Muon spin relaxation in some multiferroic fluorides

Muon spin relaxation $(\ensuremath{\mu}\text{SR})$ has been employed to investigate the local magnetic properties of multiferroics ${\text{K}}_{3}{\text{Fe}}_{5}{\text{F}}_{15}$, ${\text{K}}_{3}{\text{Cr}}_{2}{\text{Fe}}_{3}{\text{F}}_{15}$, and ${\text{K}}_{3}{\text{Cu}}_{3}{\text{Fe}}_{2}{\text{F}}_{15}$. In all investigated systems $\ensuremath{\mu}\text{SR}$ revealed a linear $\text{F-}{\ensuremath{\mu}}^{+}\text{-F}$ ``hydrogenlike'' bonded complex with the F-F distance of $2.43(2)\text{ }\text{\AA{}}$ in the paramagnetic phase. Two consecutive magnetic transitions were found in all systems, in agreement with the magnetic susceptibility measurements. Whereas in the paramagnetic phase the local magnetic fields at muon sites are about 20 G, they strongly increase below the N\'eel temperature ${T}_{N}$ and reach values of several kilogauss. The $\ensuremath{\mu}\text{SR}$ results further suggest that the system is inhomogeneous below ${T}_{N}$ with magnetically ordered clusters being embedded into a disordered matrix. This scenario is endorsed by our electron paramagnetic resonance results.