Thermal expansion and spontaneous magnetostriction of RCo5 intermetallic compounds

Physica B 172 (1991) 517-525 North-Holland

Thermal expansion and spontaneous magnetostriction of RCo 5 intermetallic compounds A.V. A n d r e e v 1 Natuurkundig Laboratorium, University of Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands

S.M. Zadvorkin Permanent Magnets Laboratory, Ural State University, 620083 Sverdlovsk, USSR Received 5 February 1991

Thermal expansion of RCo 5 compounds (R = Pr, Nd, Sm, Gd, Tb, Dy, Ho) is investigated by X-ray diffractometry on single crystalline samples in the temperature range 5-1300 K. All compounds undergo large spontaneous magnetostrictive deformations, the volume effect re~tches 3.2 x 10 -2 at 5 K in HoCo5 5. The contributions to the volume effect from Co-Co and R - C o exchange interactions are determined by comparison with YCo 5, Y being the nonmagnetic analogue of R.

1. Introduction

The phases RCo 5 take in an outstanding place among the intermetallic compounds of the rareearth elements (R) with the metals of the iron group. Firstly, the discovery of a large magnetocrystalline anisotropy in these compounds in 1966 [1] soon led to the production of powerful permanent magnets on their basis, widely used at present. The application of rare-earth magnetic materials has begun with the RCo 5 compounds in particular. Secondly, a variety of magnetic properties, in combination with the simple crystal structure, made the RCo 5 intermetallics very apt model objects for the study of magnetism in (4f-3d)-alloys. These highly anisotropic ferro(in cases of nonmagnetic or light R) and ferri(with heavy R) magnets can be prepared relatively easily as single crystals with sufficiently large dimensions for magnetometric measurements. Therefore, their magnetic characteristics 1 Permanent address: Permanent Magnets Laboratory, Institute of Physics and Applied Mathematics, Ural State University, 620083 Sverdlovsk, USSR.

(the values and orientation of the magnetic amounts of the R and 3d sublattices, the magnetocrystalline anisotropy constants, the temperatures of the magnetic phase transitionsmagnetic ordering and spin reorientations) have been studied in great detail [2-4]. At the same time, the magnetoelastic properties of these alloys have received less attention, due to several methodological reasons, connected mainly with the large magnetocrystalline anisotropy. Huge magnetostrictive deformations, typical for the rare-earth compounds, make the investigation of the thermal expansion of RCo 5 intermetallic compounds and its anomalies at the magnetic phase transitions possible by X-ray dilatometry. Recently, we have studied Y(COl+xNix) 5 [5], CeCo 5 [6], ThCo 5 [7], and UCo 5 [8] compounds, using single-crystalline samples in the temperature region 5-1300K. Since Y, Ce, Th and U are nonmagnetic in these compounds, the results of refs. [5-8] are relevant to the Co sublattice and show that influence of the effective valency of R (3+ for Y, 4+ for Th and Ce, 6+ for U) on the magnetoelastic properties of that sublattice. Also, they show a de-

0921-4526/91/$03.50 © 1991- Elsevier Science Publishers B.V. (North-Holland)

518

A.V. Andreev, S.M. Zadvorkin / Properties o f RCo 5 intermetallic compounds

pendence of the spontaneous magnetostriction on dilution of the Co sublattice by Ni, which is nonmagnetic in these compounds. The data on the thermal expansion of RCo 5 compounds with magnetic R were given only for the temperature ranges of the spontaneous spin-reorientation phase transitions with R = Nd [9], Tb, Dy, Ho [10], Pr [11], where only anisotropic magnetostriction reveals itself. In the present paper, the thermal expansion of RCo 5 intermetallic compounds with R = Pr, Nd, Sm, Gd, Tb, Dy and Ho is investigated in a wide temperature range (5-1300K), which includes both the paramagnetic and the magnetically ordered states.

