Magnetism of californium metal

Physica 130B (1985) 225-227 North-Holland. Amsterdam

MAGNETISM OF CALIFORNIUM METAL* S.E. NAVE't, J.R. M O O R E t , M.T. S P A A R t , R.G. HAIRE** and Paul G. H U R A Y t * * "t Physics Department. The University o[ Tennesee, Knoxville, TN 37996, USA ** Transuranium Research Laboratory, Oak Ridge National Laboratory, Oak Ridge, TN 37830, USA Magnetic susceptibility measurements have been made on samples of californium-249 metal having the dhcp crystal structure. At temperatures between 100 and 34(1K and at fields up to 50 kilogauss, the samples exhibit Curie-Weiss behavior. Previous measurements extending only to 1.6 kilogauss gave a magnetic moment per atom of /z~n = (10.7 + 0.2)p,r~ and paramagnetic Weiss temperatures, Or,, in the range of - 2 to -16 K for two .samples. These values of P-¢H are in good agreement with the value expected (10.62/.q0 for a free-ion 5f'~ configuration based on an I.-S coupling scheme and Hund's rule. In this work extended to higher fields, two additional samples give the values P~.tv = (9.7 + 0.2)gn and Or, = - 4 0 K. At low temperatures the samples exhibit an ordered magnetic transition to a state with a saturated moment of 6.Ira,n/atom when extrapolated to infinitely high field. The low temperature ordered phase exists at temperatures below T, = (51 ± 2) K as determined from constant magnetization plots.

1. Introduction Previous measurements [1] of the magnetic properties of dhcp californium-249 metal indicated the presence of three regions of differing magnetic character: 1) For temperatures above 160K a paramagnetic Curie-Weiss behavior was observed in two samples (sample A of mass 142 p,g and sample B of mass 53.2 p,g) at 1650 and 800 gauss. A linear least squares fit of the inverse magnetic susceptibility versus temperature for data in this range yielded values for the effective magnetic moment of (10.7 +0.2)/xB and (10.8 + 0.2)V,u per atom and paramagnetic Weiss temperatures, Op, of - 2 and - 1 6 K, respectively. 2) For temperatures between 45 and 66 K an antiferromagnetic transition was observed in sample A in low fields (52 and 200 gauss). The transition exhibited substantial hysteresis in the * Research sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy, under contracts DE-AS05-79ERI034g with The University of Tennessee (Knoxville) and DE-AC0584OR21400 with Martin Marietta Energy Systems. The californium-249 used in this work was made available through the same offices through the transplutonium element pr(xiuction facilities at the Oak Ridge National Laboratory.

observed values of X for decreasing temperature when compared with those of increasing temperature. This transition was interpreted to be a magnetic property of pure dhcp californium metal. 3) At temperatures below 45 K the magnetic behavior was consistent with either ferromagnetism or ferrimagnetism but, due to the limitation of fields below 1600 gauss, magnetic saturation could not be achieved and a distinction between the two alternatives could not be made for either sample.

2. Results More recent measurements on californium249 metal have been made on two other samples (sample C of mass 73.0/xg and sample D of mass 98.0/~g) in the same temperature ranges but in applied fields up to 50 kilogauss. The results of these measurements are: 1) For temperatures in the range between 100 and 3 4 0 K , a Curie-Weiss behavior has been observed in samples C and D for several applied fields as seen in fig. 1 for sample C. Here we have plotted the inverse magnetic susceptibility, X-I, versus temperature for applied fields of 0.80, 1.65, 5, 10, 15, 25, 30, 40, and 50 kilogauss. The curves

