We investigate the structural and magnetic properties of two molecule-based magnets synthesized from the same starting components. Their different structural motifs promote contrasting exchange pathways and consequently lead to markedly different mag
netic ground states. Through examination of their structural and magnetic properties we show that [Cu(pyz)(H$_{2}$O)(gly)$_{2}$](ClO$_{4}$)$_{2}$ may be considered a quasi-one-dimensional quantum Heisenberg antiferromagnet while the related compound [Cu(pyz)(gly)](ClO$_{4}$), which is formed from dimers of antiferromagnetically interacting Cu$^{2+}$ spins, remains disordered down to at least 0.03 K in zero field, but shows a field-temperature phase diagram reminiscent of that seen in materials showing a Bose-Einstein condensation of magnons.
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 functi
on of $B$, with a sharp ``elbow transition to a constant value at a field $B_{rm c}$. Monte-Carlo simulations including a finite interlayer exchange energy $J_{perp}$ quantitatively reproduce the data; the concavity indicates the effective dimensionality and $B_{rm c}$ is an accurate measure of the in-plane exchange energy $J$. Using these values and Neel temperatures measured by muon-spin rotation, it is also possible to obtain a quantitative estimate of $|J_{perp}/J|$. 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.
We describe instrumentation designed to perform millimeter-wave conductivity measurements in pulsed high magnetic fields at low temperatures. The main component of this system is an entirely non-metallic microwave resonator. The resonator utilizes pe
riodic dielectric arrays (photonic bandgap structures) to confine the radiation, such that the resonant modes have a high Q-factor, and the system possesses sufficient sensitivity to measure small samples within the duration of a magnet pulse. As well as measuring the sample conductivity to probe orbital physics in metallic systems, this technique can detect the sample permittivity and permeability allowing measurement of spin physics in insulating systems. We demonstrate the system performance in pulsed magnetic fields with both electron paramagnetic resonance experiments and conductivity measurements of correlated electron systems.
