Magnetic ordering processes in the quasi-one-dimensional metamagnets ${\mathrm{RbFeCl}}_3$⋅2aq and ${\mathrm{CsFeCl}}_3$⋅2aq (aq=${\mathrm{H}}_2$O,${\mathrm{D}}_2$O)

The static and dynamic properties of the quasi-one-dimensional canted Ising antiferromagnets ${\mathrm{RbFeCl}}_3$⋅2aq and ${\mathrm{CsFeCl}}_3$⋅2aq (aq=${\mathrm{H}}_2$O,${\mathrm{D}}_2$O) are investigated in the ordered phases by neutron diffraction and magnetization measurements. For 2 K≲T≲3 K the magnetic behavior in varying field H may be explained by a sequence of collective reversals of all moments in separate long-range-ordered chains, each chain reversal being initiated by a thermally excited moment reversal at one of the chain ends. The behavior at low T is characterized by nonexponential relaxation towards a partly disordered array of long-range-ordered chains. The properties for 4.2 K<T<$T_N$ were studied for ${\mathrm{RbFeCl}}_3$⋅${2\mathrm{D}}_2$O only. Two different ferrimagnetic (FI) structures were observed in distinct regions of the (H,T) phase diagram, separated by a boundary at T≊7 K. Along the phase boundary $H_{c1}$(T) between the FI and the antiferromagnetic phase a crossover from first-order to second-order behavior occurs at T≊6.5 K. Along the boundary $H_{c2}$(T) between the FI and pseudoferromagnetic phase the crossover occurs at T≊9.0 K.