Quasi-two-dimensional $S=1/2$ magnetism of Cu[C$_6$H$_2$(COO)$_4$][C$_2$H$_5$NH$_3$]$_2$

TL;DR

This study investigates the structural and magnetic properties of a quasi-two-dimensional $S=1/2$ quantum antiferromagnet, revealing a weakly anisotropic, non-frustrated square lattice with no long-range order down to 1.8 K.

Contribution

The paper provides detailed experimental and theoretical analysis of a new Cu-based quantum antiferromagnet with a unique non-trivial dependence of magnetic couplings on Cu--Cu distances.

Findings

Establishes a quasi-2D weakly anisotropic square lattice with $J_a/J_c \,\simeq 0.7$

No long-range magnetic order observed down to 1.8 K

Magnetic couplings mediated by organic anions show non-trivial Cu--Cu distance dependence

Abstract

We report structural and magnetic properties of the spin-$\frac12$ quantum antiferromagnet Cu[C$_6$H$_2$(COO)$_4$][C$_2$H$_5$NH$_3$]$_2$ by means of single-crystal x-ray diffraction, magnetization, heat capacity, and electron spin resonance (ESR) measurements on polycrystalline samples, as well as band-structure calculations. The triclinic crystal structure of this compound features CuO$_4$ plaquette units connected into a two-dimensional framework through anions of the pyromellitic acid [C$_6$H$_2$(COO)$_4$]$^{4-}$. The ethylamine cations [C$_2$H$_5$NH$_3]^+$ are located between the layers and act as spacers. Magnetic susceptibility and heat capacity measurements establish a quasi-two-dimensional, weakly anisotropic and non-frustrated spin-$\frac12$ square lattice with the ratio of the couplings $J_a/J_c\simeq 0.7$ along the $a$ and $c$ directions, respectively. No clear signatures of the long-range magnetic order are seen in thermodynamic measurements down to 1.8\,K. However, the gradual broadening of the ESR line suggests that magnetic ordering occurs at lower temperatures. Leading magnetic couplings are mediated by the organic anion of the pyromellitic acid and exhibit a non-trivial dependence on the Cu--Cu distance, with the stronger coupling between those Cu atoms that are further apart.