2. Experimental The RCo 5 alloys were prepared in an induction furnace in alumina crucibles under a helium protective atmosphere from the consistent metals of 99.8% (R) and 99.99%(Co) purity. Since these intermetallics have a wide homogeneity region within which the magnetic properties can change considerably, and the R-rich boundary of this region is close to the exact 1:5 stoichiometry, we used for the investigation alloys containing 2-3% of the R2Co 7 phase, according to the metallography. Thus, we fixed the R-rich end of the homogeneity region of the main RCo 5 phase and the compounds with R = Pr, Nd, Sm and Gd were found to be practically stoichiometric. For the compounds with heavy R, beginning from Tb, the range of existence of the stable phase with the CaCu 5 type of structure narrows and the composition moves to the Co-rich direction, so that the R-rich boundary of the homogeneity region corresponds to the compositions TbCos.1, DyCos. 2, HoCo5.5, ErCos. 9 and TmCo 6 [12]. For R = T b , Dy, Ho, we obtained almost singlephase alloys (2-3% of R2Co7). But the alloys with Er contained up to 50% of Er2Co 7 and Er2Co17, and for Tm the desired phase was not obtained at all, because the temperature range of stability of this compounds is very narrow, the melting and the eutectoid-decomposition tem-

peratures being practically equal [12]. Thus, our study was carried out on samples with R = P r Ho. Single crystals with linear dimensions of about 2-3 mm were cut from the grains of the ingots after X-ray orientation. The surfaces were polished parallel to the (0 0 1), (1 0 0) and (1 1 0) atomic planes (the hexagonal structure of the CaCu 5 type belonging to the P6/mmm space group, in which the RCo 5 compounds crystallize, is shown in fig. 1). In the samples used for the measurements, the misorientation of the subgrains was less than 3° and the orientation of the surfaces was better then 1-2 ° . The magnetization measurements were carried out along the main crystallographic axes [0 0 1], [1 0 0] and [2 1 0] in a vibrating-sample magnetometer in static fields up to 2 T. The thermal expansion was investigated by X-ray diffractometry in a cryostat, cooled by liquid helium, in the temperature range 5-340 K, and in a high-temperature chamber between 290 and 1300 K. The samples were placed in vacuum ranging from 0.1 Pa at the highest temperatures to 0.001 Pa at room temperature, further improving with decreasing temperature. Radiation was chosen to provide values of the diffraction angle 20 exceeding 140°. The lattice parameter a was determined from the /3-(4, 0, 0) reflection in KCr radiation and the angle 20 changed between 141 ° and 152° depending on the temperature and

I Co

O

a(la)0 (2c) Q (]~)

Fig. 1. T h e crystal structure of R C o 5 intermetallic compounds.

A.V. Andreev, S.M. Zadvorkin / Properties of RCo 5 intermetallic compounds

the type of R. The parameter c was determined from the al-(0, 0, 4) reflection in K-Fe radiation (148°< 20 < 156°). The relative error in the determination of the lattice parameters was limited to 1 x 10 -4. Special attention was paid to the reversibility of the a, c ( T ) curves at increasing and decreasing T, to exclude any influence of some possible shift of the composition during the several hours of the measurements at high temperatures. Absence of reversibility was observed only for the Er compound, which for this reason is not considered in the present paper. Exposure to 1100-1300 K during one hour has not led to changes of the room-temperature lattice parameters after cooling, not even for the most unstable compounds with Dy and Ho. The error in the temperature determination was 1 K at low temperatures ( 5 - 3 4 0 K ) , in the high-temperature chamber it increased to 10 K at 1300 K. For the determination of the magnetostrictive deformations, the temperature dependences of the lattice parameters and the cell unit volume V = a2c~/3/2 were extrapolated from the paramagnetic state to the range of magnetic order. The experimental curves of a, c and V versus T between 1100 to 1300 K were used for extrapolation. The beginning of this interval was chosen to lie 100 K above the Curie temperature to eliminate any influence of the short-range magnetic ordering on the thermal expansion. The method of the extrapolation has been described in refs [5, 13]. The values of the Debye temperature in the RCo 5 compounds, necessary for extrapolation, are approximately equal to 3 0 0 K , as determined from acoustic measurements [5, 6, 11]. Since the crystal structure of the R C o 5 compounds is of lower symmetry than that of the cubic Laves phases studied in a similar way [14, 15], and since they have very high Curie temperatures, some test of the correctness of such extrapolation ought to be done. This test was carried out on the paramagnetic compounds YNi5, YNi4Co , PrNis, PrNigCo [5, 11] with the same crystal structure, similar lattice parameters and analogous thermal expansion. This test showed the correctness of the determination of the p h o n o n contribution to the thermal expansion.

