Percolation limited magnetic order in Eu1-xCaxB6

Abstract

Abstract.We report the results of measurements of the low-temperature specific heat Cp(T ) and the ac susceptibility χac(T ) in low applied magnetic fields for a series of samples of Eu1−xCaxB6. The anomalies in Cp(T ), together with the results for χac(T ) and M (H), confirm the onset of phase transitions to long range magnetic order for x < 0.7 and provide evidence that for x ≥ 0.7, the Eu moments, which are captured in large magnetic clusters with magnetic moments of the order of 260 µB, adopt a spin-glass type ground state. The data set allows to establish the low-temperature [T ,x]

Magneto-Optical Evidence of Double Exchange in a Percolating Lattice

“…The electrical transport properties seem to be governed by percolation-type phenomena across the Eu 1ÿx Ca x B 6 series [14,15]. At about x 0:7, which coincides with the site-percolation limit in a simple cubic lattice, the long-range order disappears and for 0:7 < x < 0:9 a spin-glass type ground state is adopted [15].…”

“…The Ca substitution leads to significant changes of the magnetic and electronic properties. The FM transition temperature decreases with increasing Ca content and stoichiometric CaB 6 exhibits no magnetic order [14,15]. Evidence was also established for a spin-filter effect in the charge transport and dynamics for x 0:4 [12,13].…”

“…vanish. Figure 3 displays the variation of the normalized Drude spectral weight (i.e., SW Drude =SW TOT ) as a function of x, in comparison with the variation of T C for the Eu 1ÿx Ca x B 6 series [14,15]. The ranges on the vertical axes were chosen such that T C x 0 coincides with the renormalized changes of the Drude spectral weight, inserting SW TOT for either 0 or 7 T. For both values of SW TOT , we obtain the same type of variation with x. SW Drude decreases sharply between x 0 and 0.3, reaching zero at approximately 50% Ca content.…”

“…Near and above the percolation threshold the number of disconnected Eu-rich magnetic clusters becomes significant. Even though the tendency should be towards ferromagnetism, it is not surprising that the regime above x * 0:7 (beyond percolation, and at very low temperatures) seems to be characterized by glassy magnetism [15], on account of the possible presence of superparamagnetic clusters and competing dipolar interactions at such extreme dilutions of the magnetic moments [24].…”

“…Eu 1ÿx Ca x B 6 single crystals of high structural quality were prepared by solution growth from Al flux, using the necessary high-purity elements as starting materials [14,15]. The optical reflectivity R!…”