Controlling Magnetic Order and Quantum Disorder in Molecule-Based Magnets

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 magnetic ground states. Through examination of their structural and magnetic properties we show that $[\mathrm{Cu}(\mathrm{pyz})({\mathrm{H}}_2\mathrm{O})(\mathrm{gly}{)}_2]({\mathrm{ClO}}_4{)}_2$ may be considered a quasi-one-dimensional quantum Heisenberg antiferromagnet whereas the related compound $[\mathrm{Cu}(\mathrm{pyz})(\mathrm{gly})]({\mathrm{ClO}}_4)$, which is formed from dimers of antiferromagnetically interacting ${\mathrm{Cu}}^{2+}$ 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.

Figure 1

(a) Cu-(pyz)-Cu chains on which $[\mathrm{Cu}(\mathrm{pyz})({\mathrm{H}}_2\mathrm{O})(\mathrm{gly}{)}_2]({\mathrm{ClO}}_4{)}_2$ is based. Water and gly groups that coordinate with the Cu ions are also shown. (b) The material viewed along the $c$ axis showing the packing of the chains with noncoordinating ${\mathrm{ClO}}_4$ groups. $[\mathrm{Cu}(\mathrm{pyz})(\mathrm{gly})]({\mathrm{ClO}}_4)$ viewed along (c) the $a$ axis and (d) the $c$ axis showing pairs of ${\mathrm{Cu}}^{2+}$ ions strongly coupled through pyz ligands to form alternating dimers. (Dimers are shaded, and the ${\mathrm{ClO}}_4$ ions have been removed for clarity).

Figure 2

(a) Magnetization of chainlike $[\mathrm{Cu}(\mathrm{pyz})({\mathrm{H}}_2\mathrm{O})(\mathrm{gly}{)}_2]({\mathrm{ClO}}_4{)}_2$ measured at 0.5 K. Inset: magnetic susceptibility measured in an applied field of 100 mT. (b) ZF ${\mu }^{+}\mathrm{SR}$ spectra measured at 0.9 and 0.02 K. Inset: Relaxation rate and nonrelaxing amplitude (right) as a function of temperature showing the onset of the magnetic transition.

Figure 3

(a) Static magnetic susceptibility data for $[\mathrm{Cu}(\mathrm{pyz})(\mathrm{gly})]({\mathrm{ClO}}_4)$ measured in a field of 5 mT. A fit is shown to the Bleaney-Bowers model of noninteracting dimers. (b) Heat capacity showing magnetic transitions in applied magnetic fields. (c) Magnetization at 0.58 K and (d) dynamic susceptibility $dM/dH$ at several temperatures showing sharp features at the phase transitions (data offset for each temperature for clarity). (e) Example ZF ${\mu }^{+}\mathrm{SR}$ spectrum measured at 32 mK showing no signature of long-range magnetic order. (f) Relaxation rate at 20 mK as a function of transverse applied magnetic field showing a transition around 1.7 T.

Figure 4

The $B\text{−}T$ phase diagram for $[\mathrm{Cu}(\mathrm{pyz})(\mathrm{gly})]({\mathrm{ClO}}_4)$, showing quantum critical points at $g{\mu }_B{B}_c=2.5(1)$ and 9.0(2) K. Dashed line shows the expected behavior of the phase boundaries with exponent $\varphi =2/3$, predicted for the BEC of magnons model.