Magnetically driven electronic phase separation in the semimetallic ferromagnet EuB6

Abstract

From measurements of fluctuation spectroscopy and weak nonlinear transport on the semimetallic ferromagnet EuB6 we find direct 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 'poorly conducting' background. These different parts of the conducting network are probed separately by the noise spectroscopy/nonlinear transport and the conventional linear resistivity. We suggest a comprehensive and 'universal' scenario for the MP percolation, which occurs at a critical magnetization either induced by ferromagnetic order at zero field or externally applied magnetic fields in the paramagentic region.

Lattice Strain Accompanying the Colossal Magnetoresistance Effect inEuB6

“…1(b) we show the relative length change, ∆l/l, of EuB 6 as the difference (T ) = (∆l/l) EuB6 − (∆l/l) ph , corresponding to a spontaneous strain, normalized to the extrapolated value at zero temperature 0 . Our data uncover the onset of the negative strain at a temperature around T * ∼ 35 − 40 K. This is about the same temperature below which indications for bound MPs have been observed [16,[18][19][20], suggesting that their formation is accompanied by a lattice distortion. Upon further cooling, the lattice contraction strongly increases at the percolation transition temperature T c1 , and then displays an order paramater-like behavior below T c2 (solid line).…”

“…It has been proposed that the large negative MR in EuB 6 at T c1 is a percolation-type transition resulting from the overlap of MPs, which causes a delocalization of the hole carriers [14,16,19]. Upon cooling, the polaronic clusters percolate at T ≤ T c1 , and finally merge at T ≤ T c2 , where bulk FM order sets in -a scenario in accordance with recent magnetic [20] and transport data [14,18]. Given the pronounced lattice distortions accompanying arXiv:1404.1693v1 [cond-mat.str-el] 7 Apr 2014 the formation of MPs in the manganites, it is natural to look for similar effects also in EuB 6 .…”

“…Despite its simplicity, the system shows a rich phenomenology. The inverse magnetic susceptibility, χ −1 , of the Eu moments shows a linear temperature dependence for T 20 K with a paramagnetic (PM) Curie temperature of Θ p ≈ 15.6 K [13,14] and the FM state is reached via two consecutive transitions at T c1 = 15.3 K and T c2 = 12.6 K [15][16][17][18]. Moreover, the PM-FM transition is accompanied by a drastic reduction of the resistance at zero applied field as well as a CMR effect.…”

Magnetic Stray Field Detection as Guidance for Electronic Transport Measurements in the B-T Phase Diagram of MnSi

“…In order to perform more complex electronic transport measurements -aiming e.g. to determine the low-frequency dynamics of charge carriers in magnetic systems [12] -related to specific magnetic phases or phase transition lines, it is essential to determine the exact position in the magnetic phase diagram. In the following we show that for this purpose electronic transport measurements can be conducted on a sample the stray field of which is detected simultaneously by a micro-Hall-magnetometer.…”

Reentrant Metal-Insulator Transition in Ca_{1-x}Eu_{x}B_{6}

“…The scenario of magnetic polaron percolation, which is used to explain the temperature driven metal-insulator transition (MIT) in EuB 6 [4] and the exponential decrease of resistivity under applied magnetic eld in Ca 0.4 Eu 0.6 B 6 [5], fails to describe the onset of hole-like conductivity and huge CMR enhancement under transition into the insulating ground state in Eu-rich solid solutions (x < x M IT ≈ 0.8) [3]. In this respect the study of charge transport in Ca 1−x Eu x B 6 (x < x M IT ) is of great importance for understanding the nature of CMR in this strongly correlated electron system.…”