CRYSTAL-FIELD AND EXCHANGE ANISOTROPY IN THE MAGNETIZATION OF CexY1-xSb AND CexLa1-xSb

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CRYSTAL-FIELD AND EXCHANGE ANISOTROPY IN THE MAGNETIZATION OF CexY1-xSb AND

CexLa1-xSb

B. Cooper, O. Vogt

To cite this version:

B. Cooper, O. Vogt. CRYSTAL-FIELD AND EXCHANGE ANISOTROPY IN THE MAGNETIZA-

TION OF CexY1-xSb AND CexLa1-xSb. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-1026-

C1-1027. �10.1051/jphyscol:19711367�. �jpa-00214404�

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JOURNAL DE PHYSIQUE Colloque C 1, suppliment au no 2-3, Tome 32, Fkvrier-Mars 1971, page C 1

-

1026

CRYSTAL-FIELD AND EXCHANGE ANISOTROPY IN THE MAGNETIZATION OF Ce,Y,-,Sb AND CeJ,a,-,Sb

B. R. COOPER

General Electric Research and Development Center Schenectady, New York, 12301, U. S. A.

and 0. VOGT

Laboratorium fur Festkorperphysik, ETH Zurich, Switzerland

R6sum6. - Pour une concentration en Ce d'environ 30 % dans CezY1-Sb ou 60 % dans CezLa~-~Sb, I'axe de facile aimantation (H allant jusqu'a 100 kOe en champ pulsk) passe de

<

100

>

comme dans CeSb pur a

<

111

>

ainsi que le prkvoit la thkorie du champ cristallin. Nous interprktons ceci comme une dilution de I'kchange anisotrope, mais qui laisse l'intensitk du champ cristallin presque inchangke lors du remplacement de Ce par Y ou par La. De plus, le com- portement anormal de la susceptibilitk inverse pour CeSb (minimum pour Tn ⁼16 OK, maximum, deuxikme minimum pour T plus bas) devient normal pour une concentration en Ce rkduite. I1 semble que l'interaction d'khange anisotrope soit trks sensible A l'espacement du rkseau.

Abstract. - At a Ce concentration of about 30 % in C ~ , Y I - ~ S ~ or 60 % in Ce,Lal-,Sb, the easy axis of magnetization (H up to 100 kOe pulsed field) changes from

<

100

>

as in pure CeSb to

<

11 1

>

as predicted by crystal-field theory. We understand this on the basis of diluting anisotropic exchange, but leaving crystal-field strength almost unchanged on replacing Ce with Y or La. Also, the anomalous inverse susceptibility behavior for CeSb (minimum at TN = 16 ^{O K ,} maximum, second minimum at lower T) becomes normal for lowered Ce concentration. There is evidence that the anisotropic exchange interaction is quite sensitive to lattice spacing.

Curves of magnetization versus applied field for of strongly anisotropic exchange favoring the

<

100

>

CeSb (NaCI-structure) at 1.5 OK show a series of alignment and overcoming the tendency of the crystal- sharply defined steps [I]. The easy direction is

<

100

>,

field to favor

<

^{11 1}

>

alignment.

and after the final step at 40 kOe, the

<

100

>

magne- With these ideas in mind, we have studied the tization approaches the free ion value of 2.14 y,. change in magnetic behavior of CeSb on diluting the The low field susceptibility behavior is also un- Ce with both Y and La. Single crystals of both usual [2]. The inverse susceptibility, 1 / ~ , goes through a Ce,Y, -,Sb and Ce,La, -,Sb have been made across minimum at T, = 16 OK, then has a maximum a t the entire composition range from pure CeSb to pure T ^E11 OK, and finally goes through a second mini- YSb or LaSb. This study was made with two primary

mum at 6.5 OK. motivations. First was to study the role of anisotropic

Wang and Cooper [3] have explained this 1 / ~ exchange, particularly in competition with crystal-field behavior as having its origin in crystal-field effects. A anisotropy, in determining the highly anisotropic cubic crystal-field splits the six-fold degeneracy of the magnetizafion of CeSb. Second, was to see whether free Ce3+(J = 512) ion into two levels, T,, and

r,,

the anomalous 1 / ~ behavior would become normal with two-fold and four-fold degeneracy respectively. as the strength of exchange relative to crystal-field was The behavior of 1 / ~ for CeSb can be understood if weakened by dilution of Ce, as expected from the the

T7

doublet is the crystal-field ground state.

h here

theory of Wang and Cooper [3].

is strong evidence [4, 51 that this is the case for CeP With Y or La dilution of Ce, there is a remarkable and CeAs.) According to the theory [3], the anomalous change to a

<

11 1

>

easy direction as predicted by 1 / ~ behavior arises because there is a range of exchange crystal-field theory. This is shown in figure 1 for field for which the splitting within the

r,

ground state Ce,La, -,Sb. At a Ce concentration of about 60

%

in doublet decreases with increasing field until the doublet Ce,La, -,Sb or about 30

%

in Ce,Y, -,Sb the high levels cross. There is no anomalous 1/x behavior

if

the (pulsed) field magnetization is almost isotropic. Above

r,

quartet is the ground state. this Ce concentration, there is large anisotropy with Rainford and Turberfield [5] have measured the a

