Journal of Magnetism and Magnetic Materials 109 (1992) 185-190 North-Holland

Magnetic properties of Pr(Ni l_xCox ) 5 compounds

A.R. Ball, D. f3ignoux, B. Gorges , D. Schmit t and A. Tari ~

Laboratoire Louis N~el, CNRS, 166X, 38042 Grenoble Cedex, France

Received 5 August 1991; in revised form 4 November 1991

Five single crystals of the hexagonal Pr(Ni~_.~Co.,.) 5 compounds with x_< 0.20 have been investigated by means of magnetization and susceptibility measurements. The introduction of cobalt leads to the appearance of ferromagnetism with a Curie temperature and cobalt moment which vary as x ~ and x respectively. Cobalt magnetism as well as the crystal field effects on Pr are discussed in the light of these results.

I. Introduction

The study of the Haucke phase compounds RNis, where R is a rare earth, is essential for the fundamental understanding of the magnetism in the scientifically interesting and technologically important RCo.~ compounds. In these compounds where both R and Co atoms are magnetic, the magnetization and anisotropy of the two sublat- tices are comparable in magnitude. It is, there- fore, not possible to separate the contributions of R and Co atoms to the magnetic properties of RCo 5. In the RNi 5 series, however, the 3d-shell of Ni is almost full and hence it is not magnetic. In this series, therefore, one is closest to studying the properties of the R ions in isolation.

PrNi 5 is perhaps the most interesting member of the RNi 5 series. Although there is a non- negligible exchange interaction between the Pr ions in PrNi 5 [1-3], unlike the rest of the com- pounds in the RNi 5 series, it does not order down to very low temperatures because of the non-magnetic singlet ground state due to the

~ Ot~ sabbatical leave from King Fahd University of Petroleum and Minerals KFU?M, I27 ,hran/Saudi Arabia.

crystal field (CF) effects. However, this state is not well separated from the higher excited states and this leads to a large Van Vleck susceptibility normal to the c-axis of about 7.26 × 10 -2 emu/mol at T = 1.5 K [2] which exhibits a very pronounced maximum at about 15 K. Further- more the low temperature magnetic isotherms show that this compound undergoes a metamag- netic-type transition in a field of about 120 kOe applied along the b-axis of the orthohexagonal cell ([120] direction of the hexagonal cell). In this field the two lowest CF levels become very close (in the case of the field applied along the a-axis of the orthohexagonal cell[100] direction of the hexagonal cell they actually cross), thereby in- creasing the mixing between these two levels thus increasing the induced moment on Pr.

The large Van Vleck susceptibility causes a large hyperfine (hf) enhancement factor 1 + K of (12.6 + 0.5) [4]. Here K = AXvv/~r~gNtxBgj, where A is the hf coupiing constant for Pr ~ . Consequently, there is a relatively strong indirect exchange interaction between the nuclear mo- ments mediated by the electronic exchange and the hf interaction: H = -~ygylHa(1 + K), where H~, is the applied field and I the nuclear spin of Pr. This leads to a nuclear ferromagnetic order-

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186 A.R. Ball et al. / Magnetic properties of Pr(Ni ~ _ xCox)s compounds

ing of (0.40 + 0.02) mK in PrNi.s [5]. Because of this hf enhancement factor PrNi 5 has been used as a nuclear refrigerant [1,4].

In PrNi 5 both Pr and Ni contribute to the magnetic properties of the compound. The latter in the form of an exchange enhanced Pauli sus- ceptibility of (20.1 +_ 1.0) × 10 -4 emu/mol which, however, becomes significant only at T > 50 K [2].

The properties of the rare earth compounds are anisotropic, principally because the rare earth ions have a large orbital moment tied to their non-spherical charge distribution. It is, therefore, necessary to investigate single Crystals in order to study their properties along the principal crystal axes. In this respect too PrNi 5 proves to be an ideal candidate. It crystallizes in the simple hexagonal CaCus-type structure and melts con- gruently with about 1% tolerance in the concen- tration of Ni around the stoichiometry [6,7].

