| Multiferroic composite materials, which simultaneously possess at least two of the ferroelectric, ferromagnetic, or ferroelastic orders, have recently attracted a great deal of attention in research for the magnetoelectric coupling effect. Bi Fe O3(BFO) is one of the most important multiferroics, owning to having multiferroic properties at room temperature. However, though it owns a large polarization ~ 100 μC/cm2, BFO has the week ferromagnetic for the special spiral magnetic structure. Aiming at the problem, many ferromagnetic materials were introduced in the composite system, such as Fe3O4, Co Fe2O4, Ni Fe2O4 and La0.65Sr0.35 Mn O3. Since multifunctional coupling, BFO based composite multiferroics can provide an extra degree of freedom in the design of new multistate memory devices, integrated circuit and functional sensors.In this work, Bi0.89Tb0.11Fe1-xTMxO3/Co Fe2O4(BTFTMO/CFO) thin films were prepared by the chemical solution deposition method. Effects of multi-elements co-doping and Co Fe2O4 layer on the crystal structure, the leakage current density, the valence state, dielectric, ferroelectric and ferromagnetic properties of the BFO thin films were investigated systematically and the results are as follows:(1) The influences of Tb doping on the crystal structure, leakage current density, dielectric, ferroelectric properties and the valence state of the double BFO film was investigated. The results show that there is the structural transition from the rhombohedra to the biphasic structure in the Bi0.89Tb0.11 Fe O3 film as well as less Fe2+ ions. The leakage current density is reduced and the ferroelectric properties have been significantly improved. All the results give rise to the superior ferroelectric properties(Pr ~ 88.1 μC/cm2, Ec ~ 541.1 k V/cm).(2) The crystal structure, defect chemistry, electric and magnetic properties of Bi0.89Tb0.11Fe1-xTMxO3(BTFTMO) films were characterized systematically. Tb and transition metal elements(Cr, Mn and Co) co-doping induce the structural transformation of the BFO film from the rhombohedral to the pseudo cubic phase with triclinic distortion. The well-distributed fine grains and the reduction of Fe2+ ions play important roles in the decreasing the leakage current. The Pr and Ec of the BTFCo O are 127.8 μC/cm2 and 468.1 k V/cm, which are higher than those of the BTFCr O(Pr ~ 61.3 μC/cm2, Ec ~ 718.2 k V/cm). Furthermore, the enhaced magnetic property of BTFCo O film(Mr ~ 0.8 emu/cm3) is twice of the BTFCr O and BTFMn O. In the co-doping system, Co2+ and Mn2+ doping is demonstrated dramatical improvement of ferroelectric and ferromagnetic behaviors.(3) Detailed investigations were made on the influences of B-sites co-doping(Mn-Co, Mn-Ni and Mn-Cu) on the structure change and the properties of the BFO films. It is revealed that a phase transition from rhombohedral to the pseudo cubic phase with triclinic distortion occurs in the B-sites co-doping system. The B-sites co-doping gives rise to the reduction of Fe2+ and the appearance of Mn3+, which decreases the defect concentration. It is confirmed by the low leakage current. The superior ferroelectric property is the greatest strength in various doping. Mn-Co co-doped BFO film owns the huge Pr ~ 197.8 μC/cm2. Being taken the effects of the leakage current into consideration, the accurate Pr is 146.2 μC/cm2, after leakage compensation.(4) With the introduction of the Co Fe2O4 layer, the Bi Fe O3/Co Fe2O4 film remains the same crystal structure of Bi Fe O3 phase in the top layer by the investigation of the structure. Owing to the low breakdown field strength and the high leakage current, the unsaturated ferroelectric hysteresis loops are observed. However, the Bi Fe O3/Co Fe2O4 film shows a well-saturated hysteresis loop with MS ~ 76.4 emu/cm3, Mr ~ 40.3 emu/cm3, Hc ~ 1350 Oe. The magnetism of the composite film is more 100 times than the single Bi Fe O3 film.(5) The structure, leakage current, ferroelectric and ferromagnetic behaviors of the Tb and transition metal elements(Cr, Mn and Co) co-doped BTFTMO/CFO composite films were investigated. The structural analysis reveals that the BTFTMO layers also own the pseudo cubic structure. The leakage current density of the BTFTMO/CFO film appears to be two orders lower than that of the BFO/CFO. Meanwhile, the ferroelectric properties of the BTFTMO/CFO films are far better than those of the BFO/CFO. The BTFMn Co O/CFO film shows the best multiferroic properties, with the Pr ~ 142.5 μC/cm2 and the Mr ~ 64.6 emu/cm3 at room temperature. Furthermore, a comparative study on the magnetic and electrical properties of Tb-Mn-Co co-doped BTFMn Co O and BTFMn Co O/CFO films was made. BTFMn Co O/ CFO film shows the low leakage current. In addition, BTFMn Co O and BTFMn Co O/CFO films also own the superior ferroelectric properties with Pr ~130.8 μC/cm2 and 173.0 μC/cm2, respectively. Simultaneously, their Mr are 0.7 emu/cm3 and 59.5 emu/cm3. |
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