| Multiferroics, owing to their various domain structures, coupling of charge, orbits, spin and crystal lattice and coexistence of (anti)ferromagnetic, (anti)ferroelectric and magnetoelectric (ME) properties, have widely prospective applications in storage fields and functional electronic devices, and are becoming hot spots in the research of materials and condensed matter physics in recent years. In this paper, the giant magnetoresistance, giant magnetostrictive ferromagnetic materials La0.7Ba0.3MnO3 (LBMO) and ferroelectric materials BaTiO3 (BTO) with the same perovskite structure were sintered together to prepare the multiferroic composited ceramics. With the increasing of sintering temperature, the ceramics transformed from the coexistence of LBMO and BTO to the solid solutions structured in hexagonal and tetragonal phase and then to the single tetragonal solid solutions. The paper mainly focused on the ionic composited solid solutions and the structural composites, discussing the origin of multiferroics and the mechanisms of coupling effects, respectively. The main conclusions were as follows.With the increasing content of LBMO, the electrical insulating property of the single tetragonal solid solutions decreased. The samples with hexagonal phase exhibited relative large ME effects. For the xLBMO-(1-x)BaTiO3 (x=0.08 wt.) solid solution ceramic sintered at 1475?, the ?Pm and ?Pr were-12.3% and-11.7%, respectively, with the measured maximum ME coefficient was 48.6mV/cmOe. The further research indicated that with the reduction of A site ion radii, the mole ratios of hexagonal phase were reduced (34%,18%,10% for L4MO-BTO (A=Ba?Sr?Ca), respectively), however, the ferroelectric, dielectric and magnetic features of the three solid solutions were enhanced. The three solid solutions exhibited intrinsic MD effects, the values at 10 MHz frequency were 3.37%?3.21% and 2.65%, respectively.On the basis of the experimental results, combined with the first-principles calculations, the coexistence mechanism of structure asymmetry and B site magnetic ions in solid solutions was deeply discussed. The first-principles calculations indicated that when the hexagonal solid solution formed, Mn occupied the asymmetric site preferentially, which satisfied the structural conditions of multiferroics that the asymmetry of structure and B site magnetic ion should coexist. Both the ferroelectric and magnetic properties were originated from B site ion, exhibiting intrinsic ME effects. The further calculations indicated that with the reduction of A site ion radii, the asymmetry of coplanar octahedrons aggravated, lead to the increment in electric dipole moment and enhancement in ferroelectric and dielectric features, which were corresponding to the experimental data.The structural particulate-based xLBMO-(1-x)BaTiO3 (x=0.10 wt.) composite ceramics exhibited a higher ME effects (?Pr=-21.1%, ?Ec=-9.2%) than the solid solutions, and negative magnetic resistance (MR) and (MD) effects as well. The core-shell structured LBMO@BTO ceramics which prepared by the soft-chemistry method exhibited the strongest ME effects (?Pr=-39.4%,?Ec=-16.2%) belonging to the higher contents of FM phase and tighter interface coupling in the core-shell structure. And the core-shell structure sample exhibited positive MR and negative MD effects.Regard to the structural composited multiferroics, the ME effects was originated from the stress coupling. And the more compact interface can induce larger ME effects. For the particulate-based LBMO-BTO composite ceramics, tunneling mechanism of carrier dominated.When the magnetic field applied, both the Spin-polarized Tunneling Effect at grain boundary and the Double-Exchange effect between Mn3+and Mn4+in the LBMO grain caused the reduction of sample's impedance, resulted in the negative MR, MD and ME effects. For the core-shell structured sample, the magnetic field induced increment of conductance at higher frequency was similar to the particulate-based sample, which owing to the Double-Exchange effect between Mn3+and Mn4+. However, the reduction of conductance at lower frequency was due to the restriction of electron tunneling effects and the domination of scattering mechanism of carriers in the core-shell structure. Becauses of the magnetostriction, the strain of interface and the deformation of the grain were induced, resulted in the enhancements of rough interface scattering and acoustic phonon scattering, which reduced the conductance, and led to the positive MR and negative MD, ME effects. |
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