We report measurements of weakly nonlinear electronic transport, as measured
by third-harmonic voltage generation V3ω, in the low-carrier density
semimetallic ferromagnet EuB6, which exhibits an unusual magnetic ordering
with two consecutive transitions at Tc1=15.6\,K and Tc2=12.5\,K. Upon cooling in zero magnetic field through the ferromagnetic
transition, the dramatic drop in the linear resistivity at the upper transition
Tc1 coincides with the onset of nonlinearity, and upon further cooling is
followed by a pronounced peak in V3ω at the lower transition
Tc2. Likewise, in the paramagnetic regime, a drop of the material's
magnetoresistance R(H) precedes a magnetic-field-induced peak in nonlinear
transport. A striking observation is a linear temperature dependence of
V3ωpeak(H). We suggest a picture where at the upper transition
Tc1 the coalescing MP form a conducting path giving rise to a strong
decrease in the resistance. The MP formation sets in at around T∗∼35\,K below which these entities are isolated and strongly fluctuating, while
growing in number. The MP then start to form links at Tc1, where
percolative electronic transport is observed. The MP merge and start forming a
continuum at the threshold Tc2. In the paramagnetic temperature regime
Tc1<T<T∗, MP percolation is induced by a magnetic field, and the
threshold accompanied by charge carrier delocalization occurs at a single
critical magnetization.Comment: to appear in J. Kor. Phys. Soc (ICM2012 conference contribution