The substitution of Co for Ni in PrNi 5 results in direct competition between the Pr and Co anisotropies as the former prefers the b-axis while the latter (i.e. Co) prefers the c-axis in the RCo 5 compounds. On reaching the critical concentra- tion for magnetism, cobalt should contribute to the exchange field acting on Pr, thereby driving it towards the high magnetic state mentioned above.

We have, therefore, been investigating single crystals of the series Pr(Ni~_.,.Co.,.).~ in the con- centration range x < 0.20 in order to understand better the appearance of magnetism on cobalt and the role of the latter in the magnetic proper- ties of RCo 5 compounds. For this task we have chosen PrNi 5 as the starting compound and have substituted Co for Ni, systematically, and investi- gated the properties of each compound in detail by means of magnetization, susceptibility and specific heat measurements.

2. Experimental details

We have prepared five single crystals with x = 0.00, 0.01, 0.05, 0.10 and 0.20. They were grown from 99.9% pure Pr and 99.999% pure Ni and Co by Czochralsky-pulling from molten buttons of about 80g-mass in a cold crucible induction fur- nace. Their crystal axes were determined by the

Laiie back-reflection method and were subse- quently cut into cubes of about 3 mm 3 in dimen- sion. The lattice parameters of the crystals were determined using the Debye-Scherrer method. The parameter a is weakly concentration depen- dent and increases linearly from 4.976/~ for the compound with x = 0 to 4.980 /~ for the com- pound with x = 0.20. The paramet, e r c is found to be almost constant at c = 3.976 A over the con- centration range of our investigation.

Magnetic measurements were made in the temperature interval 1.5-300 K and in fields of up to 80 kOe, in a magnetometer using the ex- traction method.

3. Experimental results

The variation of magnetization M, at 1.6 K, with the internal field H i (applied field corrected for the demagnetization field) applied along the b-axis of the compounds is shown in fig. 1. The compounds with x = 0.00 and x = 0.01 are clearly paramagnetic. The remaining three compounds are ferromagnetic at this temperature as indi- cated by the rapid rise in M at low fields. Above about 20 kOe the M - H graphs are almost paral- lel. The high field superimposed susceptibility of these compounds is therefore not modified by the cobalt content to any significant degree. Its value being about 8.38 × 10 -2 emu/mol .

Shown in fig. 2 is the variation of M with Hi, at T = 1 . 6 K, for the a, b and c axis of the orthorhombic crystallographic directions of the four compounds containing cobalt. In all four, the easy axis is b and ¢ remains as the hard axis. As the figure shows, there is no anisotropy in the basal plane in low fields because of the symmetry considerations, as in these compounds Pr experi- ences a CF of hexagonal symmetry. However, at high fields there is an anisotropy which increases with increasing cobalt content. This figure con- firms the fact that the compounds with x >_ 0.05 are ferromagnetic at low temperatures.

Fig. 3 shows the M vs. H i graphs of the compounds with x = 0.10 and x = 0.20 at various temperatures overlapping with their ordering temperatures. In fields larger than 10 kOe the

A.R. Ball et al. / Magnetic properties o f Pr(Ni t -., Co.,. )5 compounds 187

1

Pr (Ni~_xCOx) 5 X = 0.20 b-r.axis .~.._---

2.5 T =1.61~ ~

2.(1 ~

~ ~ X=0.10

~1-' N

1'0

0"5

,

20 ~0 g0 80 Hi(~Oe)

Fig. 1. Variation of the magnetization ~er formula unit at 1.6 K with ~he internal field a~lied along the easy axis ~ of the orthohexagonal cell for ~he five compounds investigated.

magnetization of the compound with x = 0.10 at T = 12 K is higher than that at T = 1.5 K (see below.)

