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Strongly Anisotropic Magnetic Phase Diagram of CeAu2Ge2

CeAu2Ge2 single crystals (tetragonal ThCr2Si2 structure) have been grown in Au-Ge flux (AGF) as well as in Sn flux (SF). X-ray powder-diffraction and EDX measurements indicate that in the latter case Sn atoms from the flux are incorporated in the samples, leading to a decrease of the lattice constants by ~ 0.3% compared to AGF samples. For both types of samples, a strong anisotropy of the magnetization M for the magnetic field B parallel and perpendicular to the c direction is observed with M||/M^{\bot} ~ 6 - 7 in low fields just above 10 K. This anisotropy is preserved to high fields and temperatures and can be quantitatively explained by crystal electric field effects. Antiferromagnetic ordering sets in around 10 K as previously found for polycrystalline samples. From the magnetization data of our single crystals we obtain the phase diagrams for the AGF and SF samples. The magnetic properties depend strongly on the flux employed. While the AGF samples exhibit a complex behavior indicative of several magnetic transitions, the SF samples adopt a simpler antiferromagnetic structure with a single spin-flop transition. This effect of a more ordered state induced by disorder in form of Sn impurities is qualitatively explained within the ANNNI model, which assumes ferromagnetic and antiferromagnetic interactions in agreement with the magnetic structure previously inferred from neutron-scattering experiments on polycrystalline CeAu2Ge2 by Loidl et al. [Phys. Rev. B 46, 9341, (1992)].