| Multiferroics (MFs) which contain more than two type of ferroic orders (ferroelastic, ferromagnetic,ferroelectric,ferrotorodic) have been a hot research spot for a long time for their magnetoelectricity effect(ME). They are considered to have great protentials in fabrication of new memory devices such as multi-states as well as magnetic sensors with supreme sensitivity. The realization of high speed writing using electric field along with non-destructive high speed reading using magnetic field will be a revolutionary invention in information storage industry. Besides, Manganites representing strongly correlated electron systems (SCES) in which phase changes originate from competition between various ground states coupled to the spin, charge and orbital degrees of freedom of the Mn 3d electrons play an predominante role in research on spintronics. So far, how to promote ME effects in single crystal Manganites or compositional materials in room temperature and their related prototype devices are one of the major concerns in MFs study.Good material research relies on effective experimental measurements. ME measurements can be grouped into two aspects. One is magnetical characterization using SQUID and VSM solutions. The other one is measuring electrical transportation under bias magnetic field through resistance, dielectric constant, conductance and so on. The dynamic ME coupling measurement requires a d.c. bias along with an a.c.. magnetic field to obtain the dE/dH coefficient. Since most of the ME effect in MFs is observed under a cryogenic environment, how to measure ME effect in low temperature under electric/magnetic field is required in MFs research.On the other hand, although there are mature means for magnetic moment detection, electrical characterization under bias magnetic field is complicated when different samples, measurement goals and means are considered. Taking MTJ as an example, several current, voltage source are required to work together in order to finish the test. Thus how to combine equipments effectively to meet the flexible needs in measurements is necessary for MFs research.But combining different equipments together is not as simple as just connecting them using calbes in most cases. There are large errors unseen in many seemly well-built experimental platforms DIY by researchers. In the case of MFs'research, such errors mainly originate from insufficient accuracy in low current measurement. How to analysize the source of experimental errors along with solutions to remove them are critical issues for researchers.Based on those MF characterization techniques mensioned above, we first rebuilt the alternative electrical measurement platform based on Physical Property Measurement System(PPMS, Quantumn Design). This part of work paved the way for the dielectric measurement of multiferroic ceramic Bi5Ti3Fe1-xCrxO15 (BTFCO) under low temperature. Then the relation of the orientation of La0.7Sr0.3MnO3LSMO) thin films with their electric and magnetic properties was discussed in detail. Then we tried to fabricate Mott field transistor based on electron-doped manganite Ca1-xCexMnO3(CCMO) and its ME effect was characterized in detail with the help of the same alternative electrical measurement platform built from PPMS.Last but not the least, High angle annular dark filed scanning transmission electron microscopy (HAADF-STEM) provides a more direct way to interprete MF superlatice and material phase structure information. It can also provide chemical composition information, which with the help of Electron Energy Loss Spectroscopy (EELS) provides a solid support for MF research. The analysis of HAADF images can also provide direct interpretation such as polarization mapping, thus making it a very important tool in MFs' characterization.The main work and results are as follows:1 Discussing the possible origins of experimental errors in detail for alternative electrical measurement platform based on PPMS(QD, USA) with valid solutions to solve those errors.Many labs utilized PPMS combined with other measuring instruments to meet varied measurement needs. But there are few papers reporting the uncertainties in such experiments. Errors and solutions for test diviations especially in low current and dielectric measurement were discussed.2 The dielectric behavior of Cr-doped Bi5Ti3FeO15 (BTFCO) ceramics was systematically studied by temperature-dependent dielectric/impedance spectroscopy using PPMS combined with a LCR meter. Such work aimed at revealing the inherence between the charged defects caused by doping and dielectric responses.Two dielectric relaxation processes were found in grain interiors of the BTFCO ceramics. Both of them showed similar activation energy. This brought a great challenge to discern the different mechanisms of the conductivity and/or dielectric responses in Fe-Cr-based materials. Nevertheless the origins of the two relaxations were unambiguously determined by adjusting the Cr doping contents. One relaxation at high temperature region was proposed to be associated with the localized electron hopping between Fe3+ and Fe2+ whereas another one at lower temperatures was assigned to the localized hole transfer between Cr3+ and Cr6+ inside the grains. Such assumption is further verified using X-ray photoelectron spectroscopy (XPS) data.3 The magnetic and magnetotransport properties of La0.7Sr0.3MnO3 (LSMO) thin films prepared on (100), (110), and (111) oriented SrTiO3 substrates by chemical solution deposition method were systematically studied. The differences in structural, magnetic and magnetotransport properties of these samples related with the orientations of the films were revealed.X-ray diffraction 0-20 and /?-Scan measurements results showed that all the films had perfect crystalline orientation and in-plane alignment. Both the Curie temperature and metal-insulator transition temperature were almost the same for all the three orientations, about 339 K. The magnetoresistance value for the films reached maximum near the Curie temperature. Both the magnetization and MR value for (100) direction LSMO were the largest, and meanwhile for (111) direction were the smallest. The saturation magnetization decreased approximately proportional to T2 at low temperature (T<TC/2) for all the oriented film. Inversely, the resistivity increased proportional to T2 in the same temperature range, which can be attributed to the electron-electron scattering.4 In order to study more about real applications concerning MFs, Cai_xCexMnO3 (CCMO)with x= 0.05 was used as the channel of field effect transistor with an electric double layer(EDLT) covered with ionic liquid. With the help of electric double layer formed from ionic liquid, the modulation of CCMO maganetic phases using electric field was realized, thus achieving the goal of ME effect in CCMO-EDLT.Experiment data gained from PPMS alternative electrical measurement platform showed that the structure of CCMO-EDLT can control the metal-insulating transition of CCMO successfully. Such research provided an effective way to fabricate ME devices using magnities.5 A modified peaks finding method which can extract structural and chemical information was proposed. The quantification of the structure and chemical information of multiferroic HAADF images was realized.As an attempt to analysize the microstructure of multiferroics quantitatively, an automatic segmentation algorithm combined with a self-adaptive integration was performed in order to interprete the structural and chemical information from a single HAADF image. Consequently, the polarization map and strain distributions were rebuilt from a HAADF image of the rhombohedral and tetragonal BiFeO3 interface. A MnO2 monolayer in LaAlO3/SrMnO3/SrTiO3 heterostructure was discerned using the same method. Those works provided a new alternative tool for experimental data analysis. |
PDF Full Text Request