Field dependence of the magnetization of the two-dimensional antiferromagnetK2MnF4

Both the up and down sublattice magnetization and the net magnetization of the nearly two-dimensional antiferromagnet ${\mathrm{K}}_2$Mn${\mathrm{F}}_4$ have been measured and analyzed in detail as a function of temperature in external magnetic fields up to the spin-flop transition. The experimental method has been tracking the NMR frequencies of the ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ nuclei adjacent to the magnetic ions. Spin-wave theory with inclusion of Oguchi-type renormalization, as well as semiempirical scaling of the temperature variation of the magnon energy gap, is in excellent agreement with the data up to 18 K for all fields below ∼ 40 kG, decreasing to 13 K at 50 kG, i.e., up to a line in the ($H,T$) diagram where the thermal decrement of the sublattice magnetization is only 7%. Near $T=0\mathrm{}$ K, the spin flop has been observed to occur at 54.5 ± 0.5 kG. Renormalization is found to become increasingly important with field, and apparently its field-dependent part is well described by first-order spin-wave interactions. It appears that in a field the magnetization of the sublattice with the magnetic moments in the direction of the external field initially decreases more rapidly with temperature than is the case in zero field. The effect is due to the preferential excitation of long-wavelength magnons of the low-energy magnon branch, which by virtue of Bogolyubov coupling reside on both sublattices. The reduction of $T_N$ with field has been found to be 1.1 ± 0.5 K at 30 kG and 3.5 ± 0.5 K at 50 kG. Finally, the z component of the transferred hyperfine constant of the in-layer ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ was deduced to be - 51 ± 1 MHz.