Neutronscattering study of magneticfieldinduced transitions in a twocomponent antiferromagnetic system with competing spin anisotropies
Abstract
Neutronscattering experiments have been performed on the quasitwodimensional antiferromagnet K_{2}Mn_{0.978}Fe_{0.022}F_{4} in order to study the coexisting three and twodimensional (d=3 and d=2) magnetic order. The measurements were carried out in the temperature range 2 K<T<60 K, and in external magnetic fields up to H=50 kOe applied perpendicular to the layers, i.e., parallel to the easy axis of magnetization. From temperature scans at constant field, and field scans at constant temperature, we have obtained an HT phase diagram consisting of four phases, namely, the paramagnetic P phase, the antiferromagnet axial A phase, an antiferromagnetic intermediate I phase, and the spinflop or planar phase. Coming from the P to the A phase, d=3 and d=2 ordered subsystems coexist, whereas in the l phase the d=2 longrange order (LRO) gradually changes into the d=3 LRO. Upon entering the planar phase all d=2 LRO disappears and there is no longer a division in two subsystems. After leaving the planar phase the complete spin system remains fully d=3 ordered, as long as the P phase is not reached. The three ordered phases are further characterized by differences in domain structures. The HT phase diagram can be explained by assuming that in this twocomponent antiferromagnet with competing spin anisotropies (namely, the axial dipolar anisotropy of the Mn^{2+} and Fe^{2+} ions and the planar singleion anisotropy of the Fe^{2+} ions) at H=0 a mismatch occurs in the correlations along the c axis between xy components in d=2 ordered clusters around the Fe^{2+} ions and the z components of the d=3 ordered Mn^{2+} spins in adjacent layers. Applying a sufficiently strong field forces all the moments to lie in the planes and consequently the mismatch in correlation is removed. The observed HT diagram differs from that found for a weakly anisotropic Heisenberg antiferromagnet, in that the firstorder spinflop line in the latter case has become split up in two secondorder transition lines, which encompass an I phase, and consequently yield a tetracritical point. The HT diagram for K_{2}Mn_{0.978}Fe_{0.022}F_{4} can be explained by constructing an xHT diagram for K_{2}Mn_{1x}Fe_{x}F_{4}, using as a basis the earlier found xT diagram for H=0 and the HT diagram for a weakly anisotropic Heisenberg antiferromagnet (x=0). In the xHT diagram a tetracritical line occurs, and the intermediate phase becomes a threedimensional region.
 Publication:

Physical Review B
 Pub Date:
 May 1979
 DOI:
 10.1103/PhysRevB.19.4741
 Bibcode:
 1979PhRvB..19.4741B