structure and magnetic properties of arc-melted r2fe16sicx (x = 0∼2.0) compounds with r = gd, y...
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Pergamon
Solid State Communications, Vol. 95, No. 5, pp. 301-306, 1995 Ekvier science Ltd
Printed in Great Britain
~38-1o98(95)00263-4 0038-1098/95 S9.Xh.00
STRUCTURE AND MAGNETIC PROPERTIES OF ARC-MELTED R2Fq&iCx (I = a-2.0)
COMPOUNDS WITH R = Gd, Y AND Sm
Bing Liang, Bao-gen Shen, Zhao-hua Cheng, Jun-xian Zhang. Hua-yang Gong,
Fang-wei Wang, Shao-ying Zhang and Wen-shan Zhan
State Key Laboratory of Magnetism, Institute of Physics, Chinese Academy of Sciences,
P.O.Box 603, Beijing 100080, People’s Republic of China
(Received 20 January 1995; accepted 9 March 1995 by Z Can)
The structural and magnetic properties of compounds in the series RzFeuSiC, (R=Gd,
Y and Sm) have been investigated by X-ray diffraction (Mu)) and magnetization
measurements. Samples with x= 0, 0.5, 1.0, 1.5 and 2.0 were prepared by arc-melting.
XRD patterns indicate that RzFeuSiC, are single phase with the hexagonal ThzNir,-type
or the rhombohedral ThzZnl7-type structure except for SmPer&iCz.o , which contains a
small amount of a-Fe as a secondary phase. Both the Curie temperature T, and unit cell
volume v are found to increase monotonically with increasing C concentration x. The
saturation magnetization is almost independent of C content. Samples of Sm2Fe,aSiC,
with x20.5 exhibit an easy c-axis anisotropy at room temperature. The anisotropy field
HA increases with increasing x from 65 kOe for x=0.5 to about 1 IO kOe for x=1 5. The
Si substitution in the 2: 17-type carbides is found to not only help the formation of single
phase, but also enhance the anisotropy of the Sm sublattice.
Llntroduction
R2Fer7 intermetallic compounds have not been used as
permanent magnets due to their low Curie temperature and
room-temperature planar anisotropy. A lot of attempts
have dealt with improving their magnetic properties by
either substituting a third element such as Co, Al, Si and
Gau4’ or introducing the interstitial atoms such as H N
and Ct’-“t. However , the carbon content that is allowed in
arc-melted R2Fe& is much lower, and the higher-carbon
R2Fe& compounds prepared by gas-solid reaction are
unstable at high temperatures. Shen et al.“2-‘J’ have
were stable at high temperatures at least up to IOOO”C. But
a synthesis of single-phase compounds Sm2Fe& with
xBl.5 was still difficult. Recently, it was discovered that
the substitution of Ga, Al and Sin’-‘*’ for Fe in the
R2Fe& compounds could help the formation of rare-
earth-Fe compounds with high C concentration. However,
a larger number of third nonmagnetic element normally has
a detrimental effect on both the saturation magnetization
and the Curie temperature. For the purpose of application,
it is necessary to reduce substituted amount of
nonmagnetic elements. In this paper, we studied the
formation and magnetic properties of arc-melted
successtilly prepared the high-carbon R2Fellcx compounds R2FeuSiC, ( x = O-2.0 ) compounds with R = Gd, Y and
by melt spinning method, and found that these carbides Sm.
301
302 PROPERTIES OF ARC-MELTED &Fe,,SiC, Vol. 95, No, 5
Z.Experimental 5 I Gd2Fer6SiC2
Alloys with composition RzFe&iC, (R=Gd, Y and Sm;
x=0,0.5. I .O. I .5 and 2.0) were prepared by arc-melting raw
materials of at least 99.9% purity under highly purified
argon atmosphere. The ingots were remelted at least three
times to ensure their homogeneity. As-cast alloys were
sealed in steel tubes and heat treated at temperatures close
to l400K for four days and then rapidly cooled to room
temperature. Powder X-ray diffraction (XRD) with Cu-k,
radiation was used to determine phase purity, crystal
structure and lattice parameters. The Curie temperature
was obtained from the temperature dependence of
magnetization measured by a vibrating sample
magnetometer in a magnetic field of 700 Oe. The
saturation magnetization was measured by an extracting
sample magnetometer in a field of 65 kOe. Oriented
powder samples of cylindrical shape for anisotropy field
measurements were prepared by mixing the powder with
epoxy resin and then aligning in a field of IO kOe at room
temperature. The anisotropy field was determined from
magnetization curves along and perpendicular to the
orientation direction by using the extracting sample
magnetometer with a magnetic field of up to 65 kOe.
