Journal of the Korean Physical Society, Vol. 32, Feburary 1998, pp. S380∼S381

A New Modification to the Sawyer-Tower FerroelectricHysteresis Loop Tracer

G. Ornelas-Arciniega and J. Reyes-Gomez

Centro de Investigacion en Ciencias Basicas Universidad de Colima,CP 28000 Colima Col, Mexico

A. G. Castellanos-Guzman

Laboratorio de Investigacion en Materiales, DIP-Cucei, Universidad de Guadalajara,CP 44281 Guadalajara Jal, Mexico

A new modified Sawyer-Tower circuit has been developed for obtanining undistorted ferroelectrichysteresis loops. The proposed instrument is equipped with circuits for compensating both, the lowconductance and linear component of the capacitance of the ferroelectric sample under study.

The original Sawyer-Tower circuit [1] for obtaininghysteresis loops of ferroelectric crystals has been in usefor more than sixty years. It is a very practical circuitto characterize ferroelectrics and to study fundamentalphenomena such as spontaneous polarization, coercitivefield and polarization reversal mechanisms in general. [2]In the original circuit, however, no provision was taken toeliminate and/or compensate the different contributionscurrently superimposed on the ferroelectric response tothe applied field, such as non-linear conductivity of thesamples under study, signal phase differences etc. Thesecontributions deform the hysteresis loops to some extentand in some cases have even led to misinterpretation ofthe ferroelectric character of some crystals. [3] A rela-tively simple modification of the original Sawyer-Towercircuit is presented here, in which we also have incorpo-rated some features proposed in the past by Sinha [4],Mestner [5] and Tanaka et al. [6] enabling thus our cir-cuit to compensate the insulation resistance and linearcapacitance of the ferroelectric sample. With this newversion, which is simpler and less expensive than thosementioned above, one is able to read spontaneous polar-ization values directly from the oscilloscope.

Figure 1 shows a simplified block diagram of the mod-ified ferroelectric hysteresis loop tracer. The oscillatorgenerates a sinusoidal voltage (at a frequency of 100Hz). The signal amplitude can be adjusted and there isprovision for applying external signals of different waveshapes. The selected voltage, Vcr, is applied to threedifferent points: (1) to one side of the electroded ferro-electric sample which , in turn, is placed in a modifiedhot stage Leitz 350, or any other thermically-controlledcontainer, (2) to the network designed for compensatingboth, the insulation resistance and linear capacity of theferroelectric sample, and (3) to the amplifier/attenuator,whose output is directly connected to the X axis of an

oscilloscope. The other side of the ferroelectric sampleis connected to an Op-Amp being thus at zero volts po-tential (virtual ground). Vcr denotes the voltage acrossthe ferroelectric sample. This voltage produces a chargeacross the ferroelectric sample equal to CxVcr, where Cxdenotes the sample’s capacitance.

Since the Op-Amp has, ideally, an infinite inputimpedance, the charge for the ferroelectric sample shouldcome from Cm. i.e. the capacitor connected between theamplifier’s output and the non-inverted terminal. Thecharge from Cx, assuming the sense of the charge move-ment as shown in Fig. 1, goes into Cm producing thusa voltage which is equal to q/Cm, so that the Op-Ampoutput voltage is -Vm and since the charge should be thesame in both capacitors we have Vm = −(Cx/Cm)Vcr.

This voltage, which is proportional to the polarizationof the ferroelectric sample, is inverted by an amplifierand it is then applied to the Y axis of the oscilloscope.

Compensation of the linear conductivity of ferroelec-tric samples is realized in this circuit by injecting acharge of the opposite sign to that produced by Vcr acrossthe sample resistance. This current is generated by in-verting Vcr, multiplying it by an attenuation factor Kr

and applying it to Rc. Cancellation is attained whenKrVcr/Rc = Vcr/Rx, or Kr = Rc/Rx. Kr can be ad-justed in the range 0 ≤ Kr ≤ 1/50. Similarly, the currentthrough the linear capacitance of the sample is cancelledby applying a voltage to Cc such that Kc = Cx/Cc . Theconstant Kc has the same range that Kr as mentionedabove.

In our circuit Rc and Cc have values of 24.9 kohm and4700 pF respectively. These values allow the compensa-tion of the ferroelectric sample resistance and linear ca-pacitance in the following ranges: 1245 kohm ≤ Rx ≤ ∞,and 0 ≤ Cx ≤ 94 pF.

In practice, the current going into the Op-Amp in-

-S380-

A New Modification to the Sawyer-Tower Ferroelectric Hysteresis Loop Tracer – G. Ornelas-Arciniega et al -S381-

Fig. 1. Block diagram

Fig. 2. Uncompensated hysteresis loop (BaTiO3) at roomtemperature.

puts terminals is different from zero. The Op-Amp isa composite Op-Amp with a discrete JFET front endstage thus the Op-Amp inputs are connected to the gateof a FET transistor so the current is a kind of a leak-age current. These currents are of low frequency andsomehow impredictables. The leakage current chargesthe capacitor Cm so it introduces an error in the chargemeasurement. The autozero circuit cancels the low fre-quency charge (the leakage current so to speak) in suchaway that the circuit behaves as a high-pass filter withcutoff frequency of 0.2 Hz thus the DC output voltage(Y axis) is zero. When the internal voltage is applied tothe ferroelectric sample, its frequency is so high (100 Hz)that the autozero circuit does not have any real effect onthe measurements. Provision has been taken by meansof a switch to disable this circuit when using an externalvoltage with a frequency lower than 10 times the cutofffrequency of the autozero circuit. As an example of theapplication of this modified Sawyer-Tower circuit hys-teresis loops have been obtained on a sample of bariumtitanate doped with iron. Figures 2-4 give examples ofa hysteresis loop for a lossy sample Fig. 2 as well as the

Fig. 3. Compensated hysteresis loop (BaTiO3) at roomtemperature.

Fig. 4. Compensated hysteresis loop (BaTiO3) at hightemperature T= 353 K.

correct loop after using compensation at room tempera-ture (Fig. 3), and with the sample at a temperature of 80◦C (Fig. 4). The values of the ferroelectric parameters ofthese samples it i.e. spontaneous polarization and coerci-tive field were evaluated from the compensated loop andwere found to be in good agreement with values reportedin the literature. [7]

The authors wish to thank Mrs. Nelida Figueroa-Perez for her technical assistance in constructing he pro-totype and final version of the circuit. Support fromConacyt-Mexico through Project 3981E is gratefully ac-knowledged.

REFERENCES

[1] C. B. Sawyer and C. H. Tower, Phys. Rev. 35, 269 (1935).[2] K. Yoo and Seshu B. Desu, Phil. Mag. B 69, 461 (1994).[3] M. E. Lines and A. M. Glass, Principles and Applications

-S382- Journal of the Korean Physical Society, Vol. 32, Feburary 1998

of Ferroelectric and Related Materials (Clarendon Press,Oxford, 1977), p. 103.

[4] J. K. Sinha, J. Sci. Instr. 42, 696 (1965).[5] J. Mastner, J. Sci. Instr. (J. Phys. E) Ser. 2 1, 1249 (1968).[6] Jun Hatano, Tsuneo Watanabe and Raymond Le Bihan,

Ferroelectrics 126, 311 (1992).[7] Jack C. Burfoot and George W. Taylor, Polar Dielectrics

And Their Applications (The Macmillan Press Ltd., Lon-don, 1979), p. 165.