Structure, thermal stability and magnetostrictive properties of PrFex (1.5 ≤ x ≤ 3.0) alloys

Article abstract of DOI:10.1016/j.jallcom.2006.10.013

Polycrystalline alloys PrFe_x (x = 1.5, x = 1.8, x = 1.9, x = 2.0, x = 3.0) have been prepared by arc-melting and subsequent high-pressure synthesis method. Their structure, thermal stability, easy magnetization direction and magnetostriction are investigated. Almost single Laves phase (MgCu₂-type) is found in sample with x = 1.9. Samples with x = 2.0, x = 3.0 consist of PrFe_1.9 phase with MgCu₂-type cubic structure and α-Fe phase. The Curie temperature (238 °C) and the decomposed temperature (408 °C) of PrFe_1.9 alloys are derived from M-T curves. The easy magnetization direction of PrFe_1.9 Laves phase lies along [1 1 1] axis at room temperature which is verified by Mo?ssbauer spectrum. Furthermore, PrFe_1.9 alloy shows a large magnetostriction λ_|| - λ_⊥ = 1081 × 10^-6 at a magnetic field of 13 kOe.

Full text of DOI:10.1016/j.jallcom.2006.10.013

Journal of Alloys and Compounds 443 (2007) 11–14
Structure, thermal stability and magnetostrictive properties
of PrFex (1.5 ≤ x ≤ 3.0) alloys
Y.G. Shi, S.L. Tang∗, Y.J. Huang, B. Nie, B. Qian, L.Y. Lv, Y.W. Du
National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China
Received 13 August 2006; received in revised form 29 September 2006; accepted 2 October 2006
Available online 13 November 2006
Abstract
Polycrystalline alloys PrFex (x = 1.5, x = 1.8, x = 1.9, x = 2.0, x = 3.0) have been prepared by arc-melting and subsequent high-pressure synthesis
method. Their structure, thermal stability, easy magnetization direction and magnetostriction are investigated. Almost single Laves phase (MgCu2-
type) is found in sample with x = 1.9. Samples with x = 2.0, x = 3.0 consist of PrFe1.9 phase with MgCu2-type cubic structure and ␣-Fe phase. The
Curie temperature (238 ◦C) and the decomposed temperature (408 ◦C) of PrFe1.9 alloys are derived from M–T curves. The easy magnetization
direction of PrFe1.9 Laves phase lies along [1 1 1] axis at room temperature which is verified by Mo¨ssbauer spectrum. Furthermore, PrFe1.9 alloy
shows a large magnetostriction λ|| – λ⊥ = 1081 × 10−6 at a magnetic field of 13 kOe.
© 2006 Elsevier B.V. All rights reserved.
PACS: 75.50.Bb; 75.80.+q
Keywords: Rare earth compounds; High-pressure; Magnetostriction
1. Introduction
and systemically study its structure and magnetic properties. It
has been reported that PrFe2 alloy which contained an amount
of second phase but showed a remarkable magnetostriction con-
stant λ − λ = 1030 × 10−6 at room temperature at a magnetic
