Magnetic ordering in the randomly mixed quadratic-layer antiferromagnet with competing anisotropies K2CoxFe1-xF4.

Abstract

The magnetic ordering in the randomly mixed quadratic-layer antiferromagnet with competing anisotropies ${\mathrm{K}}_{2}$${\mathrm{Co}}_{\mathrm{x}}$${\mathrm{Fe}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{F}}_{4}$ is studied with neutron scattering. For x=0.18, just below the multicritical concentration ${x}_{c}$=0.22, the critical fluctuations are suppressed and the sublattice magnetization increases only slowly with decreasing temperature, indicative of a crossover from two-dimensional (2D) Ising to 2D XY behavior near ${x}_{c}$. For x=0.27, two successive transitions are observed, at ${T}_{N}$=64.4\ifmmode\pm\else\textpm\fi{}0.5 K and ${T}_{L}$=27\ifmmode\pm\else\textpm\fi{}2 K. Between ${T}_{N}$ and ${T}_{L}$ the spin components along the tetragonal axis order. At ${T}_{L}$ the oblique antiferromagnetic phase is entered, without appreciably affecting the axial order. The transition at ${T}_{L}$ exhibits unusually slow critical fluctuations. The systems with x=0.12 and 0.55, at considerable distance from ${x}_{c}$, acquire the ${\mathrm{K}}_{2}$${\mathrm{FeF}}_{4}$ and ${\mathrm{K}}_{2}$${\mathrm{CoF}}_{4}$ magnetic structures, respectively.