Evidence for magnetic phase separation in colossal magnetoresistance compound EuB5.99C0.01

• We present electric, magnetic, thermal and SANS studies of EuB5.99C0.01.
• We provide evidence about mixed magnetic structure in EuB5.99C0.01.
• Carbon rich regions play role of “spacers” in EuB5.99C0.01.
• Spacers prevent percolation of magnetic polarons and formation of bulk FM state.
• Spacers are responsible for differences between EuB5.99C0.01 and pure EuB6.

EuB5.99C0.01 is a low-carrier density ferromagnet that is believed to be intrinsically inhomogeneous due to fluctuations of carbon content. In accordance with our previous studies, electric transport of EuB5.99C0.01 close above the temperature of the bulk ferromagnetic (FM) ordering is governed by magnetic polarons. Carbon-rich regions are incompatible with FM phase and therefore they act as spacers preventing magnetic polarons to link, to form FM clusters, and eventually to percolate and establish a (homogoneous) bulk FM state in this compound, what consequently causes additional (magneto) resistance increase. Below the temperature of the bulk FM ordering, carbon-rich regions give rise to helimagnetic domains, which are responsible for an additional scattering term in the electrical resistivity. Unfortunately, there has not been provided any direct evidence for magnetic phase separation in EuB5.99C0.01 yet. Here reported results of electrical, heat capacity, Hall resistivity and small-angle neutron scattering studies bring evidence for formation of mixed magnetic structure, and provide consistent support for the previously proposed scenario of the magnetoresistance enhancement in EuB5.99C0.01.