MAGNETIC SPECIFIC HEAT OF CoCl2.2 PYRAZINE BELOW 2 K.

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MAGNETIC SPECIFIC HEAT OF CoCl2.2

PYRAZINE BELOW 2 K.

D. Gonzalez, J. Bartolomé, R. Navarro, F. Greidanus, L. de Jongh

To cite this version:

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JOURNAL DE PHYSIQUE

_Colloque

_C6,

_{supplkment ou no}

_8,

_Tome

_39,

_aolit

_1978,

_page

_{~ 6 - 7 6 2}

MAGNETIC SPECIFIC HEAT

O F

CoC112 PYRAZINE

BELOW2

K,

2

D. Gonzalez, .I. Bartolom6, R. Navarro, F.J.A.M.

rei id anus+

and L.J. De .Jongh+

pepartamento de Fisica Fwzdrmrental, University of Zaragoza, Spain

KamerZingh Onnes Laboratory, Univerity of Leiden,The IVetherZmd.9

Rlsum6.- Nous avons mesurd l a c h a l e u r s p d c i f i q u e magngtique de CoC12.2 pyrazine en-dessous de 2 K .

Une fnomalie lambda a 6 t 6 trouv6e B Tc = 0.855 K . Les r 6 s u l t a t s s o n t b i e n d g c r i t s par l e modgle s . c . S =

-,

₂ XY, avec I.Jl/k = 0,42 K.

A b s t r a c t . - The magnetic s p e c i f i c h e a t of CoC12.2 pyrazine below 2 K is r e p o r t e d . A lambda anomalz is

d e t e c t e d a t Tc = 0.855 K . The d a t a appear t o be w e l l d e s c r i b e d by p r e d i c t i o n s f o r t h e s.c., S = p.

XY model, with / J ) / k = 0.42 K.

H e t e r o c y c l i c l i g a n d s , b r i d g i n g metal i o n s i n polymeric t r a n s i t i o n metal complexes, have been shown/l/ t o be e f f e c t i v e a s exchange i n t e r a c t i o n p a t h s (through t h e a s s o c i a t e d X - o r b i t a l s ) . For i n s - t a n c e , i n Cu(N0,). pyrazine the cu2+ i o n s a r e cou- p l e d v i a t h e p y r a z i n e s , and t h e compound behaves m a g n e t i c a l l y a s an a n t i f e r r o m a g n e t i c Heisenberg li- n e a r c h a i n ( J / k =

-

3.7 K)/2/. Furthermore, compounds of g e n e r a l formula M ~ + ( N c s ) , ( t r i a z o l e ) ,

( w i t h M = 3 d m e t a l i o n ) a r e found t o have magnetic o r d e r i n g temperatures t h a t a r e a l s o i n t h e l i q u i d ' ~ e range/3,4/. Here we p r e s e n t s p e c i f i c h e a t d a t a on CoC12.2 p y r a z i n e which compound has t h e t e t r a g o -

0

n a l u n i t c e l l (space group I 4 / m ) / 5 / (ao = 7.12 A

0

and c = 10.63 A) shown i n f i g u r e I a . I n t h i s s t r u c t u r e each CO,+ i o n is connected t o f o u r n e a r e s t magnetic neighbours w i t h i n t h e (001) l a y e r s by py- r a z i n e r i n g s (exchange J ) ; t o e i g h t neighbours i n

1

a d j a c e n t l a y e r s v i a CO-Cl-Cl-CO p a t h s i n v o l v i n g bond a n g l e s of 95.7O (exchange J 2 ) and t o two a d d i t i o - n a l neighbours a l o n g t h e c-axis v i a n e a r l y c o l l i n e a r CO-Cl-Cl-CO p a t h s (exchange J 3 ) . A schematic view of t h e t h r e e magnetic i n t e r a c t i o n s i s given i n f i - gure 1 b , whereas t h e bond a n g l e s and bond d i s t a n c e s f o r t h e corresponding superexchange p a t h s a r e sum- marized i n f i g u r e s I c-e. I t i s c l e a r t h a t , depen- d i n g on t h e r a t i o ' s of t h e t h r e e i n t e r a c t i o n s J

1 ' JZ,J3, t h e e f f e c t i v e number of e q u i v a l e n t magnetic neighbours may range from z _{e f f}= 2 ( i f :

)

J 3 ) > > ) J 2 ) ,

1 ~ ~ ) ) t o z e f f = 14 ( i f : ) J , )

=

) J ~ )

=

1 ~ ~ ) ) .

The s p e c i f i c h e a t was measured using an adia- b a t i c demagnetization a p p a r a t u s d e s c r i b e d elsewhe- r e / 6 / . Data taken i n t h e range 0.2 K < T < 2 K on a powdered sample a r e shown i n f i g u r e 2. A magnetic o r d e r i n g anomaly i s observed a t Tc = 0.855

+

0.015K.

F i g . 1 : a ) S t r u c t u r e of CoC12.2paz. Only pyrazine r i n g s i n &he b a s a l p l a n e a r e shovn. D i s t a n c e s a r e given an A . b) S p a t i a l arrangement of t h e exchange i n t e r a c t i o n s . c ) , d ) , e ) superexchange p a t h s a s s o c i a - t e d with J J and J3, r e s p e c t i v e l y .

