Manipulation of Nanoscale Charged Polar States in Manganites
published: Jan. 18, 2008, recorded: October 2007, views: 434
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The complicated interplay among charge, spin and lattice degrees of freedom in manganites is believed to induce the unexpected magnetic and transport phenomena, such as the colossal magnetoresistance (CMR). Manganites display also a variety of useful multiferroic properties such as colossal magnetocapacitance effect and high dielectric constant. In multiferroics ferromagnetic order can be controlled by an electric field, or ferroelectric order can be controlled by a magnetic field. Among them, La1-xSrxMnO3 is the most attractive candidate for multiferroic applications because of a combination of desirable properties. In this work we report the observation of the high contrast of electric field induced charged polar states after the local application of the electric field to the surface of samples via several Scanning Probe Microscopy (SPM) techniques in La0.9Sr0.1MnO3 and La0.89Sr0.11MnO3 single crystals. The electric-field-induced contrast is observed in Kelvin mode (KFM) confirming local modification of the surface properties of manganites. Piezoelectric effect of the induced states is assessed using Piezoresponse Force Microscopy (PFM). These results are complemented by the measurements of piezoresponse hysteresis and surface potential hysteresis loops at some area in standard pulse dc mode. The induced polar charged states relax with characteristic time constant of about 80 hours at room temperature, which exceeds Maxwell relaxation time by many orders of magnitude. The mechanisms of the observed phenomena are discussed along with the possible instrumentation effects. The origin of the effect can be related to the nanoscale charge and spin dynamic inhomogeneities appearing in manganites due to a delicate balance of charge, lattice and magnetic order. The injection of the additional charge carriers in the induced area promotes the appearance of the polar charged states. The long relaxation time for the induced charged state may be explained by the existence of the intrinsic inhomogeneous states. All these results show that the existence of the stable areas with the increased charge concentration is possible and thus it confirms the tendency towards charge segregation in manganites.
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