We observed that, at annealing temperatures below 670◦C, there is crystallization of soft phase Fe3B and a small amount of hard phase Nd2Fe14B.. At annealing temperatures above 670◦C, cr
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Crystalline evolution and large coercivity in Dy-doped (Nd,Dy)2Fe14B/α-Fe nanocomposite magnets
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2007 J Phys D: Appl Phys 40 119
(http://iopscience.iop.org/0022-3727/40/1/001)
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Trang 2J Phys D: Appl Phys 40 (2007) 119–122 doi:10.1088/0022-3727/40/1/001
Crystalline evolution and large coercivity
nanocomposite magnets
N D The1,2, N Q Hoa1,3, S K Oh3, S C Yu3, H D Anh1, L V Vu1and
N Chau1,4
1 Center for Materials Science, College of Science, Vietnam National University Hanoi,
334 Nguyen Trai Road, Hanoi, Vietnam
2 Department at Physics and Astronomy, University of Glasgow, Glasgow C12 8QQ, UK
3 Department of Physics, Chungbuk National University, 361-763 Cheongju, Korea
E-mail: chau@cms.edu.vn
Received 6 April 2006, in final form 9 November 2006
Published 15 December 2006
Online atstacks.iop.org/JPhysD/40/119
Abstract
Nanocomposite hard magnetic materials (Nd,Dy)4.5Fe77.5B18(No 1) and
(Nd,Dy)4.5Fe76B18Nb1.2Cu0.3(No 2) have been prepared by crystallizing
amorphous ribbons, fabricated by single roll melt-spinning The evolution
of a multiphase structure was monitored by an x-ray diffractometer and by
thermomagnetic measurement We observed that, at annealing temperatures
below 670◦C, there is crystallization of soft phase Fe3B and a small amount
of hard phase Nd2Fe14B At annealing temperatures above 670◦C,
crystallization of α-Fe and probably Dy2Fe14B phases with large
magnetocrystalline anisotropy led to a drastic enhancement in the hard
magnetic properties of the materials The maximum value of HCis found to
be 4.2 kOe for sample No 1 For sample No 2, with co-doping of Nb and
Cu, nanostructure refinement yields a strong enhancement in exchange
coupling between the component phases Thereby, we obtained high
reduced-remanence of 0.78, high remanence of 1.15 and a high (BH)max
value up to 16.2 MGOe
(Some figures in this article are in colour only in the electronic version)
1 Introduction
Nanocomposite exchange-spring magnets provide an
alterna-tive way of producing high remanence magnetic materials,
which can be used to make resin bonded magnets Additional
merit is in the cost reduction owing to the low consumption
of rare-earth elements Nanocomposite magnets have been
studied for compositions like (Pr,Nd)2Fe14B/Fe3B [1,2] and
(Pr,Nd)2Fe14B/α-Fe(Co) [3 8] Some of the recently reported
new kind of nanocomposite magnets are self-assembled FePt
[9,10], melt-spun nanocomposite magnets FePtB [11,12]
