Structure and magnetic properties of Sm2(Fe1−xCox)16GaC and Sm2(Fe0.8Co0.2)16GaCy compounds

Journal of

A~©YS

A N D COMPOUNDS ELSEVIER

Journal of Alloys and Compounds 257 (1997) 1-4

Structure and magnetic properties of Sm2(Fel_xCOx)16GaCand Sm2(Feo.8Coo.2) 16GaCy compounds Shao-ying Zhang*, Bao-gen Shen, Zhao-hua Cheng, Jun-xian Zhang, Bing Liang, Fang-wei Wang, Hua-yang Gong State Key Laboratory of Magnetism, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, China

Received 3 March I996; receivedin revised form 3 April 1996

Abstract The effect of the substitution of Co for Fe on the formation, structure and magnetic properties of Sm2FelrGaCycompounds were studied. Compounds with composition Sm2(F%_xCox)~rGaC (x=0-0.3) and Sm2(Feo.sCOo.2)arGaCy(y=0-2.0) were prepared by arc melting. X-ray diffraction shows that these carbides are almost single phase with the rhombohedral Th2ZntT-type structure. The unit cell volume (v) decreases monotonically with increasing Co concentration (x). The Curie temperature (To) increases with x from 592 K for x=0 to 816 K for x=0.3. The room-temperature saturation magnetization and the anisotropy field are 126.4 emu g-~ and 117 kOe respectively when the Co concentration x reaches 0.3. Addition of carbon results in the decrease of saturation magnetization and has a small effect on the Curie temperature and anisotropy field. © 1997 Elsevier Science S.A. Keywords: Smz(Fe~_~Cox)lrGaCycompounds:structure, magnetic properties

1. Introduction It is known that the rare-earth (R) iron-rich RzFe17 intermetallic compounds are not suitable for permanent magnets because their low Curie temperature and roomtemperature plane magnetic anisotropy. The magnetic properties of R2Fet7 can be considerably improved by interstitial nitrogen [1,2] or carbon [3,4]. However, it is regretful that the poor high-temperature stability of Sm2F%vNy or Sm2Fe17Cs compounds prepared by gassolid reaction restricts their possible application as permanent magnets. Our previous studies have shown that the heavy rare-earth iron compounds R2FeIvCy with high carbon concentration up to y=3.0 having a high stability at least up to 1000 °C can be obtained by melt-spinning [5]. But it was still difficult to synthesize SmzFe17Cy compounds with y->l.5. Recently, it was discovered that the highly stable SmzF%7Cy with y--<3.0 can be formed by the substitution of Ga or Si for Fe [6,7]. It was found that the arc-melted SmzFe~4Ga3Cy compounds with y-->1.5 show Curie temperatures higher than 600 K and room-temperature anisotropy fields higher than 90 kOe. The drawback of Sm,_(Fe, M)ivCy (M=Ga, A1 and Si) compounds is relatively low saturation magnetization due to the introgCorresponding author. 0925-8388/97/$17.00 © I997 ElsevierScience S.A. All rights reserved. PII S0925-8388 (97)00009- I

duction of non-magnetic M atoms. A study on Sm2(Fe l_xC%) 17Nycompounds has shown that the substitution of Co for Fe increases the Curie temperature and saturation magnetization as well as the anisotropy field [8]. In this paper, we report on the influence of the substitution of Co for Fe on the magnetic properties of SmzFe16GaCs.

2. Experimental Iron and carbon were first melted into Fe-C alloy in an induction furnace. Samples with the composition Smz(Fel_xCox)16GaC (x=0, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30) and Sma(Feo.sCoo.z)16GaCy (y=0.05, 0.10, 0.15 and 0.20) were prepared by arc-melting in an argon atmosphere of high purity. The elements used were at least 99.9% pure. The ingots were melted at least four times to ensure homogeneity. An excess of 4.5% Sm was added to compensate for the evaporation loss of Sm during melting. After melting, the ingots were then annealed in a steel tube in a highly purified argon atmosphere at 1450 K for 4 days followed by quenching into water. X-ray diffraction analyses on powder samples were performed using a Rigaku D/max-2400 diffractometer and Cu Kc~ radiation to determine the crystallographic structure. The room-temperature saturation magnetization was measured by an ex-

2

S.-y. Zhang et al. / Journal of Alloys and Compounds 257 (1997) 1 - 4

820

Sm2(Feo.ssCo0.15)16GaC

.< >

810 8OO 12,55

.<

12.50

0

12.45

8.70 .< v

t~

8,65 8.60 0.0

i

1

I

30

40

50

0.i

0.2

0.3

x

60

Fig. 2. Lattice parameters a and c and unit cell volume v of Sm2(Fe ~_~Co~),rGaC compounds as functions of Co concentration.

