Structural and magnetic properties of rare earth – iron –

cobalt – vanadium intermetallic compounds (R = Tb, Dy)

D. Hadjiapostolidoua, M. Gjokab, C. Sarafidisa, E.

Pavlidoua, T. Bakasc, O. Kalogiroua,*

aDept. of Physics, Aristotle University of Thessaloniki, 54 006 Thessaloniki,

Greece

bInstitute of Materials Science, NCSR “Demokritos”, 153 10 Ag. Paraskevi, Attiki,

Greece

cDept. of Physics, University of Ioannina, 45110 Ioannina, Greece

Abstract

Starting with the Nd3(Fe,Ti)29 stoichiometry [Tb3(Fe1-

xCox)27.4V1.6 and Dy3(Fe1-xCox)27.8V1.2; x= 0.6, 0.8, 1.0] two

novel series of R-Fe-Co-V intermetallic compounds with a

disordered variant of the hexagonal Th2Ni17-type structure

were formed. The cell parameters decrease and the Curie

temperature increases with increasing Co content. XRD

patterns of magnetically aligned powder samples revealed

the presence of planar magnetic anisotropy.

1

Keywords: R-TM intermetallics (A); Magnetically ordered

materials (A); Powder metallurgy (B); X-ray and γ-ray

spectroscopy (D); Magnetic measurements (D)

*Corresponding author:

Tel.: +30 231 0 99 8148, Fax: +30 231 0 99 8003

E-mail address: orestis.kalogirou@physics.auth.gr

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1. Introduction

It is known that the substitution of Fe for Co in

R3(Fe,T)29 intermetallic compounds has a remarkable effect

on the magnetic properties of this class of materials. Shah

et al. [1] have reported the introduction of Co in Pr3(Fe1-

xCox)27.5Ti1.5 (x0.3) and Huo et al. the corresponding Co-

substituted 3:29 compounds with R= Gd for x0.4 [2].

Recently, we have reported on the structural and magnetic

properties of R3(Fe1-xCox)29-yTy (R= Nd, Tb, Dy; T= Ti, V and

x= 0 – 0.4) extending the occurrence of the 3:29 phase in

the R¯Fe¯Co¯T system for R= Nd [3] and Dy, Tb [4]. The

Nd3(Fe,Ti)29-type (3:29) compounds crystallize in the

monoclinic system with the A2/m space group [5]. Like the

Th2Ni17- (2:17) and ThMn12-type (1:12) structures, the

Nd3(Fe,Ti)29-type structure can be derived from the CaCu5-

type structure by replacement of a fraction of the R sites

in the CaCu5 structures by pairs of Fe atoms (dumb-bells).

In the hexagonal variant of the 2:17 phases with the

Th2Ni17-type structure (2:17H), this substitution may induce

a substantial degree of disorder, leading to non-

3

stoichiometric compounds and mixed stacking. It is

particularly interesting that the Nd3(Fe,Ti)29-type

structure can be considered as a combination of

rhombohedral Th2Zn17- (2:17R) and tetragonal ThMn12-type

structural blocks [5]. As a consequence, the 3:29 phase

cannot be formed for heavy rare earths, which in the case

of the 2:17 phase crystallize in the hexagonal Th2Ni17-type

structure. It is not yet clear whether the 3:29 phase can

be formed for more than 40% substitution of Fe by Co atoms.

Up to now there is only one work reporting the formation of

a 3:29 compound with more than 40 at.% substitution of Fe

by Co atoms; it concerns a Gd3(Fe1-xCox)25Cr4 compound with 60

at.% substitution (x= 0.6) [6].

The aim of this work was the synthesis, investigation of

the structure and magnetic properties of alloys with

nominal compositions Tb3(Fe1-xCox)27.4V1.6 and Dy3(Fe1-xCox)27.8V1.2

for x= 0.6, 0.8, 1.0. It is shown that instead of the 3:29

phase, a disordered variant of the hexagonal Th2Ni17-type

structure is formed. Such a behavior has also been observed

for the x= 0.4 member of the Dy3(Fe1-xCox)27.8V1.2 series [4].

4

2. Experimental

Starting ingots with a nominal stoichiometry Tb3(Fe1-

xCox)27.4V1.6 and Dy3(Fe1-xCox)27.8V1.2 (x= 0.6, 0.8, 1.0) were

prepared by arc-melting high-purity elemental constituents

several times in a high-purity Ar atmosphere. The samples

were subsequently wrapped with Ta foil, sealed in evacuated

quartz tubes, annealed at Τ= 1323 Κ for 72 h, and then

quenched in water. X-ray powder diffraction (XRD) (Cu Kα

radiation and Rietveld analysis) was employed to determine

the nature of the phases and their unit cell parameters.

