Investigation of the crystal and magnetic structures of the trigonal multiferroic iron-boracite Fe3B7O13(OH)

We have investigated the crystal and magnetic structures of the trigonal iron-boracite Fe3B7O13X with X = OH by neutron diffraction. Neutron diffraction enables us to locate the hydrogen atom of the hydroxyl group and determine the magnetic ground state of this member of the multiferroic boracite family. No evidence was found for a monoclinic distortion in the magnetic ordered state. The magnetic symmetry allows for magnetoelectric and ferroelectric properties. The N/eel tempera- ture TN of 4.86(4) K confirms the general trends within the boracites that TN decreases from X = I > Br > Cl > OH. Surprisingly while Fe3B7O13OH exhibits the largest frustration with $|theta/T_N| = 5.6$ within the Fe3B7O13X series, no reduction of the magnetic moment is found using neutron diffraction.

Quantum magnetism of perfect spin tetrahedra in Co$_{4}$B$_{6}$O$_{13}$

Co$_{4}$B$_{6}$O$_{13}$ contains undistorted tetrahedral clusters of magnetic Co$^{2+}$ ions. The high-field magnetization of this magnet exhibits a periodic undulation indicating quantization of the total spin number per cluster. Measurements of magnetic susceptibility and specific heat reveal that the ground state is composed of several different singlet states, reflecting the high symmetry of the spin tetrahedron. An exact diagonalization calculation taking account of single-ion type anisotropies and Dzyaloshinsky-Moriya interactions reproduces the expretimental results very well.