Magnetic and crystallographic properties of ternary rare earth compounds of the type R2Co14B

Journal of Magnetism and Magnetic Materials 51 (1985) 211-217 North-Holland, Amsterdam

211

MAGNETIC AND CRYSTALLOGRAPHIC PROPERTIES OF TERNARY RARE EARTH C O M P O U N D S O F T H E T Y P E R2CoI4B

K.H.J. BUSCHOW, D.B. de M O O I J Philips Research Laboratories, 5600 JA Eindhoven, The Netherlands

S. S I N N E M A , R.J. R A D W A N S K I * and J.J.M. F R A N S E Natuurkundig Laboratorium, University of Amsterdam, 1018 XE Amsterdam, The Netherlands

Received 13 March 1985

Compounds of the type RzCo14B having the tetragonal Nd2Fe14B structure were found to exist for R = Y, La, Pr, Nd, Sm, Gd and Tb. The lattice constants of all these compounds were determined. Differential scanning calorimetry was used to determine the Curie temperatures of these materials, ranging from Tc = 955 K for La2Co14B to Tc = 1050 K for Gd2Co14B. The saturation moment and anisotropy fields were studied at 4.2 K on magnetically aligned powder samples in magnetic fields up to 35 T. Estimated values of the anisotropy fields range from 6 T for LaECo14B to 75 T for Pr2Co14 B. The 3d sublattice anisotropy favours an easy magnetization direction perpendicular to the c axis whereas the crystal field induced anisotropy of the 4f sublattice corresponds to the second-order Stevens factor a s of the R component. As in R2Fe14B, the 3d sublattice moment couples antiparallel with the spin moment of the 4f sublattice.

1. Introduction In a previous paper [1] we reported on the magnetic properties of the series of R2Fe14B compounds. The interest in these c o m p o u n d s stems primarily from the discovery that some of its members can serve as starting materials for the attainm e n t of powerful permanent magnets [2]. The crystal structure of these c o m p o u n d s is fairly complex, the tetragonal unit cell containing 58 atoms [3-5]. C o m p o u n d s of similar crystal structure are also formed when R metals are c o m b i n e d with Co and B [6]. It will be shown in this paper that the occurrence of tetragonal R2Co14B c o m p o u n d s is m o r e restricted than in the case of REFe14B compounds. The purpose of the investigation reported here was to determine the magnetic properties of the R2Co14B compounds, the emphasis being on the magneto-crystalline anisotropy. This anisotropy * On leave of absence from Dept. of Solid State Physics, Academy of Mining and Metallurgy, Cracow, Poland.

arises out of two separate contributions due to the 4f sublattice and 3d sublattice, respectively. In the R 2 Fea4B series the 3d sublattice anisotropy favours an easy magnetization direction parallel to the c-axis. The 4f sublattice anisotropy dominates in all cases where the orbital m o m e n t of the R comp o n e n t is non-zero, but the associated easy magnetization direction is parallel to the c-axis only in those c o m p o u n d s where the asymmetric 4f charge cloud of the R c o m p o n e n t has a prolate shape. It will be shown here that the 4f sublattice anisotropy in RECo14B c o m p o u n d s is qualitatively the same as in R 2 F e I 4 B compounds, but that the behaviour of the 3d sublattice anisotropy is completely different.

2. Experimental F o r the preparation of the samples we used 99.9% pure starting materials which were melted together under an argon arc in purified argon gas. After arc melting the samples were wrapped in

0 3 0 4 - 8 8 5 3 / 8 5 / $ 0 3 . 3 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)

212

K.H.J. Buschow et al. / Ternary rare earth compounds of the type R2CoI4B

Ta-foil, sealed into an evacuated quartz tube and vacuum annealed at 900°C for two weeks. X-ray diffraction was performed by means of a standard powder diffractometer equipped with a graphite crystal monochromator using CuK~ radiation. The low-field magnetic measurements (B ~< 2 T) were made on a PAR vibrating sample magnetometer. The temperature dependence of the magnetization o ( T ) was studied on an automatic o - T recorder, based on the Faraday method. The high-field magnetization measurements with B up to 35 T were performed at 4.2 K in the high-field magnet at the University of Amsterdam [7]. For all measurements we used powdered samples after aligning the powder particles in a magnetic field and fixing their direction with epoxy resin. High-field isotherms were recorded with the external field either parallel (oji) or perpendicular (o±) to the alignment field. The anisotropy fields BA were derived from the intersection points of the extrapolated oll(B ) and o ± ( B ) curves measured at 4.2 K. Values for the saturation magnetization os were derived from the high-field part of the o(B) curves by extrapolating to B = 0. The Curie temperatures (T~) were determined by means of differential scanning calorimetry performed under purified argon gas, using a heating rate of 20 K / m i n .

