Non-volatile field effect modulation of magnetism in dilute magnetic oxide thin films

Abstract

In this work I studied the electric-field manipulation of transport and magnetic properties in La₀.₆₆Sr₀.₃₃MnO₃ (LSMO) and Zn₀.₉₂Mn₀.₀₈O thin films, through the geometry of field-effect transistors. In such field-effect devices, the application of electric field through the gate dielectric attracts or repels charge carriers in the ferromagnetic channel, leading to charge accumulation and depletion at the gating interfaces and hence modulating the transport and magnetic properties of the channel layer. Usually the strength of modulation depends on the thickness of the ferromagnet and the electrostatic screening depth within the channel. In this thesis, the modulation of magnetic properties in the ferromagnetic layer, as induced by the application of gate voltage via ferroelectric polymer or ionic liquid gating, were described. Several related investigations were performed in this thesis. Firstly, the electric-field manipulation of carrier transport and magnetism in the LSMO (7.5 nm) channel was studied, using ferroelectric copolymer of poly[(vinylidenefluoride-co-trifluoroethylene] [P(VDF-TrFE)] as the dielectric gate. Upon the polarization reversal of P(VDF-TrFE), the LSMO channel exhibited a 15 % change in resistance at room temperature (and less than 1% change at 20 K). Experimental results in the resistance-temperature (R-T) measurement showed insignificant change of Curie Temperature (TC) of LSMO upon switching the polarization direction of P(VDF-TrFE), which was attributed to the short screening depth in the LSMO channel. Apart from controlling the LSMO behavior by reversal of P(VDF-TrFE) polarization, electric-field manipulation of the transport and magnetic properties of LSMO films was also examined through the application of low-voltage pulse chains across the P(VDF-TrFE) gate, using the same device structure. With the increase of positive-pulsing cycles from 0 to 36 k, the LSMO channel exhibited a gradual decrease in TC from 280 K to 265 K and showed suppression of low-field magnetoresistance at 20 K. The results also indicated that TC and magnetoresistance at 20 K could be reversibly controlled through the application of gate pulses of different polarities, as well as the number of pulses applied. X-ray photoelectron spectroscopy (XPS) and transport measurements in the LSMO channel under different gas environment revealed that low electric-field pulse switching with positive gate pulses favored oxygen vacancy (Ov) creation in LSMO layer under vacuum environment, while the Ov annihilation in LSMO channel was more favorable under oxygen environment. By comparing the pristine and gated samples, a change in the ratio between Mn³⁺ and Mn⁴⁺ ions was observed. Such results indicated that the manipulation of transport and magnetic properties in LSMO during the low-voltage-pulse cycles were due to electrochemical reactions (namely Ov creation/annihilation) in the LSMO, rather than electrostatic accumulation or depletion of charge carriers. The final study involved the electric-field manipulation of transport and magnetic properties in the dilute magnetic semiconductor (DMS) of Zn₀.₉₂Mn₀.₀₈O, using an electric-double-layer gating through ionic liquid. In addition to modulation of ZnO channel resistance, the electrostatically-controlled Zn₀.₉₂Mn₀.₀₈O films exhibited a tunable magnetoresistance below 100 K, indicating that the application of gate voltage effectively manipulated the charge carriers of the ZnO channel. Moreover, anomalous Hall effect measurements revealed an enhanced ferromagnetic state as the gate voltage switched from -2V to 2V, thereby indicating the controllable electron carrier density also altered the ferromagnetism in the material.