and nanocomposite (Nd,Dy)(Fe,Co,Nb,B)5.5 /α-Fe multilayer
magnets [13] So far, high-performance nanocomposite
mag-nets have not been obtained with low rare-earth content because
4 Author to whom any correspondence should be addressed.
high coercivity has not been reached Nonetheless, the effect
of doping elements on the microstructure and magnetic proper-ties of nanocomposite magnets has shown something remark-able [8,12–15] With a small amount of Cr and Co doping, a special microstructure, namely the cellular structure, was
ob-served in α-Fe(Co)/Nd2Fe14B nanocomposite magnets [8] In fact, the formation of a cellular structure resulted in high shape anisotropy of nano-grains, which contributes to the total mag-netic anisotropy of the material Thereby, a high-performance nanocomposite magnet was obtained with a very low concen-tration of Nd (4.5 at.%) Hence, the role played by the cellular structure could be an important ingredient that should be taken into account in producing high-performance magnets with low rare-earth content Therefore, in this article, we investigate fur-ther the effect of substituting a small amount of Dy for Nd and the role of Nb and Cu in microstructural refinement
Trang 32 Experimental
The amorphous precursors with the composition of (Nd,Dy)4.5
Fe77.5B18(No 1) and (Nd,Dy)4.5Fe75.5B18.5Nb1.2Cu0.3(No 2)
have been fabricated by the rapid-quenching technique in an Ar
atmosphere in an Edmund Buehler melt-spinner with a linear
speed of 30 m s−1 Subsequently, we put the amorphous flakes
in a quartz tube, evacuated to a high vacuum state, then filled
the tube with highly purified Ar and finally annealed them
isothermally at appropriate temperatures
The crystalline evolution of as-cast samples was
monitored using a differential scanning calorimeter (TA
Instruments model 2960) The structure of the samples
was examined by an x-ray diffractometer (Bruker model
D5005) with Cu–Kα radiation Microstructural observation
was carried out by a scanning electron microscope (JEOL
model 5410 LV) Magnetic characteristics were measured by
a vibrating sample magnetometer (Model DMS 880) with
the maximum applied field of 13.5 kOe, and demagnetization
curves were measured using a hysteresisgrapher (Walker
model AMH 25) The demagnetizing factor of the specimens
was approximately corrected
3 Results and discussion
Figure1(a) displays differential scanning calorimetry (DSC)
results for amorphous ribbons with a heating rate of
20◦C min−1 The curves exhibit three clearly exothermal
peaks, which are related to the formation of a magnetic phase
in the thermal process According to Li et al [16], the
crystalline evolution of (Nd,Dy)FeB amorphous ribbons could
be expressed as:
Amorphous→ Amorphous’ + o-Fe3B→ Amorphous” +
t-Fe3B + (Nd,Dy)2Fe14B→ t-Fe3B + (Nd,Dy)2Fe14B + α-Fe.
However, structural examination by an x-ray
diffractome-ter (XRD (figure1(b))) shows a different result It can be
described as follows:
• The first peak corresponds to the crystallization of the
Fe3B and Nd2Fe14B phases, which is similar to that of
other NdFeB-based amorphous ribbons [5,8];
• The second peak, occurring at a slightly higher
temperature, is related to the formation of α-Fe, and seems
to be a (Nd,Dy)2Fe14B phase (suggestion);
• In sample No 2, the exothermal peaks shift to lower
temperature (see figure1) because of the doping of Cu with
low melting temperature and a high diffusion coefficient
as the nucleation is accompanied in crystallization [17]
A multiphase structure is also confirmed by measuring
the thermomagnetic curve of the annealed samples (see
figure2) Obviously, the curves exhibit Curie temperatures
of the Nd2Fe14B and Fe3B phases As seen in figure 2,
the thermomagnetic curve of the sample annealed at 650◦C,
which is lower than the temperature at the second exothermal
peak, exhibits Curie temperatures of the Nd2Fe14B and Fe3B
phases within the measuring temperature range Meanwhile,
the Curie temperature of a (Nd,Dy)2Fe14B phase can be found
in the thermomagnetic curve of the sample annealed at 670◦C,
which is the onset crystallization temperature of the second
exothermal peak
Figure 1 DSC curves of as-cast samples with the heating rate of
20◦C min−1measured in flowing Ar gas (a) and XRD results for sample No 1 at different annealing temperatures (b).
Figure 2 Temperature dependence of magnetization of annealed
sample No 1 measured in 100 Oe applied field.
Therefore, we suggest that the crystalline evolution process in our materials is as follows:
Amorphous→ Amorphous’ + Fe3B→ Amorphous” +
Fe3B + Nd2Fe14B→ Amorphous”’ + α-Fe + (Nd,Dy)2Fe14B
→ α-Fe + Fe3B + (Nd,Dy)2Fe14B
120
Trang 4Figure 3 Magnetic parameters as a function of annealing
temperature for sample No 1 (annealing time of 5 min).
