Antiferromagnetic properties of (DIMET)2SbF6

Synthetic Metals, 19 (1987) 425 430

ANTIFERROMAGNETIC PROPERTIES OF

425

(DIMET) 2 SbF6

R. LAVERSANNE, C. COULON and J. AMIELL Centre de Recherche Paul Pascal, CNRS, Domaine Universitaire de Bordeaux I, 33405 Talence C4dex

(France)

J.P. MORAND E.N.S.C.P.B.,

351Cours

de la Lib4ration,

33405 Talence C~dex

(France)

ABSTRACT A new antiferromagnetic organic salt,

(DIMET) 2 SbF6

is presented. The peculiar,

strongly dimerized structural arrangement of this salt allows its description in terms of quasi-isolated dimers.

In this modelization the exchange coupling cons-

tants are deduced from transfer integrals and the anisotropy energy is calculated from dipolar interactions. The good agreement found between our description and experimental results is an asset for discussion about the origin of antiferromagnetism in organic conductors.

INTRODUCTION Several recent studies in organic conductors have been devoted to antiferromagnetism

[1]. The origin of antiferromagnetism in organic compounds, which is of

critical importance to understand the competition between the different possible ground states is still under discussion

[2]. Unfortunately,

TMTCF salts the balance between the bandwith repulsion

U

t

prevents any simple modelization

in the well known

and the intrasite electronic [2,3]. We present inlthis commu-

nication a new antiferromagnetic

salt, the

ethylenedithiotetrathiafulvalene

[4]. As shown below, the peculiar structural ar-

rangement of this compound

(DIMET)2SbF 6 where DIMET is dimethyl-

which exhibits a well dimerized packing, allows a

simple modelization in terms of quasi isolated dimers.

ELECTRONIC AND STRUCTURAL CHARACTERIZATION (DIMET) 2SbF 6 has been prepared by electrochemical oxidation

[4] from which

elongated single crystals have been obtained. Electrical conductivity measurements evidenced semi-conducting behavior with a room temperature conductivity of about 2 x i0-3 ~-icm-1

0379-6779/87/$3.50

[4d].

© Elsevier Sequoia/Printed in The Netherlands

426

b

AH(G,:uss)

Xp/×p(3OOK) •o

•

• •

÷ •

,

÷

S

÷

•

•

•

+ ÷ + ÷

(J) ~

÷

f

~'"t-.. ~0

Fig. I. Schematic view of the projection of the structure of (DIMET) SbF perpendicular to the long axis molecule. The definition of the different transfer integrals is also ~iven (from ref. 5).

o~ th~

~0 - 0 T (K) Fig. 2. E.S.R. linewidth (+) and spin susceptibility (e) of (DIMET)2SbF 6

The room temperature structural arrangement has been determined and presented at the YAMADA conference [51 . The most striking feature of the structure was the well pronounced

dimerization of the stacks, clearly visible on projection per-

pendicular to the long axis of the molecule as shown in fig. I. The temperature dependence of the E.S.R. signal is shown in fig. 2. The spin susceptibility behavior is the signature of a chain of localized spins [6] with a maximum at 80K leading to an intrachain exchange of about 60K.

This behavior

is in contrast with the temperature dependence of the spin susceptibility Xp of other organic conductors like the TMTTF salts for which Xp is a slightly decreasing function of the temperature [Ib]. This result is in agreement with a low value of the electrical conductivity and may be related with the strong dimerization of the structure. At low temperature we observe a broadening of the E.S.R. linewidth while the E.S.R. susceptibility vanishes around 1] K. (g-factor)

Moreover the position of the signal

is shifted in this temperature range. Such behavior suggests the occu-

rence of an antiferromagnetic ordering of spins, which has been confirmed by antiferromagnetic resonance (AFMR).

ANTIFERROMAGNETIC RESONANCE ON (DIMET)2SbF 6 AFMR experiments have been performed using a previously described technique E7]. The rotation patterns presented in fig. 3 have been obtained at 2.5K

rota-

ting the crystal around two simple axes as shown in the insert of fig. 3. The

427 fits of the curves have been made using the Nagamiya theory [8] and are also given in fig. 3. Since the experimental temperature is low compared with the transition temperature (IIK),

H,(kG)

(~

AI C~ b

only the zero T limit has been used to obtain thesefit~

H,(kGI ®

1C

e

9 8

71

7

6

' 2'6 O(deg)

6'

= O(deg)

Fig. 3. Antiferromagnetic rotation patterns obtained at 2.5K rotating the sample around A I and A 2 (these axes shown in the insert are perpendicular to the crystal faces). Also given in this figure are typical fits obtained using the Nagamiya theory with : ~+ ffi 12.4 kG ~- = 9.5 kG. The rotation axes are : (a) the hard axis, (b) the intermediate axis. The "bubble" like curve given in fig. 3a is characteristic of a rotation in a plane close to the easy -intermediate plane, with an experimental frequency lower than ~_, the lowest zero-field resonance frequency

[7]. The rotation pattern

given in fig. 3b has been obtained in a plane perpendicular to the previous one and is compatible with a rotation into the easy-hard plane. The extrema of the two rotation patterns lie close to the b direction which can be identified with the easy axis, the hard and intermediate axes corresponding to the A 2 and A 1 directions respectively (see fig. 3). The obtained parameters are ~_ = 9.4 ± 0.I kG;

~+ ffi 12.5 ± 0.2 kG

In the zero T limit ~_ is equal to the spin-flop field.

