Enhanced conduction band density of states in intermetallic EuTSi₃ (T=Rh, Ir)

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We report on the physical properties of single crystalline EuRhSi₃ and polycrystalline EuIrSi₃, inferred from magnetisation, electrical transport, heat capacity and ¹⁵¹Eu Mössbauer spectroscopy. These previously known compounds crystallise in the tetragonal BaNiSn₃-type structure. The single crystal magnetisation in EuRhSi₃ has a strongly anisotropic behaviour at 2 K with a spin-flop field of 13 T, and we present a model of these magnetic properties which allows the exchange constants to be determined. In both compounds, specific heat shows the presence of a cascade of two close transitions near 50 K, and the ¹⁵¹Eu Mössbauer spectra demonstrate that the intermediate phase has an incommensurate amplitude modulated structure. We find anomalously large values, with respect to other members of the series, for the RKKY Néel temperature, for the spin-flop field (13 T), for the spin-wave gap (≃ 20-25 K) inferred from both resistivity and specific heat data, for the spin-disorder resistivity in EuRhSi₃ (≃ 35 μOhm.cm) and for the saturated hyperfine field (52 T). We show that all these quantities depend on the electronic density of states at the Fermi level, implying that the latter must be strongly enhanced in these two materials. EuIrSi₃ exhibits a giant magnetoresistance ratio, with values exceeding 600 % at 2 K in a field of 14 T.