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-

Neutron scattering is used to study NiCl2−2xBr2x·4SC(NH2)2, x = 0.06, a bond-disordered modification of the well-known gapped S = 1 antiferromagnetic quantum spin system NiCl2·4SC(NH2)2. The magnetic excitation spectrum throughout the Brillouin zone is mapped out at T = 60 mK using high-resolution time-of-flight spectroscopy. It is found that the dispersion of spin excitation is renormalized, as compared to that in the parent compound. The lifetime of excitations near the bottom of the band is substantially decreased. No localized states are found below the gap energy ∆ 0.2 meV. At the same time, localized zero wave vector states are detected above the top of the band. The results are consistent with a more or less continuous random distribution of bond strengths, and a discrete, possibly bimodal, distribution of single-ion anisotropies in the disordered material.