Neutron diffraction studies of CeAl2 at low temperature

(1)

HAL Id: jpa-00218892

https://hal.archives-ouvertes.fr/jpa-00218892

Submitted on 1 Jan 1979

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Neutron diffraction studies of CeAl2 at low temperature

B. Barbara, J. Boucherle, J. Buevoz, M. Rossignol, J. Schweizer

To cite this version:

B. Barbara, J. Boucherle, J. Buevoz, M. Rossignol, J. Schweizer. Neutron diffraction studies of CeAl2 at low temperature. Journal de Physique Colloques, 1979, 40 (C5), pp.C5-321-C5-322.

�10.1051/jphyscol:19795109�. �jpa-00218892�

(2)

JOURNAL DE PHYSIQUE Colloque C5, supplément au n° 5, Tome 40, Mai 1979, page C5-321

Neutron diffraction studies of CeAl ² at low temperature

B. Barbara (*), J. X. Boucherle (**), J. L. Buevoz (°), M. F. Rossignol (*) and J. Schweizer (**°)

(*) Laboratoire Louis-Neel, C.N.R.S., 166X, 38042 Grenoble Cedex, France (**) D.R.F.-D.N., Centre d'Etudes Nucleases, 85X, 38041 Grenoble Cedex, France (°) Institut Laue-Langevin, 156X, 38042 Grenoble Cedex, France

Résumé. — Des mesures de neutrons polarisés sur CeAl

2

montrent que le magnétisme est essentiellement dû à des ions Ce

3 +

et mettent en évidence un couplage anormal entre les spins de Ce et la polarisation des électrons de conduction. D'après les mesures de neutrons sur poudre et sur monocristal nous avons démontré que la struc- ture magnétique est modulée sinusoïdalement jusqu'à r

N

/10, résultat incohérent avec la dégénérescence de Kra- mers. Ces deux points particuliers peuvent être reliés au comportement Kondo à haute température.

Abstract. — Polarized neutron measurements on CeAl

²

show that the magnetism is essentially due to a Ce

^{3 +}

ion and indicate an anomalous coupling between the Ce spins and the polarization of conduction electrons.

From powder and single crystal neutron measurements we have demonstrated that the magnetic structure is sinusoi'dally modulated down to r

N

/10, a result which is incompatible with Kramers' degeneracy. These two particular results can be related to the high temperature Kondo behaviour.

1. Introduction. — At high temperature, CeAl

²

is f "(HB/Ce) known to be a Kondo compound [1]. In this paper H « ° _* we summarize the results of neutron diffraction I r ??,'«"

j * M ««

studies carried out in Grenoble, which clarify the „ J'V* „ nature of the magnetism in CeAl

2

at low tempe- .

0 4

»°!l 73 <=

rature. f~ »s

° : -" sS

2. Individual behaviour of cerium atom. — Polarized • ; ; " ^ » neutron diffraction allows to access to the magne- , • * " r 2 5

tization densities. Such experiments have been done ° ' '> SS, on CeAl

²

at T = 1.5 K on the ferromagnetic compo- "°-

^{2 s T}

* " J _«

nent induced by an applied field of 48 kOe using the "» * « »

^s

"° ; *o

facilities of the diffractometer_D5 at the I.L.L. For a ° . • t • * " ? s field applied along the [Oil] axis, the magnetic » « i*

amplitudes show a substantial and unusual dispersion S» ^ • f |

which can be explained by the anisotropy of the 4f ZJL?- • '*

¹

° * ' •

^sin9

f

^A

(*"

¹

) form factor : a calculation based on a Ce

^{3 +}

model, " 5 i,'*

^a

!|g * J 1 including crystal and exchange fields [2] shows a very <° «S ^, , , , good agreement with the experiment (figure 1). =?£ sg 5 | Thus it is obvious that the greatest part of the magne-

⁵

SS » tism is due to a C e

3 +

moment.

