Role of t2g versus eg interactions in the physical properties of A2OBO3 (A = Mn, Fe)

From electron spin resonance (ESR), magnetization, and thermoelectric power on the isostructural Mn2OBO3 and Fe2OBO3, we have been able to establish important differences between the magnetic properties of these two compounds. These differences are rationalized on the basis of t(2g) versus e(g) electronic interactions, orbital occupancy, and structure. For Mn2OBO3, we propose the possibility of fast electron transfer between the Mn2+ and Mn3+ cations, in a similar way to the 3+/4+ Zener pairs in manganites. In the Fe2OBO3 system, our main findings are the observation of a magnetization reversal at low field and the observation of the spatially inhomogeneous short-range charge-order state that becomes truly long-range only below the Neel temperature. The phase diagram of both compounds is reconsidered on the basis of these results.