and Magnetic Materials 239 (2002) 450–452
Spacer layer effect and microstructure on multi-layer
[NdFeB/Nb]n films
Jai-Lin Tsaia,*, Yeong-Der Yaoa, Tsung-Shune Chinb, H. Kronmu.llerc
a Institute of Physics, Academia Sinica, Nankang, Taipei 115, Taiwan
b Materials Science & Engineering, National Tsing Hua University, Hsinchu 300, Taiwan
c Max-Planck-Institut fur Metallforchung, Stuttgart, Germany
.
Abstract
Spacer layer effect on multi-layer [NdFeB/Nb]n films has been investigated from the variation of magnetic properties
and microstructure of the films. From a HRTEM cross-section view observation, the average grain size of [NdFeB/Nb]n
multi-layers was controlled by both annealing temperature and thickness of NdFeB layer. Selected area diffraction
pattern indicated that the structure of Nb spacer layer was amorphous. The grain size and coercivity of [NdFeBx/Nb]n
films change from 50 nm and 16.7 kOe to 167 nm and 9 kOe for films with x ¼ 40 nm, n ¼ 10 and x ¼ 200 nm, n ¼ 2;
respectively. r 2002 Elsevier Science B.V. All rights reserved.
Keywords: Coercivity; Selected area diffraction pattern; Hard magnet
1. Introduction
Single-layer Nd–Fe–B films prepared at high tem-
perature or subsequently annealing have been published
in our previous work [9]. In this article, we report our
experimental studies of the spacer layer effect on multi-
layer [NdFeBx/Nb]n films, especially for inter-diffusion
between magnetic and non-magnetic layers in Nb spacer
layers as post annealing at high temperature.
Rare-earth transition (RE-TM) metal and interme-
tallic compounds have been extensively studied in
application of permanent magnets due to their high
magnetocrystalline anisotropy [1]. Microstructure varia-
tion is an interesting and important subject in process of
RE-TM permanent magnets. RE-TM thin film magnets,
for example, structure of Nd2Fe14B and Sm2Co17,
Sm2Fe17N3, etc. prepared by sputtering method [2,3].
Different underlayers Cr, W, Ta, etc. have been used to
control the texture of NdFeB, SmCo films [4,5]. The
study of finite grain size in RE-TM films has been
studied by controlling annealing temperature and film
thickness [6]. The realization of granular rare-earth
compound in a non-magnetic matrix, NdFeB/W and
SmCo/W has been reported [7]. RE-TM isolated grains
in a non-magnetic matrix by deposition NdFeB/W in a
multilayer form and then breaking the multiplayer
structure by proper annealing process. Granular multi-
layer [CoPt/C, Ag,B]n films have also been studied [8] to
isolate CoPt grains in C, Ag, B matrix, respectively.
2. Experimental
Multi-layer [NdFeBx/Nbz]n films were prepared by an
ion beam sputtering method with a high-purity argon
source (99.999%). The base pressure of the vacuum
system is 1.5 ꢀ 10ꢁ8 Torr and the working pressure was
7.5 ꢀ 10ꢁ5 Torr. The thickness of hard magnetic layer
NdFeB varied from 25 to 200 nm. Nb was used as spacer
layer z ¼ 5 nm and the total thickness of NdFeB layers
were fixed at 400 nm. Commercially available sintered
Nd15Fe77B8 magnet was used as the target. Si(1 1 1)
wafer cleaned with standard IC process were used as the
substrates. All the films were deposited at room
temperature and then subsequently annealed. As sput-
tered films were put in quartz tube and pumped to a
*Corresponding author. Fax: +886-02-27899636.
E-mail address: tsaij1@phys.sinica.edu.tw (J.-L. Tsai).
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 6 2 4 - 2

J.-L. Tsai et al. / Journal of Magnetism and Magnetic Materials 239 (2002) 450–452
vacuum of 2.3 ꢀ 10ꢁ7 Torr. The annealing temperature
451
varied from 5501C to 7001C for 20 min.
The structure of films was identified by an X-ray
diffractiometer (XRD) with a SIMENS D5000. The
microstructure of films was investigated by a high-resol-
ution electron microscopy (HRTEM) (JEOL400FX).
Chemical analysis of films was performed by
inductively coupled plasma (ICP, Spectroflame).
