SPECIFIC HEATS OF QUASI-ONE-DIMENSIONAL MAGNETS UNDER MAGNETIC FIELD

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SPECIFIC HEATS OF QUASI-ONE-DIMENSIONAL

MAGNETS UNDER MAGNETIC FIELD

K. Takeda, Y. Yoshino, T. Haseda

To cite this version:

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JOURNAL DE PHYSIQUE

_{Colioque C6, suppl6ment au no 8,}

_{Tome 39, aoiit}

_{1978, page}

_C6-729

SPECIFIC HEATS O F QUASI-ONE-DIMENSIONAL MAGNETS UNDER MAGNETIC

FIELD

K.

Takeda, Y. Yoshino and T. Haseda

Department of Material Physics, Faculty of Engineering Science,

Osaka University, Toyonaka, Osaka, Japan

Rdsum6.- La chaleur spdcifique et la tempdrature de Ndel des mono-cristaux quasi

uni-dimensionnels

E C H ~ ) ~

NH]

Cl3 2H20; EcH~),, N

]

M

n

Cl3 et Cu

-

benzoate,

3H20 ont dt6 mesur6es en fonctron du champ extdrieur. La tempdrature de Ndel

decroft pour l'ion Co2+ et croft pour l'ion Mn2+ sous l'effet d'un champ magnC-

tique appliquG de.20 kG. Pour le dernier sel la chaleur spdcifique prdsente un

large maximum autour de 0-7 K dans un champ d e = 12 kG

.

Les rdsultats sont dis-

cutQs

1

partir de considerations sur la dimensionalit6 d'un systsme de spins

sous champ.

Abstract.- The magnetic heat capacity and the Neel temperature for single crystals

of quasi-one-dimensional magnets,

[I

(CH3)

3

NH]coc13.

2H20,

[

(cH~)

, + N I M ~ C ~ ~

and Cu-

benzoate 3H20, have been observed as a function of external magnetic field. The

Neel temperature decreases for the co2+ salt, and increases for the ~

n

salt with

~

+

increasing magnetic field up to 20 kG. The heat capacity gives a broad maximum

around 0.7

K under H212 kG for the last salt. The results are discussed considering

the effect of the external field on the spin dimensionality in one-dimensional sys-

tem.

I. INTRODUCTION.- The transition temperature T

N

for a quasi-one-dimensional magnet can be determi-

ned by a formula kTN=zJ1

.Sld(TN), where

zT'

is the

interchain interaction and 5

ld(TN) is the spin

correlation length along an isolated chain at T

N. The aim of the present paper is to investigate

the dependence of correlation function on the ex-

ternal magnetic field by the observation of the

heat capacity and TN under the magnetic field.

One-dimensional Heisenberg antiferromagnet (1d-H-

AF), in appropriate magnetic field, may effective-

ly turn its symmetry of spin interaction into X-Y,

or Ising type /l/. This can be reflected on the

increase of TN and on the magnetic heat.

capacity

as well. For a comparison between Ising and Hei-

senberg spin interaction, we selected three one-

dimensional compounds, ~ c H ~ )

3 NH]COC~~.

2H20

(TMCoC)

,

EcH~)

1 + g M n ~ 1 ~

(TMMC) and Cu-benzoate.

3H20.

11. EXPERIMENTAL RESULTS AND DISCUSSION.-

l.

TMCoC

The magnetic heat capacity Cm is shown in Figure

1

for variou; values of the external magnetic field

H parallel to c-axis (easy axis). When H=O, Cm is

consistent with the result by D.B. Losee et al.

can be reproduced, including the sharp peak at

TN

=

4.135 K, by the Onsager's exact solution for

the two-dimensional Ising net with JV/J=1/100

(J/k=15 K) 121. As H increases, the peak looses its

strength of singularity, gradually shifting to the

lower temperature side and finally vanishes for

H2Hc (790

G)

leaving the characteristic curve of

one dimensional system including the broad maximum

in the higher temperature region.

I C 6

1 !

I

i

C L C I 0 2 3 1 ( H c = 7 9 0 O e ) I 0.2 ;

-

Thcor

l

i

l T I K I

j

c- L 5 , o 2 3 1 ( H c = 7 9 0 O e ) I 0.2 ;

-

Thcor

Fig.

1 :

The magnetic heat capacity of TMCoC in

magnetic field. The dotted line corresponds to the

2-d Ising net with J'/J=I/IOO.

The solid line in-

dicates the theoretical curve for the l-d Ising

chain in magnetic field.

