Low Temperature Magnetotransport Properties of Polycrystalline Ca 3 Co 4 O 9
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Low Temperature Magnetotransport Properties of Polycrystalline Ca3Co4O9 David J. Magginetti1, Shrikant Saini1, Ashutosh Tiwari1 1
Nanostructured Materials Research Laboratory, Department of Materials Science and
Engineering, University of Utah, Salt Lake City, Utah 84112, USA
ABSTRACT Ca3Co4O9 (CCO) is a promising material for thermoelectric applications; however, this layered oxide shows a large number of physical features that complicate understanding and systematically improving its properties. A significant component of CCO’s behavior is its magnetotransport properties, particularly in the low temperature region where an incommensurate spin density wave affects its band structure. In order to improve understanding in this area, we perform low temperature magnetoresistance (MR) measurements on a bulk CCO sample. Field-less resistivity measurements confirm the conventional behavior of our sample, with a metal-to-insulator transition at approximately 70 K, and a shoulder indicating ferrimagnetism at 14 K. Resistivity vs. temperature under applied magnetic field show significant MR below around 35 K. INTRODUCTION Since it has become known that that layered cobaltites such as Ca3Co4O9 (CCO) show promising thermoelectric properties of large thermopower and low resistivity at room temperature, a great deal of study has gone into studying these materials, both in terms of improving their thermoelectric figure of merit as well as understanding their fundamental physics. For instance, significant research has gone into improving CCO’s figure of merit through elemental doping [1-3], carrier concentration optimization [4], and practical device design [5]. The thermoelectric performance of CCO has been attributed both to strong spin entropy [6] or carrier localization [7], and the debate over the exact mechanism is ongoing. CCO is also well known to possess a rich temperature-dependent resistivity profile, with a relatively large number of charge transport transitions transpiring at low temperatures [8]. As the efficiency of the Seebeck effect is highly dependent on these transport properties, understanding and quantifying them is an important step in developing CCO as a thermoelectric material. Cobalt (Co) also plays a significant role in creating unique magnetic properties in this material, and while study has been made on the magnetotransport properties of CCO at high magnetic fields [9], no study has been made on these properties at low magnetic fields. Here we report on CCO’s electrical transport and magnetotransport properties at low temperature and low magnetic fields. Structural characterization was performed on bulk CCO pellets using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Analysis of the material’s transport properties was performed by low temperature electrical measurements with and without the application of magnetic field.
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