The magnetic-field-assisted melt-spinning (FAMS) Nd10.5Fe72Co11B6.5 ribbons were spun in an external magnetic field of 3 kG. The prepared ribbons were ball-milled in xylene solvent for 30 minutes followed by adding in and mixing with the high-temperature binder HTB-1 (3 wt%) for further 10 minutes. The mixed powder was dried and in-mold aligned in a magnetic field of 18 kOe and hot-compacted at 200 oC. The parameters of the FAMS ribbon preparation, ball-milling process, hot-compaction and anisotropic bonded magnets’ fabrication were optimized leading to the magnet’s energy product (BH)max of 10 MGOe. The microstructure and magnetic properties of prepared ribbons and bonded magnets will be discussed in details

Truong, Nguyen Xuan

Vuong, Nguyen Van

Vietnam Academy of Science and Technology: Journals Online

Communications in Physics, Vol. 26, No. 1 (2016), pp. 59-66
DOI:10.15625/0868-3166/26/1/7686
MAGNETIC FIELD ASSISTED MELT–SPINNING Nd-Fe-B RIBBONS AND
ANISOTROPIC BONDED NdFeB MAGNETS PREPARED THEREOF
NGUYEN XUAN TRUONG AND NGUYEN VAN VUONG†
Institute of Materials Science, Vietnam Academy of Science and Technology,
18 Hoang Quoc Viet Street, Cau Giay District, Hanoi, Vietnam
†E-mail: vuongnv@ims.vast.ac.vn
Received 08 January 2016
Accepted for publication 14 April 2016
Abstract. The magnetic-field-assisted melt-spinning (FAMS) Nd10.5Fe72Co11B6.5 ribbons were
melt-spun in an external magnetic field of 3.2 kG. The prepared ribbons were ball-milled in xylene
solvent for 30 minutes followed by adding in and mixing with the high-temperature binder HTB-1
(3 wt%) for further 10 minutes. The mixed powder was dried and in-mold aligned in a magnetic
field intensity of 18 kOe and hot-compacted at 200˚C. The parameters of the FAMS ribbon prepa-
ration, ball-milling process, hot-compaction and anisotropic bonded magnets’ fabrication were
optimized leading to the magnet’s energy product (BH)max of 10 MGOe. The microstructure and
magnetic properties of the prepared ribbons and bonded magnets will be discussed in details.
Keywords: field-assisted melt-spinning (FAMS), magnetic properties, anisotropic magnets.
Classification numbers: 61.05.cp, 75.30.Gw, 75.50.Ww , 75.60.Ej.
I. INTRODUCTION
By the conventional melt-spinning technique, the Nd2Fe14B ribbons are formed with the
microstructure where the grains are randomly oriented. These ribbons are utilized for making
isotropic bonded magnets by using an organic binder. Because of the random orientation of easy
axes, the remanent magnetization Mr of ribbons is ∼ 0.5Ms = 8 kG. It is understood that the
microstructure of aligned grains is desired to enhance the remanence which, in turn, can improve
the squareness and the energy product (BH)max of ribbons and magnets prepared thereof, if the
coercivity of ribbons and magnets should be well controlled.
Several routes have been explored with the aim of producing anisotropic NdFeB hard
nanostructured materials by HDDR treatments [1–5]. Different techniques were applied to pre-
pare anisotropic bulk magnets like the die-upset or hot deformation methods [6–9]. However,
these methods are difficult to be employed for massive production of magnets.
c©2016 Vietnam Academy of Science and Technology
60 NGUYEN XUAN TRUONG AND NGUYEN VAN VUONG
Recently, the nanocomposite hard/soft magnets are developed from bottom-up. The
nanocomposite core/shell grains are the building blocks of this kind of magnets. Normally, the
configuration of hard core/soft shell is used for the nanostructured nanocomposite hard/soft mag-
net development. We proved in Ref. [10] that this configuration can be easily prepared by the
magnetic-field-assisted melt-spinning (FAMS) technique. For Nd-Fe-B systems, during the FAMS
process, the α-Fe seeds are formed initially on the wheel surface, the hard magnetic Nd-Fe-B
grains are then grown on the seeds along the temperature gradient perpendicular to the ribbon
