We investigate the phase diagram of TmB4, an Ising magnet on a frustrated Shastry-Sutherland lattice, by neutron diffraction and magnetization experiments. At low temperature we find Néel order at low field, ferrimagnetic order at high field, and an intermediate phase with magnetization plateaus at fractional values M/M_(sat)=1/7,1/8,1/9,... and spatial stripe structures. Using an effective S=1/2 model and its equivalent two-dimensional fermion gas we suggest that the magnetic properties of TmB4 are related to the fractional quantum Hall effect of a 2D electron gas.

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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 MS were observed as a function of applied magnetic field B c. From additional neutron scattering experiments on TmB4, 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.PACS numbers: 75.30.Kz, 75.25.+z IntroductionRare earth tetraborides REB 4 crystallize in a tetragonal structure with the space group P 4/mbm, where the RE ions map to a Shastry-Sutherland type geometrically frustrated lattice (SSL) in the c-plane. It was shown that all heavy REB 4 (RE = Tb, Dy, Ho, Er, Tm) exhibit a strong Ising-like anisotropy which orients the RE magnetic moments along the c-axis, and complex phase dia-