Using NMTO-{ \it downfolding} technique, we explore and establish the origin of ferromagnetism in the pyrochlore system, Tl$_{ 2}$Mn$_{ 2}$O$_{ 7}$. It is found to be driven by hybridization induced spin-polarization of the delocalized charge carriers derived from Tl-$s$ and O-$p$ states. The mean-field estimate of the ferromagnetic transition temperature, T$_c$, estimated using computed exchange integrals are found to be in good agreement with the measurements. We find an enhancement of T$_{ c}$ for moderate doping with nonmagnetic Sb and a suppression of T$_{ c}$ upon application of pressure, both in agreement with experimental findings.
Categories: cond-mat.mtrl-sci cond-mat.str-el