plane. This crystalline growth process creates (00l) texture of ribbons. The magnetic field in-
creases the volume energy of seeds and thus decreases their critical size and, consequently, de-
creases the average grain size [10–12]. These textured ribbons should become a good starting
material for preparing anisotropic magnets.
In this work, we present the results of preparing the anisotropic nanocomposite
Nd2(Fe,Co)14B/α-Fe ribbons by using the FAMS process. The FAMS ribbons consist of nanocom-
posite hard core/soft shell grains and were used for producing anisotropic bonded magnets.
II. MATERIALS AND METHODS
The pre-alloy of composition of Nd10.5Fe72Co11B6.5 was arc-melted under argon protection
atmosphere by using the starting high-purity (99.9%) metals of Nd, Fe, Co and FeB alloy. The
ingot was melted thrice to obtain a high homogeneity. In a typical procedure, the batch amount of
∼ 20g of this pre-alloy was melt-spun onto a copper wheel under 0.05 MPa argon atmosphere from
a quartz tube. The copper wheel has been equipped with the magnetic field oriented perpendicular
to the wheel surface. The field strength measured directly on the wheel surface was 3.2 kG. The
quartz tube orifice diameter was fixed at 1.0 mm, the distance between the nozzle and the wheel
surface was kept constant by 5 mm. The copper wheel speed was varied in the range of 8 – 16
m/s, which was lower than that chosen in [10]. The composition and structure were examined by
the X-ray diffraction (XRD) patterns carried out by using SIEMENS D5000 X-ray diffractometer
with Cu-Kα radiation. The phase composition was analyzed by means of Rietveld refinement
of XRD patterns for all the diffraction peaks taken in the 2θ angle ranged from 25˚ to 85˚ by
using the Crystal Impact Software for Chemists and Material Scientists. The texture of magnets
was determined by using the alignment degree of the (00l) peaks appeared on the XRD patterns
taken on the magnet surface [13]. The morphology of ribbons was studied by using the Hitachi-
S4800 field emission scanning electron microscopy (FESEM). The hysteresis loops of ribbons
were measured by the pulsed field magnetometer (PFM) under an applied field up to 60 kOe.
III. RESULTS AND DISCUSSION
The field-assisted melt-spinning ribbons
The wheel speeds ranged from 8 to 16 m/s are found to be optimal to form hard/soft
core/shell grains aligned parallel to the direction of the assisted magnetic field. This range of
wheel speeds optimizes two requests: the growth of grains with suitable sizes to maintain a high
coercivity and the alignment of easy axes of grains along the direction of the magnetic field assist-
ing the melt-spinning process.
The XRD patterns of Nd10.5Fe72Co11B6.5 ribbons are shown in Fig. 1. The magnetic texture
of the free surface of ribbons were clearly observed by the significant increase of intensities of
MAGNETIC FIELD ASSISTED MELT–SPINNING ND-FE-B RIBBONS AND ANISOTROPIC ... 61
(004), (006), (008) and (00,10) peaks. This crystallographic preferred orientation along c-axis
was caused by the shape anisotropy of Fe/Co seeds formed at the beginning step of crystal growth
under action of an assisting field [13]. The texture was enhanced in the ribbon melt-spun at the
lowest used speed v = 8 m/s and is due to the synchronal effect of two forces creating the texture,
the field assisted [10, 11, 14] and the temperature gradient oriented perpendicularly to the ribbon
plane. This gradient was established by the rapid cooling of the melt dropped on the rotating wheel
surface [14]. The formed texture helped to enhance the squareness γ of the ribbon magnetization
loops which can be, for the Nd-Fe-B based ribbons with high crystalline anisotropy, evaluated by
using the ratio between two coercivities, γ =bHc/iHc. The values of γ are 0.81, 0.75, 0.73, 0.70
and 0.71 for the FAMS ribbons melt-spun in 3.2 kG at v = 8, 10, 12, 14 and 16 m/s, respectively.
 