The composition was determined on polished samples by a

scanning electron microscope (SEM), equipped with an

electron microprobe analyzer (EDAX). The Curie temperature

(TC) was determined from high-temperature magnetization

measurements carried out in a field of 0.1 T, using a

vibrating sample magnetometer (VSM). XRD patterns of

magnetically aligned powder samples with the alignment

direction normal to the sample holder were recorded in

order to study the magnetic anisotropy of the compounds.

5

57Fe Mössbauer spectra were recorded at 85 K on a

conventional constant acceleration spectrometer with a

57Co(Rh) source.

3. Results and discussion

The X-ray diffraction patterns of all the alloys could

be indexed in the hexagonal system, and not in the

monoclinic system expected from the nominal stoichiometry

3:29. Characteristic scanning electron microscopy

micrographs taken on polished R= Tb, x= 0.6 and x= 0.8

samples are shown in Fig. 1. Black spots correspond to

unreacted Fe, Co and V, and white spots to unreacted rare

earth metal. Electron microprobe analysis yielded a Tb:

(Fe,Co,V) ratio from 1:9.1 to 1:9.7 for the light gray

main phase, in fair agreement with the 3:29 stoichiometric

ratio (1:9.66), as well as with a 2:17 non-stoichiometric

ratio. For R= Tb, x= 0.6, a ratio 1:12 was obtained for

the dark gray secondary phase (see Fig. 1a), in accordance

with the XRD pattern of this sample. In all cases, the

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ratio between the Fe and Co atoms was the same as that of

the nominal stoichiometry within the experimental error. A

detailed Rietveld analysis (Fig. 2) showed that in all

samples the main phase crystallized with a disordered

variant of the hexagonal Th2Ni17-type structure (2:17H).

Small amounts of secondary phases, like the rhombohedral

Th2Zn17-type (2:17R), the monoclinic Nd3(Fe,Ti)29-type

(3:29), the tetragonal ThMn12-type (1:12) compounds and α-

Fe(Co), were found (see Table 1). In the case of R= Tb, x=

1.0, a large amount of the 2:17R phase, 43.3 wt.%, was

found.

The unit cell parameters and the composition of the

samples, as obtained by the Rietveld analysis, are given

in Table 1. As it is shown in Fig. 3, the cell parameters

of the 3:29 phase decrease with increasing Co

concentration. Such a lattice contraction has been

observed in all R3(Fe1-x-yCoxTy)29 (R= Pr, Gd, Nd) compounds

[1-4] and is attributed to the fact that Fe atoms are

substituted by smaller Co atoms.

It is well known [7] that the real crystal structure of

R2Fe17 compounds with a heavy R ion is a partially

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disordered Th2Ni17-type hexagonal structure (space group

P63/mmc). In this structure dumb-bells of Fe atoms replace

some R ions at the 2b position, resulting in a change from

2:17 stoichiometry to a Fe-rich composition. Two models of

the disordered structure have been proposed. In order to

describe the structure of Lu2Fe19, Givord et al. [8] have

proposed a model where a dumb-bell Fe site (4e) is formed

around the 2b R site, whereas an additional R site (2c) is

introduced at the center of the 4f dumb-bell. At the same

time the 12j Fe site splits into three, and the 12k Fe

site, into two sites. In their refinement of the structure

of Y2Fe17, Moze et al. [7] used a model where a dumb-bell Fe

site (4e) is also formed around the 2b R site and an

additional Y site (2c) is introduced at the center of the

4f dumb-bell. In this model the 12j Fe site is, however,

split into two sites.

For the compounds under study, the optimum R-factors

were found when applying the model proposed by Givord et

al. [8]. The occupation numbers and the refined

composition as obtained by the Rietveld and EDAX analyses

are given in Table 2. The rare earth to transition metal

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ratio calculated from the Rietveld analysis is in good

agreement with the corresponding ratio obtained from

electron microprobe analysis. It is worth noting that from

the refined formula of the 2:17H phase in the x= 0.6

sample, a composition of 9.3 at.% Tb (90.7 at.% Fe-Co-V)

and 9.5 at.% Dy (90.5 at.% Fe-Co-V) is obtained,

respectively. This composition is very close to the

nominal composition of the samples, 9.4 at.% Tb(Dy) and

90.6 at.% Fe-Co-V, which is the stoichiometry of the 3:29

structure. This means that the 3:29 phase and the Fe-rich

disordered Th2Ni17-type hexagonal structure can practically

have the same composition regarding the ratio between the

R and transition metal ions. In the case of the x= 1.0

samples, the calculated formulae are closer to the

stoichiometric 2:17 composition, whereas for x= 0.8

intermediate values are obtained.

