En las fronteras de la Materia Condensada

The simultaneous presence of submicrometer ferromagnetic metallic and charge ordered and/or paramagnetic insulating regions observed in some manganese-oxide-based compounds, the Phase Separation (PS) phenomena, is a major discovery in the study of strongly correlated electron systems. There are conclusive evidences, both from theoretical and experimental works, showing that the high values of low field magnetoresistance achieved in PS manganites are due to the unique possibility of unbalancing the amount of the coexisting phases by the application of low magnetic fields.
We have studied magnetic and transport properties on different PS compounds: polycrystalline La_5/8-yPr_yCa_3/8MnO₃ (y=0.3, grain size 2µm) and La_1/2Ca_1/2Mn_1-zFe_zO₃ (0.02<z<0.05, grain size ~ 0.6µm), both of them synthesized by the liquid mix technique, and single crystalline La_5/8-yPr_yCa_3/8MnO₃ (y~0.35), grown by the travelling floating zone method.
All the studied compounds exhibit PS features below the ferromagnetic ordering temperature T_C. Pronounced time dependent effects observed close below T_C indicate that the fractions of the coexisting phases are dynamically changing in this temperature range. As a distinctive fact, we found that different amounts of the relative phase fractions can be easily tuned by application of low magnetic fields ( < 1 T), an effect that is persistent after the magnetic field is turned off. This non-volatile memory, imprinted on the actual amount of the ferromagnetic phase, can be read through transport measurements.
The effect, which is achieved in a delimited temperature range, is due to the confluence of two factors. On one hand, the existence of a true PS equilibrium state with definite equilibrium fractions of the coexisting phases. On the other hand, the slow dynamics displayed by the systems when they are driven to metastable states by an external stimulii. The fact that the same global features were found in polycrystalline and single crystalline samples suggests that this effect is not produced by extrinsic properties related to grain boundaries, but a distinctive feature of the studied PS compounds.