The magnetic properties of the layered compound ${\left({\mathrm{C}}_2{\mathrm{N}}_2{\mathrm{H}}_{10}\right)}_{1∕2}\mathrm{Fe}\mathrm{P}{\mathrm{O}}_4\left(\mathrm{O}\mathrm{H}\right)$ have been studied using dc magnetization and ac susceptibility measurements. The compound orders as a canted antiferromagnet at $T_c=29.6\mathrm{K}$. The crossover in critical exponent of magnetization from ${\beta }_1=0.19$ to ${\beta }_2=0.30$ is observed and attributed to the change of magnetic lattice dimensionality from 2 at low temperature to 3 just below $T_c$. The formation of net magnetic moment and magnetic domains below $T_c$ is evidenced by an irreversible behavior of the dc magnetization, the presence of absorption component in the ac susceptibility, and complicated relaxation phenomena observed down to $15\mathrm{K}$. Applying Cole-Cole analysis to the frequency dependences of dispersion ${\chi }^{\prime }$ and absorption ${\chi }^{″}$, two relaxation processes were distinguished in the paramagnetic and ordered phases; the temperature dependences of their relaxation times were analyzed. The faster relaxation process is described by a scaling law above $T_c$, and by a steady decrease of the relaxation time in the ordered phase. This process is related to the existence of solitons in the magnetic chains forming the two-dimensional layers. The slower process follows the Vogel-Fulcher law in the paramagnetic phase and the Arrhenius law with the activation energy $E_a\approx 83\mathrm{K}$ in the ordered phase. This process is attributed to the formation of magnetic domains and domain-wall movement. The mechanism of these processes is proposed and related to the crystal structure of the material.

• Received 31 March 2004

DOI:https://doi.org/10.1103/PhysRevB.70.144405