Alternating linear-chain antiferromagnetism in copper nitrate Cu${\left(\mathrm{N}{\mathrm{O}}_3\right)}_2$.2.5 ${\mathrm{H}}_2$O

Current interest in the behavior of Heisenberg alternating antiferromagnetic quantum chains has been stimulated by the discovery of an unusual class of magnetoelastic spin-Peierls systems. Copper nitrate, Cu${\left(\mathrm{N}{\mathrm{O}}_3\right)}_2$.2.5${\mathrm{H}}_2$O, does not display a spin-Peierls transition, but its dominant magnetic behavior is that of a strongly alternating antiferromagnetic chain with temperature-independent alternation. A remarkable, simultaneous fit is demonstrated between theoretical studies and a wide variety of zero- (low-) field experimental measurements, including susceptibility, magnetization, and specific heat. The fitting parameters are $\alpha \left(\mathrm{degree}\mathrm{of}\mathrm{alternation}\right)=0.27\mathrm{}$, $\frac{J_1}{k}=2.58\mathrm{}$ K, $g_b=2.31\mathrm{}$, and $g_{\perp }=2.11\mathrm{}$. Slight systematic discrepancies are attributed to weak interchain coupling. Theoretical studies also predict a rich variety of behavior in high fields, particularly in the region involving the lower and upper critical fields, $H_{c_1}=28\mathrm{}$ kOe and $H_{c_2}=44\mathrm{}$ kOe. Experimental specific-heat measurements at $H=28.2\mathrm{} \mathrm{and} 35.7$ kOe show quantitative agreement with theory in this interesting parameter region. The fitting parameters are the same as for zero field and, again, small discrepancies between theory and experiment may be attributed to interchain coupling. The exceptional magnetic characterization of copper nitrate suggests its use for further experimental study in the vicinity of the high-field ordering region.