EuPt₂Si₂ crystallizes in the CaBe₂Ge₂-type tetragonal crystal structure and is known as a valence-fluctuating compound. From the χ(T) measurement of polycrystalline samples, it is found that EuPt₂Si₂ orders antiferromagnetically at T_N = 15 K. χ(T) at high temperatures follows the Curie-Weiss law with an effective magnetic moment of μ_eff = 7.7 μ_B/Eu, indicating that the Eu ions are essentially divalent. Here, μ_eff for the Eu²⁺ free-ion is 7.94 μ_B/Eu. On the other hand, an unusual broadening of the Mössbauer resonance line width is observed below about 100 K, suggesting that the valence is fluctuating and already deviates slightly from divalent to trivalent at room temperature. One of the intriguing findings for EuPt₂Si₂ is the -lnT dependence of ρ(T) below T ≃100 K. In addition, the magnetic entropy of S_mag ≃ 0.6Rln8 below T_N and the strongly enhanced γ ≃ 200 mJ/(K²·mol) suggest that a heavy-fermion state is realized at low temperatures in EuPt₂Si₂.

Fig. 1. High-field magnetization curves M(H) (left-hand scale) and relative changes in the magnetoresistance Δρ/ρ₀ for J // [100] (right-hand scale) in magnetic fields applied parallel to the [100] and [001] directions at 1.4 K. The M(H) data below 7 T were measured in static magnetic fields using a SQUID magnetometer at 2 K. The Δρ/ρ₀ curves below 14 T were measured using a superconducting magnet. Broken lines for Δρ/ρ₀ at high magnetic fields are guides to the eye.

Fig. 2. Comparison of the P–T phase diagrams for EuPt₂Si₂ and EuCu₂(Ge_0.4Si_0.6)₂, cited from Ref. [2]. The P–T phase diagram for EuCu₂(Ge_0.4Si_0.6)₂ was obtained by the resistivity measurements for J // [100] under pressures. PCC, BAC, and CAC denote piston cylinder cell, Bridgman anvil cell, and cubic anvil cell.

To obtain more insight into the heavy-fermion features in EuPt₂Si₂, we grew single crystals of this compound and studied the effects of a magnetic field as well as pressure on its magnetic properties.

The single crystal of EuPt₂Si₂ shows two antiferromagnetic transitions at T_N1 = 21 K and T_N2 = 16 K. The -lnT dependence of ρ(T) was observed in the temperature range from T_N1 to about 100 K, as observed in polycrystals. The effect of magnetic field on the -lnT dependence of ρ(T) is found to be very weak, with Δρ/ρ₀ of only ~0.1% at μ0H = 8 T. Even at μ₀H = 40 T, which was produced using a pulsed magnet, Δρ/ρ₀ amounted to only -20%, as shown in Fig. 1. These results suggest that the Kondo-like behavior of EuPt₂Si₂ is rather stable against the application of magnetic field, which is in contrast to the Ce-based heavy-fermion compounds.

On the other hand, the pressure markedly shifts the -ln T dependence of ρ(T) to higher temperatures, and T_N1 is gradually decreased and suppressed to zero above P ≃ 4 GPa, as indicated in Fig. 2. From the comparison of the observed magnetic, electronic, and thermal properties of EuPt₂Si₂ with those of the well studied EuCu₂(Ge_0.4Si_0.6)₂,[2] the anomalous features observed below 100 K in ρ(T), S(T), and ΔV/V for EuPt₂Si₂ are most likely to be due to the valence fluctuation of the Eu ions. As a future problem, X-ray absorption and Mössbauer experiments are desirable to confirm the temperature dependence of the Eu valence in EuPt₂Si₂.

References

• [1] T. Takeuchi et al., J. Phys. Soc. Jpn. 87, 074709 (2018).
• [2] W. Iha et al., J Phys. Soc. Jpn. 87, 064706 (2018).

Authors

• T. Takeuchi^a, T. Yara^b, Y. Ashitomi^b, W. Iha^b, M. Kakihana^b, M. Nakashima^c, Y. Amako^c, F. Honda^d, Y. Homma^d, D. Aoki^d, Y. Uwatoko, T. Kida^a, T. Tahara^a, M. Hagiwara^a, Y. Haga^e, M. Hedo^b T. Nakama^b, and Y. Ōnuki^b
• ^aOsaka University
• ^bUniversity of the Ryukyus
• ^cShinshu University
• ^dTohoku University
• ^eJapan Atomic Energy Agency