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Electronic phase separation due to magnetic polaron formation in the semimetallic ferromagnet EuB6_6 - A weakly-nonlinear-transport study

Abstract

We report measurements of weakly nonlinear electronic transport, as measured by third-harmonic voltage generation V3ωV_{3\omega}, in the low-carrier density semimetallic ferromagnet EuB6_6, which exhibits an unusual magnetic ordering with two consecutive transitions at Tc1=15.6T_{c_1} = 15.6\,K and Tc2=12.5T_{c_2} = 12.5\,K. Upon cooling in zero magnetic field through the ferromagnetic transition, the dramatic drop in the linear resistivity at the upper transition Tc1T_{c_1} coincides with the onset of nonlinearity, and upon further cooling is followed by a pronounced peak in V3ωV_{3 \omega} at the lower transition Tc2T_{c_2}. Likewise, in the paramagnetic regime, a drop of the material's magnetoresistance R(H)R(H) precedes a magnetic-field-induced peak in nonlinear transport. A striking observation is a linear temperature dependence of V3ωpeak(H)V_{3\omega}^{\rm peak}(H). We suggest a picture where at the upper transition Tc1T_{c_1} the coalescing MP form a conducting path giving rise to a strong decrease in the resistance. The MP formation sets in at around T35T^\ast \sim 35\,K below which these entities are isolated and strongly fluctuating, while growing in number. The MP then start to form links at Tc1T_{c_1}, where percolative electronic transport is observed. The MP merge and start forming a continuum at the threshold Tc2T_{c_2}. In the paramagnetic temperature regime Tc1<T<TT_{c_1} < T < T^\ast, 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

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