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Magnetic Control of the Ferroelectricity in an Organic Spin-Peierls Material

F. Kagawa, S. Horiuchi, and M. Tokunaga

Recently, magnetically controllable ferroelectrics came into a new light due to their variety of attracting novel phenomena and also potential applications for memory devices. Most of the ferroelectric magnets have been realized in frustrated magnets in which the special kinds of magnetic order break the space inversion symmetry of the crystals. In these materials, however, the observed electric polarization (P) was considerably smaller than those in typical ferroelectric materials. Consequently, the thermodynamic properties are mostly dominated by the magnetic energy.

Fig.1. Schematics of the stacks of ionic donor (D+) and acceptor (A-) in TTF-BA at above (upper) and below (lower) the TSP. The blue arrow in the upper figure indicates the spin in each cite. The spins form singlet dimers below the TSP. The orange arrow in the lower figure represents the electric polarization in the spin-Peierls state. Ionic displacement in the dimer phase breaks the inversion symmetry of the electric charge.

Fig.2 . Field dependence of the electric polarization (P) along the b-axis in a single crystal of TTF-BA in the presence of the bias voltage of 100 V. Magnetic fields were applied parallel to the electrode (normal to the b-axis). Application of magnetic fields of 56 T almost completely suppresses the P for 50 K ≤ TTSP, whereas does not cause significant change in P at 4.2 K

In this context, one-dimensional quantum magnets provide another kind of playground to study the spin-related ferroelectricity [1]. In particular, the organic charge-transfer salt TTF-BA (tetrathiafulvalene-p-bromanil) has been focused as a possible candidate of the spin-Peierls (SP) ferroelectrics (see Fig. 1), whereas the presence of ferroelectricity and the magnetoelectric effects have not been studied because of the absence of sizable crystals. We studied electric and magnetic properties on newly synthesized large crystals of TTF-BA in magnetic fields (H) generated by a pulse-magnet.

The temperature (T) dependence of P and the P-E hysteresis loops clearly indicate the emergence of the ferroelectricity below TSP = 53 K [2]. At temperatures slightly lower than the TSP, isothermal magnetization (M) curves up to 56 T showed broad but finite superlinear increases in M accompanied with the changes in the slopes of the M-H curves [2], which are characteristic of the collapse of the spin-gap states in the SP systems [3]. The effect of H shows up more clearly in the P-H curves. The change in P along the b-axis of the crystal approaches ~ 1,000 µC/m2 at 50 K in the presence of the bias voltage of 100 V as shown in Fig. 2. These results indicate that the ferroelectricity in TTF-BA originates from the instability in the quantum spin system.

On the other hand, the P shows up almost independent of H at 4.2 K. For conventional (non-ferroelectric) SP materials, the magnetic phase diagrams are known to be merged into the universal one when plotted on a reduced H-T plane. In the conventional case, the experimentally determined phase boundaries are reasonably reproduced by the Cross’s theory at low fields [4]. Our results demonstrate that the ferroelectric SP phase in TTF-BA is less sensitive to the applied field than that is expected from the Cross’s theory. This unusual stability of the SP state can be attributed to the strong spin-phonon coupling which may be characteristic of the ferroelectric SP system TTF-BA. It is interesting to clarify the nature of the possible soliton-lattice phase in high fields and at low temperatures [5], whereas it remains open in the present study.


References
  • S. Horiuchi and Y. Tokura, Nat. Mater. 7, 357 (2008).
  • F. Kagawa et al., Nat. Phys. 6, 169 (2010).
  • As one of the examples, see M. Hase et al., Phys. Rev. B 48, 9616 (1993).
  • M. C. Cross, Phys. Rev. B 20, 4606 (1979).
  • A. I. Buzdin, M. L. Kulic and V. V. Tugushev, Solid State Commun. 48, 483 (1983).
Authors
  • dCross-Correlated Materials Research Group, RIKEN
  • cDepartment of Applied Physics, The University of Tokyo
  • bAIST, Tsukuba
  • aMultiferroics Project, Erato, JST
  • F. Kagawaa, S. Horiuchib, M. Tokunaga, J. Fujiokaa, and Y. Tokuraa,b,c,d