CRITICAL BEHAVIOUR OF THE MAGNETOELECTRIC EFFECT IN Cr2O3

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CRITICAL BEHAVIOUR OF THE

MAGNETOELECTRIC EFFECT IN Cr2O3

E. Fischer, G. Gorodetsky, S. Shtrikman

To cite this version:

E. Fischer, G. Gorodetsky, S. Shtrikman. CRITICAL BEHAVIOUR OF THE MAGNETOELEC- TRIC EFFECT IN Cr2O3. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-650-C1-651.

�10.1051/jphyscol:19711224�. �jpa-00214051�

JOURNAL DE PHYSIQUE

Colloque C I, supplkinent au no 2-3, Tome 32, Fkvrier-Mars 1971, page C I - 650

CRITICAL BEHAVIOUR OF THE MAGNETOELECTRIC EFFECT IN C r 2 0 3

E. FISCHER, G. GORODETSKY (*) and S. SHTRIKMAN

The Weizmann Institute of Science, Department of Electronics, Rehovoth, Israel

Rbum6. - La dtpendance en temperature de la susceptibilite magnCtoClectrique linkaire a ( T )

M / E ( E

champ electrique appliqut, M = moment magnetique induit), a ete mesurke au voisinage de la temp6rature critique TN. Ainsi que I'on pouvait esperer, I'effet en fonction de la temperature suit une loi de puissance; I'exposant (e

0,35 & 0,Ol) en accord avec ce que l'on a pu obtenir a partir d'autres proprittts physiques.

Abstract. - The temperature dependence of the linear magnetoelectric susceptibility - a ( T )

M / E ( E

applied electric field, M

induced magnetic moment) -has been measured in the vicinity of the critical temperature, TN. AS expected, the dependence on temperature follows a power law, .and the exponent (e

0.35 & 0.01) agrees with what has been expected from the behaviour of other physical properties.

The magnetoelectric effect is strongly dependent on temperature in the range immediately below the cri- tical temperature, TN. In analogy with the critical behaviour of other physical properties [I], we expect the magnetoelectric susceptibility to behave like

E is the applied electric field (afew thousand Volts/cm), M the induced magnetic polarization, measured in the direction of the electric field, TN is the critical tempe- rature, above which the effect disappears, and

is a constant exponent.

The measurements have been carried out on a non-oriented single crystal which showed the magne- toelectric effect spontaneously. (Ordinarily, the sam- ples have to be cooled through TN with an electric and a magnetic DC field applied for showing the effect.) An electric AC field with a frequency of 1 030 cycles was applied to two opposite faces of the crystal.

The induced magnetic moment was picked up by a set of compensated pickup coils and amplified by a lock-in amplifier. To obtain good temperature homogeneity, the sample assembly was immersed in an oil bath, the temperature of which could be sweeped very slowly.

The temperature was indicated by the resistivity of a bifilarly wound copper coil, which vafies linearly with temperature in the temperature range considered.

A sensitivity of 60 mV/cm on the recorder could be obtained and was sufficient for this experiment.

The result is shown in figure 1. As can easily be seen, the temperature resolution is not limited by noise or drift of the thermometer, or by temperature fluctuations between sample and thermometer. The limitation comes from the sample itself. Above TN, a exhibits a tail. It is not yet clear if this tail is a result of sample inhomogeneity, or if it is an intrinsic effect. The curve is reproducible. By changing the direction of the temperature sweep, some lag appears, which is however small for slow temperature sweep

(*) On leave at Rurgers University, New Brunswick, N. J., USA.

and does not change the shape of the curve.

The analysis of the critical behaviour has been carried out as follows. The expected behaviour is given by formula (I). If this holds, a plot of log u ( T ) versus log AT must result in a straight line. One diffi- culty arises from the presence of the tail above TN : a direct determination of TN is impossible. We therefore made different assumptions for TN (in Fig. 1 given by the vertical lines 1, 2, 3, 4). As a result we get the curves 1, 2, 3, 4 in the log-log plot (Fig. 2).

We chose the one which showed the best linearity for

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711224

CRITICAL BEHAVIOUR OF THE MAGNETOELECTRIC EFFECT IN Cr203 C 1 - 651

small AT. In this way we determined the critical temperature for which formula (1) fits best.

The slope of the line in figure 2 is equal to

We obtain

The measurement was carried out in the tempera- ture interval

The temperature dependence of the sublattice magne- tization in Cr203, as determined by neutron diffrac- tion [2], shows the same behaviour. This is what one would expect if u can be expanded in terms of the sublattice magnetization near T,.

References

[I ] FISHER (M. E.), Rep. Progr. Phys., 1967, 30, 615, [2] SHAKED (H.) and SHTRIKMAN (S.), Solid Stare Comm.,

HELLER (P.), ibid., p. 731. 1968, 6, 425.