Heat capacity anomaly of Haldane-gap antiferromagnet NENP below 1K

PflYSICA

Physica B 194-196 (1994) 1261-1262 North-Holland

Heat capacity anomaly of Haldane-gap antiferromagnet NENP below 1K A.Koda a, T . K o b a y a s h i a, Y.Tabuchi a, K.Amayaa and T . Y o s i d a b

aFaculty of Engineering Science, Osaka U n i v e r s i t y , Toyonaka, Osaka 560, Japan bNakanihon Automotioe College, Sakahogi, G i l a 565, Japan Heat c a p a c i t y measurements o f H a l d a n e - g a p a n t i f e r r o m a g n e t NENP a r e p e r f o r m e d . An anomalous broad peak was o b s e r v e d below 1K b e s i d e s S c h o t t k y anomaly c o r r e s p o n d i n g t o H a l d a n e - g a p . T h i s broad peak e x i s t s u n d e r zero m a g n e t i c f i e l d and d i s a p p e a r s u n d e r e x t e r n a l m a g n e t i c f i e l d o f 3T, We have a l s o measured t h e h e a t c a p a c i t i e s of b o t h Cu-doped and Zndoped s a m p l e s , b e c a u s e i m p u r i t y and e n d - c h a i n s p i n s may be r e s p o n s i b l e f o r t h e o b s e r v e d anomaly as w e l l a s t h e p o s s i b i l i t y o f long r a n g e o r d e r . Heat c a p a c i t y measurements have been performed i n H a l d a n e - g a p a n t i f e r r o m a g n e t NENP u n d e r e x t e r n a l m a g n e t i c f i e l d s in order to study the p o s s i b i l i t y of long range order due to field-induced gap q u e n c h i n g . '> Though we d i d n o t f i n d long r a n g e o r d e r w i t h i n our e x p e r i m e n t a l r a n g e o f t e m p e r a t u r e s and m a g n e t i c f i e l d s , we found the heat c a p a c i t i e s corresponding to H a l d a n e - g a p . The h e a t c a p a c i t i e s a t low t e m p e r a t u r e s c o u l d be f i t t e d t o :

was o b s e r v e d i n u)SR measurements.P) We a l s o performed h e a t c a p a c i t y m e a s u r e ments i n b o t h Cu-doped and Zn-doped s a m p l e s i n o r d e r t o examine i f i m p u r i t y and e n d chain spins are responsible for the obs e r v e d anomaly. Three s a m p l e s u s i n g i n t h e p r e s e n t s t u d y a r e O.I%Cu-NENP, 1.2%Cu-NENP and Zn-NENP which i n c l u d e s a s m a l l e r amount t h a n 50ppm o f Zn2. i o n . For a l l s a m p l e s ,

exp(-~/kT) C, = aR(a/kT)2(l+expf_~/kT)) ~ .

(i) which i s a S c h o t t k y anomaly f o r a s i m p l e t w o - l e v e l system w i t h a gap e n e r g y a. We estimate the gap e n e r g y a/k=18±lE and a=0.095±0.010 u n d e r z e r o f i e l d . The d e t a i l s of t h e h e a t capacity c o r r e s p o n d i n g to Haldane-gap is reported i n our a n o t h e r p a p e r of t h i s c o n f e r e n c e . Another broad peak o f NENP i s found a t lower t e m p e r a t u r e s u n d e r zero f i e l d , as shown i n F i g . 1 . A p p l y i n g e x t e r n a l f i e l d , t h e broad peak i s s e e n t o move t o h i g h e r t e m p e r a t u r e and t o d i s a p p e a r u n d e r e x t e r n a l m a g n e t i c f i e l d of 3T. We can e x t r a c t o n l y t h e broad peak C , ' by s u b t r a c t i n g the l a t t i c e c o n t r i b u t i o n and H a l d a n e - g a p c o n t r i b u t i o n from t o t a l h e a t c a p a c i t y . Thus o b t a i n e d broad peak c , ' u n d e r zero f i e l d can be f i t t e d t o S c h o t t k y anomaly (1) w i t h a gap e n e r g y a / k e l . 0 5 K and a~0.003 as shown i n F i g . 2 . T h i s i m p l i e s t h a t the f r e e s p i n s e x i s t u n d e r i n t e r n a l f i e l d H=~/g~8. Howeve r , t h e t e m p e r a t u r e dependence of C, at t h e lowest t e m p e r a t u r e r e g i o n i s c l o s e t o T - l i n e a r than e x p o n e n t i a l dependence. This may be s p i n - g l a s s - l i k e o r d e r , c o r r e s p o n d i n g to t h e f r e e z i n g / s l o w i n g down of s p i n s which

NENP pure

0.06 --~0.04

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T(K) Fig.1

Heat c a p a c i t i e s o f p u r e sample under magnetic f i e l d s . i

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NENP pure at 0T .%

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0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved

SSDI 0921-4526(93)E1211-4

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T(K) Fig.3

T(K) Fig.4

Heat capacities of a l l samples under zero f i e l d .

the amounts of impurity ion were determined by chemical analysis. The heat c a p a c i t i e s of a l l samples under zero f i e l d are shown in Fig.3. The broad anomaly observed in pure NENP vanishes by doping a small amount of impurity and then larger heat capacity appears with increasing amounts of impurity. Thus the broad anomaly under zero f i e l d is i n t r i n s i c property in pure sample. On the other hand, we did not observe any obvious changes in the heat capacity r e f l e c t e d Haldane-gap as seen in Fig.3. For Cu-doped NENP, ESR measurement has been performed where the r e s u l t is explained by the model that the valence bonds are broken at the Cue* s i t e s resulting in s p i n - i / 2 s t a t e s at the Nie* s i t e s neighboring the Cu2*.3~ As shown in Fig.4, our heat capacity r e s u l t s for 1.2%Cu-NENP under, zero f i e l d can be f i t t e d to Schottky anomaly for a fourlevel s y s t e m with the gap energy &/k~O.42K, a2/k~l.8K and a3/k~6.lK which is close to the energy diagram suggested by the ESR measurement. The absolute value of heat capacity a=O.Oll is also consistent with 1.2% amount of doped Cu2. ion. Applying external f i e l d H , , t , a broad peak appears and moves to high temperatures with increasing f i e l d as shown in Fig.5, which is similar to pure NENP. The broad peak can be f i t t e d to Schottky anomaly ( i ) . The obtained energy gap can be explained by

0.6

0.4

F i t t i n g to Schottky anomaly f o r four level system. •

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T(K) Fig.5

Heat c a p a c i t i e s of 1.2%Cu-doped sample under magnetic f i e l d s .

Zeeman s p l i t t i n g ~=ggsH,xt with g~2.6 of free spins as shown in the inset of Fig.5. Since the energy s p l i t t i n g under zero f i e l d is smaller than Zeeman s p l i t t i n g , i t can be neglected under the present external f i e l d . Up to now, the broad peak of pure NENP under zero f i e l d is s t i l l not clear. We consider that i t is caused by free spins at chain e n d s but there s t i l l remains the p o s s i b i l i t y of long range order due to the existence of inter-chain interaction.

REFERENCES

(i) T.Kobayashi et a l . , J.Phys.Soc.Jpn.61(1992)1772. (2) B . J . S t e r n l i e b e t a l . , J.Magn.&Magn.Mater.104-107(1992)801. (3) M.Hagiwara e t a l . , Phys.Rev.Lett.65(1990)3181.