Evidence of strong antiferromagnetic coupling between localized and itinerant electrons in ferromagnetic Sr2FeMoO6

摘要

Magnetic dc susceptibility ($\chi$) and electron spin resonance (ESR)measurements in the paramagnetic regime, are presented. We found a Curie-Weiss(CW) behavior for $\chi$(T) with a ferromagnetic $\Theta = 446(5)$ K and$\mu_{eff} = 4.72(9) \mu_{B}/f.u.$, this being lower than that expected foreither $Fe^{3+}(5.9\mu_{B})$ or $Fe^{2+}(4.9\mu_{B})$ ions. The ESR g-factor $g= 2.01(2)$, is associated with $Fe^{3+}$. We obtained an excellent descriptionof the experiments in terms of two interacting sublattices: the localized$Fe^{3+}$ ($3d^{5}$) cores and the delocalized electrons. The coupled equationswere solved in a mean-field approximation, assuming for the itinerant electronsa bare susceptibility independent on $T$. We obtained $\chi_{e}^{0} = 3.7$$10^{-4}$ emu/mol. We show that the reduction of $\mu_{eff}$ for $Fe^{3+}$arises from the strong antiferromagnetic (AFM) interaction between the twosublattices. At variance with classical ferrimagnets, we found that $\Theta$ isferromagnetic. Within the same model, we show that the ESR spectrum can bedescribed by Bloch-Hasegawa type equations. Bottleneck is evidenced by theabsence of a $g$-shift. Surprisingly, as observed in CMR manganites, nonarrowing effects of the ESR linewidth is detected in spite of the presence ofthe strong magnetic coupling. These results provide evidence that the magneticorder in $Sr_{2}FeMoO_{6}$ does not originates in superexchange interactions,but from a novel mechanism recently proposed for double perovskites.