Magnetic properties were measured by a vibration
sample magnetometer (VSM, EG&G Model-PAR
4500), and by a SQUID (Quantum Design MPMS
SQUID Magnetometer) in the temperature range
5–400 K with the maximum applied field 2 and 5.5 T,
respectively.
3. Results and discussion
Fig. 1 shows the hysteresis loops of [NdFeB200 nm/
Nb5 nm]2 and [NdFeB40 nm/Nb5 nm]10 films annealed
at 630731C for 20 min. For [NdFeB200 nm/Nb5 nm]2
film, the coercivity and remanence ratio are 9.0 kOe and
0.78, respectively. But higher coercivity 16.7 kOe and
lower remanence ratio 0.65 was measured in
[NdFeB40 nm/Nb5 nm]10 films. This can be understood
that for a NdFeB film with fixed thickness, however with
increasing the number of Nb spacer layers, it will inhibit
the grain growth of Nd2Fe14B grains and enriches the
grain boundary.
The cross-sectional view of the multi-layer
[NdFeB40 nm/Nb5 nm]10 film was observed, as shown
in Fig. 2(a). Under the Cr protection layer, the 40 nm
NdFeB layers were separated by 5 nm Nb layers. In
general, the average grain size is roughly 50 nm after
analyzing hundred of grains and the interface of the film
and substrate was not a straight line (not shown in
Fig. 2) due to the inter-diffusion between Si and NdFeB
layer. It is clear that the grain growth was inhibited by
Fig. 2. (a) The cross-sectional view of
a
multi-layer
(NdFeB40 nm/Nb)10 film (b) Selected area diffraction pattern
of the (NdFeB40 nm/Nb)10 film.
the Nb layers, and this is quite different as comparing to
the polyhedron grains in NdFeB sintered magnet and
ribbons [1]. In other words, the grain growth is two-
dimensional in thin films and is limited at the thickness
direction. The various gray regions originate from the
different crystallographic orientations of the isotropi-
cally distributed grains. The faces deviate slightly from
the planar shape. Fig. 2(b) shows a selected area
diffraction pattern of the [NdFeB40 nm/Nb5 nm]10 film.
The diffraction spots of grains are not distributed
uniformly and the rings are not clear. Only one
amorphous like ring Nb(1 1 0) with some small spots
on ring can be indexed belonging to (3 1 3) plane of the
Nd2Fe14B phase. Fig. 3 shows the lattice fringe of some
grains in the [NdFeB40 nm/Nb5 nm]10 film. Diffusion
phenomenon can be visible between NdFeB grain and
Nb spacer layer with a distance about 5 nm (area A).
Two lattice parameters are estimated. One is 0.58 nm
and the other is 0.59 nm. The lattice has slightly
Fig. 1. Hysteresis loops of (NdFeBx/Nb)n films measured at
room temperature.

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J.-L. Tsai et al. / Journal of Magnetism and Magnetic Materials 239 (2002) 450–452
Fig. 4. A dark field image of the multi-layer (NdFeB200 nm/
Nb)2 film.
a consequence, two different modes happen in the
annealing process of NdFeB films. One is the nucleation
and growth of NdFeB nucleus on the amorphous type
Nb for multi-layer, the other is on the Si(1 1 1) or NdFeB
grains for single-layer.
Fig. 3. HRTEM microscopy of the (NdFeB40 nm/Nb)10 film.
distortions at the inter-diffusion area. That is due to the
formation of Nb-type second phase or Nb-rich
Nd2Fe14B grains.
Fig. 4 show the cross-sectional micrograph of a single
NdFeB layer of (NdFeB200 nm/Nb5 nm)2 film. The
average grain size observed in [NdFeB200 nm/Nb5 nm]2
film is roughly 167 nm, this is quite larger as compared
with [NdFeB40 nm/Nb5 nm]10 films. Here in multi-layer
NdFeB system, the grain size can be controlled by
thickness of the NdFeB layer, this is quite different to
the grain growth in bulk and ribbon NdFeB magnets,
which is controlled by annealing temperature only. Two
phenomena can be found in dark field image in Fig. 4.
The precipitation of the second phase NbFeB grains
(black spots, area A) with the size 4.5 nm occur within
the Nd2Fe14B grains. Another Nb-type phase (white
spots, area B) with grain size from 8 to 23 nm distributes
along the grain boundary and within grains. According
to the mechanism of nucleation and grain growth of thin
films, the nucleus generally occurred on the interface or/
and surface, and less frequent at the grain boundary. As
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