Study of the magnetic transitions in Y2BaCuO5 and Y2Cu2O5 by specific heat and Mössbauer measurements

Journal of Magnetism and Magnetic Materials 104-107 (1992) 621-622 North-Holland

Y2Cu205

Study of the magnetic transitions in Y2BaCuO5 and by specific heat and M6ssbauer measurements Y. Gros

a,

F. H a r t m a n n - B o u t r o n a, j. O d i n

b,

b,

A. B e r t o n

p. Strobel c and C. M e y e r a

" Lab. Spectrom~trie Physique 1, BP87, F-38402 St Martin d'H&es, France b C.R.T.B.T., CNRS, BP 166X, 38042 Grenoble, France " Lab. Cristallographie, CNRS, BP 166X, 38042 Grenoble, France Magnetic specific heat data on Y2BaCuO5 confirm 3D ordering at T N = 15.5 K and the presence of SRO above T N. M6ssbauer spectra of SYFe doped Y2Cu205 show the appearance of a hf field below T N = 12 K; Cu-Fe exchange seems smaller than Cu-Cu exchange.

Magnetic specific heat of Y2BaCu05 (green phase). In ref. [1] the M 6 s s b a u e r study of 57Fe d o p e d YzBaCuO5 showed that its 3D a n t i f e r r o m a g n e t i c ordering t e m p e r a t u r e is T N = 15.5 K, which is much lower that the t e m p e r a t u r e of its susceptibility maxim u m ( = 2 5 - 3 0 K) suggesting low-dimensionality effects. This T N is in a g r e e m e n t with an optical determination by Agladze et al. on E r d o p e d Y2BaCuO5 [1]. M 6 s s b a u e r results o b t a i n e d by H o d g e s a n d Sanchez on 57Fe d o p e d Y b z B a C u O 5 also yielded a similar T N [2]. Since then, an optical study has shown [3] that diluted (Y0.98DY0.02)2BaCuOs orders at 14.5 K, while concent r a t e d DY2BaCuO 5 exhibits two p h a s e transitions at 18 K (Cu ordering) and at 11 K (Dy ordering); this last result was c o n f i r m e d in ref. [4], w h e r e specific heat m e a s u r e m e n t s were p e r f o r m e d on R 2 B a C u O 5 ( R = rare e a r t h ) a n d also on the d i a m a g n e t i c isomorph YEBaZnOs. In this last c o m p o u n d the specific heat is entirely due to the p h o n o n c o n t r i b u t i o n which, according to fig. 4 of ref. [4], c o r r e s p o n d s to a D e b y e t e m p e r a t u r e O D = 390 K. T h e p r e s e n t work is devoted to m a g n e t i c specific h e a t m e a s u r e m e n t s on Y2BaCuOs, which were p e r f o r m e d on the sample already studied by M 6 s s b a u e r spectroscopy (MS), b o t h in o r d e r to confirm the Neel t e m p e r a t u r e o b t a i n e d by MS a n d in the hope of getting more i n f o r m a t i o n on the dimensionality. T h e p o w d e r has b e e n i n c o r p o r a t e d into a silver plastic adhesive with known heat capacity (Ecc o b o n d solder 56c from E m e r s o n and Cuming) [5]. M e a s u r e m e n t s of the specific heat were p e r f o r m e d b e t w e e n 1.3 and 28 K with an adiabatic technique. A l t h o u g h at 15 K the c o n t r i b u t i o n of the b i n d e r is c o m p a r a b l e to t h a t of the sample, the e s t i m a t e d accuracy is b e t t e r t h a n 1.5% in this t e m p e r a t u r e range. A lattice c o n t r i b u t i o n Cph = 0 . 3 T 3 m J / m o l K 3, corres p o n d i n g to O D = 388 K, was s u b t r a c t e d from the total specific heat C t o t in o r d e r to obtain the m a g n e t i c specific heat C m. T h e curve C m / T versus T plotted in fig. l a exhibits a very small b u m p at 15 K. T h e b u m p is t Laboratoire de l'Universit6, J. Fourier-Grenoble I, associ6

au CNRS (UA 8).

more visible in fig. l b which displays C m / T 3. T h e smallness of the 3D p e a k is consistent with the assumption of low-dimensionality effects. O n the o t h e r hand, the curves of fig. la, b do not allow to d e t e r m i n e a precise dimension. Indeed, for spins S = 7 l and with c o n v e n t i o n 2JSiS j for the energy of a pair, o n e expects that: for 3D ( M F T ) C m has a discontinuity 1.5R = 12.47 J / t o o l K at T = T c ; for 2D A F [6] C m exhibits a m a x i m u m 0.452R = 3.76 J / m o l K at T = 1.2J; for 1D A F [7] C m exhibits a m a x i m u m 0.35R = 2.91 J / m o l K 5 0 0

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Fig. 1. YzBaCuO5: analysis of the magnetic specific heat C m ~ Cto t - 0 . 3 T 3 ; ( a ) plot of C m / T vs T; (b) plot of C m / T 3 vs T.