3.Results and discussion
X-ray diffraction (XRD) measurement indicates that
the annealed RzFe&C, (R = Gd, Y and Sm) samples are
single-phase with the rhombohedral Th2Zni,-type or the
hexagonal ThzNirT-type structure except for Sm2Fei&iCz.o
which contains a small amount of a-Fe phase. Figure I
shows the XRD patterns of arc-melted RIFer&iCzo
compounds with R = Gd, Y and Sm. It has been shown
previously that the arc-melted RzFer&z.o ingots have a
multiphase structure with the rare-earth carbides, the 2:17
phase and a drastical a-Fe phase.l’6i Even if these
lWer7C2.o ingots are annealed at high temperature, the
single-phase carbides are still not formed. It is quite evident
that the substitution of Si for Fe, like Gal’5.‘“l and A11’7.‘81,
I YzFei&iCz
I Sm2Fer&iC2
L r+” &
30 40 50 t I
28 (deg.)
Fig. I. X-ray difl’raction patterns of arc-melted and heat-
treated R2Fe16SiC2 with R = Gd ,Y and Sm.
in the R2Fe& compounds with higher carbon
concentration can stabilize the crystal structure of the
2: l7-type carbides.
The lattice parameters (I and c and unit cell volumes v of
the RzFerhSiC, compounds derived from the X-ray,
diffraction analysis are presented in Table I. There is no
structure change in R2Fe&iC, compounds for R = Gd and
Sm, but for the YzFenSiC, compounds, a structural
transformation takes place from hexagonal Th2Nir7-type to
rhombohedral Th2Znr7-type with increasing carbon
concentration. This behavior is similar to those found
earlier by Sun et al 1’91 and Kou et al f2”l. For x=1, both
structure types may coexist, as shown in Table I. The
addition of the interstitial carbon atoms is found to expand
the unit cell. Figure 2 shows the unit-cell volume of
R2Fe&iC, compounds as a function of carbon
Vol. 95, No. 5 PROPERTIES OF ARC-MELTED &Fe,,SiC,
Table 1
The lattice parameters a and c. and unit cell volumes I’ of the R2Fe&iC, compounds with x = Gd, Y
and Sm.
GdsFet&iC, Y2Fet6SiC, SmaFet&C, 1
X u(A) c(A) v(A-) a(A) c(A) V(P) u(A) 3
4) 4)
0 8.524 12.422 781.6 8.454 8.296 513.5 8.554 12.419 787.1
0.5 8.566 12.429 789.7 8.479 8.303 517.0 8.587 12.424 793.2
1.0 8.592 12.436 795.1 8.508 8.312 521.1 8.611 12.434 798.4
8.578 12.420 791.4
1.5 8.638 12.446 804.2 8.608 12.436 798.0 8.63 1 12.442 802.6
2.0 8.658 12.455 808.6 8.628 12.463 803.5 8.650 12.451 806.7
concentration, For the sake of comparison with the
rhombohedral volumes, we have multiplied the hexagonal
volumes by 3/2. The unit cell volumes increase
monotonically with increasing C content. The silicon
substitution leads to a slight contraction of the unit cell
The saturation magnetization M, at 1.5 K and the Curie
temperature T, for R2FeiGSiC, compounds with R = Gd
and Y are listed in Table II, and the T, for SmPei&C, are
presented in Table III. The values of M, are found to be
almost independent of carbon concentration x, whereas the
Curie temperature T, increases monotonically with
increasing carbon content. Figure 3 shows the C-
concentration dependence of the Curie temperature of the
samples investigated here together with those of the
Y2Fei7C,[‘4.‘9’ and Gd2FeirC~‘*’ compounds for
comparison. For Gd2FeleSiC2.0 , Y2Fe16SiC2.0 and
SmzFer&SiCz.,, , the T, is 104 K, 162 K and 123 K higher
L
800 -
o R=Sm -
v R=Gd
l R=Y 770 1 I I
0.0 0.5 1.0 1.5 2.0
X
Fig.2. The unit cell volume of RzFe&iC, compounds with
R = Gd .Y and Sm as a tinction of C concentration x.
303
than those of carbon-free counterpart respectively. It is
well known that the Curie temperature of intermetallic
compounds is sensitive to the interatomic Fe-Fe distance,
the increase in T, can be attributed to the increase in the
positive Fe-Fe exchange coupling due to the addition of
carbon atoms. It is found that the substitution of Si for Fe
in the R2Fe17Cs compounds results in an increase in T, at
lower carbon concentration, and results in a slightly
decrease at higher carbon concentration.
In order to determine the type of magnetocrystalline
anisotropy of the Sm2Fe&iC, compounds, both X-ray
diffraction and magnetic measurements were used. As an
example, figure 4 presents the room-temperature XRD
patterns of magnetic-field-oriented powder samples of
Sm2FelBSiC, with x = 0, 0.5 and 2.0. Obviously, a drastic
increase in the (006) peak and the complete disappearance
of (hk0) reflection reveal that the samples of SmzFel,$iC,
with x t 0.5 have an easy-axis anisotropy, while carbon-
free sample SmlFer&i exhibits an easy-plane anisotropy.