519

3. Results 3.1. PrCo 5

Figure 2 shows the thermal expansion of PrCo 5. One can see that parameter a does not sense the magnetic ordering at T c = 910 K, the experimental and extrapolated a ( T ) curves coincide above about 450 K. In this range, the thermal expansion of PrCo 5 is similar to that of YCo 5 [5], where no magnetostrictive deformation was observed along the a-axis. However, at low temperatures some negative basal-plane deformation appears. The thermal expansion along the cdirection is similar to the YCo 5 case as well; considerable deformation appears just below To. But in this direction some additional positive deformation occurs in the same temperature range, where the negative one takes place in the basal plane, leading to an Invar effect below 400 K. The negative basal-plane contribution to the volume effect predominates, so that the spontaneous volume magnetostriction tos changes nonmonotonically with temperature.

V,nm3 0,09O

,

,

,

~

,

0.088 / a,~ o.510

O./400c'nnq

i o.508

~ .~.~ ~'~"

0.398 0.396

0.506 0.504

.// 0.502 / O 0.500 0

I

200

[

I

I

gO<) 600 800

I

1000 T,K

Fig. 2. The temperature dependences of the lattice parameters a and c and of the unit-cell volume V for PrCo5. The

dashed lines correspond to the extrapolations from the paramagnetic range into the magneticallyordered one.

A.V. Andreev, S.M. Zadvorkin / Properties o f RCo 5 intermetallic compounds

520

Table 1 a, c are the lattice parameters at r o o m temperature; T c the Curie temperature; T a the t e m p e r a t u r e of the spin-reorientation phase transition of the " p l a n e - c o n e " type; T 2 the temperature of the "cone-axis" type of spin reorientation; Aa a n d Ac are the linear s p o n t a n e o u s magnetostrictive deformations in the basal plane and along the c-axis, respectively (at 5 K); h~ '° and )t2 '° are the exchange magnetistriction constants in the basal plane and along the c-axis, respectively (at 5 K); h~ '2 and ,~2'2 are the anisotropic magnetistriction constants in the basal plane and along the c-axis, respectively (at 5 K); c~v is the volume coefficient of the t h e r m a l expansion in the paramagnetic range (at 1200 K); tos the volume s p o n t a n e o u s magnetostrictive deformation (at 5 K); rococo the contribution to the volume effect from the C o - C o exchange interaction (at 5 K); tORCo is the contribution to the v o l u m e effect from the R - C o exchange interaction (at 5 K); ncoco the magnetoelastic coupling coefficient of the Co sublattice (at 5 K); a n d nRc o is the intersublattice magnetoelastic coupling coefficient (at 5 K). R

Y

Pr

Nd

Sm

Gd

Tb

Dy

Ho

a, n m c, n m T c, K T~, K T2, K h a, 10 -3 Ac, 10 3 )t~ '°, 10 -3 h~ '°, 10 3 A1'2, 10 -3 A~'2, 10 3 av, 10 5K-1 tOs, 10 -3 rOCoCo, 10 -3 tORCo, 10 -3 ncoco, 10 3/~2 -3 -2 nRco, 10 IXB