Shown in fig. 4 is the temperature variation of the magnetization of all the five compounds stud-

ied in a field of 30 kOe, applied along the easy axis. All the compounds show a broad peak at about the same tempera ture (T m = 15 K). In PrNi 5 this has been shown to be due to the CF level scheme [2,8]. Thus the substitution of Co for Ni

1.5 Pr(Nil_ x COx) 5 x =o.ol • O-QXiS

~.o o b-ox!~ ~

O-5 - ~

0 :~,,~r~ - I _ ~ I _. t x --0.20 ~2.5_~= ~

g 2.0 l'b ~ ,

1"0 ~ ~ ~ ~ I

0-5

0 ' 0 20 40 60 80

= . -~1.5

-I.0

O5

' 0 X=0.10

~ 1.5

-~o0

Hi(kOe) Fig. 2. Variation of the magnetization at 1.6 K with the inter,.J field for the three main symmetry directions of the four compounds

containing cobalt.

188 A.R. Ball et aL / Magnetic" properties of Pr(Ni ~ _ xCox)5 compounds

2.0 12K r~(N,o.~ C oo.,)s _ , ~ ~ ~i~

,., o_ox,,

~ ~ 40K

~'0

O.

. . . . t - - ~ ~ I , I I I . . I ~ l ~

~ 0~ Pr(Nt0'8 C°°'2)5 _ ~ l S K ~2"5 b-axis _ ~ - ' ~ • _ / ~ 2 4 " 2 K

2'0

1'5

1"0

(b) 0'5

I 0 ' ~'0 ' Zo ' G'0 ~'0

Hi(k0e) Fig. 3. Field dependence of the magnetization along the b-axis of the orthohexagonal cell of the two compounds with x = 0.10

and x = 0.20 at various temperatures.

does not modify the CF energy levels to any significant degree in these compounds. The spe- cific heat of the compounds shows a Schottky anomaly at around the same temperature, the peak value of which remains about the same as in the case of the magnetization when the content of Co is varied (Tari and Kuentzler - unpub- lished). This figure also shows that the peak height of the magnetization as measured from the con- stant Van Vleck level decreases with increasing Co content but the peak itself does not disappear completely. This suggests that the exchange field acting on Pr remains below the critical field for the metamagnetic transition.

The temperature variations of the paramag- nctic reciprocal susceptibility parallel and per-

-' and X~. ~ pendicular to the e-axis, A'II re- spectively, are shown in fig. 5. The separation ~ between A'I~- ~ and XI-= decreases with increasing cobalt content. This shows that the anisotropy of Co sublattice is becoming important at around x = 0.20. Indeed in Y(Nil_~.Co.~) 5 the easy axis is already c at x = 0.20 [9], and in Pr (Ni0.4Co0.6) 5 the easy axis is [120] at T < 155 K and is e at T > 185 K [10]. In PrNi 5 Barthem et al. [2] and Nait-Saada [6] observed that the effect moments deduced from the slopes of the reciprocal suscep- tibility at 300 K are much larger than the free pr ÷3 moment. Fig. 5 shows that the slope of the inverse susceptibility decreases and hence the effective moment increases with increasing Co content of the compounds.

i t ~ Pr(Nil'xC°x)5 Z'0

1"9

i I 41o8 14 : 1 x=oeo -I.7

(39 -l.6

1"4

:;

o; 04 ~ ~ : 0 "3~_. 0.2 0.1 o~ ~5 30 45

T(K) Fi~, 4. Thermal variation of the low temperature magnetiza- tion of all the fiw compounds stud~ed Jn a field o[ 30 kOe

applied alon8 the easy axis ~ of the orthoh¢xa~onal cell.

A.R. Ball et aL / Magnetic properties of Pr(Ni t - . , Cox).s compounds 189

4. Discussions and conclusion

From the foregoing it is evident that the com- pounds with x > 0.05 are ferromagnetic at low temperatures. The variation of T¢ is quadratic in x (see fig. 6a). Thus one may write:

T¢ =/3x z, wi th/~ = 950 K, (1)

in the concentration range of our investigation. Eq. (1) would give T c = 342 K for PrNi2Co 3, whereas the experimental value is 635 K [10]. Therefore the validity of this quadratic variation of T¢ with x does not extend too far beyond the concentration range of our compounds.

