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
La5/8-yPryCa3/8MnO3 (y=0.3, grain size 2µm) and La1/2Ca1/2Mn1-zFezO3
(0.02<z<0.05, grain size ~ 0.6µm), both of them synthesized by the liquid mix technique,
and single crystalline
La5/8-yPryCa3/8MnO3 (y~0.35), grown by the travelling floating zone method.
All the studied compounds exhibit PS features below the ferromagnetic ordering temperature
TC. Pronounced time dependent effects observed close below
TC 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.
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