Magnetic phase separation in LaMn$_{1-x}$Fe$_{x}$O$_{3+y}$

We have investigated the LaMn$_{1-x}$Fe$_{x}$O$_{3+y}$ system in the whole range of 0$\le $x$\le $1, for polycrystalline samples prepared by solid state reaction in air. All samples show orthorhombic structure (space group Pnma). For x=0 the oxygen excess, estimated to be y $\sim $ 0.1, produces vacancies in the La and Mn sites and generates a fraction around 20{\%} of Mn$^{4+}$ ions (3t$_{2g})$ and 80{\%} of the usual Mn$^{3+}$ ions (3t$_{2g}$, 1e$_{g})$, with possible double exchange interaction between them. The Fe-doping in this system is known to produce only stable Fe$^{3+}$ ions (3t$_{2g}$, 2e$_{g})$. We find an evolution from a fairly strong ferromagnetic (FM) behavior, with saturation magnetization (T=2K) m$_{S}$ $\sim $ 4 $\mu _{B}$ and Curie temperature T$_{c} \quad \sim $ 160 K, for x=0, to an antiferromagnetic (AFM) behavior, with T$_{N}$=790 K, for x=1. For intermediate Fe contents a mixed phase scenario occurs, with a gradual decrease (increase) of the FM (AFM) phase, accompanied by a systematic transition broadening for 0.2$<$x$<$0.7. A calculation based on the expected exchange interaction among the various magnetic-ion types, accounts very well for the m$_{S}$ dependence on Fe doping.