Displayed in fig. 6b is the variation of the spontaneous magnetization M per formula unit, deduced from the Arrott plots, as a function of cobalt concentration. As the figure shows, the variation of M with x is quadratic. The magneti- zation per formula unit may be written as M = Mpr + 5 /ZcoX where /XCo is the Co moment in the ordered state. For Pr one may write Mpr = Xpr Hex(Co) = nco_P~XpdXco x, where nco_P~ is the coefficient of the molecular field due to the inter- action between the Pr and Co sublattices. Xp~ can

1000 i I=~'(Ni~-xC°x)s ~ c

800-~ o ~lc ~o~c . ~ ~ - ~00~ . . ~ ~ , ,

~ ~ ~+~, ~ - ~ . . _ ~00~ ~ ~.~- ' -" ~ ~ ~ ~ ~ . .--" ~ ,~

i :L ~ 0 ~ -e ~ " - ~ ~ ~ ~ ~

. . . . _ ~ _ . . - . ~ ~ = 0 ~ ~ ~ . _ . . - - ~

. I ~ + ~ +e ~ ~- I +0a- -+ .... +

~ ~ - ~ - ~ - q .~~,---- !

o~ ~ - - ~o ~,;, ~6o + a~ 0 100 ~K)

Fig. 5. Temperature variation of the reciprocal suseeptihilities parallel and perpendicular to tke c-axis +1~ i and ~ ~t, +espec-

tively, of the five compounds i~vesiigated.

X ~ 0 0;9) ...... 0:02 . . . . . _0;0_~ ....... ~04

i ~ / / I

....20 ~-~

, ,

(b) ~

~'0 ~ ~ ~

~ ~ ~ m

~ ~ ~ ~

~ ~ ~g ~

O~ ~05 04 0 0.15 0.20 X

Fig. 6. (a) Var ia t ion of ~he Cur ie ~em~era~ure Tc wifi~ x ~ and (b) var ia t ion of ~l~e s~on laneous magnet iza t ion ~er fo rmula

uni t M and g c o with x for the five c o m p o u n d s s tudied.

be a function of the total field acting on the Pr ion. However, as in the case of PrNi.s when the field is applied along the [120] direction, Mpr varies linearly with the field at least as long as the value of this field is less than that of the critical field for the metamagnetic process, we can as- sume that Xt,~ is approximately field indepen- dent. Thus:

M = ( n c o _ P r X p r + 5 ) ~ c o X , (2)

AS M = ~ x 2 we expect P, co to vary linearly with x. The coefficient U = (nc:o_prXp~ + 5) may be estimated (--7.8) from the data of PrNi.~Co 3 [10], where M = 7 . 8 5 and #co = 1.68Ix B. From the knowledge of U and M one can deduce the variation of ~co as a function of the cobalt con- centration x for the compounds studied. This is displayed in fig. 6b which shows that in the corn-

190 A.R. Ball et al. / Magnetic properties of Pr(Ni t - .~.C°x )s compounds

pound with x = 0.20, Co is far from having its full moment because the value of /zoo is 0.85/~B in this compound while in the RCo 5 compounds the value of the Co moment is found to be 1.9/z a [11] irrespective of its rare earth partner. This linear variation of /Zco with x is not valid over the entire concentration range because it would lead to a Co moment of 2.5~ a for x = 0.6 whereas the measured value is 1.68/za [10].

When magnetization varies linearly with the concentration, which is the case in compounds with a well defined and constant moment such as gadolinium intermetallics, T c varies linearly with x. The quadratic variation of Tc with x is the consequence of the linear variation of/ZCo with X.

We conclude that in all five compounds stud- ied the easy direction of magnetization is [120] and the hard axis remains as c. In low fields the magnetization is isotropic in the basal plane but some anisotropy appears at high fields. The sub- stitution of Co for Ni and the concomitant in- crease in the strength of the exchange field acting on Pr is not sufficient to drive Pr into a high magnetic state i.e. above the metamagnet~: tran- sition mentioned above and does little by way of modifying the CF parameters and hence the CF level scheme.

Acknowledgement

This work has been partly supported by Euro- pean Commission with its Research and Develop- ment Programme BIREM-BREU-0068.

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