Micron-size Crystallites of CrO2 with insulating surface layer

Abstract

CrO2 is a well-known half metallic ferromagnet with TC ~ 400K. In granular form, surface layer CrO2 contains another oxide Cr2O3, which is an insulator. Due to the presence of insulating surface layer, granular CrO2 is known to exhibit activated transport, as metallic grains are separated by insulating surface layer, leading to tunneling and consequently a large tunneling magnetoresistance. Here we show that depending on the crystallite size and connectivity, (i) activated transport region along with a metallic region is observed in the electron transport measurement. (ii) In addition to a large tunneling magnetoresistance, significantly large electro resistance has also been observed in these samples. Thus, the electrical transport is not only influenced by ultra-thin surface layer of Cr2O3, which is antiferromagnetic and insulating, but also data suggest that the magnetoelectric character of Cr2O3 plays a significant role in transport. It is noteworthy that the % MR in 5 Tesla field is similar in magnitude to electro-resistance in few mA of current applied. The influence of magnetoelectric and antiferromagnetic nature of grain boundary is further corroborated by the changes in lattice parameters observed in X-ray diffraction data, which have been analyzed using Rietveld profile refinement. Rietveld Profile Refinement data of X ray diffraction data has been obtained in the temperature range from 300 K to 100 K. This enables us to plot lattice parameters of both CrO2 and Cr2O3 as a function of temperature. These data reveal an anomaly in lattice parameters of Cr2O3 in the vicinity of the magnetoelectric transition of Cr2O3. These data bring out the role of magnetic state of insulating Cr2O3 in electronic transport. The specific heat data in the low temperature region on the granular CrO2 is also been analyzed for obtaining electronic contribution to specific heat. This thesis extensively examines the mesoscopic and well-defined crystallites of CrO2, placing a central focus on employing X-ray diffraction (XRD) techniques. The analysis of XRD data utilizes Rietveld refinement, offering in-depth insights into the structural characteristics of the sample. Additionally, this thesis highlights the importance of size and interface effects in granular CrO2, especially the observation of electro-resistance. This has the potential to propel the development of magnetic tunnel junction technology, with additional tunability.