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Current-induced suppression of short range ordering phenomena in La0.67Ca0.33MnO3 microbridges

Oct. 13, 2010 (past)
12:10 p.m. to 1:30 p.m.
60 St. George Street, MP 606
Christianne Beekman speaker details
Christianne Beekman

Abstract:

Doped manganites are strongly correlated electron systems which show a large variety in physical properties. The material La0.67Ca0.33MnO3(LCMO) for instance shows a transition from a paramagnetic insulator to a ferromagnetic metal (M-I) around TMI = 250 K and the well known colossal magnetoresistance (CMR) effect. The physics is mainly determined by the competing interactions, trapping of the electrons in Jahn-Teller (JT) distortions (polarons) and the itinerancy of the electrons in the Double-Exchange mechanism when spins become polarized. This competition signals that only small free energy differences exist between a variety of different possible phases of the system. As a result the phase of the material can be tuned easily by various external perturbations, such as magnetic and electric fields and strain. Furthermore, the susceptibility of the M-I transition to disorder (doping disorder, oxygen nonstoichiometry, defects from strain relaxation, twinning, and grain boundaries) can lead to the coexistence of the metallic and insulating phases on a variety of length scales. Electrical transport in these correlated electron systems is therefore a complex phenomenon, which has hardly been probed on small length scales. Here I address such transport in the mesoscopic regime, by investigating microbridges made in LCMO ultrathin films, grown in a strained or unstrained state on SrTiO3 (STO) or NdGaO3 (NGO) substrates. I show currentvoltage characteristics as function of temperature and in high magnetic fields and with varying film thickness. For strained films, in warming from the metallic to the insulating state, I find non-linear effects in the steep part of the transition characterized by a differential resistance with a strong peak around zero applied current, and saturating at higher currents after resistance drops up to 60 %. As a possible explanation I use the concept of a phase of glassy polarons which is formed in the M-I transition, assisted by the strain, and which is very sensitive to the injection of charge carriers, leading to current-induced melting of the newly forming insulating state.

0.67Ca0.33MnO3(LCMO) for instance shows a transition from a paramagnetic insulator to a ferromagnetic metal (M-I) around TMI = 250 K and the well known colossal magnetoresistance (CMR) effect. The physics is mainly determined by the competing interactions, trapping of the electrons in Jahn-Teller (JT) distortions (polarons) and the itinerancy of the electrons in the Double-Exchange mechanism when spins become polarized. This competition signals that only small free energy differences exist between a variety of different possible phases of the system. As a result the phase of the material can be tuned easily by various external perturbations, such as magnetic and electric fields and strain. Furthermore, the susceptibility of the M-I transition to disorder (doping disorder, oxygen nonstoichiometry, defects from strain relaxation, twinning, and grain boundaries) can lead to the coexistence of the metallic and insulating phases on a variety of length scales. Electrical transport in these correlated electron systems is therefore a complex phenomenon, which has hardly been probed on small length scales. Here I address such transport in the mesoscopic regime, by investigating microbridges made in LCMO ultrathin films, grown in a strained or unstrained state on SrTiO3 (STO) or NdGaO3 (NGO) substrates. I show currentvoltage characteristics as function of temperature and in high magnetic fields and with varying film thickness. For strained films, in warming from the metallic to the insulating state, I find non-linear effects in the steep part of the transition characterized by a differential resistance with a strong peak around zero applied current, and saturating at higher currents after resistance drops up to 60 %. As a possible explanation I use the concept of a phase of glassy polarons which is formed in the M-I transition, assisted by the strain, and which is very sensitive to the injection of charge carriers, leading to current-induced melting of the newly forming insulating state.