0.4952 0.3955 940 0 6.8 0 6.8 0 0 4.3 6.8 6.8 0 2.5 0

0.5031 0.3985 920 120 -3.8 9.9 -3.2 8.7 -1.2 2.4 4.1 2.4 6.5 --4.1 2.4 --1.1

0.5033 0.3978 930 235 295 -3.2 7.8 -3.6 8.5 -1.0 2.0 4.1 1.4 6.5 --5.1 2.4 --0.9

0.4987 0.3981 1000 0.8 4.6 -0.2 6.5 1.4 -2.8 4.1 6.2 6.5 --0.3 2.4 -

0.4979 0.3972 1020 0 6.1 0 6.1 0 0 4.1 6.1 6.5 --0.4 2.4 -0.1

0.4957 0.3979 980 397 410 1.0 5.3 0.4 6.6 -1.9 3.8 4.2 7.3 6.7 0.6 2.5 0.1

0.4924 0.3986 970 300 370 -3.7 33.4 -4.3 34.6 -1.8 3.7 6.3 26.0 10.1 15.9 3.7 1.2

0.4920 0.3979 960 45 170 -2.5 37.2 -2.7 37.7 -0.7 1.4 7.2 32.4 11.4 21.0 4.2 1.6

All main characteristics of the RCo 5 compounds investigated are collected in table 1. Here )ta and Ac correspond to the linear spontaneous magnetostrictive deformations in the basal plane and along the hexagonal c-axis, respectively. All these deformations were determined as the relative differences between experimental and extrapolated curves, and are given in table 1 for the temperature 5 K. As one can see, % is considerably lower in PrCo 5 than in YCo 5 [5]. The PrCo 5 compound undergoes a spinreorientation transition of the "cone-axis" type at about 120 K, but this does not reveal itself in the thermal expansion within our experimental error, because the maximum value of angle 0 between the c-axis and the easy direction is only 23 ° at 5 K , and the O(T) dependence is not sharp. As we showed in ref. [11], this transition can be observed in the thermal expansion if the cone will open up to 0 = 90 ° by the dilution of the Co sublattice with Ni or in another way.

3.2. N d C o 5

It can be seen from fig. 3 that the thermal expansion of NdCo 5 has the same peculiarities as that of PrCo 5. The only difference is the reflection of spin reorientation in the a, c(T)-dependences at 235-295 K. On the scale of fig. 3, the anomaly in the basal plane is practically invisible, but it exists and is negative. A detailed investigation of the spin-reorientation range of this compound is presented in ref. [9]. 3.3. S m C o 5

SmCo 5 (fig. 4) differs from the previous ones by the change of sign of the basal-plane deformation, by a lower value of Ac, the absence of the Invar effect along the c-axis and, consequently, a monotonic temperature dependence of V. Despite the fact that the absolute values of the linear deformations are smaller, since they have

A.V. Andreev, S.M. Zadvorkin I Properties of RCo 5 intermetallic compounds

521

V,.m~

V, nm

0. 090

I0.O~8

3

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0.396

0.508

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-

O.

50(

0.396 O.496

0.506 0.)% 4,49 0

200

qOO

600

800

1000

T, K

0.50@

Fig. 5. Same as in fig. 2, but for GdC%. 0. 502

3.4. I

I

200

I

zlO0

600

I 800

0.500

i

1000

T,K

Fig. 3. Same as in fig. 2, but for NdCo 5.

the same sign, we observe an increased volume effect in comparison with NdCo 5, and tos is practically equal to the value in YCo s. The spinreorientation transition is absent in this compound because the Sm and Co sublattice have both uniaxial anisotropy.

,

,

,

i

i

V, n m 3

GdCo 5

The thermal expansion of GdCo 5 (fig. 5) is very similar to that of YCo 5, characterized by the absence of basal-plane deformation and similar values of h c and tos.

