Magnetic structure of equilibrium, zero-field, and non-equilibrium in-field phases in TmB4.

TmB4 belongs to a family of metallic rare earth tetraborides in which the rare earth moments form an orthogonal dimer network corresponding to the 2D Shastry-Sutherland lattice (SSL).  In the case of Tm3+ (J=6), the crystal field produces a pseudo-doublet ground state with dominant mj=±6 components and large (6.6 μB) Ising-like magnetic moments. Competition between the RKKY and dipolar interactions on the SSL leads to three ordered phases in zero field and the emergence of fractional magnetization plateaus at 1/8 and ½ of the saturation magnetization. While the magnetization plateaus appear to be non-equilibrium effects, the structures that exist within them may well be related to those of the zero-field ordered phases.  Here we present  results of single crystal neutron diffraction to determine the magnetic structures of the zero field three phases, i.e. commensurate AFM phase with k=(1,0,0), low-temperature incommensurate AFM phase ( k’=(1+delta,0,0)) and higher-temperature incommensurate phase (k”=(1+delta,delta’,0)), and report on our efforts to investigate the out-of-equilibrium phases of TmB4 using neutron diffraction combined with in-situ measurements of magnetization and dilatometry.