Exchange parameters of copper-based quasi-two-dimensional Heisenberg magnets measured using high magnetic fields and muon-spin rotation

Pulsed-field magnetization experiments (fields $B$ of up to 85 T and temperatures $T$ down to 0.4 K) are reported on nine organic Cu-based two-dimensional (2D) Heisenberg magnets. All compounds show a low-$T$ magnetization that is concave as a function of $B$, with a sharp ``elbow’’ transition to a constant value at a field $B_{\mathrm{c}}$. Monte-Carlo simulations including a finite interlayer exchange energy $J_{\perp }$ quantitatively reproduce the data; the concavity indicates the effective dimensionality and $B_{\mathrm{c}}$ is an accurate measure of the in-plane exchange energy $J$. Using these values and Ne'el temperatures measured by muon-spin rotation, it is also possible to obtain a quantitative estimate of $\mathrm{\mid }{J}_{\perp }\mathrm{/}J\mathrm{\mid }$. In the light of these results, it is suggested that in magnets of the form [Cu(HF${}_2$)(pyz)${}_2$]X, where X is an anion, the sizes of $J$ and $J_{\perp }$ are controlled by the tilting of the pyrazine (pyz) molecule with respect to the 2D planes.