Photopyroelectric calorimetry applied to the study of phase transitions down to 10K

A. Oleaga1, A. Mendioroz1 and A. Salazar1

1Departamento Fisica Aplicada I, Escuela Tecnica Superior de Ingenieria, Universidad del Pais Vasco, Bilbao, Spain

Keywords: thermal diffusivity, low temperature, phase transition
property: thermal diffusivity
material: solids

The photopyroelectric (PPE) calorimetry has been widely used to measure the thermal properties of solids and liquids. In the standard back configuration, where an opaque sample is periodically illuminated on one side while the other side is in contact with the pyroelectric detector, thermal diffusivity (D) and thermal effusivity (e) can be obtained from simple linear relations. From the constitutive relation K = ρcD, (where ρ is the density), thermal conductivity (K) and specific heat (c) can be retrieved. PPE calorimetry has two main advantages: the temperature dependence of D, K and c can be obtained from one single heating and/or cooling run at a fixed frequency; and a good signal to noise ratio is obtained with very small light intensities. This means that small temperature gradients are induced in the sample and therefore this calorimetry is especially suited to study phase transitions. In the last years we have been using different PPE calorimeters (some of them designed at our lab), to characterize the thermal properties of magnetic and ferroelectric samples around their transition temperature in the range 10-600 K but there were severe experimental limitations in the range 10-77K due to lack of thermal contact and the presence of mechanical vibrations in the cold head. In this paper we present a recent development of the PPE calorimetry below 77K, down to 10K, obtaining high resolution measurements, overcoming the problems that the previous system presented: The sample is in a helium atmosphere, which implies that thermal contact between the sample and the detector through the coupling thermal grease is assured at all temperatures, and the system is designed so as to eliminate mechanical vibrations at the sample/detector location (they would introduce electrical noise in the signal, as the photpyroelectric detectors are also piezoelectric). This has been achieved adapting a commercially available Mossbauer closed-cycle He cryostat which has a very specific mount designed to avoid vibrations. Besides, it allows performing slow cooling/heating runs down to 10 mK/, with a temperature resolution of 1 mK, which makes it very suitable to study phase transitions in detail. Lastly, as there is no refrigerant consumption, very long measurements can be performed. We present as an example the study of the critical behaviour of the antiferromagnetic to paramagnetic transition in the family of hexagonal manganites RMnO3 (R=Lu, Yb, Tm, Er, Ho, Y), thus completing our previous works on orthorhombic manganites. We acknowledge the support given by Grupos de Investigacion del Gobierno Vasco IT351-10.

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