Multiferroizität in niedrigdimensionalen magnetischen Systemen mit unterschiedlichen Spins

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This German-Russian project aims at investigating magnetoelectric and multiferroic phenomena in the class of low-dimensional mixed spin systems of the howardevansite structure type. This rich and practically uninvestigated family of metal oxides includes, in particular, Fe7(PO4)6, NaFe7(PO4)6, LiCuFe2(VO4)3, and NaCuFe2(VO4)3 which will be studied in beginning of the project while new materials will be addressed furtheron. Our preliminary investigations indicate the presence of significant magnetodielectric coupling in the magnetically ordered state of some of these compounds. At present, magnetically low-dimensional materials such as the howardevansite-type ones at hand with sequences (chains) of different spins are rather rarely studied. The appearance of spontaneous electric polarization in these systems is associated with the formation of non-collinear long-period magnetic structures. In the frame of this project, the synthesis by means of hydrothermal and solid-state methods as well as a variety of sophisticated experimental studies will be applied. To be specific, in addition to standard characterization techniques we will study the specific heat, thermal expansion and magnetostriction, magnetization, pulsed field magnetization, dielectric permittivity, and the dielectric polarization. Application of magnetic field up to 18 T (static) and 35 T (pulsed) will enable constructing the magnetic phase diagrams. In addition, the magnetic ground state will be studied by means of high-frequency Electron-Spin Resonance-Spectroscopy and Muon-Spin-Rotation. This rich variety of methods will provide all relevant information on the magneto-elastic-dielectric couplings, thereby enabling in collaboration with our theory partners to elucidate the mechanisms of magnetelectric coupling and multiferroicity in this class of compounds. As a side activity, the electrochemical properties of Li-based materials will be studied and in-situ SQUID magnetometry will be performed in order to investigate the effect of delithiation on the magnetic properties. We believe that the project will not only yield several particular examples of materials with strong magneto-dielectric coupling realized by a new mechanism in materials with unusual sequences of different spins but will also open an avenue towards a new family of multiferroics and magnetoelectrics.