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Submitted on 1 Jan 1979
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Magnetization behaviour and the valence of uranium in (UxTh1-x)Sb
B. Cooper, O. Vogt
To cite this version:
B. Cooper, O. Vogt. Magnetization behaviour and the valence of uranium in (UxTh1-x)Sb. Journal
de Physique Colloques, 1979, 40 (C4), pp.C4-66-C4-67. �10.1051/jphyscol:1979420�. �jpa-00218816�
JOURNAL DE PHYSIQUE Colloque C4, supplLment au no 4, Tome 40, avril 1979, page C4-66
Magnetization behaviour and the valence of uranium in (U,Th,-,)Sb
B. R. Cooper and 0. Vogt (*)
Dept. of Physics, W. Virginia University (**), Morgantown, W. Va. 26506, U.S.A. and Francis Bitter National Magnet Laboratory (t), MIT, Camb., MA 02139, U.S.A.
(*) Laboratorium fiir Festkorperphysik, ETH, CH-8093 Zurich, Switzerland
Rdsum6. - On a effect& des mesures d'aimantation en champ fort (jusqu'h 100 kOe) sur des monocristaux de la solution solide (U, Thl-,)Sb. Pour (Uo,sTho,z)Sb nous observons un comportement ferromagnktique avec une anisotropie trks importante selon ( 111 ). Pour une concentration importante de Th, I'aimantation est faible, 0,58 ~ E L B pour (Uo,,Tho,,)Sb B 91 kOe avec une faible anisotropie, ce qui sugg5re qye 1'&?t fondamental de champ cristallin est un singulet, mettant en Bvidence un changement de valence de U B U lors de la dilution par le thorium.
Abstract. - High field (up to 100 kOe) magnetization measurements have been performed on single crystal samples within the solid solution pseudobinary (U, Thl-, )Sb. In (Uo sTho SSb ferromagnetic behaviour is found with strong anisotropy favoring ( 111 ) alignment. At high thorium concentration, the small magnetization of (Uo.,Tho.9)Sb which is paramagnetic at 4.2 K, 0.58 pB at 91 k0: with little anisotropy, calls for a singlet crystal-field ground-state, pointing to a valence change from U to u4+ on dilution by thorium.
USb is antiferromagnetic ( T , = 241 K, ordered moment = 2.85 pB), and the wave function beha- viour necessary for compatibility with the neutron scattering behaviour [I] seems to definitely elimina- te a 5f2(U4') configuration for uranium in USb, and strongly points toward a 5f3 ionic state (U3'). With this in mind, the results of our high field (up to 100 kOe) magnetization measurements on single crystal samples within the solid solution pseudobina- ry (U, Th,-,)Sb are very interesting.
The behaviour found can be understood when one considers two effects of the dilution of U by Th.
First, is the fact that one reduces the opportunity for interaction between uranium ions. Thus at high thorium concentration, (U,.,Th,.,)Sb is paramagnetic down to 1.5 K. More interesting is the second ef- f ect , the fact that thorium invariably appears with the tetravalent state, Th4'.
For low thorium concentrations, the primary ef- fect of substitution of thorium for uranium is appa- rently to add one conduction electron per substituted thorium, thereby changing the nature of the magne- tic interaction between the remaining uranium ions from antiferromagnetic toward ferromagnetic. In fact, there is a striking resembTa ce between the effect of low concentration (say y p to 20 or
hi;
30 %) Th substitution for U in USb an4that of low concentration substitution of Te for Sb in USb. Thus both (U,,Th,,)Sb, as shown in figure 1, and [2]
U(Sbo,Teo.2) are ferromagnetic with anisotropy strongly favoring ( 11 1 ) alignment.
(**) Permanent address, supported in part by NSF Grant NO. DMR 77-06721.
(+) Supported by the National Science Foundation.
Fig. 1.
-
Magnetization of (UosTho dSb at 4.2 K.For (U,,Th,,,)Sb, magnetic saturation is reached at somewhat lower field at 20 K than at 4.2 K (as shown in figure I), and the saturation moment, and corresponding large remanent moment, is 2.86 p B per U atom. From figure 1 we see that the ( 100) saturation magnetization and remanent moment is reduced by almost exactly a factor of
m.
The( 110 ) moment is reduced by a factor of
m.
Thisclearly shows that strong anisotropy holds the ma- gnetization along ( l l 1 ).
The presence of ferromagnetism in (U,,,Th,.2)Sb is indicated by the very large remanent moment, but the magnetization process is clearly unusual. Per- haps the domain alignment is initially by domain rotation rather than by wall motion, and this is the basis for the unusual behaviour. It would, however, be most valuable to have neutron diffraction studies to complement the present magnetization studies.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979420
MAGNETIZATION BEHAVIOUR AND THE VALENCE OF URANIUM IN (U,Th,-,)Sb C4-67
At high uranium concentrations the behaviour is compatible with the U3'(5f3) configuration found for USb, but the magnetization at 100 kOe for parama- gnetic (U,,Th,.,)Sb at 4.2 K, as shown in figure 2, is much less than that expected for any crystal-field configuration for Sf3 (by at least a factor of 2 and typically by a factor of 4 or 5). For example, the magnetization is about a factor of 4 less than that expected with crystal-field parameters for U3' de- duced from the neutron scattering measurements [I]
in USb.
moment within a crystal-field model. This in turn implies that the high thorium concentration has forced a valence change of uranium to U4'(5f3.
With such a singlet ground-state, reasonable crystal-field splittings to the first excited state give agreement with the experimental magnetization at 100 kOe. The size of crystal-field splitting required varies from about 90 K, for no exchange, to a few hundred K for increasing ferromagnetic exchange.
The present data and state of analysis are insuffi- cient to independently fix the crystal-field splitting and exchange. However, if we use the shift in 1/x between (U,,,Th,,)Sb and (U,,Th,dSb to crudely estimate the ferromagnetic exchange, and include this by taking Heff = H
+
A,, M with A,, isotropic, we get the behaviour shown by the long dashed curves in figure 2. If we attach significance to the small anisotropy in magnetization found experimen- tally, a corresponding anisotropy in exchange is indicated, with A,, for the ( 11 1 ) direction changing to the value given for the short dashed curve. The detailed nature of the magnetization curves indicates the presence of higher order anisotropic exchange interactions.H (kOe)
Acknowledgments. - We have benefited from
Fig. 2. -Magnetization of (U, ,ThO9)Sb at 4.2 K . discussion with John Faber, Jr., in which he sug- gested that the strong tendency of thorium to select the tetravalent state might explain the valence chan- ge in uranium at high thorium concentration. H.
On the other hand, the magnetization behaviour of Heer and R. Siemann have provided valuable help (U, ,Th,,)Sb in figure 2 can easily be understood if with the numerical computations. We have had there is a
r,
singlet crystal-field ground-state. In valuable discussions with S. Foner, A. Furrer, fact, this is the only explanation possible for the low G. H. Lander, and M. H. Mueller.References
[I] LANDER, G. H., MUELLER, M. H., SPARLIN, D. M. and VOGT, O., Phys. Rev. B 14 (1976) 4035.
[2] B u s c ~ , G., VOGT, O . , DELAPALME, A. and LANDER, G. H . , to be published (1978).
