Two-Dimensional Antiferromagnetism in Mn(HCOO)2·2H2O

Long-range two-dimensional magnetic correlations have been observed in Mn${\mathrm{}\left(\mathrm{HCOO}\right)\mathrm{}}_2$·2${\mathrm{H}}_2$O by the quasi-elastic scattering of neutrons. A direct indication of the two-dimensional magnetic character of the system is the occurrence of the scattering near "lines" in the reciprocal lattice. The "lines" are parallel to $a^{*}$, indicating that long-range correlations are occurring in planes of atoms parallel to (100). The two-dimensional character is observed below $T_N=3.6\mathrm{}$ K and persists to at least $2T_N$. Observation of the $q$ dependence of the scattering above $T_N$ is consistent with a decreasing two-dimensional correlation length as the temperature increases. Another interesting property of this crystal is the different degree of ordering exhibited by two types of Mn moments which occur in the primitive cell at two pairs of inequivalent positions $A$ sites and $B$ sites). The $A$ sites and $B$ sites form alternating layers of atoms parallel to (100) and strong intraplanar antiferromagnetic coupling exists only for the $A$ sites. Measurements of magnetic Bragg peaks below $T_N$ indicate that $〈{\mu }_A〉$ is an order of magnitude larger than $〈{\mu }_B〉$, which is in agreement with a molecular field model with very small interplanar coupling. The sublattice magnetization $〈{\mu }_A〉$ was found to vary as ${\mathrm{}(3.62-T)\mathrm{}}^{0.23\pm 0.01}$ from 2.00 to 3.48°K, the initial slope of the magnetization just below $T_N$ being larger than that obtained in isotropic three-dimensional systems.