20 (deg.) Fig. 1. X-ray diffraction patterns of Sm2(Feo.8~Gao.l~)lrGaC compound; (a) non-aligned samples, (b) aligned samples.

3. Results and discussion

tracting sample magnetometer in a field of 65 kOe. The Curie temperatures were determined from the temperature dependence of magnetization measured by a vibrating sample magnetometer in a magnetic field of 700 Oe. The aligned samples for anisotropy field measurements were prepared by mixing the powder with epoxy resin and then aligning in a magnetic field of 10 kOe. The anisotropy field was determined from magnetization curves measured along and perpendicular to the orientation direction by using the extracting sample magnetometer with magnetic fields up to 65 kOe at room temperature.

X-ray diffraction study of Sm2(Fe1_xCOx)16GaC (x=0 0.3) and Smz(Feo.sCoo.2)16GaCy (y=0--2.0) shows that the prepared samples exhibit a rhombohedral Th2Zn17-type structure. The samples contain a small percentage of a-Fe as an impurity phase in some cases. The substitution of Co for Fe does not change their crystal structare. The substitution of Ga for Fe helps the formation of high carbon rare-earth iron compounds with 2:t7-type structure [6]. The typical X-ray diffraction patterns are shown in Fig. 1. The lattice parameters derived from the X-ray diffraction patterns are listed in Table 1. The substitution of Co in

Table 1 Lattice parameters a and c, unit cell volume v, Curie temperature T c, saturation magnetization M s and anisotropy field HA at room temperature for Sma(FeI_~Co~)lrGaC and Sm2(Feo.sCoo~)lrGaCy Compound

a (.~)

c (.~,)

v (~3)

T¢ (K)

M, (emu g-l)

HA (kOe)

Sm2FelrGaC Smz(Feo.95Coo.os)14GaC Sm2(Feo.9oCoo.io) t6GaC Sm2(Feo.85Coo.a~)lrGaC Sm2(Feo.8oCoo.2o) 16GaC S m 2(Fe o,75Coo 25) ,4GaC Sm2(Feo.7oCoo.3o) ~rGaC Sm~(Feo.~Cooa) 16GaCo.5 Sm 2(Feo. sCoo.z)16 GaC1.5 Sm2(Feo.sCoo.2) arGaC~.o

8.658 8.670 8.644 8.643 8.641 8.627 8.614 8.626 8.647 8.655

12.499 12.502 12.490 12.492 t2.487 12.479 12.473 12.486 12.491 12.493

811.5 8t3.8 808.2 808.1 807.4 804.4 801.5 804.5 808.9 810.4

592 641 685 719 754 785 816 754 753 758

125.2 130.8 134.0 126,9 133.6 130.2 126.4 142.9 126.8 121.4

118 126 125 110 128 126 117 109 128 125

S.-y. Zhang er al. I Journal of Alloys and Compounds 257 (1997) 1 - 4

Sm2Fe16GaC leads to the reduction of the lattice constant and unit cell volume. An approximately linear decrease of unit cell volume with x is observed, as shown in Fig. 2. For Sm2(Fe0.sCo0.2)~6GaC s, the unit cell volume expands with increasing C concentration as can be seen from Table 1. When y=2.0, the unit cell volume expansion is about 2.1% compared with the carbon-free compound. The room-temperature saturation magnetization (M~) and the Curie temperature (T~) of Smz(F%_~C%)16GaC and Smz(Feo.sCo0.a)t6GaCy are summarized in Table 1. The Ms values of Smz(F%_~C%)16GaC are in the range of 125.2-134.0 emu g-~ and have a small dependence on Co content. When the Co concentration is constant, Ms decreases slightly with increasing carbon concentration. Fig. 3 shows the T~ of Sm2(Fet_~COx)x6GaC as a function of Co concentration. The T~ is found to increase with increasing Co concentration from 592 K for x = 0 to 816 K for x=0.3. A similar result was observed in Smz(Fet_~C%)t7Ny [8]. In previous studies it is was shown that the interstitial carbon atoms [3,4] or the substitutional Ga atoms [9] in Sm2F%7 compound led to a strong increase of the Curie temperature. The enhancement of To is mainly due to the lattice expansion. However, an increase of T~ with increasing x in Smz(Fe1_~Cox)16GaC is observed, although the substitution of Co for Fe results in a monotonic decrease of the unit cell volume. In general, in the Fe-rich rare-earth iron compounds the To is mainly determined by the Fe-Fe exchange interaction. For Sm2(F%_~C%)~6GaC, the T~ is mainly determined by the Fe-Fe, Fe-Co and Co-Co interactions. Usually, the F e Co exchange interaction is much stronger than the Fe-Fe and Co-Co exchange interactions as shown in other F e Co based alloys [10]. Accordingly, the substitution of Co for Fe results in an increase of To. The easy direction of magnetization for