sents Y, La, Pr, Nd, Sm, Gd or Tb. In all the other cases investigated (R = Ce, Dy, Ho, Er, Tm, Lu) the X-ray diagrams did not contain the typical X-ray pattern associated with the tetragonal Nd2Fe14B structure. Instead we observed X-ray patterns composed mainly of the hexagonal structure found for R2Co17 compounds. The lattice constants determined for the tetragonal R z C O l 4 B compounds are listed in table 1 and plotted as a function of the R component in fig. 1. Separate X-ray measurements were made on magnetically aligne d powders of the various R2Co]4B compounds in order to determine the direction of easy magnetization. For this purpose X-ray diagrams were taken with the plane of the X-ray beam parallel and perpendicular to the alignment direction. From the results of these measurements it was concluded that in R2Co14B the magnetic field had led to an alignment of the powder particles primarily in the crystallographic c-direction when R is Pr or Nd, whereas it had resulted in an alignment primarily perpendicular to the c-direction in the compounds in which R represents Y, La, Sm or Gd. In the case of Tb2Co14B the difference between the X-ray pattern for the two directions was less distinct. This could mean that the alignment has resulted in a direction which is only approximately parallel to

3. Experimental results The X-ray diagrams of the R 2 C o ] 4 B c o m p o u n d s were indexed on the basis of the tetragonal Nd2Fe14B structure [3-5]. This structure type was observed, however, only in cases where R repre-

R2COllB

1.20

0.89 ~

1.19

088

Table 1 Lattice constants of tetragonal (Nd 2 Fej4B structure type)

R2COl4B

compounds ,

Compound

a(nm)

c(nm)

0.87

Y2Col4 B La2Col4 B Pr2COl4 B Nd 2Co14 B SmzCo14B Gd2Col4B Tb2Co14B

0.860 0.867 0.863 0.864 0.861 0.861 0.860

1.171 1.201 1.187 1.183 1.179 1.176 1.173

0.861

1.18

c

/

°~+ I

I

I

~

I

I

I

I

I

I

LeCePr NdPmSmEuC_x:Tb l Dy

Fig. 1. Variation of the lattice constant in the series R2Co14B.

213

K.H.J. Buschow et aL / Ternary rare earth compounds of the type R 2CoI4 B 140

120

100

"E .<

8(3

. , f , J ' f

8 .~*~*

T = 42 K • -parallel ÷ - perpendicular

,../+ ~ + /

;

lb

1;

2'0

~'s

3'o

3s

Magnetic field B (T)

Fig. 2. Field dependence of the magnetization in magnetically aligned samples of PrzCo14B measured at 4.2 K in two mutually perpendicular directions parallel and perpendicular to the alignment field.

the c-direction or that the alignment is far from ideal. This is not surprising in view of the comparatively low magnetization of Tb2Co14B, which makes alignment in a field less effective. Results of high-field magnetic measurements on Pr2Co14B at 4.2 K are shown in fig. 2. In this case 1/.0

120

Ao~ 100 E

g

f

f

//./'J'J"

.......j÷J÷"

the anisotropy field obtained by extrapolation is extremely large and equals BA = 75 T. Approximately only half of this value is reached in Nd2Co]aB , for which results are shown in fig. 3. Furthermore, the value of o± in Nd2ColaB is already quite appreciable in relatively small applied fields, which feature is absent in the data taken at room temperature (fig. 4). Results of high-field magnetic measurements for the remainder of R2Co14 B compounds are shown in fig. 5. In the first two examples, Y2COlaB and La2Co14B, the anisotropy fields are comparatively modest and are equal to approximately 6 T. In the other three examples the data do not lend themselves for estimating values of BA. The field dependences of the values of Oll at 4.2 K for R2Co]aB with R = Y, La, Pr, Nd, Sm and Gd have been used to determine %. These values are listed in table 2, together with the corresponding values of

8A.