However the systematic deviations observed at f *

¹

— °

^{b t}

? ,

e r v e d ( #

]

^a

"

^{d f i c}

^

^{c u ,}

,

^{a t e d}

^ { ™

f a c t o r s f

° f C e - i o n

J

. . in CeAl

2

with an applied field along [Oil].

low sin Oil are very interesting. The calculated values of the form factor are smaller than the observed ones.

This is due to a polarization of conduction electrons

T h i g reguU J s & d i r e c t m e a s u r e m e n t o f t h e n e g a t i v e

as shown previously in the isomorphous compound

r e s Q n a n t

^ introduced to describe the Kondo NdAl

2

[3] But in CeAl

2

contrary to the results

e f l f e c t {n cedum c o m d s [ 7 ]

.

obtained for the other RA1

²

compounds [3-5] and more generally for rare earth metal and alloys [6],

this polarization tends to increase the C e

3 +

moment. 3. Collective behaviour of cerium moments. — Since J = L — S the spins S and o of Ce and conduc- Taking into account the localized magnetism on the tion electrons are opposite corresponding to a negative Ce

^{3 +}

ions, it is possible to throw some light on their constant 3 in the coupling Hamiltonian — 2 3So\ collective behaviour by studying the magnetic struc-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19795109

(3)

C5-322 B. BARBARA, J. X. BOUCHERLE, J. L. BUEVOZ, M. F. ROSSIGNOL AND J. SCHWEIZER

ture at low temperature. Given the negative results of many experiments performed on classical powder neutron diffractometer, we have used the ultra- sensitive multidetector powder diffractometer D1B of the I.L.L. Numerous very weak extra lines were observed at 1.9 K. They were indexed by means of a unique propagation vector k

⁼

112 + ^{z, 112}

^-

^z, ^112,

with z = 0.1 12. Their intensities allow to show that all the moments are along the [l 1 l] axis and sinusoidal- ly modulated. The experimental maximum value of the moment (0.89 p,) is in good agreement with a calculation [g], assuming a r7 doublet as crystal field ground state [9]. Such a ground level should lead, a t low temperature, to an antiphase structure [l01 which cannot be detected here due to :

-

the weakness of the moment which does not allow to see higher order satellites,

-

the relatively high value of the temperature : at 1.9 K (- TN/2) the thermal effect cannot be completely neglected [l l].

A study [l21 on a single crystal down to 0.4 K (- TN/lO) has confirmed the powder results : the sinusoidally modulated structure (figure 2) remains

Flg. 3.

-

Thermal variation of

z

and of the lntenslty of (171)' satellite.

frozen as shown by the absence of higher order satellites and the invariance of z value (figure 3) down to 0.4 K. The resulting reduction of most of the Ce moments is incompatible with a T7 Kramers doublet and implies a non magnetic ground level [12].

4. Conclusion.

-

Three important points stand out in these neutron studies :

-

a magnetic ordering exists in CeAl, below 3.8 K,

-

the stable modulation of the moment implies a singlet as ground state [ll]. Such a ground state cannot arise from crystal field for a Kramers' ion, but can be due to a coupling with conduction electrons.

-

this coupling constant J is negative which is a necessary condition for the existence of a Kondo singlet [13].

Numerous theoretical studies [14-171 are in progress to explain the coexistence of magnetic ordering and

Fig. 2.

-

Magnetic structure of CeAl,. Kondo effect in CeA1,.

References

[l] BUSCHOW, K. H. J., VAN DAAL, H. J., Phys. Rev. Lett. 23 BARBARA, B., ROSSIGNOL, M. F., PURWINS, H. G., WALKER, E.,

(1969) 408

; Solid State Commun.

17 (1975) 1525

;

CORNUT, B., COQBLIN, B., Phys. Rev. B 5 (1972) 4541. LOWENHAUPT, M,, STEGLICH, F., Physica 86-88B (1977) [2] EARBARA, B., BOUCHERLE, J. X., DESCLAUX, J. P,, ROSSI- 187.

