Successive phase transition at ambient pressure in CeCOSI: Single crystal studies

We succeeded in synthesizing single crystals of the tetragonal antiferromagnet CeCoSi, which shows an anomalous pressure induced ordered phase (PIOP) with an extraordinary high transition temperature of T ∼ 40 K. In this paper, we present the results of the magnetization, specific heat, and electrical resistivity at ambient pressure using single crystals. Magnetization curves up to H = 55 T were also examined on a polycrystalline sample. From the results, we found a new phase transition at T₀ ∼ 12 K above the antiferromagnetic phase transition at T_N = 9.4 K. At T₀, the magnetization shows a small kink, followed by a further increase with decreasing temperature, which is reminiscent of characteristics reported previously under pressures with respect to the PIOP. Also, the value of T₀ quantitatively coincides well with that expected by extrapolating the phase boundary of the PIOP from high pressures to ambient pressure in the P–T phase diagram. Therefore, we conclude that the phase transition at T₀ ∼ 12 K found at ambient pressure is identical to the phase transition of the PIOP. In other words, the PIOP already exists at ambient pressure from T₀ ∼ 12 K to T_N = 9.4 K. Since the space inversion symmetry is locally broken at the Ce site, so-called odd parity multipole moments could be related to the phase transition at T₀. Below T_N, the magnetization shows a steep decrease in both H ∥ 001 and 100. By taking into account for the small anisotropy of the magnetization as if the ground state is an isotropic doublet, the local environment of Ce sites, and the multipole ordering below T₀, the magnetic structure below T_N could be not so simple. We also discuss the characteristics of the crystal structure, and the Ce-4 f and Co-3d electron states.