Magnetic Structure and Phase Diagram of $TmB_4$

Magnetic structure of single crystalline $TmB_4$ has been studied by magnetization, magnetoresistivity, and specific heat measurements. A complex phase diagram with different antiferromagnetic phases was observed below $T_{N1}$=11.7 K. Besides the plateau at half-saturated magnetization (1/2 $M_S$), also plateaus at 1/9, 1/8 and 1/7 of $M_S$ were observed as a function of applied magnetic field Bparallel c. From additional neutron scattering experiments on $TmB_4$, we suppose that these plateaus arise from a stripe structure which appears to be coherent domain boundaries between antiferromagnetic-ordered blocks of 7 or 9 lattice constants. The received results suggest that the frustration among the $Tm^{3+}$ magnetic ions, which maps to a geometrically frustrated Shastry-Sutherland lattice, leads to a strong competition between antiferromagnetic and ferromagnetic order. Thus, stripe structures in intermediate field appear to be the best way to minimize the magnetostatic energy against other magnetic interactions among the Tm ions combined with very strong Ising anisotropy.