Determining the spatial and spin anisotropy of reduced-dimensionality Cu-based magnets using EPR, ultra-high-field magnetization and simulations

Pulsed-field magnetization experiments (up to 85 T) and electron paramagnetic resonance (EPR) experiments ($10-110$~GHz) are reported on a family of organic Cu-based two-dimensional (2D) Heisenberg magnets. The low-$T$ $M(H)$ relationship is concave, with a sharp transition to a saturation value at a critical field $H_{\rm c}$. Monte-Carlo simulations including a finite interlayer exchange energy quantitatively reproduce the data. Thus, one can obtain accurate values for both intra- and interlayer exchange energies. The EPR spectra show pronounced changes in effective $g$ factor, linewidth and zero- field intercept at temperatures, fields and frequencies of the same energy scale as the dominant exchange parameter. The EPR results are modeled using finite-cluster- size methods, and the data are well matched by an easy-plane spin anisotropy in the range $0.01-0.05$. Thus, EPR measurements allow the spin orientation dependence of the exchange interaction to be determined.