Exchange Bias Effect On The Single-phase Long-period Bi Containing Layer-structured Multiferroics

Layer-structured Bi-containing Aurivillius oxides, a material class consisting of alternatively stacked fluorite-like (Bi2O2)2+ units and perovskites-like [Am-1BmO3m+1]2-blocks, have attracted much attention for the coexistence of ferroelectric (FE) and ferromagnetic (FM) behaviors, potentially above the room temperature (RT). Their non-centrosymmetric structure and the inhomogeneous distribution of Fe/Ti ions in the octahedral sites accompanying with their strong interactions via oxygen ions appear to play an important role in determining their fascinating FE and FM properties. All these provide an alternative way to explore the potential multiferroics to be used in information storage, sensor and other spintronic devices.Studies have shown that the length of the perovskite blocks in these layer-structured Bi-containing Aurivillius oxides have large impact on their multiferroics properties. Short-period compounds, e.g. Bi5FeTi3O15 (m=4), Bi6Fe2Ti3O18 (m=5) and Bi7Fe3Ti3O21 (m=6), show the paramagnetic (PM) ground state at RT; while compounds with long periods, e.g. BisFe4Ti3O24(m=7) and Bi9Fe5Ti3O27 (m=8), are mostly antiferromagnetic (AFM) materials with a weak ferromagnetism at RT. Existence of the ferromagnetism in such long-period oxides may trigger the formation of glassy magnetic state due to the FM and AFM interactions, which may lead to exchange bias (EB) phenomenon in these long-period layered oxides. This thesis mainly focuses on the investigation and exploration of new multiferroic properties and mechanism in these layer-structured Bi-containing Aurivillius oxides, including:1) Fabrication of Co/Y co-substituted Bi7Fe3Ti3O21 using a improved combustion synthesis methode and the relationship investigation of the substitution content and the multiferroic performance; 2) Fabrication, structure investigations and performance studies of a new long period compound Bi10Fe6Ti3O30 as a potential single phase exchange bias material; 3) Exploring the magnetic evolution of Co substituted Bi10Fe6Ti3O30. The main results are listed as follows:Chapter 1:A brief introduction on the ferroelectric and magnetic materials, and the multiferroics, of which FE and FM behaviors are coexisted. The thesis focuses on the preparation and novel performance investigations, including multiferroic properties and EB effect, of the layer-structured Bi-containing Aurivillius phase oxides with a general formula of (Bi2O2)2+(Am-1BmO3m+1)2- (here, m is the number of the perovskite layers), which are considered as one of the most promising candidates potentially to be single-phase multiferroics, especially functioning at the RT.Chapter 2:Fabrication of yttrium-modified Bi7Fei.sCo1.5Ti3O21 Aurivillius phase oxides using a modified Pechini method instead of solid state reaction. Superexchanging interactions between high-spin states of Fe3+ and the substituted Co3+ ions were suggested to play an important role in the observed FE and FM enhancement at RT. Their multiferroic properties were investigated as a function of the Y content in the formula of Bi7-xYxFe1.5Co1.5Ti3O21. Derivative thermo-magneto-gravimetry measurement indicates that the measured magnetic properties are mainly originated intrinsically from the Y-modified Aurivillius phases within the proper Y content.Chapter 3:A brief introduction on exchange bias effect, which usually describes an exchange coupling across the interface between two magnetic materials and plays a crucial role in developing fundamental physics as well as practical applications of the spintronics. EB effect has been usually observed in the artificially made systems consisting of FM and AFM components. Recently, EB effect has been extended into a few artificial multiferroic heterostructures. In above materials, EB effect can be controlled by an electric field applied to the multiferroic phases on account of the effective magnetoelectric (ME) coupling. In fact, many important progresses have been achieved in such artificial EB materials in recent years, yet challenges remain which mostly associated with the artificial structures. Interestingly, EB was recently observed as an intrinsic property in some hole-doped perovskite oxides, such as manganite and cobaltite, with the inside phase separation leading to the EB across the FM/AFM interfaces. This research can be considered as an emerging effort to pursue future single-phase EB materials.Chapter 4:Observation of a remarkable exchange bias arising from the temperature-dependent interaction between the antiferromagnetic spins and the cluster glassy regions was firstly observed in a newly developed, single-phase multiferroic compound of BiioFe6Ti303o with the nine-layered structure. Inhomogeneous distribution of the magnetic Fe ions in this long-period oxide was experimentally identified, which confirms the presence of the short-range magnetic ordering (cluster glassy state) and the canted antiferromagnetism. Finding of this new single-phase material accompanying a remarkable exchange bias effect would be beneficial to both basic physics understanding and the potential device development. Now available models does not shed light on the mechanism responsible for the complicated factor which influence the exchange bias field.Chapter 5:Magnetic evolution from PM (AFM) to FM is obviously observed when using Co to substitute Fe. More nonequivalent positions of B sites available for Fe or Co ions in the perovskite-like blocks between the (Bi2O2)2+ layers are suggested to account for this observation. Their non-centrosymmetric structure and the inhomogeneous distribution of Ti/Fe/Co ions in the octahedral sites accompanying with their strong interactions via oxygen ions appear to play an important role in determining their fascinating FE and FM properties.Chapter 6:We summarized the content of the whole thesis and forecasted the prospect of research directions in chapter six.