  Considerable evidence exists for the failure of the traditional theory of
quantum critical points (QCPs), pointing to the need to incorporate novel
excitations. The destruction of Kondo entanglement and the concomitant critical
Kondo effect may underlie these emergent excitations in heavy fermion metals --
a prototype system for quantum criticality -- but the effect remains poorly
understood. Here, we show how ferromagnetic single-electron transistors can be
used to study this effect. We theoretically demonstrate a gate-voltage induced
quantum phase transition. The critical Kondo effect is manifested in a
fractional-power-law dependence of the conductance on temperature ($T$). The AC
conductance and thermal noise spectrum have related power-law dependences on
frequency ($\omega$) and, in addition, show an $\omega/T$ scaling. Our results
imply that the ferromagnetic nanostructure constitutes a realistic model system
to elucidate magnetic quantum criticality that is central to the heavy fermions
and other bulk materials with non-Fermi liquid behavior.