Measurements of the temperature dependence of o above room temperature showed that the Curie temperatures (To) of the R2Co14B compounds fall outside the temperature range available in our o - T apparatus. Values of T~ determined by means of differential scanning calorimetry are included in table 2. Several examples of measurements of the temperature dependence of Oil below Tc are shown in figs. 6-8. The results shown for Tb2COlaB in fig. 6 are characteristic of R 2 C O l a B compounds in which R is a heavy rare earth metal. In these cases o,

N d 2 Cov.B 120

t Nd2 CO1L B

T=42K

,;0

v

• - parallel

+ - perpendiculor

06

fo~

o

20

;

lb

1~

2'0

2s

I

magnetic field BIT) B (T)

Fig. 3. Field dependence of the magnetization in magnetically aligned samples of Nd2Co14B measured at 4,2 K in two mutually perpendicular directions parallel and perpendicular to the alignment field.

2.0

Fig. 4. Field dependence of the magnetization in magnetically aligned samples of Nd2Co14B measured at 300 K in two mutually perpendicular directions parallel and perpendicular to the alignment field.

214

K.H.J. Buschow et al. / Ternary rare earth compounds of the type R2CoHB

Table 2 Curie temperature (T~) obtained from DSC measurements and saturation magnetization (05) obtained at 4.2 K from measurements of the magnetization in fields up to 35 T in R2Col4B. The quantities M S and /~¢o represent the total saturation moment and the Co moment, respectively. The values of BA represent the field strengths where the curves Olt(B ) and o± ( B ) intersect at 4.2 K. The easy magnetization directions (EMD) are given with respect to the c-axis; aj represents the second-order Stevens factor R 2Co 14B R=

T~(K)

Y La Pr Nd Sm Gd Tb

1015 955 995 1007 1029 1050 1035

°s (Ame/kg) 107 102 124 126 (89) (32)

Ms ( # a / F . U . )

/t co (/t n )

BA(T)

EMD

aj

19.42 20.35 24.82 25.37 18.10 6.90

1.39 1.45 -

6-7 6-7 75 30

± c ± c IIc IIc ± c ± c

0 0 + 0

lie

-

-

_

decreases with decreasing T. The results shown for Pr2COlnB and Nd2Co]nB in figs. 7 and 8 are characteristic of compounds of light rare earth elements. In these cases o increases with decreasing temperature. Qualitatively the temperature dependence of o± for P r 2 C o ] a B and T h 2 C o l 4 B is the same as the temperature dependence of the corresponding air A difference in the temperature de-

110L

R2Co it. B •

-

parallel

• -perpendicu[or

~-~./..~'--~

~Ioc 9C

/~0

i

,

,

o.

~ _ + ~ ÷

:i

,-

-~

~-

~

~3o

2c 8C

,

Gd

8£ : .

a 3 ~.~.

pendence between o± and Oll may be expected as a result of a temperature-dependent magnetocrystalline anisotropy. When interpreted in this way, the minimum of the curve shown in fig. 8 would then be indicative of a maximum in the temperature dependence of the anisotropy near 120 K. Alternatively the strong rise of the o± curve at temperatures below 50 K may also point to spin reorientation in the sense that the easy magnetization direction (in zero field) is no longer parallel to the c-axis. The latter possibility is the more probable one in view of the results reported elsewhere for the isotypic compound Nd2Fe]aB [4,6,8].

Lo

/

-

-


"+-+"""

,.t÷~+~ "÷ +"+ ÷ + ÷--"-'+

~

2o

k~

7C

1C Tb2

1C • ,./

.....

C014 B

B=l.25T i

5

,

.

.

.

.

,;

.

.

.

.

.

.

i;

.

.

2;

i

i

i

25

Magnetic field B (T} Fig. 5. Field dependence of the magnetization in magnetically aligned samples of R2Co14B measured at 4.2 K in two mutually perpendicular directions parallel and perpendicular to the alignment fidd.

I

0

i

200

i

i

400 Temperature (K)

i

600

Fig. 6. Temperature dependence of the magnetization ( o ) of Tb2Col4B ( H = 1.25 T), measured along a field direction parallel (Oil) and perpendicular ( a t ) to the alignment field.

