| Nano-scale material is a natural, incidental or manufactured material containing particles, in an unbound state or as an aggregate or as an agglomerate and where, for50%or more of the particles in the number size distribution, one or more external dimensions is in the size range1nm-100nm. As nano-scale materials possess surface effect, small size effect, macroscopic quantum tunneling effect and quantum confinement effect, which are entirely different from those of macroscopic materials, the mechanical, thermal, optical and magnetic properties of nano-scale materials are distinct from those of common materials and of high research value, provoking the strong interest of researchers. As yet, industrial production has only been realized in nano-powders, such as calcium carbonate, siliceous reinforcing agent and zinc oxide, etc., and other nano-scale materials are still in the laboratory stage.Multiferroic materials are materials that exhibit more than one primary ferroic order parameter simultaneously, such as ferromagnetism/anti-ferromagnetism, ferroelectricity/anti-ferroelectricity and ferroelasticity, etc., which can be adjusted and controlled by one another. With potential applications in various fields, such as spin quantum devices, data accessors and detectors, etc., multiferroic materials have always been a hot spot in research. Presently, compared with mainstream research on ferromagnetism and ferroelectricity, as well as the mutual modulation and control in between, research on ferroelasticity is comparatively less. Normally, ferromagnetism and ferroelectricity are mutually exclusive in one material. In recent years, however, researchers have found many compounds, films and mixtures which possess the property of ferromagnetism and ferroelectricity simultaneously. Applications of multiferroic materials is still rather limited. For instance, the multiferroic effects of many materials can only be observed in extremely low temperature, or relatively obvious magnetoelectric effects only present themselves when a high external electric/magnetic field is applied.As multiferroic materials often exhibit different properties from common materials, we are dedicated to studying the magnetic, electric and multiferroic properties of nano-scale materials in recent year, expecting to discover some with potential application values. Our major objects of study are nano-scale oxides and perofskite structure compounds. Through data analysis and theoretical calculation, we have discovered electric/magnetic properties in nano-scale material which are different from those of bulk materials, and observed magnetoelectric effect in nano-structure TiO2, SrTiO3, La2Ti2O7, Pr2Ti2O7. All these interesting phenomena may arise from vacancies, defects or stress of the nano-structure.The main conclusions of our work are as follows:1. Pure ZnO nanomaterials exhibits weak ferromagnetism (FM) while Sm doped ZnO (SmxZn1-xO) could enhance the FM. When x is less than0.6, only ZnO diffraction peaks could be observed; when x is larger than0.6, the Sm2O3peaks appear. Therefore, we investigated the ferromagnetism of SmxZn1-xO when x equals0,0.2,0.4and0.6. We found that the saturated magnetism (Ms) reached a maximum value when x equals0.4, while Ms decreased when x equals0.6but still larger than the Ms of the pure ZnO, indicating that the Sm doping indeed enhanced the FM of ZnO. On the other hand, the annealing temperature (TA) also affects the FM of pure ZnO and SmxZn1-xO nano materials that the Ms decreases with increasing TA.2. The La2Ti2O7nanopowders and nanocrystalline materials prepared by sol-gel method exhibited room temperature ferromagnetism (RTFM), and the vacuum annealing enhanced the FM while a subsequent annealing in air after vacuum annealing lowered the FM (which is still larger than the FM of initial powders). The crystallization temperature of La2Ti2O7powders was about700℃, and we found that the Ms of La2Ti2O7powders decreased with increasing TA. In addition, the nanocrystalline pellets still exhibited weak FM if the annealing time was short (1000℃for1h), while the FM disappeared when the annealing time was long enough (1000℃for1.5h) or the TA was high enough (1100℃for 1h). The ferromagnetism and ferroelectricity (FE) could coexist in nanocrystalline La2Ti2O7pellets if both the TA and annealing time were proper. Moreover, we found notable magnetodielectric (MD) effect in the nanocrystalline La2Ti2O7pellet, indicating the FM and FE are coupled in the La2Ti2O7sample.3. Similar to nanocrystalline La2Ti2O7samples, we investigated the structure, ferromagnetism, dielectric constant, ferroelectricity and multiferroic properties of Pr2Ti2O7nanomaterials. We found that the prepared Pr2Ti2O7nanocrystalline pellets also presented RTFM while the FE Curie temperature Tc was high (about570K), while the MD effect was found as well. Simultaneously, we found the FM of Pr2Ti2O7nanocrystalline materials was enhanced after polarized in DC electric field, indicating a coupling between magnetic and electric properties. We believed the Pr2Ti2O7nanocrystalline materials were valuable for application due to its high Tc.4. Both peroveskite SrTiO3and CaTiO3were incipient ferroelectric materials, while their dielectric constants monotonically increased as the temperature decreased to0K. However, we unexpectedly observed dielectric anomaly in SrTiO3and CaTiO3nanomaterials prepared by sol-gel method, and the room temperature ferroeletric hysteresis loops of SrTiO3nanomaterials was obtained, which was even more interesting. For CaTiO3nanomaterials, we could not get the hysteresis loop due to the temperature where the dielectric anomaly peak appeared was far below room temperature. We concluded that these interesting phenomena were caused by the size effect or strain effect which may destroy the quantum paraelectric state in SrTiO3and CaTiO3. In addition, we investigated the FM of SrTiO3and CaTiO3nanomaterials, and the results revealed that the origin of the FM was cation vacancy which was then proved by the first principle calculations.5. In accordance to our previous work, we also conducted some research on the structure and magnetism of nanocrystalline rutile TiO2We found the TiO2nanocrystalline materials present weak ferromagnetism which may be originate from oxygen vacancies. In the meantime, we also studied the structure and magnetic property of TiO2thin film prepared by pulse laser deposition. The TiO2thin film deposited on SrTiO3substrate by PLD showed room temperature ferromagnetism while the saturated magnetic moment increased after the film was polarized by DC pate. This phenomenon indicated that the electric and magnetic properties may, to some extent, couple in the rutile TiO2film.In short, through a series of research on nanomaterials, we find that nanomaterials, whether in magnetic property or electric property, is quite different from bulk materials, and we also found magnetoeletric effect and electric induced magnetic effect in some nanocrystalline materials, which may provide some contribution and reference for the exploration of new multiferroic materials.
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