Ordering phenomena and chemical bond in bismuth based manganites
- Czech Science Foundation (2003-2005)
This project is focused to a material science of magnetic mixed oxides of transition elements. Typical examples of such materials are the perovskites with variable valence of manganese with general formula Ln1-x
, where Ln = La, Pr, Bi..., and A = Ba, Sr, Ca. The main reason for the revived interest in this kind of materials is their potential application for the construction of reading heads in magnetic recording media and as a source of spin polarized charge carriers for "spin electronics". The scope of present project is preparation and complex investigation of special manganite systems with ionic ratio Mn3+
(1:1), eventually (1:2) or (1:3), which show charge ordering. Exceptional case are compounds Bi0.5
where such ordering seems to be supported by a stereoactivity of the Bi3+
lone electron pair and leads to an unusually high charge ordering temperature (above 500 K).
Ferromagnetic phase separation in Mn-site doped perovskites (Ln,A)MnO3
- Grant Agency of the Academy of Sciences (2002-2004)
The perovskite manganites Ln1-x
(Ln = La, Y or rare earth, A = alkaline earth) become subjects of considerable research effort because of their prospects as materials of colossal magnetoresistence (CMR) or metallic conductors with spin polarized carriers. Recently, much interest was attracted to unusual phase separation processes which seem to be at the root of the CMR properties. The present project follows this trend and focuses to separation of ferromagnetic metallic regions in various non-conducting manganite systems, controlled by a small doping on Mn sites with other transition metal ions. By means of the structural, magnetic, spectroscopic and electric transport experiments, combined with the ab-initio and effective Hamiltonian based calculations, the proposed study aims to a better understanding of the microscopic interactions of dopant ions and their surroundings.
Oxidic materials with photoinduced insulator-metal transition
- Grant Agency of the Academy of Sciences (2000-2002)
The charge ordered antiferromagnetic insulating groundstate of perovskite manganites Ln1-x
in the concentration range of Mn3+
close to 1:1 can be transformed to a metallic ferromagnetic state by magnetic field, high pressure or other means. It is of great practical importance that the metallic state can be induced locally by illumination with X-rays or visible light while surrounding material remains insulating. This project is directed to novel class of manganites based on alkali metal substitutions which show similar effect. The research will involve the preparation of samples by ceramic methods and growth of single crystals from high temperature solutions. A complex physical and chemical characterization will be undertaken with the aim to prepare materials for optically switched electronic elements.
Charge ordering, magnetism and the insulator-metal transition in the Mn3+/Mn4+ perovskites
- Czech Science Foundation (1999-2001)
Comprehensive study of the manganites with the perovskite structure, in which the Mn3+
ions coexist. In dependence on the Mn3+
content, these systems exhibit a variety of phase transitions - structural, as well as magnetic, insulator-metal and charge order. Our attention will be focused on systems with the Mn3+
ratio close to 1:1 and 1:3, in which the proximity of the critical temperatures of the above mentioned phase transitions is expected to cause an unconventional behavior, leading e.g.
to the colossal magnetoresistence or to a change of the crystal structure caused by an applied magnetic field. Hitherto studies of these systems (especially with the 1:3 ratio) are relatively scarce and leave a number of questions open. Experimental part of the project includes preparation of the polycrystalline samples and the single crystals, determination of the crystal and magnetic structures using X-rays and neutrons, measurements of the magnetic moment, electric and thermal properties.
Local magnetic properties and the giant magnetoresistance in mixed-valence manganites
- Grant Agency of the Academy of Sciences (1996-1998)
Lanthanum and rare-earth manganites with heterovalent substitutions are for certain concentration of Mn4+
ions ferromagnets with metallic conductivity below Curie temperature. The electric transition is accompanied with a large magnetoresistence effect which spreads over a broad temperature window. The magnetoresistence can be enhanced, role of charge and magnetic inhomogenities, the magnetoresistence and other transport properties will be studied on quenched and annealed samples carefully checked on the oxygen stoichiometry. The experiments will be completed with detailed structural and magnetization investigations. In distinction to recent reports, the study is extended to novel systems with monovalent substitutions for the rare-earth cations.
Structure and Properties of Perovskites with Mn3+, Mn4+ Ions.
- Czech Science Foundation (1994-1996)