An excellent magnetostritive material should have low mag-
netocrystalline anisotropy and large magnetostriction at low
magnetic fields. Although the intermetallic compounds TbFe2
and DyFe2 have large magnetostriction at room temperature,
they are unsuitable as practical materials in magnetostrictive
devices and for applications because these two compounds have
high anisotropies. According to the single-ion model [1], PrFe2
should have a larger magnetostriction and much lower magne-
tocrystalline anisotropy than TbFe2 and DyFe2. Furthermore, a
magnetostrictive alloy with high-Pr content should have a good
practical prospect because Pr is much cheaper than Tb or Dy.
Thus, much effort has been paid to enhance the Pr content in
magnetostrictive materials under normal atmosphere in the past
years [2–6]. As an example, a single phase cannot be obtained
in (Pr,Tb)Fe2 system when the Pr content is over 25 at.% for
rare earths [4]. Up to now, an effective method to prepare high-
Pr content magnetostrictive materials is still desired. So, it is
interesting to synthesize PrFe2 with single cubic Laves phase
||
⊥
field of 20 kOe [7]. It is possible that the magnetostriction may
be improved if PrFe2 single phase can be obtained.
In this paper, polycrystalline alloys PrFex (x = 1.5, x = 1.8,
x = 1.9, x = 2.0, x = 3.0) were prepared by arc-melting and subse-
quent high-pressure synthesis method. Their structures, thermal
stability, easy magnetic direction and magnetostriction have
been investigated.
2. Experimental details
Ingots with stoichiometric composition of PrFex (x = 1.5, x = 1.8, x = 1.9,
x = 2.0, x = 3.0) were prepared by melting the constituent metals in a magneto-
controlled arc furnace under a high-purity argon atmosphere. The purities of
Pr and Fe are 99.9%, 99.8%, respectively. The as-cast ingots (about 10 mm in
diameter and 2 mm in thickness) were wrapped in tantalum foils and pressed into
a graphite pipe heater with the shape of cylinder. The heater was then pressed to
6 GPa by a Hall-type hexahedral anvil press and heated to different temperatures
and for various time. Conventional X-ray diffraction (XRD) analysis was carried
out using Cu K␣ radiation with a Rigaku D/Max-gA diffractrometer. The actual
composition of the samples was analyzed by the inductively coupled plasma
(ICP) spectrometry. The Curie temperature and the decomposed temperature
∗
Corresponding author. Tel.: +86 25 835 938 17; fax: +86 25 835 955 35.
E-mail address: tangsl@nju.edu.cn (S.L. Tang).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.10.013

