Electronic phase separation due to magnetic polaron formation in the semimetallic ferromagnet EuB6 — A weakly-nonlinear-transport study

Abstract

We report measurements of weakly nonlinear electronic transport, as measured by third-harmonic voltage generation V 3ω, in the low-carrier density semimetallic ferromagnet EuB6, which exhibits an unusual magnetic ordering with two consecutive transitions at Tc1 = 15.6K and Tc2 = 12.5K. In contrast to the linear resistivity, the third-harmonic voltage is sensitive to the microgeometry of the electronic system. Our measurements provide evidence for magnetically-driven electronic phase separation consistent with the picture of percolation of magnetic polarons (MP), which form highly conducting magnetically ordered clusters in a paramagnetic and less conducting background. 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 V 3ω at the lower transition Tc2. Likewise, in the paramagnetic regime, a drop of the material's magnetoresistance R(H) precedes a magnetic-fieldinduced peak in nonlinear transport. A striking observation is a linear temperature dependence of V3ωpeak. 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* ∼ 35K 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. © 2013 The Korean Physical Society