Induced Phase Separation in GdBaCo$_{2-x}$Fe$_{x}$O$_{5.5-\delta }$ by Fe Doping*

A detailed magnetic study has been carried out for the Fe doped GdBaCo$_{2-x}$Fe$_{x}$O$_{5.5-\delta }$ (x = 0, 0.02, 0.05, 0.10, and 0.20) cobaltites. Fe doping enhances the high temperature T ferromagnetism present in the Fe-free samples and it induces a second ferromagnetic order, which develops at a lower T. Also, the spin-state transition temperature shifts to higher T with increasing the Fe content. Phase separation in two magnetic phases, namely, Fe-rich and Fe-free regions, provides a phenomenological interpretation of these magnetic behaviors. The Fe-rich regions are responsible for the second ferromagnetic order present at lower T. The shift of the low temperature M(T) peak related to this second ferromagnetic order is due to the decrease in the oxygen content in the Fe-rich regions with increasing Fe doping and the presence of the antiferromagnetic Fe-Co superexchange interaction in these regions. The high temperature M(T) peak (corresponding to the Fe-free regions) and the spin-state transition temperature shift to higher temperatures with Fe doping because of the slight compression of the Fe-free regions with Fe doping.