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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