2θ (degree)
Fig. 1. XRD patterns of FAMS Nd10.5Fe72Co11B6.5 ribbons melt-spun at: a) v = 8 m/s,
b) v = 10 m/s, c) v = 12 m/s, d) v = 14 m/s; e) v = 16 m/s.
The mentioned texture of the prepared FAMS Nd10.5Fe72Co11B6.5 ribbons is approved by
the high ratio α between the remanent magnetization Mr{M(H = 0)} and the magnetization Ms
measured at H = 30 kOe (see Fig. 2). For the ribbons melt-spun at v = 8, 10, 12, 14 and 16 m/s,
the factor α is 0.66, 0.69, 0.73, 0.68 and 0.70, respectively. All these values are higher than the
value of 0.5 typical for the isotropic ribbons.
To analyze the phase composition of melt-spun ribbons the Rietveld refinement method
was used. The typical result is presented on Fig. 3. The refinement of the XRD pattern of ground
Nd10.5Fe72Co11B6.5 ribbon melt-spun in 3.2 kG and at v = 12 m/s reveals the coexistence of the
hard phase of Nd2Fe12.8Co1.2B (phase 96-151-1058), the soft phase Fe/Co and the non-magnetic
phase of Nd. The average grain size is about 100 nm which coincides with the SEM result attached
in the inset of Fig. 3.
62 NGUYEN XUAN TRUONG AND NGUYEN VAN VUONG
 
Fig. 2. M(H) curves of the FASM ribbons melt-spun at: a) v = 8 m/s, b) v = 10 m/s, c)
v = 12 m/s, d) v = 14 m/s, e) v = 16 m/s.
Fig. 3. The Rietveld refinement result of the FASM Nd10.5Fe72Co11B6.5 ribbon melt-
spun in 3.2 kG and at v = 12 m/s. Its FESEM graph is attached in the inset.
The soft phase fraction was estimated by the Rietveld refinement is about 7 wt%. The
existence of this soft phase is approved by the non-zero magnetization of the temperature de-
pendent magnetization curve M(T ) measured for the range T > 673 K (see Fig. 4). The abrupt
MAGNETIC FIELD ASSISTED MELT–SPINNING ND-FE-B RIBBONS AND ANISOTROPIC ... 63
decrease of M(T ) curve corresponds to the Curie temperature Tc, where the ferromagnetic to para-
magnetic phase transition is occurred. The enhancement of Tc in comparison with the value of
593 K of Nd2Fe14B phase is resulted by the partial substitution of Fe by Co in this 7 wt% of the
soft phase [15].
 
Fig. 4. Thermo-magnetic curve M(T) of the Nd10.5Fe72Co11B6.5 ribbon melt-spun at v=
12 m/s. The field of 0.5 kOe was applied during this measurement. The sample was
thermally demagnetized before the measurement.
Preparation of anisotropic magnets
The highly coercive ribbons melt-spun in 3.2 kG at the wheel speed of 12 m/s was chosen
for making anisotropic bonded magnets. The one-component high-melting temperature binder
was added to the ribbons in the weighing ratio of binder to ribbons of 3:100. The mixture was
ball-milled into the powders with the average size of ∼ 5 µm. The final powders were loaded into
the mold, aligned in 18 kOe magnetic field and hot-compacted at 200˚C under 4000 psi for 30
minutes.
The prepared magnet has the mass density of ∼ 6.0 g/cm3. The anisotropy of magnets
was checked by analyzing the XRD patterns (see Fig. 5A) taken on the magnet surface with the
normal vector perpendicular (perpendicular surface, curve a) and parallel (parallel, curve b) to the
direction of the aligning magnetic field.
The intensity enhancement of two peaks (004), (006) of the pattern a) of Fig. 5A) reveals the
texture enhancement of the parallel surface in comparison with that of the perpendicular one. The
ratio ζ between the two peaks, ζ = I(006)/I(410), was taken into account for estimating the texture
degree of crystalline alignment along the easy axis (00l). For the fully isotropic Nd2Fe12.8Co1.2B
powder sample (see Fig. 5B), ζ equals 0.34 which is 5.3 times lower than the value of ζ = 1.81
64 NGUYEN XUAN TRUONG AND NGUYEN VAN VUONG
calculated from the pattern b of Fig. 5A for the parallel surface, which, in turn, is about 2 times
larger than that ζ calculated from the curve a of Fig. 5A for the perpendicular surface.
 