From the minimum temperature observed in

the isentropes of the (H-T) diagram, TN(H) is

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found t o d e c r e a s e a s TN(H) / T ~ ( o ) = I-a(H/Hc)2 w i t h aa0.67 f o r H<0.3Hc. I f we t a k e t h e d e m a g n e t i z a t i o n e f f e c t i n t o c o n s i d e r a t i o n TN(H) may b e more s e n s i - t i v e t o H and h e n c e t h e v a l u e o f a i s p r o b a b l y l a r - g e r t h a n t h e above v a l u e . The system b e g i n s t o r e v e a l t h e c h a r a c t e r o f t h e i s o l a t e d l i n e a r c h a i n f o r H,Hc. The s o l i d l i n e i n d i c a t e s t h e t h e o r e t i c a l v a l u e f o r t h e I s i n g c h a i n w i t h s Z = 1 / 2 , ~ / k = 1 4 . 2 K and g,=6.42 which a r e a p p r o p r i a t e f o r t h i s s a l t / 2 , 3 / . The agreement between experiment and t h e o r y

i s r a t h e r s a t i s f a c t o r y . These r e s u l t s show t h a t t h e l o o s e l y packed one-dimensional I s i n g s p i n system r e d u c e s i t s TN(H) w i t h i n c r e a s i n g H, and t u r n s i n t o a n i s o l a t e d one-dimensional system f o r H>Hc. 2. TMMC.- Q u i t e a s m a l l i n t e r c h a i n i n t e - r a c t i o n ( 2 7 ' / ~ = 1 0 - ~ ) b r i n g s t h e system i n t o o r d e - r e d s t a t e a t TN=0.84 K, l e a v i n g I % of t h e m a g n e t i c e n t r o p y (S=5/2) below TN 141. F i g u r e 2 shows t h e r e s u l t o f t h e h e a t c a p a c i t y a t H=O and 10 kG. When

H ~ C ( e a s y = p l a n e ) , t h e peak a t TN s h i f t s t o t h e h i g h e r t e m p e r a t u r e and becomes b r o a d e r . It is w o r t h w h i l e t o n o t e t h a t t h e o v e r a l l d a t a p o i n t s f o r T>TN i n m a g n e t i c f i e l d a r e h i g h e r t h a n t h o s e i n z e r o f i e l d . The i n c r e a s e o f TN(H) may b e a s s o - c i a t e d w i t h t h e e l o n g a t i o n of Cld(T) i n m a g n e t i c f i e l d which i s due t o t h e r e d u c t i o n of s p i n dimen- s i o n a l i t y / I / . T X ' M C F i g . 2 : The h e a t c a p a c i t y o f TMMC. The s o l i d l i n e i n d i c a t e s t h e t h e o r e t i c a l v a l u e f o r Id-H-AF w i t h S=5/2 and J / k =

-

6.7 K. TN(H) o b s e r v e d up t o 12 kG i s c o n s i s t e n t w i t h t h e r e s u l t s b y NMR / 5 / and s p i n - l a t t i c e r e l a x a - t i o n experiment 161. m a g n e t i c h e a t c a p a c i t y Cm (0,T) c a n b e e x p r e s s e d by t h e t h e o r e t i c a l v a l u e f o r Id-H1AF w i t h S=1/2, and J / k = I O K, i n t h e r a n g e 0.5<T<3 K. TN i s r e p o r t e d t o b e 0 . 7 6 K by

AFMR

e x p e r i m e n t under H210 kG 171. However, a n y anomaly c a n n o t b e d e t e c t e d by t h e h e a t c a p a c i t y measurement a r o u n d TN. When H/b-axis

( e a s y a x i s ) , Cm(H,T) a r e a l m o s t t h e same a s Cm (O,D o r H21 2 kG. But when H//c-axis, Cm(H,T) becomes l a r - g e r and g i v e s a broad maximum around 0 . 7 K under H21 2 kG ( F i g u r e 3 ) .

..

.

...

.0' , p'."

.

-9 >.sea L-

,.:..-

#$g,

..:

.,F-; $??. S.... ,P

>;

99

.;3

.

.'D . ' O O .. . . H

-

0 n G H,, b ( 12 & G 1 H,, c i 12 h G ) t n e o ( J / k - - 9 . s K ) F i g . 3 : The m a g n e t i c h e a t c a p a c i t y of Cu-benzoate. 3H20 i n m a g n e t i c f i e l d . The s o l i d l i n e shows t h e t h e o r e t i c a l v a l u e f o r Id-H-AF w i t h S1112 aqd J / k = -9.8 K. ACKNOWLEDGEMENT,- One of t h e a u t h e r s (K.T.) w h i s k t o acknowledge t h e d i s c u s s i o n w i t h Dr.W.J.M.de Jonge

.

R e f e r e n c e s / l / V i l l a i n , J. and Loveluck, J.M., J . P h y s i q u e 38 (1977) L-77.

-

1 2 1 Losee, D.B., MC E l e a r n e y , J . N . , S h a n k l e , G.E., C a r l i n , R.L., C r e s s w e l l , P.J. and Robinson, W.T., Phys. Rev. (1972) 2185.

1 3 1 Spence, R.D. and Botterman, A.C., Phys. R e v . g (1974) 1993.

/ 4 / Takeda, K., Phys. L e t t . (1974) 335. De J o n g e , W . J . M . , Swiiste, C.H.W., Kopinga, K.

and Takeda, K., Phys. Rev.

B12

(1975) 5858. / 5 / Dupas, C. and Renard, J.P. S o l i d S t a t e Comun.

20 (1 976) 58 1

.

-

/ 6 / Borsa, F. and Boucher, J . P . , Phys. L e t t . (1977) 256.

/ 7 / Oshima, K . , Okuda, K. and D a t e , M., J.Phys. Soc. J a p a n

5

(1976) 475.

3. CU-BENZOATE.3H (L- We i n v e s t i g a t e t h i s 2