High field, high pressure magnetic properties of UAl2

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High field, high pressure magnetic properties of UAl2

J. Fournier, J. Beille

To cite this version:

J. Fournier, J. Beille. High field, high pressure magnetic properties of UAl2. Journal de Physique

Colloques, 1979, 40 (C4), pp.C4-145-C4-146. �10.1051/jphyscol:1979447�. �jpa-00218843�

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JOURNAL DE PHYSIQUE Colloque C4, supplément au n" 4, Tome 40, avril 1979, page C4-145

High field, high pressure magnetic properties of UA1

²

J. M. Fournier and J. Beille (*)

Centre d'Etudes Nucleaires de Grenoble, Departement de Recherche Fondamentale, Section de Physique du Solide, 85 X,

38041 Grenoble Cedex, France —

(*) Laboratoire Louis-Neel, C.N.R.S., BP 166, 38042 Grenoble Cedex, France

Résumé. — Nous avons mesuré l'aimantation de UA12 sous champ fort (jusqu'à 150 kG) et sous haute pression hydrostatique (jusqu'à 7 kbar). La compressibilité de UA12 a également été déterminée jusqu'à 4 K.

Abstract. — M e a s u r e m e n t s a r e r e p o r t e d of t h e m a g n e t i z a t i o n of UAh u n d e r high field (up t o 150 k G ) a n d high h y d r o s t a t i c p r e s s u r e (up t o 7 k b a r ) . T h e c o m p r e s s i b i l i t y of UA12 h a s a l s o b e e n d e t e r m i n e d d o w n t o 4 K .

1. Introduction. — The alloy UAI2 has been exten- sively studied as being representative of a concen- trated spin fluctuation system [1]. The physics of UA12 can be understood through the existence of a narrow 5f band at the Fermi level. We thus have undertaken a study of the magnetic and elastic properties of UA1² under pressure assuming that it should be a sensitive parameter.

2. Experimental.

— The sample was prepared by induction heating of the proper amount of the pure metals. Its quality was checked both by X-ray diffraction and by che- mical analysis.

— Magnetic measurements under pressure have been performed at the S.N.C.I., true hydrostatic pressure up to 7 kbar being obtained using helium as the transmitting medium. Fields up to 70 kG were produced by a superconducting coil and the magnetization was measured by the extraction method [2].

— Magnetization up to 150 kG, down to 1 K under atmospheric pressure has been measured at the S.N.C.I. using a water-cooled Bitter coil and the extraction method [3].

— High pressure neutron diffraction measurements have been performed at I.L.L., the quasi- hydrostatic pressure, up to 20 kbar being obtained by a clamp technique [4].

— Strain gauge thermal expansion measurements have been used to check the above results at 300 K and 77 K, up to 6 kbar.

3. Experimental results. — The magnetization re- mains linear up to 150 kG and down to 1 K. These two extreme values correspond to fx.BH/kB T = 10 and an induced moment —0.1 ju-B. This result should be explained by paramagnons models.

The susceptibility of UA1² between 4 K and 250 K is shown on figure 1 plotted versus T for different

Fig. 1. — Susceptibility of UA12 versus temperature for different pressures.

pressures. Two essential features are readily obtained from i t :

— the x(T) curves keep the same shape under pressure,

— the average pressure dependence is very important, being equal to :

^ P = - 2 5 M b a r - ' .

In order to obtain the true volume dependence of the susceptibility we have determined the compressibility /3 of UAL, ; its pressure dependence is reported on figure 2 at 300 K and 4 K, obtained from neutron diffraction (we observed that no structural change occurs down to 4 K, up to 20 kbar).

The room temperature value jS^300K = 1.65 Mbar~' obtained from the more precised strain gauge technique is rather high and gives a volume dependence of the susceptibility :

9 L° g * = +1 5 . a Log v

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979447

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C4-146 3. M. FOURNIER AND J. BEILLE

However the salient feature of figure 2 is the very important change in compressibility between 4 K and 300 K (a factor of two). Strain gauge thermal expansion measurements indicate that p is essential- ly constant above 80 K so that the change in @ takes place at low temperature.

- an increase of

1

^{8,1 :}observed, - no change of p,,, : observed.

To study the low temperature regime, one has to substract the saturated value of the high temperature regime,

X A -

¹⁰^x emu/g. Doing this, again the regime is understood by plotting 1/x -

XA

^versusT.

There is a Curie-Weiss regime below 40 K, down to 10 K (Fig. 3). Below 10 K the susceptibility again saturates. This second regime is characterized by the following constants

1

6= 0.58 M bar- I I

2704 , I

, , 1

5 10 15 2 0

P I k bar)

Fig. 2. - Compressibility of UAl, versus pressure at 300 K and 4 K.

4. Discussion.

-

We don't have yet enough results to discuss the interesting elastic properties of UA1, and will restrict ourselves to the susceptibility results.

The thermal contraction from 300 K to 4 K is equivalent to that produced by a pressure of

-

10 kbar. Thus the temperature dependence of

x

is not governed by thermal expansion and the main effect of the pressure is probably to increase the narrow 5f bandwidth.

A more careful analysis of

x(T)

should distin- guish two regimes : a high temperature regime above 50 K and a low temperature regime below.

The high temperature regime is readily understood by plotting 1/x versus T : a Pauli like susceptibility progressively transforms into a Curie-Weiss law above 100 K. It is characterized by the following constants :

A -

¹⁰^x ^emu/g

8,- -230K peff = 2'94 PB

TT

-

¹⁰⁰K (end of the Curie-Weiss behaviour)

.

This high temperature regime is probably due to local spin fluctuations giving rise to temperature- induced local magnetic moments as recently propo- sed by Moriya [S]. The local character of the spin fluctuations is indicated by the value of peff which is comparable to U'4 ones. The negative value of the paramagnetic Curie temperature indicates that the fluctuations are antiferromagnetic rather than ferro- magnetic.

The pressure effects are consistent with this inter- pretation : assuming that under pressure the Sf bandwidth increases one should have :

-

a decrease of

X A

^:^observed,

- an increase of T* : observed,

This low temperature regime is probably due to paramagnons (non-localized spin fluctuations). The non-local character is indicated by the small value of the effective moment. However one should be sure that no magnetic ordering occurs at very low temperature (below 1 K) before doing any detailed theory of this low temperature regime.

Fig. 3.

-

Inverse susceptibility of UA1, versus T (after substrac- tion of the high temperature saturated value, , y h = l o x

uem/g).

5. Conclusion.

-

The pressure dependence of the susceptibility of UA1, and the analysis we have made show that two regimes should be distinguished : a low temperature one, below 50 K, probably governed by paramagnons and a high temperature one, above 50 K, governed by localized spin fluctuations.

Further experiments are underway on this so interesting alloy.

Acknowledgments. -We wish to thank I.L.L. and S.N.C.I. for the possibility to perform the measurements reported in this article.

References

[I] TRAINOR, R. J., BRODSKY, M. B. and CULBERT, H. V., Phys.

Rev. Lett. 34 (1975) 1019.

[2] BARTHOLIN, H., FLORENCE, D. and PAUREAU, J., International Conference on High Pressure (Moscou) 1973.

[3] FERON, J. L . , HUG, G., MORIN, P., PICOCHE, J. C., REBOUIL

LAT, J. P., Revue Phys. &pl. 6 (1971) 55-58.

[4] BLOCH, D., PAUREAU, J., VOIRON, J. and PARISOT, G., Rev.

Sci. Instrum. 47 (1976) 296.

[ S ] MORIYA, T . , Solid State Commun. 26 (1978) 483.