Hyperfine interaction of 151Eu in Eu2TiO4

Volume 38A, number 6

HYPERFINE

PHYSICS LETTERS

INTERACTION

OF

151Eu

13 March 1972

IN Eu2TiO4*

CHIA-LING CHIEN and F. de S. BARROS** Department of Physics, Carnegie-Mellon University, Pittsburgh, Pennsylvania 15213, USA Received 18 January 1972 The hyperfine interaction of 151Eu in Eu2TiO4 has been measured using the M~ssbauer Effect. Using the four-fold symmetry of the Eu2+ site, it has been found that the nuclear quadrupole moment ratio of the firstexcited to the ground states of 151Eu is R = 1.34 ± 0.03, and the quadrupole interaction energy is e2qQo = -(190 ± 7) MHz. Below Tc = 7.8K, the magnetic hyperfine field Hhf points close to the direction of the fourfold axis. The zero-degree value of Hhf is estimated to be (305 ± 3) kOe.

In the past, s e v e r a l a t t e m p t s had been made to m e a s u r e the ratio R , of the n u c l e a r q u a d r u pole m o m e n t s of the 21.6 keV e x c i t e d - s t a t e and the g r o u n d - s t a t e of 151Eu. M e a s u r e m e n t s of R f r o m p u r e quadrupole s p e c t r a yielded r a t h e r d i v e r s e r e s u l t s [1, 2], p r o b a b l y owing to the l i m i t e d r e s o l u t i o n . M e a s u r e m e n t s of R f r o m c o m b i n e d magnetic and quadrupole s p e c t r a have also been r e p o r t e d by Nowik et al.[3] and Stachel et al.[4, 5] on EuIG, and Kalvius et al.[6] on s e v e r a l n o n cubic Eu 2+ compounds. In the c a s e of EuIG, f u r t h e r c o m p l i c a t i o n a r i s e s f r o m the p r e s e n c e of two Eu s i t e s with slightly different Hhf and e2qQ values. Stachel et al. obtained a value of R = 1.28 + 0.05 by fitting t h e i r data to two n e a r l y i d e n t i c a l p a t t e r n s [5]. F o r the n o n - c u b i c c o m p o u n d s u s e d by Kalvius et al., the r e p o r t e d value [6] is R = 1.30 + 0.05. It is to be e m p h a s i z e d that all the compounds u s e d by the above w o r k e r s have r a t h e r low s y m m e t r y . Thus, in o r d e r to fit t h e i r data to a H a m i l t o n i a n containing a c o n v e n ient n u m b e r of v a r i a b l e s , it was n e c e s s a r y to a s s u m e that the e l e c t r i c field g r a d i e n t t e n s o r is a x i a l ( V x x = Vvv ;77--(Vyy - V x x ) / V z z = 0 ) , and that the magn~t-ic hyperfine field, Hhf , l i e s in the d i r e c t i o u of the m a j o r axis of the e l e c t r i c field g r a d i e n t t e n s o r . We r e p o r t h e r e M ~ s s b a u e r m e a s u r e m e n t s of Eu2TiO 4 [7] which o r d e r s m a g n e t i c a l l y at a c o n v e n i e n t l y high t e m p e r a t u r e of 7.8 K; w h e r e a s all of the n o n - c u b i c compounds mentioned above have o r d e r i n g t e m p e r a t u r e s below 2 K [8 ~. B e c a u s e of the f o u r - f o l d s y m m e t r y at the Eu z+ site, the e l e c t r i c field g r a d i e n t t e n s o r i s axially s y m *Work supported in part by the National Science 'Foundation and the Office of Naval Research. **Present address: Instituto de Fisiea, Univ. Fed. Rio de Janeiro, Brazil.