0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

E Gros et al. ,/Magnetic transitions in Y2BaCuO5 and Y:Cu20.s

622

at T = 0.96J; for 1D F [7] C m exhibits a m a x i m u m 0.134R = 1.11 J / m o l K at T = 0.70J. A t low T, C m ct T z in 2D A F [8] and C m ~ T in 1D A F [9]; for 1D F, see fig. 15 of ref. [7]. In the p r e s e n t work the t e m p e r a t u r e range explored is not sufficient to draw a complete curve. However at low T, C m ~ T 2 as in 2D AF, while at 2 0 - 2 5 K, C m seems to reach values = 8 - 1 0 J, i.e. i n t e r m e d i a t e b e t w e e n 2D a n d 3D values. This absence of a definite magnetic dimensionality may be due to the fact that all Cu 2+ ions are far apart, without any obvious chain or layer structure ( t h e r e are ferromagnetic rows in the 3D o r d e r e d phase, but thc H T susceptibility is: X ~ C / ( T + 0), with O > 0 [1], while in 1D F, 6 ) < 0). Alternately, s h o r t - r a n g e o r d e r might arise from the anisotropy of the exchange interactions, itself related to the anisotropy of the Land6 g factor; indeed, in Y2BaCuO5 t h e r e are two families of Cu 2+ ions, with their principal axes almost at right angle in the ac-plane, and in the 3D o r d e r e d p h a s e their magnetic m o m e n t s are effectively almost at right angle [1]. Strong S R O above TN(3D) in such a situation has

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References

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C. Meyer et al., Solid State Commun. 74 (1990) 1339. J.A. Hodges et al., J. Magn. Magn. Mater. 92 (1990) 201. M.N. Popova et al., JETP Lett. 52 (1990) 563. R.Z. Levitin et al., J. Magn. Magn. Mater. 90&91 (1990) 536. [5] A. Berton et al., Cryogenics (1977) 584. [6] M.S. Makivic et al., Phys. Rev. B 43 (1991) 3562. [7] L.J. de Jongh et al., Adv. Phys. 23 (1974) 1 (see table 1. page 34). [8] M. Takahashi, Phys. Rev. B 40 (1989) 2494. [9] M. Steiner et al., Adv. Phys. 25 (1976) 87 (see p. 1(/8). [10] P. Bonville et al., Phys. Rev. B 18 (1978) 2196. [11] J.L. Garcia Munoz et al., Phys. Lett. A 149 (19911) 319. [12] R. Troc et al., Phys. Lett. A 125 (1987) 1987. [13] S.W. Cheong et al., Phys. Rev. B 38 (1988) 7013. [14] B.L. Ramakrishna et al., Solid State Commun. 68 (1988) 775. [15] V.V. Moshchalkov et al., J. Magn. Magn. Mater. 90&91 (1990) 535. [16] J. Aride et al., Solid State Commun. 72 (1989) 459. [17] F. Hartmann-Boutron et al., J. Magn. Magn. Mater. 104-107 (1992) 501. [1] [2] [3] [4]

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Mgssbauer study of S7Fe doped Y:Cu:O s. Y2Cu205 has an o r t h o r h o m b i c structure which can be visualized as an array of zig-zag oxygen bridged c o p p e r chains parallel to thc a-axis [11]. C o p p e r occupies in equal p r o p o r t i o n s two inequivalent sites, both of which have distorted and elongated pyramidal oxygen neighbourhoods. At low T, Y2Cu205 b e c o m e s a 3D antiferrom a g n e t with T N = 11-13 K [11-15]. Its magnetic structure is c o m p o s e d of f e r r o m a g n e t i c planes parallel to ah which are antifcrromagnetically coupled [11,16]. 1% ~TFc was i n t r o d u c e d into this c o m p o u n d . M 6 s s b a u e r spectra are p a r a m a g n c t i c doublets down 1o 12 K. Below 12 K t h e r e is a r a t h e r progressive broadening of the spectrum (fig. 2). A resolved magnetic hfs begins to a p p e a r a r o u n d 5 K and a well developped magnetic s p e c t r u m is observed only at 1.4 K. T h e R T spectra can bc d e c o m p o s e d into two q u a d r u p o l e douNets with equal intensities, isomer shifts c o r r e s p o n d i n g to Fe 3+, and c o m p a r a b l e q u a d r u p o l e splittings, I AI = 0.98 and 1.36 m m / s . T h e s e results are lower than the values c o m p u t e d by s u m m a t i o n inside a 80 A radius sphere: A = 1.82 m m / s for Fe in Cu(1) and A = 2.23 m m / s for Fe in Cu(2), the reduction being possibly due to a shift of the iron with respect to the c o p p e r position [17] or to a local distortion a r o u n d Fe. At 1.4 K, apart from a m a g n e t i c impurity which is already visible above T N, the spectrum can approximately bc d e c o m p o s e d into two subspectra with equal intensities and with hf fields; H , = 360 and 405 kOe (which is fairly low for HS Fe3+). In conclusion, the M 6 s s b a u e r study of STFe substituted into YzCu205 confirms the existence of a transition a r o u n d 12 K; the low increase of the hf field below 12 K might be due to a low C u - F e exchange coupling c o m p a r e d with C u - C u exchange.

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Fig. 2. Y2Cu:Os: thermal variation of 57Fe M6ssbauer spectra.