Studies On Magnetism And Magnetoelectric Effect In Oxide Films

Oxide materials possess enormous potential for applications in modern technology. They exhibit arrange of functional properties including electrical, magnetic and optical response. A wide variety of materials such as insulators, dielectrics, semiconductors, ferromagnets, ferroelectrics, colossal magnetoresistive materials and superconductors have been produced from oxides. Certain classes of oxides exhibit multiple simultaneous distinct functional properties; such compounds are normally referred to as "multifunctional" materials. For example, oxides such as transition metal doped ZnO, TiO2,InO3etc. can exhibit both ferromagnetic and semiconducting properties. Additionally, oxides such as Ni3V2O8, BiMnO3, BiFeO3and DyMnO3, exhibit simultaneous ferromagnetic and ferroelectric properties. Magnetoresistive materials such as Pr1-xCaxMnO3exhibit a coupled metal-insulator transition and magnetic ordering. Because multifunctional materials can often exhibit new properties, it is important to understand these systems from both scientific and technological point of view. The incorporation of these materials in device technology will lead to the development to new device applications. For example, incorporation of giant magnetorsistive (GMR) materials into modem memory devices such as read heads has highly enhanced their storage capacity.Tin oxides SnO2is an important n-type semiconductor with a wide band gap (Eg=3.6eV at300K), which has a wide range of applications, including solid gas sensors, liquid crystal displays, photovoltaic cells, and transparent conductive electrodes. A variety of methods such as sol-gel, chemical vapor deposition, magnetron sputtering, thermal evaporation, pulsed laser deposition and molecular beam epitaxy have been used to prepare SnO2film. In recent years, considerable efforts have been focused on the synthesis of magnetic impurity doped SnO2thin films in order to explore DMS properties. On the other side, rare earth ion doping in various hosts has been investigated most frequently due to their unique fluorescence properties, stability, and high emission quantum yields. It is also important to note that rare earth ions can be doped into oxides with relative ease. It was envisaged that if rare earth ions can doped into oxide based semiconductors, then band gap excitation may result in efficient energy transfer that would result in interesting optical as well as magnetic properties. It is therefore interesting from a technical point of view to dope oxide semiconductors with rare earth ions to see the possibility of coexistence of optical and magnetic properties. If found, such a coexistence will be of great importance for magneto-optical device applications.(200)oriented Sm-doped SnO2thin film has been successfully on (001)sapphire substrate at the substrate temperature of600℃, in oxygen atmosphere of1.1×10-2Pa. XRD pattern shows good crystallinity, and room temperature ferromagnetism has been observed with saturated magnetization of10.01emu/cm3and coercive field of7.222Oe. Ab inito calculations on the SnO2(200) plane with LDA+USIC method shows that, antiferromagnetic state is more stable when the system is merely doped with Sm, whereas ferromagnetic state becomes more stable than anti ferromagnetic state when Sm dopant and oxygen vacancy coexist in the system, without much change in the magnetic moment, therefore, the observed room temperature ferromagnetism in our sample should originate from the coexistence of Sm dopant and oxygen vacancy.In recent years, considerable attention has been devoted to the investigation of multiferroic materials due to their fascinating physical properties and potential applications. Especially, the magnetoelectric (ME) effect, i.e. the manipulation of magnetization by application of electric field and vice versa, has been studied the most in multiferroic materials, such as Bi-based perovskite-type compounds, Tb-based manganites, hexagonal rare-earth manganites, and the family of BaMF4(where M is a divalent transition metal ion). However, single phase multiferroic materials are generally rare because transition-metal d electrons reduce the tendency of an off-centering ferroelectric distortion. Moreover, most of the existing single-phase ME multiferroics are either antiferromagntic above room temperature, such as BiFeO3, or ferromagnetic at low temperatures, such as BiMnO3. In order to achieve room temperature single phase multiferroic material, an alternative way is to introduce room temperature ferromagnetism (RTFM) into room temperature ferroelectric oxide materials through intrinsic defects, such as cation or oxygen vacancy, to realize the coexistence of ferromagnetism and ferroelectricity at room temperatre, while various intrinsic defects can be formed in the sample prepared by PLD through the adjustment of deposition parameters.