The study of the electric and magnetic properties of PbZr0.2Ti0.8O3-BiFeO3 multilayers

The results of the electric and magnetic measurements performed on PbZr_0.2Ti_0.8O₃-BiFeO₃ symmetric structures, deposited on Pt/Si wafers, were compared for different number of layers in order to analyse the effect of interfaces over the macroscopic properties. It was found that the shape and magnitude of the capacitance-voltage characteristic, as well as the shape and parameters of the ferroelectric and magnetic hysteresis, depend on the number of interfaces in the intended multilayer structure. A temperature induced gradual transition from a magnetically disordered spin glass like phase of low temperature to an uncompensated antiferromagnetic phase at room temperature takes place in the BiFeO₃ films, under low applied magnetic fields. A partial ferromagnetic like order can be obtained at low temperatures by increasing the field. The observed changes in the electric and magnetic behaviour of the systems were related to an increased degree of disorder for electric dipoles and magnetic moments, due to the increased number of layers and crystallization treatments.     

Liquid-phase sintered bismuth ferrite multiferroics and their giant dielectric constant

Rapid liquid-phase sintering method has been modified and used to synthesize Bi_1-xLa_xFeO_3-δ compositions (0 ≤ x ≤ 0.5). The temperature and sintering procedures to obtain stable single-phase samples have been defined. This provides a significant reduction in the synthesis time, which is especially important for mass production. Furthermore, giant dielectric constants have been observed in Bi_1-xLa_xFeO_3-δ compositions, the values of which are one order of magnitude larger than those of similar Bi-based multiferroics obtained by conventional methods. It potentially makes them one of the most promising materials for modern technological applications.     

Multiferroic Bi0.7Dy0.3FeO3 thin films directly integrated on Si for integrated circuit compatible devices

Magnetoelectric multiferroic Bi_0.7Dy_0.3FeO₃ (BDFO) thin films deposited on p-type Si (100) substrate using pulsed laser deposition technique demonstrated a saturated ferroelectric and ferromagnetic hysteresis loop at room temperature. More interestingly, the observed change in electric polarization with applied magnetic field in these films indicated the presence of room temperature magnetoelectric coupling behavior. Using high-frequency capacitance-voltage measurements, the fixed oxide charge density, interface trap density and dielectric constant were estimated on Au/BDFO/Si capacitors. These results suggest the integrated circuit compatible application potential of BDFO films in the field of micro-electro-mechanical systems and non-volatile memories.     

Room-temperature magnetic control of ferroelectric polarization and enhanced ferromagnetic properties in 0.8BiFeO3-BaTiO3 ceramic

A multiferroic 0.8BiFeO₃ ??0.2BaTiO₃ (0.8BF-BaT) ceramic was prepared by solid-state reaction to study the magnetic control of its ferroelectric polarization and ferromagnetic properties. The influence of the magnetic field H on the ferroelectric polarization of 0.8BF-BaT ceramic was investigated. The maximum displacement current density Jm and the remanent polarization Pr of 0.8BF-BaT increased by 35% and 20% respectively after it being magnetized, indicating that the polarization was suppressed in the magnetic domain walls. Under a magnetic field of H?=?0.7?T, Jm and Pr decreased by 6.1% and 6.4% respectively, revealing coupling between polarization P and magnetic field H. The magnetic properties of BiFeO₃ were improved by introducing of BaTiO₃ and the magnetic Néel temperature T_N increased by 70?K because of structural effects.     

A new multiferroic ternary solid solution system: NdFeO3–Pb(Fe1/2Nb1/2)O3–PbTiO3

Ceramic materials were sintered from powders of the NdFeO₃–Pb(Fe_1/2Nb_1/2)O₃–PbTiO₃ (NF–PFN–PT) ternary system synthesized by the conventional solid reaction method and their multiferroic properties investigated. The structure, electric and magnetic properties of the ternary system have been investigated. The introduction of Pb(Fe_1/2Nb_1/2)O₃ into the NdFeO₃–PbTiO₃ binary system can effectively increase its electric properties. The ternary system exhibits enhanced piezoelectric property with optimal piezoelectric constants d₃₃=143 pC/N, reduced coercive fields E_C=5.78 kV/cm and remnant polarization P_r=12.8 μC/cm² for 0.10NF–0.56PFN–0.34PT, near tetragonal phase region. The Curie temperature (T_C) of the NdFeO₃–Pb(Fe_1/2Nb_1/2)O₃–PbTiO₃ ceramics varies in the range from 108.7 °C to 67.9 °C. The magnetic hysteresis loops show that the ternary system is paramagnetic originating from canting of paramagnetic sublattices in NF–PFN–PT, due to the rare earth ions Nd³⁺ influencing on the exchange interaction between Fe³⁺ ions at the octahedral sites.     

