On 18 August at 14:15 Kristjan Kalam will defend his doctoral thesis “Magnetic, electric and structural properties of atomic layer deposited zirconia-based nanolaminates and mixtures” for obtaining the degree of Doctor of Philopsophy (in materials science).
senior research fellow Aile Tamm, University of Tartu
research professor Kaupo Kukli, University of Tartu
reserch fellow Aarne Kasikov, University of Tartu
senior research fellow Robert Zierold, University of Hamburg (Germany)
senior research fellow Ivo Heinmaa, National Institute of Chemical Physics and Biophysics, Tallinn
The main goal was to fabricate a multiferroic nanoscale film using atomic layer deposition. Multiferroic is a material that is both ferromagnetic and ferroelectric, that is, polarizes in both magnetic and electric fields, and retains that polarization after removing the external field. Such a material could be used in novel nanoelectronics applications, such as memory devices or sensors. Atomic layer deposition was chosen to fabricate the films, because this is the method actually used in modern nanoelectronics to deposit ultrathin films, and the only method which can provide conformal films over a large substrate area and at the same time provide thickness control at the nanometer level. It was known beforehand, from literature, that a material possessing ferromagnetic and ferroelectric behavior in the same sample in the same phase will be a difficult task. This phenomenon has been observed in bulk materials and/or very low temperatures, but not in thin films and at room temperature, which are both necessary, if one wishes to consider an actual nanoelectronics application.
In various ZrO2-based thin films, it was shown that some films showed ferromagnetic hysteresis and some exhibited behavior resembling ferroelectric response. In one case, ferromagnetic and ferroelectric behavior were observed in the same material sample.
It was concluded that although one cannot speak of ferromagnetism in the traditional sense, when thin metal oxide films are studied, but in certain cases, ferromagnetism may still arise from the defects of a material, such as oxygen vacancies. Although these defects make the detection of ferroelectricity harder, a reasonable trade-off can be found between enough defects to induce ferromagnetism and not so much to overwhelm the signs of ferroelectricity completely. The author believes such as case was found, when a defective material, which was found ferromagnetic in all cases, namely ZrO2, was mixed with a less defective material, HfO2, known already in literature to be ferroelectric in some cases.