Studies On Magnetic Exchange Mechanism Of Doped BaTiO₃ And Preparation Of Magnetic Thin Films

Multiferroic materials, which exhibit magnetic and electric orderings simultaneously, offer the prospect of novel devices. Transition-metal-doped BaTiO₃ is one of the most promising systems for multiferroic research. However, there is still an intense debate on the origin of magnetism. Considering the situation mentioned above, the first part of my work has been focused on the Fe-doped BaTiO₃ system. Systematic investigations have been made on the magnetic properties and exchange mechanism by changing Fe-doping concentration, doping position, annealing atmosphere and preparation conditions. The main results obtained are as follows:1. B-site Fe-doped samples, Ba(Ti_1-xFe_x)O₃ (7≤x≤70 at.%), with room-temperature ferromagnetism were prepared by solid-state reaction and stoichiometry control. The measurements of X-ray diffraction and M(o|¨)ssbauer spectroscopy indicate that all samples are single-phase, crystallizing in a 6H-BaTiO₃-type hexagonal perovskite structure, and Fe atoms dissolve in BaTiO₃ host lattice as Fe³⁺, substituting for pentahedral and octahedral Ti⁴⁺. Thus, the relation is established between magnetic properties and Fe local environments, including oxidation state, occupational site and site ratio. The results of magnetization measurements show that the observed ferromagnetism originates from the super-exchange interactions between Fe³⁺ in different occupational sites associated with oxygen vacancies. With increasing the Fe content, the saturation magnetization is mainly influenced by two factors: the ratio of pentahedral to octahedral Ti sites and oxygen vacancies. The increase of the former has a beneficial effect on the magnetic properties, whereas that of the latter is detrimental to them.2. Considering the same valence and smaller size of Fe²⁺ compared with Ba²⁺, A-site Fe-doped samples were synthesized and the magnetic properties for different doping positions were compared. The results show that A-site Fe doping has been realized to a certain extent via controlling the stoichiometry of raw materials. The A-site doped sample is single-phase with a 6H-BaTiO₃-type hexagonal perovskite structure, and Fe ions are present in the form of Fe²⁺ and Fe³⁺, substituting for A-site Ba²⁺ and octahedral Ti⁴⁺, respectively. The double-exchange interaction between Fe²⁺ and Fe³⁺ is expected to produce room-temperature ferromagnetism. However, compared with the B-site doped sample, the A-site doped one has a remarkably reduced magnetization due to the variation of magnetic exchange mechanism. With the Fe content of 7 at.%, the saturation magnetization for A-site doping is six times lower than that for B-site doping. Moreover, the Fe-doping concentration range is relatively narrow for A-site doping. There exist secondary phases in A-site doped samples with the Fe content higher than 7 at.%.3. The B-site doped sample was post-annealed in vacuum and oxygen ambient, respectively. The effect of annealing atmosphere on magnetic properties and exchange mechanism was investigated via the control of Fe valence, occupational site and oxygen deficiency afforded by changing annealing atmosphere. The results show that the annealed Ba(Ti_0.3Fe_0.7)O₃ samples are still single-phase, and exhibit room-temperature ferromagnetism but the magnetization changes with different annealing atmospheres. After vacuum annealing, the competition between the increase of the ratio of pentahedral to octahedral Ti sites and the increase of oxygen vacancies eventually brings about the decrease of the saturation magnetization, while the magnetic exchange mechanism remains unchanged. On the contrary, O₂ annealing is an effective way to enhance the saturation magnetization due to the presence of Fe⁴⁺, an unusual valence for iron. The simultaneous existence of Fe³⁺ and Fe⁴⁺ in the O₂-annealed sample leads to the change of magnetic exchange mechanism from the interactions of Fe³⁺ ions in different occupational sites to the interactions of Fe ions with mixed valences.In order to further explore the way to improve the magnetic properties, B-site and (A,B)-site codoped BaTiO₃ was prepared and subsequently annealed in vacuum and oxygen, respectively, investigating the effects of interactions between different elements or different valences of the same element and annealing atmosphere on magnetic properties and exchange mechanism. The main results obtained are as follows:1. B-site codoped Ba(Ti_0.65M_0.05Fe_0.3)O₃ (M=Cr, Ni, Mn) samples were prepared, and post-annealed in vacuum and oxygen, respectively. The results show that all samples are single-phase with a 6H-BaTiO₃-type hexagonal perovskite structure. In the as-prepared state, Fe atoms are present as Fe³⁺, occupying tetrahedral and octahedral Ti sites, while Cr, Ni and Mn atoms take on the oxidation states of+3, +2 and +7, respectively. The exchange interactions between Fe³⁺ ions in different occupational sites and M ions determine the paramagnetism of all as-prepared samples. Nevertheless, the annealed ones are all ferromagnetic at room temperature. The origin of ferromagnetism changes with codoping elements and annealing atmosphere, including the double-exchange mechanism of M ions with mixed valences, the super-exchange mechanism of Fe³⁺ ions in different occupational sites, and the exchange mechanism of Fe ions with mixed valences.2.The dependence of magnetic properties on Mn-doping concentration was established for as-prepared Ba(Ti_0.7-xMn_xFe_0.3)O₃ (5≤x≤20 at.