Low-temperature thermodynamic and magnetic properties of clathrate-like arsenide Eu7Cu44As23

Abstract

Experimental investigation of heat capacity, CP, unit cell parameter, a, as well as the ESR absorption for a clathrate-like compound Eu7Cu44As23 has been performed in the temperature range of 2–300 K. The CP(T) function exhibits a sharp maximum at the temperature of ferromagnetic ordering, Tc ≈ 17 K, whereas the a(T) shows no anomaly at this temperature. This unusual behavior is governed by the clathrate-like structure of Eu7Cu44As23, where Eu2+ guest cations residing in oversized cavities of the host framework order ferromagnetically imposing no influence on the framework dynamics. The analysis of maximum of the Cp(T) function at 2–20 K allows determining the exchange integral, J, which is a characteristic of the magnetic interaction Eu2+ – Eu2+: J ≈ 0.27 K The analysis of the temperature dependencies of heat capacity, CP(T), and unit cell volume, V(T), in the frames of the Debye-Einstein model enabled determining characteristic temperatures of host and guest subsystems. Along with this, the contribution of two-level systems into thermodynamic properties of Eu7Cu44As23 was evaluated and related to the possibility of guest Eu2+ cations to occupy nonequivalent positions inside the cages of the host framework. The ESR of Eu2+ in Eu7Cu44As23 was studied at 5–300 K. The intensity of the ESR signal shows a sharp drop at low temperatures, which confirms the ferromagnetic character of the magnetic phase transition in Eu7Cu44As23. The experimental ESR spectrum was approximated by two Dyson lines, L1 and L2, which correspond to two g-factors: g1 = 2.081 ± 0,030 and g2 = 2,094 ± 0,016. The increase in L1 linewidth with temperature at 75–275 K is d(ΔH1)/dT = 0.027 G/K. These values of g-factors and d(ΔH1)/dT are derived the exchange integrals of the interaction of the Eu2+ local moment with conduction electrons J1* = 0.241 eV and J1** = 0.252 eV accordingly. For L2 J2* = 0.255 eV. Value J2** ≈ 0.26 eV is valid in 100–130 K interval only. The change of ΔH2(T) slope at higher temperatures may be due to crystal field effect.