engleski

Hydrothermal synthesis of multiferroic materials

The class of materials, extremely important for computer and electronic industry, are the perovskite materials that typically possess so-called ‘ ferroic’ properties: ferroelectricity, ferromagnetism and ferroelasticity. Only some of them are multiferroic materials that possess two – or all three – of the ‘ ferroic’ properties. For this kind of materials both magnetization and polarization are coupled, and thus in principle permit data to be written electrically and read magnetically. This is attractive, given that it would exploit the best aspects of ferroelectric random access memory (FeRAM) and magnetic data storage, while avoiding the problems associated with reading FeRAM and generating the large local magnetic fields needed to write A part of already established collaboration of the groups in IRB and IJS is pointed toward the syntheses and the study of perovskite multiferroic materials as BiFeO3, BiMnO3, BiCoO3. Bismuth ferrite is commensurate ferroelectric and incommensurate antiferromagnetic at room temperature, BiMnO3 is ferromagnetic and ferroelectric below TM=105 K, while BiCoO3 have antiferromagnetic ordering and giant ferroelectric polarization leading to giant magneto-electric coupling. We use the hidrothermal syntheses, that is crystallization of substances from high-temperature aqueous solutions at high vapor pressures, since it is simple and inexpensive method. Since it is hard to synthesized pure multiferroic, the optimization of hydrothermal conditions (pH, temp., time) is necessary to get pure materials with reduced dimensionality and nanometric sizes. The phases appearing during a hydrothermal reaction in the Fe– Bi– O system were investigated, with the aim to optimize the conditions for the syntheses of nanostructured multiferroic BiFeO3. The prepared samples were analyzed by X-ray powder diffraction (XRD), while the morphology, nanostructure and chemical composition were determined using high-resolution transmission electron microscopy (HRTEM) and/or scanning electron microscopy (SEM). In situ temperature dependent Raman spectroscopy measurements were shown as a tool for observation of antiferromagnetic to paramagnetic transition in BiFeO3, since the antiferromagnetic behavior of BiFeO3 can not be straight forward observed by magnetic measurements. We succeed in the syntheses of pure bismuth ferrite in the procedure where solutions containing both iron and bismuth ions were co-precipitated prior to hydrothermal processing. The phonon anomalies were observed by high temperature Raman spectroscopy around the magnetic Neél temperature ( 640 K), although this magnetic transition is not accompanied by a structural phase transition. In the future we plan in situ high temperature Raman investigation of BiFeO3 at the ferroelectric Curie temperature ( 1100 K) where a first-order structural phase transition is expected. Further optimizations of hydrothermal conditions for syntheses of BiFeO3 with reduced dimensionality as well as hydrothermal synthesis of BiMnO3, BiCoO3 are also planed. Detailed Raman investigation of temperature phase transitions in synthesized multiferroics will clarify their ferroic properties. These measurements will be possible since first class Raman instrument in IRB is equipped with various cryostats and temperature cells for in situ temperature measurement from 10 to 2000 K.

multiferroic; hidrothermal sintheses; HRTEM

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