3.'5. TbCos.1 The basal-plane deformation of TbCo5 1 above the spin-reorientation temperatures (397-410 K) is equal to zero (fig. 6). If one shifts the a(T) curve downwards, so as to exclude the influence of the spin reorientation, h a becomes negligible at low temperatures as well. As in NdCos, the

0.088 V '

,

,

,

,

! 0.088

0.086

0 ,nlV%

c,

nm

a

0.400

0 "t~SP1

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0.399

•

3. osz~

0.50@

0.398 0,502

0.397 .

0.396

~.~

0.~°~

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1

i

/

^ c 9° j..,oo

a

0.500

o. 39~

° v -

0.~98 i

i

200

400

I

600

I

800

0.#°~1

i

1000

Fig. 4. Same as in fig. 2, but for SmC%.

T, K

--~'

0

,

200

I

,

400

600

~

800

,

1000

Fig. 6. Same as in fig. 2, but for TbCos 1.

T, K

A.V. Andreev, S.M. Zadvorkin / Properties o f RCo 5 intermetallic compounds

522

considerable linear deformations at the spin reorientation are invisible in the V ( T ) curve due to the opposite signs of the linear deformations. A detailed investigation of the spin reorientation range is presented in ref. [10]. 3.6. DyCos. e

One can note from fig. 7 and table 1 that the DyCos. 2 compound differs considerably from the compounds discussed above by the values of coefficients of the thermal expansion in the paramagnetic range. While the coefficients of volume thermal expansion a v at 1200 K are practically the same for the compounds with R = Y, P r - T b , (4.2-+ 0.1) x 10 -5 K -1, the same coefficient for DyCos. 2 is 1.5 times larger. The increase of av can be understood, since it is connected with the decrease of the stability of the compounds with heavy R. But the huge value of e t l%l'n

,

i

i

,

,

o% increases 4 times in comparison with the Y, S m - T b compounds. It could even be bigger, but the linear deformations have different signs. Unlike PrCo 5 and NdCo 5, which also have a negative Aa, the difference between the extrapolated and experimental a(T)-curves appears just below T c. The spin reorientation at 300-370 K is discernible in the linear deformations (on the scale of fig. 7 the effect in the basal plane is almost invisible) and does not manifest itself in the V(T)-dependence, due to the different signs of the linear deformations. A detailed study of this transition is presented in ref. [10]. 3.7. H o C o s. s

The results for HoCos. s are similar to those for DyCo 2 (fig. 8). The spin reorientation at 45170 K influences the thermal expansion relatively weakly (for a detailed study see ref. [10]). The '

c ~ i,li'l'i

) 0 •086

O.z~O.3

0.084

0.@0'1

0 °082

0.599

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,

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O.4O3 I

"~f/

o.~1

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/

/ / 0.597

0.080

/ / /

0.595

/

/

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/ 0.595

/

f~

z

~

0.082

~

0,397 0.395 I 0.395

/

o,39~

/

/

/

/

o.o8o

1/ #

/

/ /

0

a~nwI

ap ~rtl

//

0.499

o.591

0.497

o.389

o.495

O. 987

o.493

o. 589

o.~91

O.383

0.g96

o. 589

III /

0.587

//

I/

f a / ~,/s~, i /

t / ~13

o.494

#

0, a . 9 2 0.385

i

I

I

i

i

200

400

600

800

qO00

T.K

Fig. 7. Same as in fig. 2, but for DyCos. ~.

/ I / i

200

0.490 i

z~O0

I

600

I

800

I

1000

Fig. 8. Same as in fig. 2, but for HoCos. 5.

T, K

A.V. Andreev, S.M. Zadvorkin / Properties of RCo 5 intermetallic compounds

values of av, hc and tos are somewhat bigger than in DyCos. 2. The spontaneous volume magnetostriction of HoC05. 5 can be considered as a record. While the value of % in DyCos. 2 is only equal to the volume effects in some Fe-rich compounds (R2Fel4B [16, 17], La(Fe, Al)13 [16, 18]), for HoCos. s such analogies do not exist (excluding some special cases, when the magnetic ordering is a first-order transition and accompanied by a structural transition with considerable change of the volume).