(K,Na)NbO3is one kind of potential candidate for practical lead-free piezoelectric ceramics, and (K0.5Na0.5)Nb03is ferroelectric with a high ferroelectric Curie temperature exceeding400℃; BaNb2O-based compounds are receiving great attention as a new ferroelectric tungsten bronze niobate material; nonstoichiometric BaNbO3-x could exhibit superconductivity with a Tc as high as22K, LiNbO3and LiTaO3are ferroelectric materials that have found important applications in many areas of advanced technology where optics and lasers dominate, therefore, it is of considerable importance to investigate the possibility of intrinsic defect-induced ferromagnetism and magnetoelectric effect in these systems. Here a series of PLD film samples were characterized by various techniques, and room temperature magnetic properties and magnetoelectric effect were investigated. The origin of magnetism is checked by ab inito calculations, and the results are listed below:1. For KNN nanocrystalline film on conductive Si substrate, XRD result indicates that oxygen atmosphere is prerequisite for the growth o f KNN, or the as-deposited film would be in amorphous state. FE-SEM images shows that higher substrate temperature facilitates the coalescence of grains. Magnetic measurement by AGM demonstrates the existence of ferromagnetism at room temperature, which is supposed to originate from cation (K, Na) vacancies, rather than oxygen vacancy. Ferroelectric measurement indicates that ferroelectricity is damaged and saturated P-E hysteresis loop cannot be obtained due to the increase of leakage current by the introduction of cation vacancies during deposition. MC effect measurement at room temperature shows that stronger ferromagnetism results in stronger interaction between magnetism and capacitance, and thus stronger MC effect. ME effect investigation displays that the in-plane Ms increases with increasing electric field, and the phenomenon that the orthometric application of electric field and magnetic field on the surface of film and subsequent removal leads to an enormous enhancement of in-plane saturation magnetization is a new phenomenon, which can be treated as a new kind of ME effect.2. For BaNb2O6film on LaAlO3substrate, XRD result shows that the phase of the as-deposited film is determined by the choice of substrate, with certain influence by substrate temperature and background atmosphere. Magnetic measurement by AGM demonstrates that thinner film results in stronger magnetism due to larger strain effect, the magnetism in BaNb2O6is related to oxygen vacancy, while the substitution of O atom in BaNb2O6by N atom could lead to the decrease of magnetism, and the largest Ms is obtained in vacuum at600℃. XPS measurement indicates that the change tendency of magnetism in different samples is in accordance with the asymmetric tendency of O1s spectrum, which means the magnetism in BaNb2O6is directly related to oxygen vacancy. Ab inito calculations indicate that stoichiometric BaNb2O6and that with one Ba vacancy is nonmagnetic, while Nb and O vacancy can lead to magnetism due to the spin-polarization of O2p electron and Nb s electron, respectively. For BaNb2O6supercell with double Nb/O vacancies, ferromagnetic coupling is energetically more favorable when the two Nb/O vacancies are located third-nearest-neighbored. Therefore, we can conclude that the observed room temperature FM in BaNb2O6films is predominantly induced by oxygen vacancies introduced during vacuum deposition, with certain contribution by Nb vacancies.3. For BaNbO3films deposited on Nb:SrTiO3substrate at600℃, the one deposited in oxygen atmosphere is not ferromagnetic at room temperature, but anisotropic ferromagnetism emerges after the application of an electric filed, and becomes stronger with increasing electric filed intensity. On the other side, the one deposited in vacuum shows ferromagnetism at room temperature, which is also enhanced after the application of an electric filed, but with higher critical electric field. These phenomena are supposed to originate from the electron transfer between the substrate and film at the interface, indicating that the magnetic properties of BaNbO3film can be adjusted by the density of polarized electron.4. We have performed ab initio calculations for the possibility of defect-induced magnetism in LiTaO3and LiNbO3by introducing intrinsic defects, namely, VLi0, VNb0/Ta,VO0,(Nb/Ta)Li0,(Nb/Ta)Li4+,(Nb/Ta)Li4++4VLi-cluster, and Lii0. Calculated results show that VO0is most stable among three vacancy cases, and (Nb/Ta)LiLi4++4VLi-has the lowest formation energy among all the defects. Stoichiometric LiTaO3and LiNbO3and that with (Nb/Ta)Li4+,(Nb/Ta)Li4++4VLLi-, and Lii0are non-magnetic, while VLi0and VNb0/Ta can induce magnetism due to the spin-polarization of O2p electrons, and the magnetism in LiTaO3with TaLi0originates from the spin-polarization of Ta d electron at the bottom of the conduction band, while the magnetism in LiNbO3with NbLi0and VO0comes from the spin-polarization of Nb s electron in the impurity band. For LiNbO3with VO0vacancy pair, ferromagnetic coupling is more favorable than and ferromagnetic coupling when VO0vacancy pair are located third-nearest neighboured. Therefore, ferromagnetism is supposed to be achieved in LiNbO3through the introduction of oxygen vavancy.In conclusion, we have proved the existence of ferromagnetism in ferroelectric oxide film with intrinsic defects both experimentally and theoretically, and new kind of magnetoelectric effect has been observed in (K0.5Na0.5)NbO3and BaNbO3film. Our research opens up a new path for the research of multiferroics and magnetoelectric effect at room temperature...