The intermediate-valent polymorphs $\alpha$- and $\beta -{\mathrm{YbAlB}}_4$ exhibit quantum criticality and other novel properties not usually associated with intermediate valence. Iron doping induces quantum criticality in $\alpha -{\mathrm{YbAlB}}_4$ and magnetic order in both compounds. We report results of muon spin relaxation ($\mu \mathrm{SR}$) experiments in $\alpha -{\mathrm{YbAl}}_{1-x}{\mathrm{Fe}}_x{\mathrm{B}}_4, x=0.014$ and 0.25. For $x=0.014$ we find no evidence for magnetic order down to 25 mK. The dynamic muon spin relaxation rate ${\lambda }_d$ exhibits a power-law temperature dependence ${\lambda }_d\propto T^{-a}, a=0.40\left(4\right)$, in the temperature range 100 mK–2 K, in disagreement with predictions by theories of antiferromagnetic (AFM) or valence quantum critical behavior. For $x=0.25$, where AFM order develops in the temperature range 7.5–10 K, we find coexistence of meso- or macroscopically segregated paramagnetic and AFM phases, with considerable disorder in the latter down to 2 K.

• Received 19 March 2016
• Revised 23 May 2016

DOI:https://doi.org/10.1103/PhysRevB.93.214421