1 ' 2

There a r e no i n d i c a t i o n f o r l a t t i c e - phonon c o n t r i - b u t i o n s i n t h e i n v e s t i g a t e d range, t h e l i m i t i n g high- temperature behaviour of t h e s p e c i f i c h e a t b e i n g well-described by t h e expected T-' dependence

( s e e below). The upturn observed a t t h e lowest tem- p e r a t u r e s (T < 0.35 K) probably a r i s e s from a con- t r i b u t i o n due t o t h e h y p e r f i n e i n t e r a c t i o n s between t h e c o b a l t e l e c t r o n i c and n u c l e a r s p i n s . I f we sub- t r a c t a T-, term r e p r e s e n t i n g t h i s h y p e r f i n e s p e c i - f i c h e a t , t h e remaining magnetic s p e c i f i c h e a t i s w e l l d e s c r i b e d by t h e s o l i d curves a and b drawn i n f i g u r e 2. These a r e t h e o r e t i c a l p r e d i c t i o n s f o r t h e

1

simple c u b i c ( s . c . ) , s p i n S =

7,XY

model a t tempera-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19786340

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Fig. 2

:

Experimental specific heat of CoC12.2 py-

razine. Curves a and b are theoretical predictions

for the

S.C.

XY model with S

=

(I~l!k

=

0.42 K)

as obtained from high-temperature serles expansions

(H.T.S.) and spin wave theory, respectively.

tures above and below Tc, respectively, that have

been amply discussed in previous publications/7/.

This model is described by the interaction hamilto-

nian

g=

-2J,iij,(SixSjx

+

SiySjy) and the predic-

tions a and b are derived from analyses of the high-

temperature series expansion for the specific heat

and from spinwave theory, respectively. The inter-

action constant obtained from fitting the theory to

the present experiment is

I

Jl /k

=

0.42 K. Needless

to say that the integrated experimental specific

heat yields an estimate of the magnetic entropy that

equals Rln2 within the accuracy of a few per cent.

The sign of the interaction J cannot be determinal

from our data, since for the XY model on loose-pac-

ked lattices the magnetic free energy is the same

for ferro- and antiferromagnetic exchange.

An effective spin 112 is not uncommon for

co2+ compounds, that order at low temperatures. For

an octahedrally coordinated co2+ ion, the '

T

ground

I

state is split into six Kramers doublets under the

combined action of a distortion from cubic symmetry

and of the spin-orbit coupling. Below T

-

50 K on-

ly the ground doublet will be appreciably populated.

Depending on the sign of the distortion parameter

the effective interaction Hamiltonion for the

S

=

1/2 ground doublet can be of the Ising or of the

XY form/7/. For the present compound there is unfor-

tunately no experimental information available about

the g

-

tensor. However, it is known that if the

octahedral coordination of co2+ is formed by four

equatorial oxygen or nitrogen atoms, with two C1 a-

toms at the apical positions, the crystal field term

is indeed of the form to yield a strong XY (planar)

anisotropy, as found for the present material.

Ex-

amples are CoC12.6H20/8/ and NiC12.4 pyrazole/9/.

This contrasts with the situation in which one has

four C1 atoms in the equatorial positions and

0

or

N atoms at the apices, as in CoC12.2H20/10/ or

CoC12.2 pyridine/ll/, for which the interaction is

of the Ising form.

Next we have to comment on the applicability

of the simple cubic magnetic lattice to the present

compound. Apparently, the interactions J ,J ,J are

1

2

3 such as to yield Z

=

6, if one compares the ex-

eff

perimental specific heat with a theoretical predic-

tion assuming equivalent coupling between a certain

number of magnetic neighbors. At any rate, the cor-

responding predictions for the XY model on other

lattice (e.g. b.c.c. or f.c.c.) are too much diffe-

rent to yield a comparable fit with the experiment.

Apparently, the interactions J2 and J3 are somewhat

weaker than J, or, alternatively, J2 and J could

3

be of different sigh so as to partially cancel one

another. The result is that, if equivalently coupled

neighbors are assumed, one obtains a value for

zeff in between four (if

1 J1

)*l

J21,

1 J3)) and four-

teen (if

1 ~ ~ 1

=1J21 =IJ

I).

The fact that we find

3

zeff to be so closely equal to six would then be a

coincidence. However, we cannot exclude the possi-

bility that the magnetic structure would be more

pronouncedly 2-dimensional

(

I

J

1 "

1 J2

1 ,l J3

1

)

and

1

that a spin-anisotropy in the easy (XY- plane would

be responsible in part for the observed anomaly.

The situation would then be similar to that in

CoC12.6H20/8/.

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References

/l/

Inoue,M. and Kubo,M., Coord. Chem. Rev.

2 (1976)

1

/2/ Losee,D.B., Richardson,H.W. and Hatfield,W.E.,

J. Chem. Phys.

2 (1973) 3600

/3/

Engelfriet,D.W., Haasnoot,J.G. and Groeneveld,W.L.,

2 .

Naturforsch.

32a

(1977) 783,

and to be published

/4/ Nap, G.M. et al., to be published

/ S /

Carreck,P.W., Goldstein,M., McPartin,E.M. and Unsworth,

W.D., Chem. Commun. (1971) 1634

/ 6 /

Algra,H.A., De Jongh,L.J., Huiskamp,W.J. and Carlin,

R.L., Physica

(1977) 187

/7/

Algra.H.A., De Jongh,L.J., Huiskamp,W.J. and Carlin,

R.L., Physica

(1976) 71,

Bartolome,J., Algra,H.A., De Jongh,L.J. and Carlin,

R.L., Physica, in the press

/8/

Metselaar,J.W., De Jongh,L.J. and De Klerk,D.,

Physica

(1

975) 53

/9/ Klaaysen,F.W., Reedijk,J. and Witteveen,H.T.,

2.