INFLUENCE OF THE ELECTRICAL BOUNDARY CONDITIONS ON THE N. M. R.-RELAXATION RATE OF 23Na IN NaNO2

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INFLUENCE OF THE ELECTRICAL BOUNDARY CONDITIONS ON THE N. M. R.-RELAXATION

RATE OF 23Na IN NaNO2

D. Müller, J. Petersson

To cite this version:

D. Müller, J. Petersson. INFLUENCE OF THE ELECTRICAL BOUNDARY CONDITIONS ON

THE N. M. R.-RELAXATION RATE OF 23Na IN NaNO2. Journal de Physique Colloques, 1972, 33

(C2), pp.C2-193-C2-194. �10.1051/jphyscol:1972265�. �jpa-00215001�

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JOURNAL DE PHYSIQUE Colloque C2, supplkment au no 4, Tome 33, Avril 1972, page C2-193

INFLUENCE OF THE ELECTRICAL BOUNDARY CONDITIONS ON THE N. M. R.-RELAXATION RATE OF 23Na IN NaNOz

D. MULLER and J. PETERSSON

Institut fiir Experimentalphysik I1 der Universitat des Saarlandes, Saarbriicken, Germany

R6sum6. - La relaxation N. M. R. de 23Na dans NaNOz est expliquke par une thCorie ther- modynamique. Les premiers rCsultats expkrimentaux relatifs A l'influence de diverses conditions aux limites Clectriques sur T I sont prksentkes.

Abstract.

-

The N. M. R. relaxation behaviour of z3Na in NaNOz is explained by a thermodynamic theory. First experimental results of the influence of various electrical boundary conditions on T I are reported.

In recent experimental and theoretical studies it has been shown that the quadrupolar nuclear spin lattice relaxation time T I of 23Na in NaNO, single crystals is strongly influenced by the dynamical instability of the crystal in the vicinity of the ferroelectric phase transition [I], [2]. The experimental results are interpreted by assuming a direct relaxation process relating the nuclear magnetic relaxation to the fluctuations of the electric field gradient tensor which is related to the dielectric behaviour. Furthermore it is known that the critical dielectric behaviour of this substance in the microwave region can be described by assuming an Ising model for the flipping of the NO;-groups [3].

As a consequence the magnetic relaxation is caused by the various polarization fluctuations depending on the wave vector q [I]. On the other hand the critical behaviour can be described equivalently by a macro- scopic thermodynamic theory of relaxation pheno- mena [4], [5]. Then it could be shown [2] that the range of validity of the observed logarithmic singularity of T I is considerably different in the paraelectric and ferroelectric phase because of the different Curie constants.

Since in the limit of long wave lengths the polarization fluctuations depend very strongly on the electrical boundary conditions [4], [5] one should expect an influence of these conditions on T I . Although our experimental results are not yet understood it is the purpose of this note to report some measurements of T I under different external electrical boundary conditions.

T I was measured by a saturation sequence as described in [2]. For an orientation of the crystal with the r. f. magnetic field parallel to the ferroelectric b-axis figure 1 shows the relaxation rate for a free crystal and a crystal with shorted evaporated gold electrodes.

o free crystal

-

shorted crystal-

I I

50 100 150 T

PC]--

FIG. 1. - Relaxation rate 1/Tl vs. temperature T for a free and a shorted crystal.

Near the transition in the first case the relaxation rate is considerably greater than in the latter one. At the phase transition maximal values of 18.3 s-

'

and 11.5 s-

'

are measured for T;' respectively. But at room tempera- ture the difference between these two quantities is not so great and seems to have the opposite sign. In the same way we measured T I with the ferroelectric b-axis parallel to the static magnetic field H,. Also in this orientation near Tc the relaxation rate was smaller for a crystal with shorted electrodes than for a free crystal.

But the difference is smaller than for the corresponding temperatures in figure 1. Also in this case at room temperature the relaxation rate seems to be greater for a shorted crystal than for a free crystal. Since near Tc the

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972265

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C2- 1 94 D. MULLER AND J. PETERSSON

polarization fluctuations of long wave lengths are Thus our experimental results are in contradiction to strongly enhanced by a shortening of the electrodes [4], this prediction and at the present we have no reaso- [5] one would expect a considerable increase of the nable interpretation. Further work will be done to relaxation rate when the electrodes are shortened. solve this problem.

References

111 BONERA (G.), BORSA (F.) and RIGAMONTI (A.), Phys. [4] M ~ ~ S E R (H. E.) and PETERSSON (J.), Fortschr. Phys.,

Rev., 1970, B 2,2784. 1971, 19, 559.

[2] AVOGADRO (A.), CAVELIUS (E.), MULLER (D.) and

PETERSSON (J.), Phys. stat. sol. (b), 1971, 44, 639. [51 PETERSSON

(J.1,

2. angew. Phys.2 19707 28, 261 and [3] HATTA (I.) J . Phys. Soc. Japan, 1968,24,1043 and 1970, ^2.Naturforsch., 1970,25a, 148.

28, 1266.