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Final Activity Report Summary - NEWMATCR (Synthesis of new materials based on half metallic ferromagnet CrO2 and optimization of its properties for spin-electronics)

Spintronics, where spin degree of freedom are used to store and transmit information is an important avenue of research and promises to offer a viable and possibly superior alternative to current semiconductor based electronic devices. The building blocks of these devices are magnetic materials with a high degree of spin polarization. The current surge of research has been to identify materials which can change their magnetic state not only using an applied magnetic field, but also through electric field and mechanical strain. These are generally referred to as multiferroic materials and a wide variety of ferromagnetic (FM), antiferromgnetic (AFM) and ferrimagnetic materials are currently being investigated for this purpose.

This project pertains to the half metallic ferromagnet CrO2 and the magnetoelectric/AFM Cr2O3. The former term refers to a ferromagnet in which only one of the spin split band crosses the Fermi level. These causes full spin polarised conduction, a prime requirement for a working spintronic device. Among the materials being considered for use in spintronic devices, CrO2 has experimentally shown nearly perfect spin polarisation and is therefore the most potential candidate in the fast developing area of spintronics.

A peculiar trait exhibited by this material is that the electron transport is metallic in single crystals whereas granular samples show an activated behaviour due to the insulating character of the grain boundaries (GB). GB have been identified to be AFM Cr2O3. This phenomenon is also central to the observation of enhanced magnetoresistance (MR) which makes granular CrO2 a natural source of a Magnetic Tunnel Junction (MTJ) - an assembly of two FM electrodes separated by an intervening insulator. Such applications have wide relevance in Magnetic Random Access Memory (MRAM) technology, magnetic switches and sensors. Due to the difficulties associated with the synthesis of CrO2, as it is a metastable phase which is not known to form in ambient pressures, there were no focussed studies to tune the MR by variations in grain size and GB density in granular CrO2 prior to our work.

The project aimed at (1) understanding novel features in electron transport which appeared as a result of variations in grain size and GB boundary density in granular CrO2 and (2) to fabricate new spintronic devices based on CrO2. One of the most significant results of the present studies has been to experimentally ascertain the role of AFM and ME character of the GB in electron transport and magnetization in granular CrO2. Note that, while the role of insulating GB in providing the tunnel barrier in spin polarized tunnelling was more or less established in literature, what role its intrinsic AFM and magnetoelectric character played in electron transport was not clear.

Here, the term magnetoelectricity refers to the fact that Cr2O3 can show electric field induced magnetization or magnetic field induced polarization. This phenomenon is well established for Cr2O3 in its bulk single crystal, polycrystalline and thin film forms. However when Cr2O3 appears as a GB or the surface layer outside the FM grain, the manifestations of these ME effects have been observed for the first time. Tuning of the grain size has enabled us to explore some hitherto unknown features related to GB in granular CrO2. For instance, from the measurements of low field thermoremanent magnetization, we get some experimental evidence that the natural strain at the interface due to the different crystal symmetry of the grain and the GB may lead to the generation of piezomoments.

These moments, in turn modulate the magnetisation of the FM component which yields very interesting interface effects, manifested in low field magnetisation in this material. Overall, these features, if translated to devices, could provide new avenues such as electric field and possibly mechanical strain control of devices based on CrO2. Another important outcome of this project is the filling of carbon nanotubes with these two technologically important materials (i.e. CrO2 and Cr2O3) as a 1st step to fabrication of nano-spintronic devices.

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