GNOL, M. F., SCHWEIZER, J., Crystalfield eHects

in metals

[l01 GIGNOUX, D., LEMAIRE, R., PACCARD, D., Phys. Lett. 41A

and alloys. Ed. by A. Furrer (Plenum Press New York)

(1972) 187 ;

1977 p. 168. GIGNOUX, D., GOMEZ-SAL, J.-C., LEMAIRE, R.,

DE

COMBA-

BOUCHERLE, J. X., PhD Thesis, University of Grenoble (1977). RIEU, A., Solid State Cornrnun.

21

(1977) 637.

[3] BOUCHERLE, J. X., SCHWEIZER,

J., Physica 86-88B (1977)

[l11 ELLIOTT, R. J., Phys. Rev.

124

(1961) 346.

174. [l21 BARBARA, B., BOUCHERLE, J. X., BUEVOZ, J. L., ROSSIGNOL,

[4] BARBARA, B., ROSSIGNOL, M. F., PURWINS, H. G., WALKER, M. F., SCHWEIZER, J. (to be published).

E., ref. [2], p. 148. [l31 NOZ&RES, P., J. Low Temp. Phys. 17 (1974) 31.

[5] BERTHIER, Y., DEVINE, R. A. B., BELORIZKY, E., Phys. Rev. B [l41 JULLIEN, R., FIELDS, J., DONIACH, S., Phys. Rev. Lett. 38

17 (1978) 4137. (1977) 1500.

[6] MOON, R. M,, KOEHLER, W. C., CABLE, J. W., CHILD, H. R., [l51 BREDL, C. D., STEGLICH, F., SCHOTTE, K. D., Z. Phys. B

29

Phys. Rev. B 5 (1972) 997.

(1978) 327.

[7] COQBLIN, B., RATTO, C. F., Phys. Rev.

21

(1968) 1065. [l61 BENOIT, A., FLOUQUET, J., RIBAULT, M., FLOUQUET, F., [S] BARBARA, B., BOUCHERLE, J. X., BUEVOZ,

J. L., ROSSIGNOL,

CHOUTEAU, G., TOURNIER, R., J. Physique Let?.

39

M. F., SCHWEIZER, J., Solid State Cornmun.

24

(1977) (1978) L-94.

481. [l71 BARBARA, B., CYROT, M,, LACROIX-LYON-CAEN, C., ROSSI-

[9] HILL, R. W., MACHADO

DA

Neutron diffraction studies of CeAl2 at low temperature

HAL Id: jpa-00218892

https://hal.archives-ouvertes.fr/jpa-00218892

Submitted on 1 Jan 1979

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Neutron diffraction studies of CeAl2 at low temperature

B. Barbara, J. Boucherle, J. Buevoz, M. Rossignol, J. Schweizer

To cite this version:

B. Barbara, J. Boucherle, J. Buevoz, M. Rossignol, J. Schweizer. Neutron diffraction studies of CeAl2 at low temperature. Journal de Physique Colloques, 1979, 40 (C5), pp.C5-321-C5-322.

�10.1051/jphyscol:19795109�. �jpa-00218892�

JOURNAL DE PHYSIQUE Colloque C5, supplément au n° 5, Tome 40, Mai 1979, page C5-321

Neutron diffraction studies of CeAl 2 at low temperature

B. Barbara (*), J. X. Boucherle (**), J. L. Buevoz (°), M. F. Rossignol (*) and J. Schweizer (**°)

(*) Laboratoire Louis-Neel, C.N.R.S., 166X, 38042 Grenoble Cedex, France (**) D.R.F.-D.N., Centre d'Etudes Nucleases, 85X, 38041 Grenoble Cedex, France (°) Institut Laue-Langevin, 156X, 38042 Grenoble Cedex, France

Résumé. — Des mesures de neutrons polarisés sur CeAl

montrent que le magnétisme est essentiellement dû à des ions Ce

et mettent en évidence un couplage anormal entre les spins de Ce et la polarisation des électrons de conduction. D'après les mesures de neutrons sur poudre et sur monocristal nous avons démontré que la struc- ture magnétique est modulée sinusoïdalement jusqu'à r

/10, résultat incohérent avec la dégénérescence de Kra- mers. Ces deux points particuliers peuvent être reliés au comportement Kondo à haute température.

