We have determined the full magnetic dispersion relations of multiferroic BiFeO3. In particular, two excitation gaps originating from magnetic anisotropies have been clearly observed. The direct observation of the gaps enables us to accurately determine the Dzyaloshinskii-Moriya (DM) interaction and the single ion anisotropy. The DM interaction supports a sizable magnetoelectric coupling in this compound.

Single crystals free from external strains are inevitable for research of the anisotropic intrinsic properties of BiFeO3. We have established a procedure to grow highly insulating bulk single crystals of BiFeO3 for the first time by a newly developed method called the laser-diode heating floating-zone (LDFZ) method, as well as by the control of the oxygen partial pressure in the atmosphere. The quality of the crystals is much improved, as demonstrated by the X-ray diffraction and the magnetic and the dielectric measurements. In the magnetization, no weak-ferromagnetism, localized-spin paramagnetism, or spin glass is essentially observed, and antiferromagnetism dominates. Enormous electric polarization with significant anisotropy is observed, indicating that by increasing the number of the coexisting ferroelastic variants, internal strains (structural mismatch) from the surrounding domains increase and the domain growth is constrained more. The internal strains play a crucial role in the domain switching in the bulk crystals.

We have developed the laser-diode-heated floating zone (LDFZ) method, in order to improve the broad and inhomogeneous light focusing in the conventional lamp-heated floating zone method, which often causes difficulties in the crystal growth especially for the incongruently melting materials. We have simulated the light focusing properties of the LDFZ method to make irradiated light homogeneous and restricted mostly to the molten zone. We have designed and assembled an LDFZ furnace, and have demonstrated how it works through actual crystal growth. The method is applicable to various kinds of materials, and enables stable and reproducible crystal growth even for the incongruently melting materials. We have succeeded in the crystal growth of representative incongruently melting materials such as BiFeO 3 and (La,Ba) 2 CuO 4 , which are difficult to grow by the conventional method. Tolerance to the decentering of the sample and highly efficient heating are also established in the LDFZ method.

We present neutron inelastic scattering measurements of the low-energy phonons in single crystal BiFeO3. The dispersions of the three acoustic phonon modes (LA along [100], TA1 along [010] and TA2 along [110]) and two low energy optic phonon modes (LO and TO1) have been mapped out between 300 K and 700 K. Elastic constants are extracted from the phonon measurements. The energy linewidths of both TA phonons at the zone boundary clearly broaden when the system is warmed toward the magnetic ordering temperature TN = 640 K. This suggests that the magnetic order and low-energy lattice dynamics in this multiferroic material are coupled.

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