K.H.J. Buschow et aL / Ternary rare earth compounds of the type R2CoI4B

Pr2 Co~ B B=0.3 T

120

z.O

~" 90 E

30

.-

60

20

3O

10 100

L

200

300

'

TIK)

Fig. 7. Temperature dependence of the magnetization (o) of Pr2Co14B (H = 0.3 T), measured along a field direction parallel (Oll) and perpendicular (o±) to the alignment field.

Nd2Col: B

~

3o

20

60 i

0

L

I

100

I

200

I

300

TJK}

Fig. 8. Temperature dependence of the magnetization (o) of Nd2Co14B ( H = 0.6 T), measured along a field direction parallel (oll) and perpendicular (OL) to the alignment field.

4. Discussion In the R 2 C o l 4 B compounds in which the orbital moment of the rare earth component is zero one may consider the magnetic anisotropy as being exlusively due to the 3d sublattice magnetization. It follows from the results obtained on La2Co~4B and Y2COl4B that the latter anisotropy gives rise

215

to a preferential plane perpendicular to the c-axis with an anisotropy field of about 6 T ( K 1 < 0). This result is clearly different from the 3d sublattice anisotropy of the corresponding Fe compounds (RzFe14B) which is of equal strength but leads to an easy magnetization direction parallel to the c-axis ( K 1 > 0). Inspection of the data listed in table 2 shows that the H A values are considerably larger in those R2C014 B compounds in which the R component has a non-zero orbital moment ( L = 0), single-ion crystal field induced anisotropy being here the origin of the large BA values. We assume that the anisotropy can be expressed to the lowest order by means of the free energy term F = K I sin20, where 0 is the angle between the magnetization and the c-axis. Sign and magnitude of the anisotropy constant K 1 are largely determined by the product - a s ( J - l / 2 ) A ° ( r 2 f ) , w h e r e otj is the second-order Stevens constant and where the quantity A ° reflects the electrostatic potential of the crystal field in a given series of isotypic compounds [9,10]. In compounds where ~tj of the R components has the same sign one may therefore expect that the corresponding K~ values will also be of equal sign, so that these compounds give rise to the same easy magnetization direction (EMD). The sign of aj for the various R components and data regarding E M D have been included in table 2. In all cases the EMD or the sign of K 1 is in agreement with model predictions, meaning that the 3d sublattice anisotropy is completely overruled by the 4f sublattice anisotropy. This is plausible since the value of BA found, for instance, in Pr2Cox4B is about an order of magnitude larger than the values of BA found in La2Co14 B and Y2Co14B.

If one takes account of the fact that the 3d and 4f contributions to KI have the same sign in Pr2Fe14B and NdEFe14B but are of opposite sign in the Co compounds it seems surprising that the Bg value of the Co compounds is the same, (NdECOlaB) or even larger (Pr2ColaB) than that in the Fe compounds. However, BA= 2Kilos, the comparatively high value of BA in the Co compounds then arises as a consequence of their os values being much lower than those of the corresponding Fe compounds. It is also possible that

216

K.H.J. Buschow et al. / Ternary rare earth compounds of the type R 2Col4B

the crystal field intensity, as measured by A °, is not the same in the two series of compounds. Neglecting the relatively weak exchange interaction between the 4f moments one m a y use the mean field model to express the Curie temperature by means of the relation: 3kT~ = acoco + ( acoco 2 + 4acoRaRco),1/2 ,

(1)

Table 3 Comparison of the magnetic coupling constants between the 3d moments (J3d-3d) and between 3d and 4f moments (Jg-3d) in compounds of the type R2Co14B and R2Fel,~B Compounds

J3d- 3d (10 -22 J)

JR - 3d (10 22 j)

R2Co14B R 2Fe14 B

15.8 4.7

-2.23 - 1.42

where acoco = ZJcocoSco(Sco + 1),

(2)

acoRaRCo

=Z1Z2Sco(Sco+ l)(ga-1)(J+

l)J2Rco .

