A huge renormalization of transport effective mass in the magnetic-polaronic state of EuB6

The comprehensive study of galvanomagnetic, thermoelectric and magnetic properties was carried out on the single crystals of low carrier density ferromagnetic metal EuB6 (T-C approximate to 13.9K, T-m = 15.8 K) in a wide range of temperatures (1.8-300K) and magnetic fields (up to 80 kOe). The analysis of the microscopic characteristics estimated from the data revealed a giant renormalization of the charge carriers' effective mass m(eff), which is observed in the paramagnetic state of this compound with strong electron correlations. The gradual decrease of m(eff) from the maximum of m(eff)similar to 30m(eff) detected at T* approximate to 80 K to the low temperature values of m(eff) (T <= T-C)similar to 0.2-1m(0) is discussed in terms of the phase separation with the formation of low resistive ferromagnetic nano-sized regions (ferrons) in the dielectric magnetic polaronic state (T> T-m). The observed unusual behavior of m(eff) favors recent explanation of the genesis of the metal-insulator transition scenario proposed for La-doped EuB6 systems [U. Yu, B.I. Min, Phys. Rev. Lett. 94 (2005) 117202.]. (c) 2007 Elsevier B.V. All rights reserved.