12
Y.G. Shi et al. / Journal of Alloys and Compounds 443 (2007) 11–14
of the sample were derived from M–T curves measured by a vibrating samples
magnetometer at field of 2 kOe. The 57Fe Mo¨ssbauer spectrum was collected
on a constant accelerated spectrometer with the transmission geometry at room
temperature. The source is 57Co in Pd matrix with an activity about 25 mCi. The
spectrum was calibrated with a standard ␣-Fe foil and analyzed by Lorentzian
lines in 256 channels using software Klencsar and MossWinn [8]. The linear
magnetostriction was measured using standard strain-gauge technique in direc-
tions parallel (λ ) or perpendicular (λ ) to applied magnetic fields up to 13 kOe
||
⊥
at room temperature.
3. Results and discussion
Fig. 1 shows the XRD patterns for PrFex with the different Fe
content. The samples were synthesized at 900 ◦C and 6 GPa for
30 min. It is found that samples with x = 1.5 and x = 1.8 comprise
MgCu2-type Laves cubic phase and a little amount of hcp-Pr
phase. The sample exhibits almost a single Laves phase when x
isupto1.9. TheICPspectrometryanalysisshowstheactualatom
ratio of Fe to Pr is about 1:1.9, which is in accordance with the
starting stoichiometric composition. When x increases to 2.0 and
3.0, the samples consist of PrFe1.9 with MgCu2-type cubic phase
and ␣-Fe phase. The lattice parameters of cubic laves phase
derived from XRD patterns of the samples with different Fe con-
Fig. 2. XRD patterns of PrFe1.9 alloys which were synthesized at 6 GPa and
different temperature for different time. (a) Synthesized at 600 ◦C for 30 min.
(b), (c), (d) and (e) are corresponding to the samples synthesized at 900 ◦C for
5 min, 15 min, 60 min and 90 min.
˚
˚
˚
tents are 7.532 A (x = 1.5), 7.488 A (x = 1.8), 7.479 A (x = 1.9),
˚
˚
7.468 A (x = 2.0) and 7.453 A (x = 3.0). The lattice parameters
become smaller as increasing Fe content in the alloys, therefore
we can deduce that Pr–Fe cubic Laves phase can exist within a
narrow composition range near x = 1.9. As we know, it is difficult
to obtain ␣-Fe phase coexisting with MgCu2-type cubic phase
in the RE-Fe (such as, Tb-Fe, Dy-Fe or Sm-Fe) alloys because
REFe3 and RE2Fe17 phases would appear if Fe content exceeds
67 at.% in alloys. And almost all the RE-Fe compounds, such
as REFe2, REFe3 and RE2F17 phases are mechanically brittle.
Therefore, PrFex alloys with x > 1.9, such as x = 3.0, that contain
a little amount of ␣-Fe phase, not only can keep a large mag-
netostriction (as shown in Fig. 6) but also can be much easier
machinable in comparison with brittle RE-Fe compounds with
single phase.
Fig. 2 shows XRD patterns of PrFe1.9 which were synthesized
at 6 GPa at different temperature for different time. As is shown
in Fig. 2, Laves phase can be obtained by annealing at 900 ◦C
for the time longer than 5 min and it is easier to obtain a single
Laves phase when annealed for the longer time. The sample
synthesized at 600 ◦C for 30 min presents the Laves phase and a
littleamountofhcp-Prphase. Fromresultsabove, wecanseethat
a single cubic Laves phase for Pr–Fe alloy can be synthesized by
the high-pressure synthesis method. It is thought that the atomic
size plays an important role in the synthesis of the Laves phase
compounds. For the RE-Fe2, the ideal radius ratio of RE and Fe
for the formation of a Laves phase is 1.225 [9]. Since the radius
ratio of Pr and Fe is 1.333, which is much larger than 1.225, it
is difficult to synthesis Pr–Fe cubic Laves phases in an ambient
atmosphere and it can only be synthesized by a high-pressure
method.
The temperature dependences of magnetizations of the alloys
were measured in a low magnetic field of 2 kOe to determine
the Curie temperature and the stability of PrFe1.9 alloys. As an
example, the thermal magnetic curve of PrFe1.9 which was syn-
thesized at 900 ◦C for 30 min is shown in Fig. 3. The rate of
heating and cooling is 10 ◦C/min. As the temperature increases
from room temperature, the curve is smooth and there is no trace
of Pr2Fe17 phase whose Curie temperature is near room temper-
ature. With the temperature increasing the magnetic moment of
the sample decreases sharply near 238 ◦C at which is the Curie
temperature of PrFe1.9 alloy. The following decomposed process
can be divided into two different periods: with the temperature
increasing from 408 ◦C to 512 ◦C, the sample is decomposing
into Pr and ␣-Fe which leads to the increase of the magnetic
Fig. 1. XRD patterns of PrFex with different x. All the alloys were synthesized
at 6 GPa and 900 ◦C for 30 min.