(3)

These expressions are similar to those used before [1]. But we have omitted the concentration-dependent factor so as to make the coupling constants -/CoCo and JRco concentration-independent. In the crystal structure of Nd2Fe14 B the R atoms are surrounded on the average by Z 1 = 18 3d nearneighbour atoms. The 3d atoms are surrounded on the average by Z = 10 3d atoms and Z2 = 2.5 R atoms. For La2C014 B and Y2Co14 B o n e has acoRaR¢o = 0. Using the experimental values for Tc and the experimental values for 2Sco =/ZCo listed in table 2 one obtains Jcoco = 15.8 x 10 -22 J and Jcoco = 17.8 x 10 22 j for the C o - C o coupling constants in La2Co14 B and Y2Coa4 B, respectively. As was done for the R2Fe14B compounds, we will assume that the exchange coupling between the 3d moments in R 2 C o 1 4 B is the same as in the compound with R = La. Application of eqs. (1)-(3) to Gd2Col4B, using Jcoco = 15.8 × 10 -22 J, leads to [JRCo ] = 2.23 X 10 -22 J. The sign of JRco is negative. This follows from the experimental value of a s in Gd2Co14B which can be expressed as o~ = 14/~Co - 2/-tGd. The antiparallel coupling between the spin moments of the R and Co atoms is also revealed in the temperature dependence of the magnetization of Z b 2 C o x 4 B and G d 2 C o l 4 B , where o decreases with temperature well below T~. The various exchange coupling constants for R2Col4B and R2Fe14B are compared in table 3. It is seen that J3d-3d as well as Jg-3d are substantially stronger in RECO14B than in R2Fel4B. Finally we will comment on the values of 0s found for the R E C o t 4 B compounds in which the R

component has an orbital moment. If one uses the values of M s listed in table 2 in conjunction with /~Co = 1.45#~/Co one finds that the values of the 4f moments in Pr2Co14B and Nd2Co14B are substantially smaller than the corresponding free ion values gJi,tn. This may originate from the crystal field interaction, which can lead to a magnetic ground state with an expectation value (Jz) of the total angular m o m e n t u m that differs from the free ion value Jz = - J . The R atoms in RzColaB are located at two crystallographically inequivalent sites for which the crystal field interaction is rather different [11]. At low temperatures one m a y therefore not exclude the possibility of a non-collinear magnetic moment arrangement for R and a concomitant too low value for M s. The occurrence of such a magnetic structure seems, however, not sufficient to explain the results obtained for Sm2Co14B. In this compound the value of M s is even lower than M s in the correponding Y- or La-compounds. This suggests that the Sm sublattice magnetization is antiparallel to the Co sublattice magnetization, although Sm 3÷ is an L - S state ion. This paradoxical behaviour of the 4f moment of Sm 3+ is found in other compounds too, such as SmC%, Sm2Co17 and SmFe 2. It may arise as a consequence of the combined action of exchange fields and crystal fields on the 4f electron system of Sm 3+ for which the energy separation of the ground nmltiplet J = 5 / 2 and the two excited multiplets J = 7 / 2 and 9 / 2 is comparatively low, and this may cause a sign reversal of the expectation value of Sz relative to that of L z + 2S~ [12,13]. In that case the total Sm moment behaves like that of a heavy rare earth metal, i.e. it couples antiparallel to the 3d moment rather than parallel. Additional support for this hypothesis comes from the temperature dependence of the magnetization,

K.H.J. Buschow et al. / Ternary rare earth compounds of the type R 2CoI 4 B

w h e r e w e f o u n d t h a t 011 d e c r e a s e s w i t h d e c r e a s i n g t e m p e r a t u r e , w h i c h is q u a l i t a t i v e l y t h e s a m e b e h a v i o u r as f o u n d f o r T b 2 C O l a B a n d G d 2 C o 1 4 B .

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[6] K.H.J. Buschow, H.M. van Noort and D.B. de Mooij, J. Less-Common Metals (in press). [7] R. Gersdorf, F.R. de Boer, J.C. Wolfrat, F.A. Muller and L.W. Roeland, in: High Field Magnetism, ed. M. Date (North-Holland, Amsterdam, 1983). [8] R. GrOssinger, P. Obitsch, X.K. Sun, R. Eibler, H.R. Kirchmayr, F. Rothwarf and H. Sassik, Mater. Lett. (1984). [9] J.E. Greedan and V.U.S. Rao, J. Solid State Chem. 6 (1973) 387. [10] E. Callen, Physica l14B (1976) 71. [11] K.H.J. Buschow, J.W.C. de Vries and R.C. Thiel, Physica 132B (1985) 13. [12] K.H.J. Buschow, A.M. van Diepen and H.W. de Wijn, Phys. Rev. B8 (1975) 5134. [13] H.W. de Wijn, A.M. van Diepen and K.H.J. Buschow, Phys. Stat. Sol. (b) 76 (1976) 11.