As far as we know, there is only one report on the

formation of a 3:29 compound with more than 40%

substitution of Fe by Co atoms, concerning the Gd3(Fe1-

xCox)25Cr4 (x= 0.6) compound [5]. As mentioned above, the

Nd3(Fe,Ti)29-type structure can be considered as a

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combination of rhombohedral Th2Zn17- (2:17R) and tetragonal

ThMn12- type (1:12) structural blocks. This is apparently

the reason why the 3:29 phase cannot be obtained with

heavy rare earths that form 2:17 compounds crystallizing

only in the hexagonal Th2Ni17-type structure (2:17H). In the

case of Gd, Tb and Dy both the hexagonal and rhombohedral

2:17 structural arrangements are formed [9]. It seems as

if the introduction of large contents of Co, x 0.6, in

the 3:29 stoichiometry leads to the formation of hexagonal

2:17 blocks, which prevent the formation of a Nd3(Fe,Ti)29-

type structure. In other words, it is not yet clear

whether the 3:29 phase can be stabilized for more than 40%

substitution of Fe by Co.

In Figs. 4 and 5 the magnetization versus temperature

(TMA) curves are plotted for R= Tb and Dy, respectively.

The Curie temperature of both series increases

monotonically with the Co concentration (see Table 1). The

weak ordering temperature (TC'= 1030 K) obtained in the TMA

curve for the R= Tb, x= 0.6 sample was attributed to the

small amount of the 1:12 phase found by the Rietveld

analysis. No ordering temperature corresponding to the

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presence of the rhombohedral 2:17R phase (43.3 and 12.6

wt.% of this phase were found in the R= Tb and Dy, x= 1.0

samples, respectively) was detected. An explanation could

be that the values of the Curie temperature of the 2:17R

and the disordered 2:17H phases almost coincide and cannot

be distinguished from the TMA curves. In the case of R= Tb

and Dy, x= 0.6, the TMA curves revealed another magnetic

transition temperature, T*, which could be attributed to

either spin reorientation phenomena or domain wall motion.

A spin reorientation phenomenon at relative high

temperatures has been reported in Tb2Co17-xMnx for x=0.2,

0.3, 0.4, but was not observed for x=0.1, 0.5, 0.6, 0.7

[10]. The spin reorientation was explained as the result

of the competition of the magnetocrystalline anisotropy

terms related to the Tb and 3d sublattices. We have

recently reported the presence of such a transition in

Tb3(Fe1-xCox)27.4V1.6 compounds with the Nd3(Fe,Ti)29-type

structure for low Co concentration, x= 0.1, 02, 0.3 [4].

The fact that it was not observed for x= 0.8 and 1.0 may

be due to a different temperature dependence of the

anisotropy constants for this Co content.

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XRD spectra of non-aligned and aligned powders, with the

alignment direction normal to the sample holder, are

presented in Figs. 6 and 7, respectively. In all cases the

h k 0 reflections are dramatically increased and all other

reflections practically vanished, revealing the presence

of planar type magnetic anisotropy for all compounds.

57Fe Mössbauer spectra were collected at 85 K for the R=

Tb and Dy, x= 0.6 and 0.8 alloys (Fig. 8). A fitting

procedure was made with four components corresponding to

the iron sublattices. In the case of the R= Tb and Dy, x=

0.6 alloys, a fifth component was introduced for α-Fe. A

small line broadening was allowed for each component, to

better simulate the distribution of the environments within

each component. The area ratio was constrained to

correspond to the occupancies obtained from the Rietveld

analysis, taking into account the V and Co occupancy of the

different sites. It was assumed that the V atoms enter the

4f site (dumb-bell in the P63/mmc space group), as it is

usually found for rare earth transition metal intermetallic

compounds. The neutron diffraction experiments performed on

Pr3(Fe1-xCox)27.5Ti1.5 (x= 0.1 and 0.3) [11] have shown that the

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Co atoms occupy those Fe sites that do not contain Ti