Figure 4 Annealing time dependence of magnetic parameters for
sample No 2 after annealing in 5 min.
Figures3and 4show annealing temperature dependence
of magnetic characteristics of the samples derived from
VSM First of all, coercivity and remanence of both samples
gradually increase with annealing temperature and after that
they drastically increase to large values This could be
explained as follows:
• At an annealing temperature, which is lower than the
temperature of the second exothermal peak, there is the
crystallization of the Fe3B phase and a small amount of
the Nd2Fe14B phase The volume fraction of Nd2Fe14B
increases, leading to an increase in coercivity;
• As the annealing temperature increases to the temperature
at the second exothermal peak, Dy atoms replace the Nd
ones in the crystal lattice of the 2: 14: 1 phase to form
the (Nd,Dy)2Fe14B phase In the Dy2Fe14B, which has
twice larger magnetocrystalline anisotropy than that of
Nd2Fe14B [18], there is a dramatic increase in coercivity
(see inset in figure2) Besides the increase in the volume
fraction of hard phases, a strong exchange coupling
between the soft and the hard phases leads to an increase in
Table 1 Magnetic parameters for sample No 1 at differing
annealing temperatures.
Ta(◦C) Mr(emu g−1) Mr/Mmax B Hc(Oe) (BH)max (MGOe)
Table 2 Magnetic parameters for sample No 2 at different
annealing temperatures.
Ta(◦C) Mr(emu g−1) Mr/Mmax B Hc(Oe) (BH)max (MGOe)
remanence as well as reduced remanence (see figures3and
4and tables1and2) The value 4.2 kOe for No 1 is quite
a high achievement obtained so far for nanocomposite magnets with low rare-earth contents
Microstructural observation was performed for the annealed samples Figure 5 is a typical example for this measurement We can say that, in sample No 2, the grain size is always smaller than that of sample No 1 For example,
in figure5, the average size of nano-crystallites is 45 nm for sample No 1 (after optimally annealing) whereas this value
is 27 nm for sample No 2 In sample No 2, there is a co-doping of Nb and Cu This produces a well-known effect
in that Cu promotes nucleation in the crystallization process, and Nb plays a role in retarding the growth of the crystal grains [5,17] Copper atoms form a high density of clusters prior to the crystallization reaction, which serve as nucleation sites for the bcc-Fe primary crystals Niobium added in combination with Cu induces the formation of the Nd2Fe14B and metastable phases in the second stage of the crystallization process by partitioning in it Because two phases are formed from the remaining amorphous phase, the crystal grain size
in the final microstructure becomes smaller than that of the specimen without Nb and Cu So, the co-doping of Cu and
Nb creates a grain refinement, which enhances the exchange coupling between magnetically hard and soft nano-grains (see table2) Enhancement of exchange coupling causes a highly reduced remanence up to 0.78 (for sample No 2) at the optimal annealing condition
4 Conclusion
The crystalline evolution, and magnetic properties of (Nd,Dy)2Fe14B/α-Fe nanocomposite magnets with low
rare-earth contents have been investigated A small amount of Dy substitution for Nd leads to an enhancement in the coercivity of the materials, up to 4.2 kOe This value is much larger than that
of similar compositions reported previously by other authors [16] The effect of Cu/Nb co-doping on microstructural refinement is discussed
121
Trang 5Figure 5 SEM micrographs of optimally annealed samples.
No 1 (a) and No 2 (b).
Acknowledgments
Research at Center for Materials Science, VNU, is financially
supported by the Vietnamese Fundamental Research Program
for Natural Sciences (Project 406506), and research at Chungbuk National University was supported by the Korean Science and Engineering Foundation through the Research Center for Advanced Magnetic Materials at Chungnam National University
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