Speaker
Description
Spin-$1/2$ copper (II)-based metal-organic compounds are known and extensively investigated examples of low-dimensional magnetic systems, where quantum fluctuations and particular coordination of the Cu$^{2+}$ ions determine their magnetic properties. In our study, we focused on an experimental study of system, Cu($en$)($sal$)Cl ($en=$ethylenediamine; $sal=$ salicylic acid). The structure hosts Cu$^{2+}$ ions within two uniquely distorted octahedral coordination environments: an axially elongated CuN$_4$Cl$_2$ unit formed by a pair of en ligands and a CuO$_4$Cl$_2$ unit formed by a pair of asymmetrically coordinated sal$^–$ anions. The crystal structure is unique because a chloride ion connects two copper (II) ions, creating a one-dimensional polymeric chain [1].
Specific heat measurements of powder sample were performed in a commercial Physical Property Measurement System (PPMS) in the temperature range from $0.4$ K to $30$ K in magnetic fields up to $9$ T. The temperature and magnetic field dependence of magnetization was measured in a commercial Quantum Design SQUID Magnetometer (MPMS) in the temperature range from $0.5$ to $300$ K in the magnetic fields up to $7$ T.
The specific heat studies in zero magnetic field indicated a phase transition to long-range magnetic order at $T_C = 0.82$ K, which was confirmed by the divergence ZFC and FC magnetization curves. It should be noted that at a temperature of $0.5$ K, a hysteresis loop was observed, indicating the presence of ferromagnetic interactions. Therefore, the experimental temperature dependence of the $\chi T$ product was analyzed using the $2$D Heisenberg ferromagnetic model, which is available in the form of the high-temperature series expansion (HTSE) relation provided for $S = 1/2$ [2]. The fit was performed in the temperature range $15–300$ K yielding $J/k_B\approx 2$ K and $g = 2.2$. On the other hand, the best agreement with specific heat data in zero magnetic field at temperatures above $1$ K was observed for the square lattice model with ferromagnetic intralayer coupling $J/k_B\approx 2$ K. The analysis of specific heat, magnetic susceptibility and magnetization identified the studied system as a quasi-two-dimensional $S = 1/2$ Heisenberg ferromagnet on the square lattice.
Acknowledgements
The financial support of projects VEGA 1/0132/22, APVV-18-0197 and APVV-22-0172 is acknowledged.
References
[1] S. S. Batool et al., “Crystal structure and spectroscopic characterization of a coordination polymer of Copper(II) chloride with ethylenediamine and the 2-hydroxybenzoate ion,” Journal of Structural Chemistry, vol. 57, no. 6. Pleiades Publishing Ltd, pp. 1176–1181, Nov. 2016. https://doi.org/10.1134/s0022476616060172
[2] G. A. Baker Jr. et al., “On the two-dimensional, spin- Heisenberg ferromagnetic models,” Physics Letters A, vol. 25, no. 3. Elsevier BV, pp. 207–209, Aug. 1967. https://doi.org/10.1016/0375-9601(67)90860-2