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for the l a t t e r four fields are virtually s u p e r i n l p o s c d upon one a n o t h e r and when lit to a ( ' u r i c - W e i s s function yield an e f f e c t i v e m a g n e t i c m o m e n t of ( 9 . 7 ± 0 . 2 ) / ~ n and a W e i s s p a r a m a g n e t i e temp e r a t u r e of (~p = --(41 :t- 2) K. For s a m p l e D the fit yields ~t~ = (9.7 ± ().2)/,tt~ and Pip = -(39 + 2) K. This is in d i s a g r e e m e n t with m e a s u r e m e n t s on s a m p l e s A and B at l o w e r fields as i l l u s t r a t e d in fig. 1). Here a plot of inverse susceptibility versus t e m p e r a t u r e is g i v e n for s a m p l e A at ().~ and 1.65 k i i o g a u s s , and s a m p l e B at 1.65 kilogauss. It a p p e a r s that the s a t u r a t i o n o f X ( d u e to s a t u r a t i o n of f e r r o m a g n e t i c impurities) w h i c h is o b s e r v e d in fig. 1 for s a m p l e (" has a l r e a d y been o b t a i n e d at 1.6 kilogauss for s a m p l e A. 2) The t e m p e r a t u r e r a n g e b e t w e e n 45 and 66 K has been e x a m i n e d extensively for the p r e s e n c e of antiferromagnetism in both of the new s a m p l e s for many a p p l i e d fields b e t w e e n 5 and 50 k i l o g a u s s . No c o m p a r a b l e e f f e c t was o b s e r v e d with such c h a r a c t e r . We must t h e r e fore c o n c l u d e that the e a r l i e r i n t e r p r e t a t i o n was i n c o r r e c t and that the antiferromagnetism was due to o x i d e a n d / o r nitride impurities resulting

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from the c o n t a m i n a t i o n of the m e l a l ' s s u r f a c e b~, the a t m o s p h e r e . 3) At l e m p e r a t u r e s below. 50 K we have observed a consistent ordered magnetic transition in all s a m p l e s as s h o w n for s a m p l e (' in fig. 2. Here we have p l o t t e d the a v e r a g e n l o m e n l per a t o m . cr, as a function of temp e r a t u r e for a series of applied fields. T w o effects are e v i d e n t : a) The m o m e n t per atom is b e c o m i n g more s a t u r a t e d with the application of the h i g h e r fiekls for v a l u e s of the t e m p e r a t u r e b e l o w the t r a n sition. This e f f e c t is more clearly p r e s e n t e d in fig. 3 w h e r e we have p l o t t e d lhe a v e r a g e m o m e n t per atom as m e a s u r e d al 15 K sis ;i function of applied field. Here the m o m e n l ~r is o b s e r v e d to be s a t u r a t i n g with the h i g h e r fields and when rr is p l o l l e d vs inverse field and e x t r a p o l a t e d to infinite H lhe limit v a l u e of tr,,, ( 15 K) = 6.1 P-I, is o b t a i n e d . Similar m c a s u r e m c n l s on s a m p l e D give a l o w e r v a l u e of 6.()/.Ltd. b) "l'he o n s e t of o r d e r e d m a g n e t i s m with d e c r e a s i n g t e m p e r a t u r e o c c u r s ~l| l o w e r temp e r a t u r e s for smaller applied m a g n e l i c fields.

S.E. Nave et al. / Magnetism of californium metal

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Fig. 2. Moment per atom in units of Bohr magneton versus temperature for sample C in magnetic fields between 5 and 50 kilogauss.

Isomagnetization plots more clearly identify the transition temperature, To, for the onset of o r d e r e d magnetism. In this m a n n e r we obtain a Tc (0 gauss) = (51 + 2) K.

3. Conclusions The high temperature magnetism of dhcp californium metal is characteristic of p a r a m a g netism with an effective magnetic m o m e n t in reasonable a g r e e m e n t with that of californium ions in a 5f 9 electronic configuration or a 3+ c h a r g e state. The 10.6/zR values are higher than that e x p e c t e d from an intermediate coupling m o d e l which is g i v e n as 10.2/zB by one estimation [2]. The lower value of 9.7/zrj is in

a g r e e m e n t with previous measurements on two small ( ~ 9 / z g ) samples of fcc californium metal by F u j i t a et al. [2]: however, these measurements indicated no o r d e r e d state down to the lowest measurement temperature of 22 K. The effective Weiss temperature is small but n e g a tive for all samples in this work and suggest the likelihood of a lower temperature transition to an o r d e r e d state. The low temperature transition to an o r d e r e d state is indeed observed, but, in the limit of high fields, the saturated m o m e n t per atom is only 6.1/za r a t h e r than a value near the paramagnetic m o m e n t values as expected for a ferromagnetic state. This lower v a l u e may be indicative of ferrimagnetism but is more likely due to magnetic anisotropy (crystal or exchange) coupled with somewhat crystalline samples.

References [1] Paul G. Huray, S.E. Nave and R.G. Haire, Proc. 13~mes Journ6es des Actinides, Elat, Israel, 1983, B1. [2] D.K. Fujita, T.C. Parsons, N. Edelstein, M. Noe and J.R. Peterson, Transplutonium 1975, W. Miiller and R. Lindner, eds. (North-Holland, Amsterdam, 1976), p. 173.