Transition metal modified bulk BiFeO3 with improved magnetization and linear magneto-electric coupling

At present BiFeO₃ (BFO) is the most attractive and sole example, which possesses low magnetization value, high leakage current and low polarization in ceramic form. Single-phase room temperature multiferroics are rare in nature. This paper deals with the improved magnetic and observed linear magneto-electric coupling in Co and Sm co-doped BiFeO₃ ceramics synthesized by sol-gel process at low temperature ∼600 °C. As synthesized Bi_0.9Sm_0.10Fe_0.95Co_0.05O₃ (BSFCO) showed high impurities phases (20%) over wide range of calcination temperatures. Impurity phases reduced drastically from 20% to 5% after leaching with nitric acid. However the electrical and the magnetic properties were almost the same for both phases. Well-defined magnetic hysteresis with high magnetic moment was found at room temperature. Ferroelectric polarization studies demonstrated similar values and shape as reported in literature for the pure bulk BFO. Linear magneto-electric (ME) coupling and weak ME coefficient (α) ∼ 0.6 e−10 s m⁻¹ were observed in the co-doped BFO. The origin of the strong ferromagnetic property in our samples may be due to the presence of rare earth and transition metal ions at the lattice sites of BFO or due to impurity phase, since we have not seen any change in magnetization with reduction of impurity phase the later effect is more unlikely.     

Structural evolution and coexistence of ferroelectricity and antiferromagnetism in Fe,Nb co-doped BaTiO3 ceramics

Multiferroics are materials that exhibit two or more primary ferroic properties within the same phase and have potential applications in sensors, spintronics and memory devices. Here, the dielectric, ferroelectric and magnetic properties of novel multiferroics derived from BaTi_1?x(Fe_0.5Nb_0.5)_xO₃ (BTFN, 0.01 ≤ x ≤ 0.10) ceramics are investigated. Multiferroism in these ceramics is manifested by the coexistence of ferroelectric long-range ordering and antiferromagnetism. With increasing x-value, there is a structural evolution from a tetragonal perovskite to a mixture of tetragonal and cubic phases, accompanied by a decrease in the temperature of maximum permittivity. At room temperature, ferroelectric behaviour is evidenced by the presence of current peaks corresponding to domain switching in the current-electric field loops, while the observation of non-linear narrow magnetic hysteresis loops suggests dilute magnetism. The results indicate that in the x = 0.07 composition the antiferromagnetic order is established through an indirect super-exchange interaction between adjacent Fe ions.     

Dielectric properties and magnetoelectric coupling in polar magnet (Fe,Zn)2Mo3O8

A complete understanding of magnetoelectric (ME) coupling in polar magnet Fe₂Mo₃O₈ (FMO) remains elusive. Here, we report the magnetodielectric (MD) and ME effects of (Fe_1-xZn_x)₂Mo₃O₈ (x = 0, 0.05) single crystals. The observation of MD effect in FMO shows a sharp peak once T is below T_N due to the metamagnetic transition, which is converted into an obvious step upon cooling. Meanwhile, the change of magnetization M modulated by electric field E is striking at the dielectric constant peak/step. In (Fe_0.95Zn_0.05)₂Mo₃O₈, however, the sharp peak at just below T_N is intact, while the step at much lower T_N is highly damaged. The ME response to 5% Zn²⁺ dopants at the two T-ranges shows different behavior. Combining with spin wave theory, it is possible to speculate that the inverse Dzyaloshinskii-Moriya interaction might play a dominant role in the ME nature at just below T_N, which is different from that at much lower T_N.     

Multiferroic properties of Co and Nd co-substituted Bi5Ti3FeO15 thin films

Co and Nd co-substituted Bi₅Ti₃FeO₁₅ thin films were prepared on Pt/Ti/SiO₂/Si substrates via metal organic decomposition method. The structural and multiferroic properties of the films were investigated. It was found that Co ions enter into the lattice and occupy the Fe site. The Bi_4.15Nd_0.85Ti₃Fe_0.5Co_0.5O₁₅ films simultaneously exhibit ferroelectric and ferromagnetic properties at room temperature, and its 2P_r and 2M_r are 38 μC/cm² and 3 kA/m, respectively. Moreover, substitutions create local ferromagnetic order and antiferromagnetic order depending on whether the local bonding is FeOCo or FeOFe/CoOCo, respectively. The competing interaction of the ferromagnetic and antiferromagnetic phases results in an interesting magnetic behavior of the films.     

Crystal texture selection in epitaxies of orthorhombic antiferromagnetic YMnO3 films

The orthorhombic phase of YMnO₃ has been epitaxially stabilized on SrTiO₃ single crystal substrates. Changing the substrate orientation, the out-of-plane orientation of the epitaxial films can be tuned to (001), (101), or (100). Depending on the orientation, the films present varied crystal domain structures and lattice strains. Single domain YMnO₃(100) films have been grown on SrTiO₃(110) substrates, whereas they present two and three crystal variants on SrTiO₃(001) and SrTiO₃(111), respectively. Epitaxial stress in each domain is anisotropic, tensile in one direction and compressive in the other. It has consequences in the texture selection when domains with different out-of-plane orientation can grow epitaxially. Antiferromagnetism of YMnO₃ films has been confirmed by measuring the exchange bias field induced in an underlying ferromagnetic SrRuO₃ layer.