Preparation of Small Disk-shaped Samples for Capacitance and Pyroelectric Current Measurements

May 29, 2015

Capacitance and pyroelectric current measurements are best suited to determine important dielectric properties of materials, e.g. the relative dielectric constant, εr, and the remnant electric polarisation, P, of materials, respectively.

Reliable measurements require a precise estimate of the area, A, and of the thickness, t, of the samples according to the following well-known relations:



Eq. 1



Eq. 2

where C is the sample capacitance to be measured, ε0 is the vacuum dielectric permittivity, α is the cooling rate in the pyroelectric current measurement and Ip(T) is the pyroelectric current measured at a given temperature, T. Precise measurements require well-defined shapes and a high degree of parallelism of layer-shaped samples. This is difficult to achieve by means of manual polishing methods and mechanical polishing should be preferred.

In this technical report, we demonstrate the successful use of the Leica EM TXP target surfacing system to prepare a small capacitor made of a sintered sample of the ceramic phase LaMn7O12 [1], which exhibits improper ferroelectric behaviour induced by an antiferromagnetic transition below the Néel temperature, TN = 78 K. The remnant polarisation in the sample was determined by means of a pyroelectric current measurement, as described in equation 1. This measurement consists of polarising the capacitor above TN with a high voltage up to 900 V; the sample is subsequently cooled down to liquid helium temperature. Then the polarising voltage is removed and the pyroelectric current is measured upon heating the sample at a given heating rate a.

In Figure 1a below, we show the capacitor obtained by mechanically polishing both sides of a cylinder of sintered sample. We obtained a small disk of uniform thickness t = 0.25 mm which was subsequently coated by a thin Au layer. In Figure 1b [2], we show the remnant polarisation curve, P(T), obtained by integrating the pyroelectric current curve, Ip(T), according to equation 1.

Fig. 1a: Picture of the metallized capacitor with A = 12.3 mm<sup>2</sup> and thickness t = 0.25 mm.
Fig. 1b: Polarisation curves obtained from pyroelectric current measurements at different polarising fields indicated in the legend.

Intellectual property of A. Gauzzi and Y. Klein, IMPMC, UPMC-Sorbonne Universités, CNRS, IRD, MNHN, Paris, France, F. Milton, A. Gualdi, D. Garcia, A.J.A. de Oliveira, Universidade Federal de São Carlos, São Carlos, Brasil, E. Gilioli, IMEM-CNR, Parma, Italy.

The authors acknowledge Sébastien Charron and Imène Estève (IMPMC) for their assistance in sample preparation.

References

  1. Prodi A et al.: Phys. Rev. B 79: 085105 (2009).
  2. Gauzzi A, Klein Y, Milton FP, Gualdi A, Dreifus D, Garcia D, de Oliveira AJA, and Gilioli E: unpublished (2014).

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