4. Discussion According to ref. [19], the linear spontaneous magnetostriction in the basal plane of a crystal with a main axis is equal to (limiting the analysis to second-order magnetostriction constants): Aa(T ) --- Al,°(r) + A~'2(T)(cos20 - ~) + ½h3"2(T)(2 cos20 - 1) sinE0,

(1)

where ;t~ '° is the exchange-magnetostriction constant in the basal plane, A1'2 the anisotropicmagnetostriction constant in the basal plane (the index o~ corresponds to deformations without reduction of symmetry), A3''2 the constant describing orthorhombic distortion ("gammamagnetostriction"), and O and 0 are angles between the easy magnetization direction and the [1 00] and [ 0 0 1] axes, respectively. The deformation along the c-axis can be written as: ;to(T) =

+

'2(T)(cos20 -

(2)

In the case of uniaxial magnetic anisotropy 0 = 0, so that aa(T ) = A~'°(T) + 2Al'2(T),

(3a)

hc(T ) = hE'°(T) + 2AE'2(T ) .

(3b)

For basal-plane magnetic anisotropy 0 = 90°, so that A~',°(T) - 1A~,2(T),

(4a)

hc(T ) = A~'°(T) - ½h2,2(T).

(4b)

ha(T )

=

523

In formula (4a), we have neglected some contribution to the magnetostriction from the "gamma-magnetostriction", because we found no visible orthorhombic distortion in the RCo 5 compounds with the basal-plane anisotropy 3' 2 ~ --4 (A ~ 10 ) in contradiction with the R2Co 7 and RCo 3 compounds, where a huge "gammamagnetostriction" exists (up to 6 x 10-3) [13, 20]. Earlier, we determined the anisotropic magnetostriction constants of several RCo 5 compounds from the above-mentioned anomalies of the thermal expansion at the spin-reorientation phase transitions and calculated them for the whole series at 5 K on the basis of the single-ion model [10]. Therefore, now we can determine the exchange-magnetostriction constants at 5 K using formulas (3) and (4). The results of this determination are also presented in table 1. One can see that these constants strongl), depend on the type of the rare-earth metal; h~'Vis practically always negative and much smaller than A2'°. Both constants have a minimum value in the middle of the series. In all compounds, the basalplane anisotropic and exchange magnetostriction constants h~ '° and h~ '2 are comparable in absolute value, but in the [00 1] direction the exchange constants A2'° are always much larger than the anisotropic ones. The anisotropic constants of the RCo 5 compounds follow the relation 2A1 '2 + /~2 '2 ~ 0, as found earlier in ref. [10], so that the volume effect tos is equal to the sum of the exchange constants 22t1'° + a2 '° only. For the further analysis of the exchange magnetostriction, it is preferable to consider the volume effect, because in the literature only this type of deformation is discussed. Figure 9 shows the temperature dependence of tos of the compounds studied. The spontaneous volume magnetostriction can be described as [21]: tos =

2 ncoco/*Co

+

2 , nRco/-I,R/.tco + nRR/,~ R ,

(5)

where/Zco, llzR are the magnetic moments of the Co and R sublattices, ncoco, nRR the magnetoelastic coupling coefficients in the Co and R sublattices, and nRCo is the magnetoelastic coupling coefficient of the intersublattice exchange interaction. It is known that the third term in eq.

524

A.V. Andreev, S.M. Zadvorkin / Properties o f RCo 5 intermetallic compounds i

i

i

@ PrCo 5 O NdCo 5 @

28

@SmCo 5 GdCo 5

@

@ TbCo5/I

2/4 @

@ DyCos. 2

@

@ H°C°5.5 @

20

@

@

"16

@ @ @

12

8

@

#

', ID

-2

t

II)

ID

, 0

0,2

0.~

ID !)