m e t r i c a l . ¢ F u r t h e r m o r e , as it is shown l a t e r , the a s s u m p t i o n of the a l i g n m e n t of Hhf along the m a j o r axis of e l e c t r i c field g r a d i e n t when T < 7.8K is found to be c o r r e c t within our e x p e r i m e n t a l resolution. Eu2TiO 4 c r y s t a l l i z e s with the K2NiF 4 s t r u c ture. The t e t r a g o n a l unit cell with cell p a r a m e t e r s a = 3.883.~ and C = 12.532~, where c is a four-fold axis, c o n t a i n s two f o r m u l a units [7]. It may be v i s u a l i z e d as stacking a l t e r n a t e l a y e r s of rock salt EuO and p e r o v s k i t e EuTiO3 [9]. However, unlike EuO and EuTiO3, the s y m m e t r y at the Eu 2+ site is no longer cubic. The M ~ s s b a u e r g a m m a - r a y beam was obtained from a Sm203 source m a i n t a i n e d at room t e m p e r a t u r e . All m e a s u r e m e n t s were p e r f o r m e d in the u s u a l t r a n s m i s s i o n geometry. The s a m p l e used was in fine powder f o r m with about 2 0 m g / c m 2 of m a t e r i a l sealed in a lucite sample holder. F o r T< 4.2 K, m e a s u r e m e n t s w e r e made by pumping the liquid h e l i u m in contact with sample. F o r T >4.2 K, a l o w - p r e s s u r e e x c h a n g e - g a s Andonian dewar was used with t e m p e r a t u r e s t a bility of about + 0.1 K. C a l i b r a t i o n of the velocity scale was r e g u l a r l y made using the 57Fe Mt}ssbauer Effect in o~-Fe203. The data at 300, 77, 4.2 and 2.2 K a r e shown in fig.1. Like in most p o l y c r y s t a l l i n e - p r o d u c e d Eu 2+ compounds, there was a s m a l l amount of Eu 3+ p r e s e n t in our sample. As it can be seen in the 300 and 77 K s p e c t r a , this Eu 3~" i m p u r i t y is r e s p o n s i b l e for the r e s o n a n c e line at n e a r In the course of this investigation, we have also examined the tetragonal seheelites EuMoO4 and EuWO4. None of these compounds, however, show magnetic ordering above 1.7 K. 427

Volume 38A, number 6

PHYSICS LETTERS

i

,

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o

300 K l

v

-115

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-I01

-Sl

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Fig.1. Mdssbauer spectra of Eu2TIO 4 at 300, 77, 4.2 and 2.2 K. The solid curves are the best fitted results. The broken curves indicate Eu 3+ peaks. zero velocity. If one a s s u m e s the s a m e D e b y e W a l l e r factor for both Eu 2+ and Eu 3+, the amount of Eu 3+ is e s t i m a t e d to be about 5%. The data were l e a s t - s q u a r e fitted by using a c o m b i n e d magnetic and e l e c t r i c - q u a d r u p o l e Hamiltonian. The v a r i a b l e s were Hhf, R , e2qQo, and the a m p l i t u d e s and the full widths at halfm a x i m u m of the Eu 2+ and Eu 3+ r e s o n a n c e s . All t r a n s i t i o n s w e r e a s s u m e d to have the L o r e n t z i a n shape. The solid c u r v e s i n fig.1 a r e the b e s t fitted r e s u l t s . The b r o k e n c u r v e s indicate the a m o u n t of a b s o r p t i o n due to Eu 3+. The m e a s u r e d r e s o n a n c e line width (FWHM) i s about 2 . T m m / s e c for low t e m p e r a t u r e and i n c r e a s e s slightly to about 2.9 m m / s e c for high t e m p e r a t u r e . The i s o m e r shift r e l a t i v e to E u 2 0 3 at r o o m t e m p e r a t u r e i s (-12.8 ± 0.I) m m / s e c for 428

