Stabilization of the collinear plateau phase by thermal fluctuations in the disordered triangular lattice antiferromagnet Rb$_{(1-x)}$K$_{x}$Fe(MoO$_4$)$_2$

Abstract

The triangular lattice antiferromagnet RbFe(MoO$_4$)$_2$ orders antiferromagnetically in a planar 120$^\circ$-structure below $T_\textrm{N}\approx 4$ K. A striking feature of RbFe(MoO$_4$)$_2$ magnetic phase diagram is the presence of collinear ``1/3-plateau'' magnetic phase, which is stabilized by thermal and quantum fluctuations at magnetization $M\approx \frac{1}{3} M_\textrm{sat}$. Static disorder caused by impurities is predicted to act against the effect of fluctuations and to suppress collinear plateau phase (Maryasin and Zhitomirsky, PRL 111, 247201 (2013)). Balance between ``dynamic'' thermal and quantum fluctuations and ``static'' impurity-induced disorder is temperature-sensitive, which allows thermal fluctuations to take over the effect of static disorder and leads to the revival of the fluctuation-stabilized ``1/3-plateau'' phase on heating. Here we present experimental results directly confirming this prediction and demonstrating re-establishment of the plateau-like phase in the diluted Rb$_{(1-x)}$K$_{x}$Fe(MoO$_4$)$_2$ sample at moderate dilution level $x=15$\% on heating as the effect of thermal fluctuations increases.