Capacitance Torquemeter Study of Magnetic Phase Transitions

A triangular lattice antiferromagnet is a well-known example of a magnetically frustrated system. Such a lattice does not allow to simultaneously minimize all pairwise interactions in a collinear antiferromagnetic structure. In a work by P. W. Anderson (1973), it was suggested that a fully disordered spin-liquid state (RVB-state) would occur on a triangular lattice. However, further studies have shown that a Heisenberg antiferromagnet on a triangular lattice develops a non-collinear antiferromagnetic ordering in zero field, in which neighboring spins are at a 120° angle. This type of ordering is observed in many real-life systems. The properties of the triangular lattice antiferromagnet also proved to be unique. Quantum and thermal fluctuations are essential for the magnetic phase selection. A prime example of the effect of fluctuations is the stabilization of a collinear "magnetization plateau" phase. In certain magnetic field intervals, the magnetization remains equal to a third of the saturation magnetization. In this phase, the magnetization vectors of two sublattices align with the external magnetic field while the third opposes it. The RbFe(MoO4)2 compound is a well-studied example of a triangular lattice antiferromagnet. In this system, the 120°-structure ordering is observed at temperatures below 4 K, and the "magnetization plateau" phase is observed in fields of approximately 4–7 T. The aim of this work is to study the resilience of the "magnetization plateau" phase to random disorder in the exchange bonds. In a work by Maryasin and Zhitomirsky (2013), it was predicted that such quenched disorder could suppress the effect of quantum and thermal fluctuations and limit the collinear phase region on the H–T diagram. This type of disorder can be induced in RbFe(MoO4)2 by substituting a portion of rubidium ions with potassium ions. In this work, an experimental study was conducted of the magnetic phase diagrams of Rb(1-x)K(x)Fe(MoO4)2 compounds with x = 0, 7.5%, 15% at temperatures down to 380 mK and in a magnetic field up to 12 T. The obtained phase diagram of a pure compound aligns well with the literature data. In samples with impurity concentrations of x = 7.5% and x = 15%, vague phase transitions into the collinear phase were observed. In the sample with x = 7.5%, the "magnetization plateau" phase was observed down to the lowest temperatures (0.38 K) and in the sample with x = 15%, no traces of this phase were found below a certain temperature. The obtained results support the hypothesis about the suppression of the fluctuation-stabilized collinear phase by the impurity-induced disorder in the exchange bonds.