Magnetic fluctuation and first-order transition in trillium lattice of Euptsi observed by ¹⁵¹Eu Mössbauer spectroscopy

We report the results of ¹⁵¹Eu Mössbauer spectroscopy on EuPtSi with a chiral structure belonging to the P2₁3 (#198) space group at zero magnetic field. The paramagnetic single absorption forms a magnetic splitting profile directly below T_N = 4.0 K and the spectrum at 3.9 K consists of the sum of the paramagnetic single absorption and the magnetic splitting absorption, which indicates a first-order transition at T_N = 4.0 K. The temperature dependence of the effective hyperfine fields H_hf at the Eu nucleus follows a power law of H_hf = H₀(1 − T=T_N) ^β with H₀ = 25.6 ± 0.4 T and β = 0.16 ± 0.01 indicates a full moment of Eu²⁺ ions. The relative integral intensity I of the ¹⁵¹Eu Mössbauer spectroscopy for EuPtSi is enhanced using a Debye calculation below T₀ ’ 15 K, which indicates the development of magnetic correlations. The line width Γ of the ¹⁵¹Eu Mössbauer spectra increases below T_FD (’ 8 K) close to the Weiss temperature θ_p and shows a sharp peak at T_N. The magnetic behavior of EuPtSi can be divided into three magnetic regions: the paramagnetic (PM) phase, the fluctuation-disordered (FD) phase and the antiferromagnetic phase with fluctuation disordering (AF with FD). The magnetic fluctuation is realized around the first-order transition between the paramagnetic and helimagnetic states in EuPtSi.