atoms, i.e. the dumb-bell sites. On the other hand,

Mössbauer spectroscopy has revealed that in the RFe11-xCoxTi

(R= Y, Dy, Er; x= 0, 3, 6, 8) compounds, the Co atoms avoid

the 8i sublattice (dumb-bell) and prefer the 8f and 8j sites

[12]. The values of the average 57Fe hyperfine field

decrease with the Co content from 29.1 T for x= 0.6 to 28.1

T for x= 0.8 in the case of Tb, and from 30.0 T for x= 0.6

to 29.2 T for x= 0.8 in the case of Dy. This is consistent

with the observation that the introduction of high

concentrations of Co in rare earth - iron metal

intermetallic compounds causes a decrease of the saturation

magnetization [13].

4. Conclusions

Although the alloys of the present study were annealed

only at a particular temperature (Τ= 1323 Κ), it seems that

the existence of R3(Fe1-xCox)29-yTy with the Nd3(Fe,Ti)29-type

structure at high Co concentration, x 0.5, is doubtful.

13

For the rare earths beyond Gd this could be attributed to

the fact that they form either both the rhombohedral and

the hexagonal 2:17 phase, or only the hexagonal one,

whereas the Nd3(Fe,Ti)29-type structure is a combination of

the rhombohedral 2:17 and the tetragonal 1:12 phases.

However, our recent results show that in the case of the

light rare earths Pr and Nd, high Co concentration leads to

the formation of mixtures of the 2:17R and 1:12 phases in a

relatively large range of annealing temperatures. It is

interesting to note that Sun et al. [14], in trying to

synthesize Pr2Co17-xMnx compounds at high Mn concentration

(x= 13), obtained a 3:29 phase with the Nd3(Fe,Ti)29-type

structure (Pr3Co6Mn23), which is paramagnetic above 5 K.

This suggests that the key factors for the formation of R-

Fe-Co-T alloys with the Nd3(Fe,Ti)29-type structure at high

Co concentration are the nature and the amount of the

stabilizing atoms.

14

References

[1] V.R. Shah, G. Markandeyulu, K.V.S. Rama Rao, M.Q.

Huang, K. Sirisha, M.E. McHenry, J. Magn. Magn. Mater. 190

(1998) 233.

[2] Guoyan Huo, Zhiyu Qiao, Guanghui Rao, Jingkui Liang,

Baogen Shen, J. Alloys Comp. 285 (1999) 216.

[3] O. Kalogirou, C. Sarafidis, M. Gjoka, T. Bakas, M.

Giannouri, J. Alloys Comp. 325 (2001) 59.

[4] O. Kalogirou, C. Sarafidis, M. Gjoka, G. Litsardakis,

J. Magn. Magn. Mater. 247 (2002) 34.

[5] O. Kalogirou, V. Psycharis, L. Saettas, D. Niarchos,

J. Magn. Magn. Mater. 146 (1995) 335.

[6] Dong Yang, Jianli Wang, Ning Tang, Yupin Sheng, Fuming

Yang, Appl. Phys. Lett. 74 (1999) 4020.

[7] O. Moze, R. Caciuffo, B. Gillon, G. Calestani, F.E.

Kayzel, J.J.M. Franse, Phys. Rev. B 50 (1994) 9293.

[8] D. Givord, R. Lemaire, J.M. Moreau, E. Roudaut, J.

Less-Common Met. 29 (1972) 361.

[9] W.E. Wallace, Prog. Solid State Chem. 16 (1985) 127.

15

[10] Z.G. Sun, H.W. Zhang, S.Y. Zhang, J.Y. Wang, B.G.

Shen, J. Appl. Phys. 87 (2000) 8666.

[11] V.G. Harris, V.R. Shah, G. Markandeyulu, K.V.S. Rama

Rao, M.Q. Huang, K. Sirisha, M.E. McHenry, IEEE Trans.

Magn. 35 (1999) 3286.

[12] J.J. Bara, B.F. Bogacz, A.T. Pedziwiatr and R.

Wielgosz, J. Alloys Comp. 307 (2000) 45.

[13] M. Katter, J. Wecker, C. Kuhrt, L. Schultz, J. Magn.

Magn. Mater. 114 (1992) 35.

[14] Zhi-gang Sun, Hong-wei Zhang, Jing-yun Wang, Bao-gen

Shen, Appl. Phys. Lett. 75 (1999) 3850.