3

Smz(F%_xCox)a6GaC and Sm2(Fe0.sCoo.2)16GaCy was identified from the diffraction patterns of field-aligned samples. All samples studied here are found to have a strong easy c-axis anisotropy at room temperature. This can be clearly seen from the X-ray diffraction patterns of magnetically aligned powder samples, as shown Fig. 1, for Smz(Fel_xCox)i6GaC with x=0.15. The strong enhancement of the (0, 0, 6) reflection and the absence of (h, k, 0) reflections indicates that the c-axis is the easy direction of magnetization. The strong uniaxial magnetocrystalline anisotropy of the Sm-sublattice in the Smz(Fel_xCOx)16GaCy can result from the effect of both Ga and C additions. In our previous study it was shown that the substitution of Ga for Fe in SmzFe17 leads to a change of magnetocrystalline anisotropy from the basal plane to the c-axis at room temperature [9]. A number of investigations have demonstrated that the introduction of carbon atoms into SmzFet7 leads to strong uniaxial anisotropy of the Sm-sublattice because the interstitial carbon atoms cause a more negative crystal field parameter A20 [11]. Fig. 4 shows an example of the magnetization curves measured along and perpendicular to the aligned directions at room temperature. The magnetocrystalline anisotropy field HA of Smz(Fel_xCOx)16GaC and Sm2(Feo.sCoo.2)i6GaCy estimated fl'om magnetization curves is listed in Table 1. The HA is found to be in excess of 110 kOe, which is 30 kOe higher than that of Nd2F%4B. For Sm2(Feo.sCoo.~)~6GaC, the room-temperature anisotropy field is 128 kOe, its Curie temperature and saturation magnetization are 754 K and 133.6 emu g-~, respectively. The present study shows that these Co containing carbides have the excellent intrinsic magnetic properties and are promising candidates for permanent magnets.

L

I

I

I

I

L

150

//

120 / ' ~ -

I

900 ~D

800

9O

J

0)

"~

60

700

30

600

18Gac ~.o

Sm2(Fel_=Co x)ieGaC 0

0 500 0.0

S m z ( F e o . ~ o C ° o.zo)

J 0.1

' 0.2

0.3

X

Fig. 3. Curie temperature T~ of Sm2(Fe~_~Cox)~6GaCcompounds as a function of Co concentration.

i

i

i

r

i

i

1

2

3

4

5

6

7

:H (T) Fig. 4. Magnetization curves of the orientated Sm2(Feo.8oCoo.2o)~GaC sample measured along and perpendicular to the aligned directions at room temperature.

4

S.-y, Zhang et al. / Journal of Alloys and Compounds 257 (1997) 1-4

Acknowledgments This work was supported by National Natural Science F o u n d a t i o n o f China. The authors wish to express their gratitude to T.S, N i n g and M. Hu for their assistance in the X-ray diffraction experiments, to K.C. Jia and S.Y. Fan for his assistance in the magnetic parameter measurements.

References [1] J.M.D. Coey, H. Sun, J. Magn. Magn. Mater. 87 (1990) L251. [2] H. Sun, J.M.D. Coey, Y. Otani, D.RF. Hurley, J. Phys.: Condens. Matter 2 (1990) 6465. [3] X,R Zhong, R.J. Radwansld, F.R. de Boer, T.H. Jacobs, K.H.J. Buschow, J. Magn. Magn. Mater, 86 (1990) 333.

[4] L.X. Liao, X. Chen, Z. Altounian, D.H. Ryan, Appt. Phys. Lett. 60 (1992) 129. [5] L. Cao, L.S. Kong, B.G. Shen, J. Phys.: Condens. Matter 4 (1992) L515. [6] B.G. Shen, L.S. Kong, F.W. Fang, L. Cao, Appl. Phys. Lett. 63 (1993) 2288. [7] B.G. Shen, L.S. Kong, F.W.Wang, L. Cao, W.S. Zhan, J. Appl. Phys. 75 (1994) 6253. [8] M. Katter, J. Wecker, C. Kuhrt, L. Schultz, R. Gr/Sssinger, J. Magn. Magn. Mater. 114 (1992) 35. [9] B.G. Shen, F.W.Wang, L.S. Kong, L. Cao, J. Phys.: Condens. Matter 5 (1993) L685. [10] F.E. Luborsky, in: E.R Wohlfarth (ed.) Ferromagnetic Materials, North-Holland Publishing, Vol. 1, 1980, p. 491. [11] T.H. Jacobs, M.W. Dirken, R.C. Thiel, L.J. de Jongh, K.H.L Buschow, J. Magn. Magn. Mater. 83 (1990) 293.