<

100

>

easy direction ; below this Ce concentration

r,-r,

crystalfield splitting, A ^E6 W, by neutron there is large anisotropy with a

<

11 1

>

easy direc- inelastic scattering. The crystal-field splitting was not tion. We understand this on the basis of diluting well determined. However, i t was possible t o put an anisotropic exchange, but leaving crystal-field strength upper limit on A of 25 OK. The best fit to the neutron almost unchanged. (The lattice constant for LaSb is energy distribution gives a value for A of 17.4 OK. about 1.2

%

greater than that of CeSb, and that of There is an interesting contrast between the expected YSb is about 4.5

%

less than that of CeSb. If the behavior for the magnetization on the basis of a point charge model holds as it seems to for other rare crystal-field scheme with the

r,

doublet as ground earth-group V compounds, then A

--

l/a5, and we state, and the M vs H behavior observed [I] in CeSb expect A for very dilute Ce in LaSb to be about 6

%

at 1.5 OK. This is that the crystal-field-only theory has less than for pure CeSb, and A for dilute Ce in YSb

<

111

>

as the easy direction, while it is

<

100

>

in to be about 26

%

greater. Both these changes would CeSb at high H. This led us to suspect the presence imply only a small change in the crystal-field-only M

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711367

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CRYSTAL-FIELD AND EXCHANGE ANISOTROPY IN THE MAGNETIZATION C1

-

¹⁰²⁷

FIG. 1. - Magnetization per Ce ion for CezLal-zSb at 1.5 ^OK.

versus H curves.) Also, the I/x behavior becomes more normal for lowered Ce concentration. There is strong evidence that La dilution is more effective than Y dilution in decreasing exchange. At low Ce concentration, both CexLa,-,Sb and CexY,-,Sb have l / ~ versus T, and high field M vs H behavior at low T, close to that given by crystal-field-only theory.

We have c ~ m p a r e d the magnetization per Ce ion for the x ⁼0.1 samples to the crystal-field-only theory for A ^EZ6 W = 20 OK, and the agreement is reasonably good. Presumably the difference between the experi- ment for x = 0.1 and crystal-field-only theory can be explained by the presence of a small exchange field together with the experimental uncertainties. (The biggest experimental uncertainty is probably accurate determination of the Ce concentration. Such accurate determination is very important since all magnetiza- tions used in the analysis are normalized per Ce ion.) Clearly for exchange effects to be responsible for the easy axis changing from

<

11 1

>

to

<

¹⁰⁰

>

with increasing Ce concentration, the exchange must be highly anisotropic. For a cubic material, a linear (with magnetization) exchange field has to be isotropic. Therefore, we consider the effect of a cubic exchange field,

where M is the magnetization per Ce ion and D < h , k , I >

differs with direction of M . As shown by the dashed curves in figure 2 such an exchange field scaled linearly with concentration of Ce (corresponding to constant

FIG. 2. - Theoretical magnetization per Ce ion for CezLal-+Sb at 1.5 OK.

microscopic exchange interaction between Ce ions independent of dilution) describes the general trend of anisotropic magnetization behavior. However, the cr opening up N of the anisotropy in the M vs H curves as x departs in either direction from the value for isotropic behavior is much less than the experimental behavior. The most probable reason for the rapid

<( opening up ^Dexperimentally is that the microscopic

anisotropic exchange interaction has a strong concentration dependence (because of lattice parameter variation). TO give an idea of how such a concentration dependence of the exchange interaction would affect the magnetization behavior, we have arbitrarily let the M~ term in He,, vary as x3. This gives the solid curves in figure 2 which more closely resemble the experimental behavior. (In both sets of curves in figure 2, we have assumed that whatever antiferromagnetic arran- gement holds at H = 0, is quickly overcome by the applied field aligning the moments. Since [6] TN = 3 OK for x = 0.7 and TN

<

1.6 OK for the smaller x values shown, this is probably reasonable.)

We hope to refine our experiments and analysis to make more quantitative our suggestions about the lattice spacing dependence of the anisotropic exchange in the (Ce, La, Y) Sb system.

Acknowledgements. - We are grateful to Miss E. Kreiger for her aid with the numerical calculations, and to Miss I. Schneider for her aid in preparing the crystals and performing the measurements.

References

111 B u s c ~ (G.) and VOGT (O.), Phys. Letters, 1967, 25A, [3] WANG (Y. L.) and COOPER (B. R.), to appear in Phys.

449. Rev., October, ^1970.

121 The numbers quoted here are from the present study. [4] JONES (E. D.), Phys. Letters, 1966, 22, 266.

Qualitatively similar effects were observed by [5] RAINFORD (B.), TURBERFIELD XK. C.), BUSCH (G.) and TSUCHIDA (T.) and WALLACE (W. E.), J. Chem. VOGT (O.), J. Phys. C , 1968, 1 , 679.

Phys., 1965, 43, 2885. [6] LANDOLT (M.), VOGT (0.) and COOPER (B. R.), to be published, ^1970.