Magneto-capacitive coupling and hybridization effects in Spinels

P. Lemmens (TU Braunschweig) and
V. Tsurkan (Chisinau, Moldova)





The aim of our study is to clear up the coupling mechanism and the application of magneto-capacitive effects in spinels. Multiferroics, with coexistence and mutual coupling of magnetic, dielectric and lattice degrees of freedom are in the centre of present interest. While practical questions involve the maximisation of both coupling constants and characteristic temperature regimes, there are also fundamental issues related to the existence of small energy scales, the effect of local distortions, and the dynamic variation of the electronic structure. Within this project we will perform Raman scattering, ultrasound spectroscopy and preparation of single crystals. Raman scattering is especially sensitive to local polar distortions. Using ultrasound spectroscopy structural and electronic relaxation processes will be studied. Samples will also be doped and optimized with respect to applications.

More activities of the applicants in the field of magneto-capacitive effects:

  • Composites of ferroelectric/magnetic components are studied in collaboration with Mats Johnsson (Stockholm) and Reinhard Kremer (MPI-Stuttgart) with support from Vinnova, "Materials by Design". Our aim is a large and bistable, application relevant coupling at room temperature and in small magnetic fields.



Ternary transition-metal chalcogenides with spinel structure AB2X4 form a group of compounds with highly correlated structural and electronic degrees of freedom. A rich spectrum of physical effects and unconventional ground states are observed, like metal-insulator transitions (CuIr2S4, HgCr2S4), superconductivity (CuRh2S4, Cu1-xZnxIr2S4), colossal magnetoresistance (FeCr2S4, HgCr2S4), charge and orbital ordering (CuIr2S4,  FeCr2S4), spin-orbital liquid, and orbital glass state (FeSc2S4, FeCr2S4).  In addition to the challenges these materials represent with respect to a fundamental understanding of their physical properties, an enormous growth in interest to magnetic spinels is caused by various possible applications in modern spin-electronics, named spintronics. These applications will utilize charge, spin and orbital degrees of freedom to control the devices characteristics and to enhance their functionality.

Recently, a “colossal” magneto-capacitive coupling (change of the dielectric constant in a magnetic field) has been discovered in the CdCr2S4 spinel, a compound with a simple ferromagnetic structure [hem05, lun05]. This property is a “hot” topic in recent solid state research [spa05] as the mutual control of these order parameters offers the possibility to design a new generation of magneto-electronic and magneto-optical devices. As an example the electrical switching of a magnetic RAM cell is suggested. Other multiferroic materials that are based on magnetic perovskite rare-earth manganites [kim03, got04, hur04] work only at low temperatures and in the direct proximity of the magnetic instability. In contrast, spinel systems show evidence for a pronounced fluctuation regime with high potential for further improvements.

It is the intention of the present proposal to strengthen the experimental data base in spinel research using inelastic light and ultrasound spectroscopy and to shift and enlarge the fluctuation regimes with large magneto-capacitive coupling to higher temperatures. To realize this, the applicants use their expertise in crystal growth and doping experiments. Despite strong activities in the 70th, no present inelastic light scattering activities with state-of-the art equipment exist. The present proposal will change this situation based on the expertise of the involved groups and collaboration partners.



[kim03] T. Kimura, et al, Nature (London) 426, 55 (2003).

[got04] T. Goto, et. al., PRL 92, 257201 (2004).

[hur04] N. Hur, et. al., PRL 93, 107207 (2004).

[hem05]  J. Hemberger, et al., Nature (London) v. 434, p. 364- 367 (2005).

[hem06a] J. Hemberger, et al., PRL 97, 087204 (2006).

[hem06b] J. Hemberger, et al., cond-mat/0607811 and PRL (2007).

[lun05]  P. Lunkenheimer, et al., Phys. Rev. B 72, 060103(R) (2005).

[tsu03]  V. Tsurkan, et al., Phys. Rev. B 68, 134434 (2003).

[tsu05]  V. Tsurkan, et al., Journ. Phys. Chem. Solids  66, 2036 (2005).

[tsu06] V. Tsurkan, et al., Phys. Rev. B 73, 224442 (2006).


Details about the project:

Presently we perform doping studies and grow larger single crystals of different compounds to allow ultrasound experiments and Raman scattering. Such data is needed to evaluate lattice coupling phenomena on the dielectric constant.



Impressum - Back to the Webpages of Peter Lemmens -  28.09.2007