The structural and magnetic properties of BaMn2Si2O7 have been investigated. The magnetic susceptibility and specific heat, measured using single crystals, suggest that the quasi-one-dimensional magnetism originating from the loosely coupled Mn2+ cha
in carrying S=5/2 is present at high temperatures, which is similar to the other quasi-one-dimensional barium silicates, BaM2Si2O7 (M: Cu and Co). The Neel temperature (TN=26 K) is high compared to the magnetic interaction along the chain (J = -6 K). The neutron powder diffraction study has revealed that the magnetic structure is long-ranged with antiferromagnetic arrangement along the chain (c) direction and ferromagnetic arrangement along the a and b axes. The detailed structural analysis suggests that the interchain interaction via Mn-O-Mn bond along the a axis is relatively large, which makes the system behave more two-dimensionally in the ac plane and enhances TN.
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 determ
ine the Dzyaloshinskii-Moriya (DM) interaction and the single ion anisotropy. The DM interaction supports a strong magneto-electric coupling in this compound.
We consider a higher dimensional gravity theory with a negative kinetic energy scalar field and a cosmological constant. We find that the theory admits an exact cosmological solution for the scale factor of our universe. It has the feature that the u
niverse undergoes a continuous transition from deceleration to acceleration at some finite time. This transition time can be interpreted as that of recent acceleration of our universe.