Abstract. — Polarized neutron measurements on CeAl

show that the magnetism is essentially due to a Ce

ion and indicate an anomalous coupling between the Ce spins and the polarization of conduction electrons.

From powder and single crystal neutron measurements we have demonstrated that the magnetic structure is sinusoi'dally modulated down to r

/10, a result which is incompatible with Kramers' degeneracy. These two particular results can be related to the high temperature Kondo behaviour.

1. Introduction. — At high temperature, CeAl

is f *"(HB/Ce) known to be a Kondo compound [1]. In this paper H « ° _ we summarize the results of neutron diffraction I r ??,'«"

studies carried out in Grenoble, which clarify the „ J'V* „ nature of the magnetism in CeAl

at low tempe- .

»°!l 73 <=

rature. f~ »s

2. Individual behaviour of cerium atom. — Polarized • ; ; " ^ » neutron diffraction allows to access to the magne- , • * " r 2 5

tization densities. Such experiments have been done ° ' '> SS, on CeAl

at T = 1.5 K on the ferromagnetic compo- "°-

* " J _«

nent induced by an applied field of 48 kOe using the "» * « »

"° ; *o

facilities of the diffractometer_D5 at the I.L.L. For a ° . • t • * " ? s field applied along the [Oil] axis, the magnetic » « *i

amplitudes show a substantial and unusual dispersion S» ^ • f |

which can be explained by the anisotropy of the 4f ZJL?- * • '

° * ' •

f

(*"

) form factor : a calculation based on a Ce

model, * " 5 i,'

!|g * J 1 including crystal and exchange fields [2] shows a very <° «S ^, , , , good agreement with the experiment (figure 1). =?£ sg 5 | Thus it is obvious that the greatest part of the magne-

SS » tism is due to a C e

moment.

However the systematic deviations observed at f *

— °

? ,

]

"

^

,

^ { ™

° f C e - i o n

. . in CeAl

with an applied field along [Oil].

low sin Oil are very interesting. The calculated values of the form factor are smaller than the observed ones.

This is due to a polarization of conduction electrons

as shown previously in the isomorphous compound

^ introduced to describe the Kondo NdAl

[3] But in CeAl

contrary to the results

.

obtained for the other RA1

compounds [3-5] and more generally for rare earth metal and alloys [6],

this polarization tends to increase the C e

moment. 3. Collective behaviour of cerium moments. — Since J = L — S the spins S and o of Ce and conduc- Taking into account the localized magnetism on the tion electrons are opposite corresponding to a negative Ce

ions, it is possible to throw some light on their constant 3 in the coupling Hamiltonian — 2 3So\ collective behaviour by studying the magnetic struc-

C5-322 B. BARBARA, J. X. BOUCHERLE, J. L. BUEVOZ, M. F. ROSSIGNOL AND J. SCHWEIZER

112 + z, 112

z, 112,

the weakness of the moment which does not allow to see higher order satellites,

the relatively high value of the temperature : at 1.9 K (- TN/2) the thermal effect cannot be completely neglected [l l].

A study [l21 on a single crystal down to 0.4 K (- TN/lO) has confirmed the powder results : the sinusoidally modulated structure (figure 2) remains

Flg. 3.

Thermal variation of

and of the lntenslty of (171)' satellite.

frozen as shown by the absence of higher order satellites and the invariance of z value (figure 3) down to 0.4 K. The resulting reduction of most of the Ce moments is incompatible with a T7 Kramers doublet and implies a non magnetic ground level [12].

Neutron diffraction studies of CeAl ² at low temperature

is f "(HB/Ce) known to be a Kondo compound [1]. In this paper H « ° _* we summarize the results of neutron diffraction I r ??,'«"

facilities of the diffractometer_D5 at the I.L.L. For a ° . • t • * " ? s field applied along the [Oil] axis, the magnetic » « i*

which can be explained by the anisotropy of the 4f ZJL?- • '*

model, " 5 i,'*

112 + ^{z, 112}

^z, ^112,