Quantum Criticality and Inhomogeneous Magnetic Order in Fe-doped alpha-YbAlB4

TL;DR

This study investigates how iron doping affects quantum criticality and magnetic order in intermediate-valent YbAlB4 compounds using muon spin relaxation experiments, revealing complex magnetic behaviors and deviations from existing theories.

Contribution

It provides new experimental insights into magnetic phases and quantum criticality in Fe-doped YbAlB4, highlighting disorder effects and coexistence of magnetic states.

Findings

No magnetic order in x=0.014 down to 25 mK

Power-law temperature dependence of muon relaxation rate

Coexistence of paramagnetic and antiferromagnetic phases in x=0.25

Abstract

The intermediate-valent polymorphs $\alpha$- and $\beta$-YbAlB$_4$ exhibit quantum criticality and other novel properties not usually associated with intermediate valence. Iron doping induces quantum criticality in $\alpha$-YbAlB$_4$ and magnetic order in both compounds. We report results of muon spin relaxation ($\mu$SR) experiments in the intermediate-valent alloys $\alpha$-YbAl$_{1-x}$Fe$_x$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 $\lambda_d \propto T^{-a}$, $a = 0.40(4)$, 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, where we find coexistence of meso- or macroscopically segregated paramagnetic and AFM phases, with considerable disorder in the latter down to 2 K.