16

Table 1

Unit cell parameters, concentration and Curie temperature of the compounds in Tb3(Fe1-xCox)27.4V1.6 and

Dy3(Fe1-xCox)27.8V1.2 samples

x R Phase (wt.%) a = b (Å) c (Å) V (Å3) Rp (%) Rwp (%) Rexp (%)TC (K)0.6 Tb 2:17H

1:123:29α-Fe(Co)

73.514.010.1 2.4

8.389(1)

8.291(1)

505.3

7.05 10.59 5.47 976

0.8 Tb 2:17H 100 8.362(1)

8.231(1)

498.5

7.09 10.75 6.23 1012

1.0 Tb 2:17H2:17Rα-

Fe(Co)

49.343.3 7.4

8.344(1)

8.165(1)

492.3

5.92 8.57 5.41 1026

0.6 Dy 2:17Hα-Fe

96.2 3.8

8.375(1)

8.282(1)

503.1

4.06 5.97 5.25 992

0.8 Dy 2:17H 100 8.353(1)

8.222(1)

496.9

6.58 8.86 5.59 1043

1.0 Dy 2:17H 86.4 8.333(1 8.155(1 490. 6.42 10.91 6.24 1063

17

2:17Rα-Fe(Co)

12.6 1.0

) ) 4

18

Table 2

Occupancy factors of the 2:17H crystallographic sites and the resulting chemical formulae from

Rietveld and EDAX analysis for

Tb3(Fe1-xCox)27.4V1.6 and Dy3(Fe1-xCox)27.8V1.2 samples

R = Tb R = Dyatom

site x= 0.6 x= 0.8 x= 1.0 x= 0.6 x= 0.8 x= 1.0

R2b 0.636 0.730 0.735 0.660 0.753 0.8132d 1.000 1.000 1.000 1.000 1.000 1.0002c 0.150 0.170 0.218 0.155 0.140 0.147

Fe

4e 0.364 0.270 0.265 0.328 0.247 0.1504f 0.850 0.830 0.782 0.845 0.860 0.8536g 1.000 1.000 1.000 1.000 1.000 1.00012k

10.655 0.665 0.430 0.602 0.644 0.550

12k2

0.345 0.335 0.570 0.398 0.355 0.450

12j1 0.677 0.788 0.740 0.725 0.725 0.720

19

12j2 0.314 0.163 0.241 0.206 0.179 0.21012j3 0.009 0.049 0.002 0.068 0.096 0.070

Rietveld Tb2(Fe,Co,V)1

9.49

Tb2(Fe,Co,V)18

.06

Tb2(Fe,Co,V)17

.51

Dy2(Fe,Co,V)19

.11

Dy2(Fe,Co,V)18

.21

Dy2(Fe,Co,V)17

.33

EDAX Tb2(Fe,Co,V)1

9.41

Tb2(Fe,Co,V)18

.33

Tb2(Fe,Co,V)18

.18

20

(a)

(b)

Fig. 1. SEM micrographs of Tb3(Fe1-xCox)27.4V1.6 alloys with(a) x= 0.6

and (b) x= 0.8.

21

22

Fig. 2. Observed (crosses) and calculated (solid line)

X-ray powder diffraction patterns (Cu Kα) of Tb3(Fe1-

xCox)27.4V1.6 and Dy3(Fe1-xCox)27.8V1.2 alloys.

23

Fig. 3. Cell parameters of the 2:17H phase in Tb3(Fe1-

xCox)27.4V1.6

and Dy3(Fe1-xCox)27.8V1.2 alloys versus the Co content.

24

Fig. 4. Magnetization versus temperature curves forTb3(Fe1-xCox)27.4V1.6 alloys.

25

Fig. 5. Magnetization versus temperature curves forDy3(Fe1-xCox)27.8V1.2 alloys.

26

Fig. 6. X-ray diffraction patterns (Cu Kα) of non-aligned and magnetically aligned powder samples Tb3(Fe1-

xCox)27.4V1.6.

27

Fig. 7. X-ray diffraction patterns (Cu Kα) of non-aligned and magnetically aligned powder samples Dy3(Fe1-

xCox)27.8V1.2.

28

Fig. 8. 57Fe Mössbauer spectra recorded at 85 K forTb3(Fe1-xCox)27.4V1.6

and Dy3(Fe1-xCox)27.8V1.2 alloys with (a) x= 0.6 and (b) x=0.8.

29