“Triangular Antiferromagnet” RbFe(MoO 4 ) 2 with the Replacement of Nonmagnetic Ions

The magnetization curves, as well as electron spin resonance and nuclear magnetic resonance spectra of 87Rb+ ions, have been measured in the quasi-two-dimensional antiferromagnet RbFe(MoO4)2 on a triangular lattice with the random modulation of the exchange bond network. Random modulation has been performed by means of the partial substitution of nonmagnetic Rb+ ions for K+ ions. It has been shown that random static disorder thus created at an impurity ion concentration of 15% drastically changes the spin structure. A noncollinear structure with significant sublattice magnetization components transverse to the magnetic field occurs in the doped compound. At the same time, the spin structure in the pure compound has the same magnetic moment, but transverse spin components disappear near a magnetic field of one third of the saturation field (three-sublattice structure with two upward sublattices and one downward sublattice). The revealed doping-induced drastic rearrangement of the spin structure of the triangular antiferromagnet has been explained by the interplay between the contribution of dynamic fluctuations, which makes maximally collinear states favorable in free energy, and the contribution from freeze-in disorder, which ensures the energy gain for the maximally noncollinear arrangement of sublattices.