Neutron-scattering study of magnetic-field-induced transitions in a two-component antiferromagnetic system with competing spin anisotropies
Abstract
Neutron-scattering experiments have been performed on the quasi-two-dimensional antiferromagnet K2Mn0.978Fe0.022F4 in order to study the coexisting three- and two-dimensional (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 H-T phase diagram consisting of four phases, namely, the paramagnetic P phase, the antiferromagnet axial A phase, an antiferromagnetic intermediate I phase, and the spin-flop 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 long-range 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 H-T phase diagram can be explained by assuming that in this two-component antiferromagnet with competing spin anisotropies (namely, the axial dipolar anisotropy of the Mn2+ and Fe2+ ions and the planar single-ion anisotropy of the Fe2+ ions) at H=0 a mismatch occurs in the correlations along the c axis between xy components in d=2 ordered clusters around the Fe2+ ions and the z components of the d=3 ordered Mn2+ 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 H-T diagram differs from that found for a weakly anisotropic Heisenberg antiferromagnet, in that the first-order spin-flop line in the latter case has become split up in two second-order transition lines, which encompass an I phase, and consequently yield a tetracritical point. The H-T diagram for K2Mn0.978Fe0.022F4 can be explained by constructing an x-H-T diagram for K2Mn1-xFexF4, using as a basis the earlier found x-T diagram for H=0 and the H-T diagram for a weakly anisotropic Heisenberg antiferromagnet (x=0). In the x-H-T diagram a tetracritical line occurs, and the intermediate phase becomes a three-dimensional region.
- Publication:
-
Physical Review B
- Pub Date:
- May 1979
- DOI:
- 10.1103/PhysRevB.19.4741
- Bibcode:
- 1979PhRvB..19.4741B