%) samples. The results suggest a sigle-phase structure of all samples and the presence of Mn⁴⁺ besides Mn⁷⁺ when the Mn content is increased. Thus, a double-exchange mechanism based on the Mn⁴⁺-Mn⁷⁺ pairs is allowed, responsible for the conversion of paramagnetism into ferromagnetism. With the increase of the Mn⁴⁺-Mn⁷⁺ pairs, the saturation magnetization is enhanced. The facts mentioned above further confirm the conclusion that the ferromagnetism of the vacuum-annealed Ba(Ti_0.65Mn_0.05Fe_0.3)O₃ sample is attributed to the double-exchange interaction of Mn ions with mixed valences.3.(A,B)-site codoped (Ba_0.8R_0.2)(Ti_0.3Fe_0.7)O₃ (R=K, Sr) samples were prepared, and post-annealed in vacuum or oxygen. The results show that all samples are single-phase, crystallizing in a 6H-BaTiO₃-type hexagonal perovskite structure. The paramagnetism for the as-prepared state is determined by the super-exchange interactions between Fe³⁺ ions in tetrahedral and octahedral Ti sites. Both O₂-annealed K/Fe and vacuum-annealed Sr/Fe codped samples exhibit room-temperature ferromagnetism, originating from the exchange couplings of Fe ions with mixed valences and of Fe³⁺ ions in different occupational sites, respectively.4. The dependence of magnetic properties on Sr-doping concentration was established for vacuum-annealed (Ba_1-xSr_x)(Ti_0.3Fe_0.7)O₃ (20≤x≤75 at.%) samples. The results show that all samples have a sigle-phase structure, and the lattice parameters gradually decrease with increasing the Sr content due to the smaller size of the Sr²⁺ ion compared with the substituted Ba²⁺ ion. At the same time both the ratio of pentahedral to octahedral Ti sites and the concentration of oxygen vacancies increase. The two competing effects finally result in the gradual enhancement of the saturation magnetization.Considering the realization of practical devices, Fe-doped BaTiO₃ thin films were prepared by pulsed-laser deposition. The high Fe-doping concentration and ferromagnetism have been obtained under appropriate preparation conditions. Meanwhile, Fe₃Si magnetic thin films were also fabricated, and the structure and properties were studied by changing preparation conditions. The main results obtained are as follows:1. Ba(Ti^1-xFe_x)O₃ (20≤x≤90 at.%) thin films have been deposited on SrTiO₃(100) substrates. The results show that the pulsed-laser-deposited thin films are single-phase, crystallizing in a tetragonal instead of hexagonal perovskite structure, different from the samples prepared by solid-state reaction, and quasi-epitaxially grow along a axis. With increasing the Fe content, the lattice parameters gradually increase due to the presence of oxygen vacancies introduced by Fe³⁺ and the larger size of the Fe³⁺ ion compared with the substituted Ti⁴⁺ ion. The magnetization measurements performed with a superconducting quantum interference device magnetometer indicate that Fe doping brings about ferromagnetic ordering at 2 K.2. Fe₃Si thin films were directly deposited on Si(100) substrates to form a ferromagnet-semiconductor structure, and different states of order were developed by changing substrate temperature. For the first time, the surface morphology, crystal structure, atomic configuration, magnetic and photoluminescent properties were studied on Fe₃Si/Si(100) heterostructures, establishing the dependence of magnetic and photoluminescent properties on structural order. The results show that over the whole range of substrate temperatures considered all films are of Fe₃Si single phase, highly oriented along the (220) plane, with no non-magnetic interface formed between films and substrates. With increasing the substrate temperature, the structural order type changes from A2 through B2 to DO₃ and the order degree gradually increases. The softest magnetic properties are obtained in the room temperature grown film with the highest saturation magnetization, the highest ratio of the residual magnetization to the saturation magnetization, and the lowest coercive force, where the saturation magnetization value nearly equals that of bulk DO₃-Fe₃Si. Meanwhile, Fe₃Si thin films exhibit strong fluorescent peaks in the ultraviolet wavelength range. With the increase of structural order degree, the ultraviolet peak positions remain unchanged while the intensities are gradually enhanced.