Y.G. Shi et al. / Journal of Alloys and Compounds 443 (2007) 11–14
13
Fig. 5. Mo¨ssbauer spectrum at room temperature for PrFe1.9 alloy which was
synthesized at 6 GPa and 900 ◦C for 30 min.
Fig. 3. Magnetization of PrFe1.9 synthesized at 900 ◦C for 30 min as a function
of temperature in the magnetic field 2 kOe.
relative areas were confined by 1:3 and a static Hamilton model
with hyperfine interaction and quadrupole splitting interaction
was used to obtain the possible relative orientation of hyperfine
field with Vzz. As PrFe1.9 has a cubic Laves phase structure,
the Fe atoms occupy only one crystallographic site with space
moment; with further increasing temperature from 512 ◦C, the
magnetic moment decreases because Pr and Fe is reacting with
each other and forming Pr2Fe17. These results are also illustrated
by XRD pattern in Fig. 4. The samples synthesized at 900 ◦C for
30 min were wrapped in tantalum foils and vacuum annealed in
sealed quartz capsules at 450 ◦C and at 550 ◦C for 2 h. It can be
seen that the sample annealed at 450 ◦C mainly contains hcp-Pr
and ␣-Fe phase and the one annealed at 550 ◦C mainly contains
¯
group of 3m, and the asymmetry factor, η = (Vxx − Vyy)/Vzz, was
confined as zero, where Vxx, Vyy, and Vzz denote the diagonal
components of the EFG tensor. The relative angles of hyper-
fine fields to Vzz (β) were calculated as 0◦ and 74.9◦ for the two
inequivalent sitesrespectively, whicharecloseto theidealvalues
of 0◦ and 70.5◦ and is in good agreement with the expected easy
magnetization direction of [1 1 1]. The corresponding parame-
ters are listed in Table 1.
hcp-Pr and Pr2Fe17 phase.
57
¨
Fe Mossbauer spectrum for the PrFe1.9 alloy was recorded
at room temperature, as shown in Fig. 5. The profile of the mea-
sured points is similar to that of cubic Laves phase compounds of
Ho0.6Tb0.4Fe2 (above 150 K), which indicates that the easy mag-
netization direction may parallel to the [1 1 1] direction [10]. The
assumption was examined by the resolving the gross spectrum
into two sextets assigned to the two inequivalent types of iron
sites due to different orientation of the spin direction with respect
to the principle axis (Vzz) of electric field gradient (EFG). The
The room-temperature magnetostriction of the PrFex alloys
with different Fe content is shown in Fig. 6. All the sam-
ples were synthesized at 6 GPa and 900 ◦C for 30 min. It
can be seen that PrFe1.9 alloy shows a large magnetostric-
tion value (λ − λ = 1081 × 10−6) at 13 kOe. With decreas-
||
⊥
ing Fe content, the magnetostrictions of samples (x = 1.5 and
x = 1.8) decrease owing to the second phase Pr in the sam-
ples. With increasing Fe content, the magnetostriction of the
sample is not dropped dramatically. The sample shows a large
magnetostriction λ − λ = 900 × 10−6 at 13 kOe even the Fe
||
⊥
content increases to x = 3.0. As we know, the high-pressure
technique is also an effect method to fabricate nanocrystalline
and nanocomposite system [11], it is hopeful that PrFe1.9/␣-
Fe nanocrystalline can be prepared by combining melt-spinnng
and high-pressure technique and the magnetostriction of the
nanocrystalline Pr–Fe alloys at low magnetic fields is hopeful
Table 1
Mo¨ssbauer parameters of PrFe1.9
Parameters
CS
Vzz
Hhf
β (◦)
Line width
Ratio (%)
Site 1 −0.06(0)
Site 2 −0.08(5)
1.3(2)
7.5(4)
17.4(3)
13.7(6)
0
74.9
0.40(4)
0.36(6)
75
25
The CS, linewidth are given in mm/s, Hhf is given in T, Vzz is given in 1021 V/m2,
CS is relative the spectrum center of ␣-Fe.
Fig. 4. XRDpatternsofPrFe1.9 alloyswhichwereannealedat450 ◦Cand550 ◦C
for 2 h.

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Y.G. Shi et al. / Journal of Alloys and Compounds 443 (2007) 11–14
a large magnetostriction λ − λ = 1081 × 10−6 at 13 kOe at
||
⊥
room temperature. Containing cheaper rare-earth Pr, PrFe1.9
alloyshouldbeapotentialcandidateforuseasamagnetostrictive
material.
Acknowledgements
Project BK2006125 supported by the Natural Sciences Foun-
dation of Jiangsu and Project 2005CB623605 supported by the
National Key Project of Fundamental Research of China.
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PrFex (x ≥ 1.9) alloys with Laves phase coexisting with ␣-Fe
phase have been synthesized by arc-melting and subsequent
high-pressure synthesis method. PrFe1.9 with single Laves phase
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