This indicates that the addition of C atoms plays an
important role in the heightening of the rare earth sublattice
anisotropy. Previous investigations have shown that the net
Table II
The Curie temperature T, and saturation magnetization M,
at 1.5K of R2FeibSiC, compounds with R = Gd and Y.
GdPeisSiC, YsFe&iC,
X T,(K) Ms(emu/g) T,(K) M,(emu/g)
0.0 535 80.8 431 152.9
0.5 573 82.4 500 155.8
1.0 603 78.0 545 154.7
1.5 619 78.6 579 152.9
2.0 639 80.7 593 150.1
304 PROPERTIES OF ARC-MELTED Iy;e16SiC, Vol. 95, No. 5
Table III
The Curie temperature T, , room-temperature saturation
magnetization M. and anisotropy field HA of Sm2Fer&iC,.
compounds T,(K) M.(emu/g) HA(kOe) EMD SmtFe&i 482 114. I plane SmzFer&iG.s 530 115.4 65 c-axis SmzFed4Cl.t~ 560 114.2 90 c-axis SmzFer,$iCr,1 592 116.5 110 c-axis Sm2Fer6SiC2.0 605 116.4 110 c-axis
anisotropy in rare-earth-Fe intermetallics was determined
by the sum of the Fe sublattice and the rare earth sublattice
anisotropies. In the R2Fel, compounds, the magnetization
of the Fe sublattice exhibits a planar anisotropy. The rare
earth sublattice anisotropy is approximately characterized
by the product of the second-order Stevens coefficient, aJ ,
which reflects the form of the 4f charge distribution of the
rare earth ions, and the second-order crystal field
parameter, A2’.t2” A negative product aJA2” gives a
uniaxial contribution from the rare earth sublattice to the
600 - (a)
650
g
600
0 550 p
500 l Gd2Fe,,C,,
I I I
o SmaFe,,SiC x /i I I I
40 50 t
28 (deg.)
D
Fig.3. C-concentration dependence of the Curie Fig.4. Room-temperature XRD patterns of Sm2Fei6SiCX (x
temperature of R2FeuSiC, (R = Gd, Y and Sm) and = 0, 0.5 and 2.0) samples aligned in an applied magnetic
RzFerTC, (R = Gd and Y) compounds. field of 10 kOe.
total anisotropy. In the case of SmzFei7 compounds, a
positive aJ and a negative AZ’ produces a negative product
a,Az’, which is responsible for an uniaxial anisotropy of the
Sm3’ sublattice. However, the Sm sublattice anisotropy
does not match against that of the Fe sublattice at room
temperature, consequently, Sm2FeiT exhibits a room-
temperature planar anisotropy. The introduction of the
interstitial carbon atoms greatly enhances the uniaxial
anisotropy of the Sm sublattice, as a result. it is able to
compensate the Fe sublattice anisotropy even at room
temperature when carbon content attains an appropriate
value. Moreover. the substitution of Si for Fe also leads to
the enhancement of the Sm sublattice anisotropy. In fact,
early studies on the ‘?m Mdssbauer spectrum of
Tm2Fe& ‘22i and TmzFe& ~1 have demonstrated that
both the carbon introduction and the silicon substitution in
s SmzFeibSiCs
Vol. 95, No. 5 PROPERTIES OF ARC-MELTED we,,SiC, 305
H(kOe)
Fig.5. Magnetization curves of SmzFelsSiC, with x=1.5
and 2.0 at 300 K along and perpendicular to the aligned
direction.
the RzFe,, compounds could lead to a marked increase in
the magnitude of the negative value of AzO, which
produced a larger KI value, and consequently, the
occurrence of a transition from an easy-plane anisotropy to
an easy-axis one. Figure 5 illustrates an example of the
magnetization curves measured along and perpendicular to
the aligned direction at room temperature for the
Sm2FerSiC, compounds with x = 1.0 and 2.0. The
anisotropy field HA estimated from the measurement curves
and the room-temperature saturation magnetization M, are
also summarized in Table III. The M, values of the
SmzFe&iC, are approximately constant at about I 15
emu/g as x increases from x = 0 to 2.0. The values of
anisotropy field H,., are found to increase with increasing
carbon concentration from 65 kOe for x = 0.5 to I IO kOe
for x t 1.5. A similar result was observed in the
Sm2Fe,&C, compounds”“. It is note worthy that the
relatively larger easy axial anisotropy field and saturation
magnetization at room-temperature combined with the
relatively higher Curie temperature provide the possibility
applied as a powerful permanent magnet material.
Acknowledgments--This work was supported by the
National Natural Science Foundation of China. The authors
wish to express their gratitude to T.S.Ning and M.Hu for
their assistance in the X-ray difiaction experimental.
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