DISCUSSION To discuss AFMR results, two energies have to be considered : the exchange and the anisotropy energies

[8]. The first one is related to the intermolecular

transfer integrals and determines the magnetic superstructure in the ordered phase. The minimization of the anisotropy energy gives the orientation of the spins. As already suggested by Torrance

[2] the anisotropy is most likely due to

the dipolar interactions between spins lying on neighbouring dimers. We show in the following analysis that this assumption is in agreement with our results. Most significantly,

the strongly dimerized structure of (DIMET)2SbF 6 allows a

428 quantitative approach to both these energies, assuming that the spins are localized on each dimer.

Exchange energy The exchange energy J is usually taken as the splitting between singlet and triplet states of a system of two spins on neighbouring

sites. In our case the

sites are the weakly interacting dimers and J may be evaluated when one considers the interdimer transfer integrals as perturbations

on the states of isolated di-

mers [9]. Four different transfer integrals ta, t~ in the a and ~ directions and t+ and t_ along b have to be considered as shown in fig. I. The corresponding Hamiltonian reads +

Ha = - t

~g (b~ a._ + aj~bi~) jo

or + Hb =- t+ Eg (bl aj~ + ajgbi~) - t_ Zg (blgbjg + b ~ b i ~

+ + a~gajg + a~ ai~)

where ~ corresponds to a or ~, i and j are the indices of two neighbouring dimers and a, b, a +, b + are the anihilation and creation operators on each monomer of a given dimer. In the a and ~ directions the resulting exchange is [9] J

=

2 ( t )2 / U

(The definition of J implies an antiferromagnetic

coupling for a positive J). The

result is more complex in the b direction. Due to the competition between t+ and t_ we obtain [10] Jb = ((t+)2 /4t ) { - F (U/4t) + G (U/4t)(l + 2t_/t+) 2 } where t is the intradimer overlap and F and G are increasing and decreasing positive functions of U/4t respectively. of Jb which becomes negative

The effect of a non zero t_ is a decrease

(ferromagnetic)

when t_ becomes large (t_ = t+/2).

The values of the transfer integrals may be evaluated from Hfickel calculations [II]. These calculations have been

done for (DIMET)2SBF 6 by Ducasse

With the values found by this author (t a = 85, t 6= 10,t+ =25, t _ = - 5

[5].

and t = 175

meV) and taking reasonable value of U we obtain a small and negative value of Jb" This suggests that the coupling along b is ferromagnetic while the couplings along the a and 6 directions are antiferromagnetic. dering of the spins with a (2a,b) superstructure.

Such couplings induce an

or-

The direction of the spins in

the ordered phase are then found by minimization of the anisotropy energy.

Anisotropy energy The anisotropy energy is obtained by calculating the dipolar interactions between spins lying on neighbouring dimers. Figure 4 gives a schematic representa-

429 tion of the structure, with different spins orientations,

assuming the super-

structure mentioned above. A simple examination of this figure clearly show3 that the interactions are all attractive in case a. Two of them are attractive (along a and 6) and the third one repulsive

(along b) in case b, and all of them

are repulsive in case c. (The dipolar energy is negative for side by side antiparallel dipoles and for parallel dipoles lying along the same axis). As a consequence the easy direction should be close to the b axis (case a), the intermediate axis should be close to the elongated axis of the molecules hard axis should be perpendicular

_a.

(case b) and the

to the mean plane of the molecules

_b_

(case c).

_c_

Fig. 4. Schematic view of the magnetic ordering along the principal magnetic directions : (a) along the easy axis b, (b) along the intermediate axis, (c) along the hard axis. ~

This determination

is confirmed numerically,

minimizing

the dipolar energy of

a system of four dimers, assuming the spin localized on the sulfur atoms. This orientation of the magnetic axes is in good agreement with the experimental resuits. The anisotropy energies Wei (in the easy-intermediate

plane) and Weh (in

the easy-hard plane) can be ~sed to estimate ~± using the formula (N~_)2 = 2JWel. where

= 2JWe h

J = 60 K has been previously deduced from ESR data. The obtained values

(Wei = 5mK, ~_ =

(~+)2

[7] :

12kG

Weh = llmK)

lead to :

~+ = 18kG

in good agreement with the experimental

data. The weak difference between expe-

riment and calculation may reveal a slight delocalization of the spins. CONCLUSIONS In the approximation of weakly interacting dimers, cribed as a system of quasi-isolated

(DIMET)2SbF 6 can be des-

spins. Under these conditions a perturbation

technique is efficient to deduce the exchange couplings between dimers from the

430 transfer integrals. Moreover the localization of the spins allows the simple calculation of the anisotropy energy. This calculation is expedient for explaining the AFMR results since both the orientation of the magnetic axes and the magnitude of ~± are deduced from our model. The generalisation of this discussion to other organic conductors should be done with care, since strong dimerization is one ingredient of our description. However some conclusions of this paper will remain valid at least when strong electronic localization occurs at low temperature [10]. ACKNOWLEDGEMENTS The authors acknowledge B. Gallois and L. Ducasse for communication of their results prior to publication. REFERENCES I

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