| Charge and spin are two intrinsic properties of electrons.The vigorous development of semiconductor technology（based on charge）and spintronics（based on spin）has played a key role in contemporary information dissemination and storage.Magnetic semiconductors combine magnetic and semiconductor properties in a single material,which makes them to be one of the most interesting semiconductor-spintronics materials.This dissertation focus on studying on such a ZnO-based magnetic semiconductor,and mainly studies the following three aspects:spintronics material preparation,spin-related transport mechanism,and the electric field control.1)Apart from the stable semiconductor characteristics,magnetic semiconductors need to have a Curie temperature above room temperature and sufficiently magnetization for practical device in applications.However,the Eu-based chalcogenides in the 1960s,the dilute-doped ⅡVI magnetic semiconductors in the 1980s,and the demonstration-type dilute magnetic semiconductors GaMnAs and InMnAs in the 1990s,the Curie temperatures of them all are much lower than room temperature.Until 2000,room temperature dilute oxide magnetic semiconductors doped with transition metal elements are finally discovered.However,the low magnetic ion solid solubility leads to weak magnetization and extremely low spin polarization,which greatly reduces the research interest of the diluted oxide magnetic semiconductor.On the other hand,this dissertation focuses on the key issue of magnetic semiconductor materials with a high-concentration（≤45%）magnetic Co ions,which were substituted into the ZnO wide-gap semiconductor,thus,room temperature magnetic semiconductor（Zn1-xCox）O single crystal thin films were successfully prepared by oxygen plasma assisted low-temperature molecular beam epitaxy.The saturation magnetization o（Zn1-xCox）O film reaches 270 emu/cm3 at room temperature（approximately equal to 1/4 of the magnetization of metal Co）,and anomalous Hall effect and magnetoresistance effect with high signal-to-noise ratios have been observed at room temperature,which provides detailed experimental evidence for carriermediated ferromagnetism at room temperature.2)The physics map for anomalous Hall effect in the low conductivity range（σxx<104 Ω1cm-1）has not yet clearly understood,it’s origin and mechanism are still unsolved.The（Zn1xCox）O magnetic semiconductor with high Co concentration prepared in this dissertation provides a suitable material system to study this problem.Together with doping non-magnetic Ga without changing the magnetization,it has made the signal observations of the anomalous Hall effect with high signal-to-noise ratios,and also the realization of large-scale independent control of the carrier concentration.In this dissertation,by using the measured hysteresis loops and the model of two carrier characteristics of electrons and holes in（Zn1-xCox）O magnetic semiconductor,the non-linear Hall resistivity from the anomalous Hall resistivity has been clearly separated,and a square scaling relationship between the anomalous Hall conductivity and longitudinal conductivity as σxyAHE ∝ σxx2 has been observed.By changing the magnetization of the sample,sign reversal of the anomalous Hall resistivity has been observed fro the first time.These two characteristics（conductivity square scaling relation and the anomalous Hall effect sign reversal）are well consistent with the intrinsic anomalous Hall effect in the multi-d-orbital tight bonding model proposed by H.Kontani et al.3)The electric field control of Rashba and Dresselhaus spin-orbit coupling is an effective all-electric method for controlling spin-dependent currents.It has been extensively and deeply studied in non-magnetic semiconductor heterostructures.However,the spin-dependent current controlling by electric field has not been experimentally demonstrated in room temperature magnetic semiconductors so far.In this dissertation,high concentration Co-doped（Zn1-xCox）O/PMN-PT multiferroic heterojunction was prepared by molecular beam epitaxy,and nonvolatile control of Rashba-Dresselhaus spin-orbit coupling of（Zn1-xCox）O has been realized by controlling the polarization of PMN-PT with gate voltage.Using the magnetoresistance measurement,it is estimated that the spin polarizability is 21%at the accumulation state and 28%at the depletion state.Using the anomalous Hall effect measurement,the anomalous Hall resistivity adjusted by the ferroelectric polarization is estimated to be ΔρyxSOC/RsM=55%.The electric field controlling of the spin current of magnetic semiconductors at room temperature is expected to provide reference experimental data for spin field effect transistor applications. |
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