 
Fig. 5. A) XRD pattern of the magnet taken on the: a) perpendicular surface, b) par-
allel surface; B) Standard calculated XRD pattern for the fully isotropic powder of
Nd2Fe12.8Co1.2B (phase 96-151-1058).
The magnet’s anisotropy observed in Fig. 5A is supported by the discrepancy of the mag-
net’s demagnetization curves measured with the measuring field oriented parallel and perpendicu-
lar to the alignment direction. These two curves are presented in Fig. 6. For these measurements,
MAGNETIC FIELD ASSISTED MELT–SPINNING ND-FE-B RIBBONS AND ANISOTROPIC ... 65
the samples were cut from the magnet with the aspect ratio larger than 5 to avoid the demagneti-
zation effect.
 
Fig. 6. The magnetization loops of the Nd10.5Fe72Co11B6.5 magnet measured in the field
oriented perpendicular (•) and parallel () to the alignment direction.
The remanent magnetization Mr of the parallel sample is 8.8 kG which is in 1.2 kG larger
than that of the perpendicular sample. The quantitative alignment degree of the magnet can be
described by the average misalignment angle, ψ = arctan[2Mr⊥/Mr‖], where Mr⊥ and Mr‖ are
the remanences measured for the perpendicular and parallel samples, respectively [16]. Typically,
ψ = 0 for the perfect alignment and 90˚ for the non-alignment. In our case, ψ equals ∼ 56˚,
corresponding to the moderate alignment. This alignment degree allows the intrinsic coercivity
iHc =7.0 kOe and (BH)max = 10 MGOe.
IV. CONCLUSIONS
High-performance Field-Assisted Melt-Spining (FAMS) Nd10.5Fe72Co11B6.5 ribbons were
melt-spun in 3 kG at the optimal wheel speed of 12 m/s. The nanocomposite microstructure of rib-
bons was proved by the XRD, FESEM, M(H) and M(T ) measurements. These high-performance
ribbons were used to produce anisotropic bonded magnets. The good alignment degree of prepared
magnets was revealed by the XRD patterns measured on the perpendicular and parallel surfaces
of magnets. The moderate misalignment angle ψ ∼ 56˚ was determined by using the remanences
measured for the perpendicular and parallel samples. The best value of the energy product (BH)max
of the prepared magnets is 10 MGOe, which meets the technical standards required for bonded
magnet applications. The investigation is being carried out for further enhancement of the perfor-
mance of anisotropic magnets.
66 NGUYEN XUAN TRUONG AND NGUYEN VAN VUONG
ACKNOWLEDGEMENTS
This research is funded by the Key National Laboratory of the Institute of Materials Sci-
ence, under Grand CST02.14. The authors also gratefully thank Holger Putz, who created match
version 3.0.
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Magnetic field assisted melt–spinning Nd-Fe-B ribbons and anisotropic bonded NdFeB magnets prepared thereof

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Magnetic field assisted melt–spinning Nd-Fe-B ribbons and anisotropic bonded NdFeB magnets prepared thereof

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