13 March 19q2

Eu 2+, and r e m a i n s constant, within the e x p e r i mental e r r o r , throughout the t e m p e r a t u r e r a n g e of this work. The best values of R and e2qQ o from the lowt e m p e r a t u r e (T<4.2 K) m e a s u r e m e n t s a r e R = 1.34 :L 0.03, e2qQo = (-189 +7) MHz. F o r the 77 and 300 K m e a s u r e m e n t s ~ when R is fixed to be 1.34, we obtain a value e qQo = -190 MHz. We take this as evidence that, when Eu2TiO 4 is m a g n e t i c a l l y o r d e r e d , the Hhf axis l i e s v e r y close to the m a j o r axis of the e l e c t r i c field gradient t e n s o r . It is of i n t e r e s t to d e t e r m i n e the extrapolated z e r o - d e g r e e hyperfine field H o. However, the lowest t e m p e r a t u r e achieved in this work was 2.2 K, which is only about 25% of the o r d e r i n g t e m p e r a t u r e . We e s t i m a t e d Ho to be about (305 ± 3) kOe; the m e a s u r e d value of Hhf at 2.2 K is 295 kOe. F o r the S - s t a t e Eu2+-ion, one can a s s u m e the hyperfine field Hhf to be p r o p o r t i o n a l to the magnetization. At t e m p e r a t u r e s close to the m a g n e t i c - p h a s e t r a n s i t i o n , T £ Tc, our m e a s u r e d v a l u e s of Hhf is found to obey the equation Hhf/Ho = B(1 - T/Tc)fl , with a c r i t i c a l index/3 close to ~ [10]. Using this e x p r e s s i o n , and fi = 1 ~, the e x t r a p o l a t e d o r d e r i n g t e m p e r a t u r e ot Eu2TiO 4 is found to be (7.8 + 0.2) K. At 4.2 K and below, we have found that the quadrupole i n t e r a c t i o n energy r e d u c e s to 85 MHz when a 2-kOe magnetic field is applied. This value is about half of that o b s e r v e d for e2qQ in the a b s e n c e of applied field, and i m p l i e s that the Eu 2+ magnetic m o m e n t s in Eu2TiO 4 a r e highly s u s c e p t i b l e to e x t e r n a l magnetic fields. This b e h a v i o r suggests a f e r r o m a g n e t i c o r d e r i n g in Eu2TLO4 • We thank Dr. G . J . McCarthy of P e n n s y l v a n i a State U n i v e r s i t y , M a t e r i a l R e s e a r c h L a b o r a t o r y for kindly providing us with the s a m p l e s used in this investigation.

References

[1] G. W. Du[aney and A. F. Clifford, in M~ssbauer effect methodology, ed. I. Gruverman (Plenum Press, lnc., New York, 1969)VoLS, p. 65. [2] A. Z. Nozik, M. Kap[an and A. I. Weiss, Bull. Am. Phys. Soc. 13 (1968} 894. [3] I. Nowik and S. Ofer, Phys. Rev. 132 (1963) 241. [4] M. Stache[, S. Hufner, G. Crecelius and D.Quitman, Phys. Letters 28A (1968) 188. [5] M. Stache[, S. Hufner, G. CreceHus and D.Quitman, Phys. Rev. 186 (1969) 355. [6] G.M. Kalvius, G.K. Shenoy, G.J. Ehnholm, T.E. Katila, O.V. Lounasmaa and P. Reivari, Phys. Rev. 187 (1969) 1503. [7] G.J. McCarthy, W.B. White and R. Roy, J. Inorg.

Volume 38A, number 6

PHYSICS

Nucl. Chem. 31 (1969) 329. [8] G. M. Kalvius, T. E. Katila and O. V. Lounasmaa, in M6ssbauer effect methodoIgy, ed. I. Gruverman (Plenum P r e s s , Inc., New York, 1969) Vol.5, p.231. p.231.

LETTERS

13 March 1972

[9] T. R. McGuire, M.W. Sharer and R. J. Joenk, J. Appl. Phys. 37 (1966) 981. [10] r-.Heller, Rep. P r o g r . Phys. 30 (1967) 731.

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