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PARAMAGNETIC SUSCEPTIBILITY OF UPt AND
CePt
John Huber
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 8, Tome 39, aotit 1978, page C6-78 1
PARAMAGNETIC SUSCEPTIBILITY OF UPt
AND
CePt John G.Hubert
and Carlos A Luengo ttt Physics Department, hrfts University, Medford, Massachusetts, 02155, USA
tt
Institute de Fisica "CZeb Wataghin", Universidade Estudual de CampinasCampinas, SE0 Pauzo, BrasiZ
R6sumB.- On rend compte de mesures de susceptibilite magngtique sur les compos6s de mzme structure UPt et CePt.
On observe un comportement de Curie-Weiss pour les deux composgs au-dessous de leur tempgrature d'ordre ferromagngtique, dO probablement aux configurations 5f1 et 4f l modif ides par le champ cris-
tallin.
Abstract.- Magnetic susceptibility measurements on theisos$ructural compounds UPt and CePt are re- ported. Curie-Weiss behavior, due presumably to 5f' and 4f configurations modified by crystalline electric fields, is observed for both compounds above their ferromagnetic ordering temperatures.
INTRODUCTION.- In the periodic table, U and Ce stand at the beginnnings of series of elements (with par- tially filled 5f and 4f electronic shells,respecti- vely) which, for the most part, sustain local magne-
tic moments in metallic compounds and alloys. They,
themselves, are of ambivalent character. While U and Ce both even superconduct in elemental form (under pressure) ; they can, too, support moments in metal- lic magnets.Where magnetism occurs, U ordering tem- peratures are in general considerably higher than
those for Ce. This is attributed to a lesser degree of 5f electron localization than of 4f. The com- pounds UPt and CePt (both orthorhombic CrB structu-
re) provide a unique opportunity to study the ferro- magnetic ordering of U and Ce in the same metallic
environment. Ferromagnetic ordering of UPt occurs at
30 K /l/; of CePt, at 6 K /2/. Magnetization measu- rements on both compounds /1,2,3/ yield saturation moments significantly less than those expected from
~und's rules considerations alone. In the case of UPt this has been postulated to result from 5f delo- calization, but with novakncy (i.e., 5f occupancy) for the U specified/l,2/ .With CePt, the Ce is tri-
valent (4f1 configuration) from unit cell volume considerations 141, and the reduced saturation mo- ment presumably reflects the presence of crystalline electric fields. Consistent with these interpreta
-
tions, the magnetics entropies determined from heat7
b u p p o r r e v unaer contracc URIC 1 > - v ~ 4 Y 4 f f - Partially supported by CNPq under contract2222.0570/77 and by FINEP under contract HIDROGENIO/ UNICAMP/FINEP/ IF/271.
capacity measurements /5,6/ are considerably smal- ler than those given by potential Hund's rules de- generacies. Specifically, the ordering entropy per
U atom in UPt is even less than kglnZ /5/, the doublet ground state value which is found per Ce atom in CePt 161. Low temperature magnetization measurements under pressure indicate a softer na-
ture of the U moments in these isostructural com- pounds. By 20 kbar
,
the limit of measurement, the U moment is reduced continuously and reversibly tobelow 10% of its zero pressure value 131; out to 7 kbar, again the limit, the Ce moment even increa-
ses slightly 1 2 1 .
EXPERIMENTAL.- We report here magnetic susceptibi- lity data for UPt and CePt fromjustabove their ordering temperatures to beyond and to room tem- perature. The samples were arc melted, then annea- led at 900°C and 700°c, respectively, for 10days in an inert atmosphere. The UPt data were taken in a superconducting susceptometer with a measuring field of 150 gauss /7/; those for CePt were taken in a Faraday magnetometer at 8 Kgauss 181. Both sets are plotted in. Figure 1 as inverse suscepti- bility versus temperature.
RESULTS AND DISCUSSION.- Figure 1.shows clearly the Curie-Weiss behavior of both U and Ce moments at temperatures sufficiently larger than their ferromagnetic ordering temperatures in the metal- lic, isostructural, equiatomic compounds UPt and CePt. The high temperature slopes (solid lines in the figure) yield effective moments of 2.62 and
Fig.1 : Plot of inverse susceptibility versus tenr perature.
2.43 Bohr magnetons(pB), respectively. It is signi- ficant that, within experimental error, these cor- respond to the Hund's rules value for either a 5f l or a 4f1 configuration, 2.54 pg. For comparison, other possible U configurations are 5f2, 5f and 5f4 for which the Hund's rules effective moments
are 3.58 pB, 3.53 p~ and again 2.54 pB.Theheatca- paci ty masurements of hengo et a1./5/, pointing to an
effective doublet ground state for UPt, would seem to obviate the even 5f occupancy configurations, and the susceptibility measurements reported here argue against 5f 3 .
CONCLUSIONS.- We conclude, then, that in their equiatomic compounds with Pt, U and Ce have 5f1 and 4f1 configurations, respectively ; but that the Hund's rules J = 5/2 six-fold degeneracies for these configurations are lifted by crystalline electric field4 giving a ground state doublet and four exci- ted states. While the excitation energies are not known, the curvatures of the inverse susce~tibili- ties (see figure 1.) may indicate a greater level splitting for W t .
the University of California, San Diego for opening their laboratory facilities to him.
References
/ l / Matthias B.T., Chu C.W., Corenzwit E.and Wohlleben D., Proc.Nat1. Academy of Sciences 64 (1969) 459
-
/2/ Huber J.G., reported at the International Con- Ference on Magnetism (Amsterdam,l976) and to be published.
/3/ Huber J.G., Maple M.B. and Wohlleben,D. ,J. Magn. 8 Magn. Mater. l(1975) 58.
/4/ Unit cell volumes for the equiatomic rare earth platinum compounds can be calculated from the crystallographic data of Dwight A.E., Conner R.A., Jr. and Downey J.W., Acta Crysta.E (1965) 835. LaPt and CePt have CrB structure whereas PrPt and beyond have the related or-
thorhombic FeB structure.
/5/ Luengo C.A., McCallum R.W., Huber J.G. and Maple M.B., Low Temperature Physics
-
LT14, M.Krusius and M.Vurio, eds. (North Holland, Amsterdam,l975),3,
p.192 and Luengo C.A., Huber J.G. and Maple M.B., J.Magn & Magn. Mater3m (1976) 305.
-
/6/ Ramsden J., Holt B.J. and Huber J.G., to be published.
/7/ S.H.E.Corporation.ll661 Sorrento Valley Road, San Diego, CA 92121, USA.
/8/ Wohlleben D., Ph.D.Thesis, University of Cali- fornia, San Diego (1968).