L @

0.6

I

II)

4}

Oi

0.8

T/T

Fig. 9. T h e t e m p e r a t u r e d e p e n d e n c e s of t h e v o l u m e s p o n t a neous magnetostriction % and the intersublattice magnetoelastic c o u p l i n g coefficient nRc o in R C o 5 i n t e r m e t a l l i c compounds.

(5) is negligible in comparison with the others [15, 22]. The first contribution to tos can be obtained from results on YCo 5 [5]. The calculation of the second contribution was made on the following assumptions: 1. The magnetic m o m e n t of Co in RCo 5 is the same as in YCos, and the magnetic moment of R is equal to the difference between the molecular magnetic moments of RCo 5 and YCo 5. 2. The coefficient ncoco in RCo 5 is the same as in YCos, if the difference in compressibility is negligible. 3. The compressibility is proportional to the volume thermal expansion coefficient av, so that we can take into account the difference in compressibility by means of the relative difference between the a v values in R C o 5 and YCo 5.

The obtained values of the magnetoelastic coupling coefficients at 5 K are given in table 1, and the temperature dependence of the nRCo coefficients is presented in fig. 9. The ncoco coefficients are assumed to be temperature independent, as found for YCo 5 [5]. For SmCos, we could not obtain a value for nsmco due to the low magnetic m o m e n t of Sm and the consequently big error in its determination as the difference between the molecular moments of SmCo 5 and YCo 5. The nRCo coefficients are weakly temperature dependent. The dependence of nr~co on the atomic number throughout the rare-earth series is rather surprising. At first sight, nRC o should correlate with the Curie temperatures, i.e. it should be maximal in GdCo 5 and decrease on both sides of G d due to the weakening of the R - C o exchange interaction. But from fig. 9 and table 1, negative nac o values emerge for light R, positive ones for heavy R and nRc o -~-0 for those R, which are expected to have maximal interaction (Gd, Tb). A similar situation was observed in the R2Co 7 compounds [13]. The difference in the signs of nRc o for light and heavy rare-earth metals correlates with the different types of magnetic o r d e r i n g - f e r r o m a g netic in R C o 5 with light R and ferrimagnetic with heavy R. The absolute values of nRc o are several times smaller than ncoco , nevertheless a considerable contribution to the volume effect results from the R - C o exchange interaction, due to the large magnetic moments of R. This is the main difference found between the Co-rich and Fetich rare-earth intermetallics. In the Fe-rich compounds R2Fe17 [23], R2Fe14B [17, 24], R F e n T i [25, 26], we have observed practically the only F e - F e exchange interaction contribution to the thermal expansion anomalies.

5. Conclusion By this study of the thermal expansion of RCo 5 compounds, we have finished the investigation of the spontaneous magnetostriction of binary R - C o intermetallic compounds with lowsymmetry crystal structure ( R a C o [22], RCo 3 [20], R2Co 7 [13], RCos). The remaining corn-

A.V. Andreev, S.M. Zadvorkin / Properties o f RCo 5 intermetallic compounds

p o u n d s , R2Col7 , c a n n o t b e i n v e s t i g a t e d in the s a m e way, d u e to too high C u r i e t e m p e r a t u r e s a n d , c o n s e q u e n t l y , too small t e m p e r a t u r e differences b e t w e e n the m e l t i n g a n d the C u r i e temp e r a t u r e s to p r o v i d e a reliable basis for the e x t r a p o l a t i o n of the a, c, V ( T ) - d e p e n d e n c e s f r o m the p a r a m a g n e t i c r a n g e into the r a n g e of magnetic ordering.

Acknowledgements T h e a u t h o r s are grateful to Professor P.F. de Ch~tel for h e l p f u l discussions. O n e of the aut h o r s (A.V.A.) t h a n k s the D u t c h F o u n d a t i o n " F u n d a m e n t e e l Onderzoek der Materie" ( F O M ) for financial s u p p o r t .

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