Electric Field And Spatial Confinment On The Physical Properties Of Perovskite Manganite Films

Connected by the octahedral consisted of the oxygen and manganese ions,the spin?orbital degrees of freedom are coupled with the lattice,result in a series of interactions like the double exchange interaction,coulomb repulsion,electron-phonon interaction in similar energy magnitude and various electronic and magnetic phases with different spin,charge,orbital ordering.Due to the energy degeneracy of the different phases,They are very sensitive to external stimuli and phase transition can be easily induced by external fields.Among the different external fields,electric field is considered the basic and most accessible in device integration.Carriers density could be modulated by applying the field effect transistor principle on manganite.Lattice distortion of manganite,especially the distortion and the rotation of the oxygen octahedral could be altered via the strain field given by the ferroelectric substrate with the inverse piezoelectric effect.The influence of electric field on the magnetic and electronic properties of manganite is studied via different means that the electric field control one or multi degrees of freedoms in manganite.The main result are as follows:1.La_0.875Sr_0.125MnO₃ was chosen as the model system to study the electric field control the carriers density in the thin film via the field effect device with ionic liquid as dielectric.Transistor with ionic liquid as dielectric was built and the influence on the ferromagnetic metallic phase,the ferromagnetic insulating phase and the charge orbital ordering with it was studied by altering the carriers density.It was found that the resistance of the film increases with the injection of electrons with ferromagnetic metallic phase suppressed and charge orbital ordering phase enhanced.While the injection of holes result in the decrease of the resistance with the ferromagnetic metallic phase enhanced and charge orbital ordering phase suppressed.Meanwhile the cooperative Jahn-Teller distortion induced abnormal resistance jump which was rarely observed in thin films was seen with electrons injected while lost with holes injected.Our result indicate that that the carrier modulation via electric field approach can change the proportion of the Jahn-Teller active Mn³⁺and the distorted oxygen octahedral staggered in space to reduce the elastic energy.As a result,the cooperative Jahn-Teller distorted phase appeared.2.Pr_0.7Ca_0.18Sr_0.12MnO₃/PMN-PT heterostructure was chosen to study the effect of spatial confinement and its influence on electric field control the transport properties and phase separation.On the?001?oriented heterostructure,we fabricated film strips with different width and studied the effect of the gate electric field on the transport properties of the manganite film strips.It was found that the electric field modulation gradually turn from a field polarity independent to polarity dependent as the width of the strip decreases.Two distinct mechanism was proposed to explain the electric field modulation phenomena,the lattice strain effect from the inverse piezoelectric effect of the PMN-PT substrate and the polarization field effect that adjust the proportion of the ferromagnetic metallic phase and the charge orbital ordering phase.Due to the spatial confinement,the electric field is not uniformly distributed in the strips but assemble at the edge of the strips with a in plane component.Such a distribution of electric field could cause the change of the relative fraction of the ferromagnetic metallic phase and the charge orbital ordering phase.As the width of the strips decreases,the field induced phase transition gradually dominates the modulation of the transport properties in the strip.As a result,the electric field modulation gradually turns from polarity independent to polarity dependent.In?011?oriented heterostructure we find an anisotropic modulation and a non-volatile resistance modulation in the in plane?100?direction.This unusual modulation is due to the abnormal distribution of the electric field in the ferroelectric substrate and induce a special distribution of ferroelectric polarization domains.3.?001?oriented Pr_0.7Ca_0.18Sr_0.12MnO₃/PMN-PT heterostructure was chosen to study the double gate modulation of the transport properties and phase separation combining a top gate with ionic liquid as the top dielectric and PMN-PT substrate as the bottom gate.The ionic liquid gating alone could cause the increase or decrease of the resistance of the film with a positive or a negative gate,indicating that a positive field favor the formation of the ferromagnetic metallic phase and suppressed the charge orbital ordering phase while the negative field does the opposite.Except for the volatile electrostatic effect that change the relative proportion of the ferromagnetic metallic phase and the charge orbital ordering phase,a new mechanism that is nonvolatile is found and is believed to be due to the field induced oxygen ion movement.When a4kV/cm back gate was applied,the effect of the positive top field is greatly enhanced and the film turn into a field induced insulating state with the same top gate bias.A opposite field could induced the metal-insulator transition from the insulating film induced by the positive field.Such an enhancement of the suppression of the formation of the ferromagnetic metallic phase could be due to the strengthen of the ability of the positive gate bias to induce the oxygen ions to move out of the film.With the formation of more oxygen vacancies in the film,the proportion of the ferromagnetic metallic phase is always below the critical percentage to form the percolation conduction passage as the temperature changes.As a result,the film remains insulating in the entire temperature range.