Y. Ding

Magnetoelectric coupling and phase transition in BiFeO3 and (BiFeO3)0.95(BaTiO3)0.05 ceramics

In situ high-resolution synchrotron x-ray diffraction reveals a local minimum in rhombohedral distortion angle αR (associated with an inflection in the lattice constant aR) near 400 and 350 °C in BiFeO3 (BFO) and (BiFeO3)0.95(BaTiO3)0.05 (BFO–5%BT), respectively. It suggests a coupling between ferroelectric and magnetic parameters near the antiferromagnetic–paramagnetic transition, which is responsible for the broad frequency-dependent dielectric maxima. A rhombohedral (R)–orthorhombic (O)–cubic (C) transition sequence takes place near 820 and 850 °C in BFO upon heating. BFO–5%BT exhibits a R–C transition near 830 °C. The BaTiO3 substitution can enhance dielectric and ferromagnetic responses and reduce electric leakage. The dielectric loss of BFO–5%BT remains less than 0.04 below 150 °C.

Structure, magnetic, and dielectric properties of (1-x)BiFeO3-xBaTiO3 ceramics

Structure, magnetization, and dielectric permittivity of (1-x)BiFeO3-xBaTiO3 (BFO-BT) ceramics have been studied as a function of BT content (x = 0.0, 0.1, 0.2, and 0.3). In situ synchrotron x-ray diffraction result of BFO reveals a rhombohedral (R)–orthorhombic (O)–cubic (C) phase transition near 820 and 850 °C upon heating. BFO-10%BT and BFO-20%BT exhibit a R–C transition near 760 and 740 °C, respectively. A C(R)–C transition takes place near 680 °C in BFO-30%BT. C(R) represents that a minor R phase coexists in the C matrix. A local minimum of R distortion angle αR occurs upon heating and implies ionic displacements. This anomaly is likely resulted from the antiferromagnetic (AFM)–paramagnetic (PM) transition and is responsible for the broad frequency-dependent dielectric maximum. BFO and BFO-10−30%BT ceramics exhibit a similar AFM behavior with magnetic susceptibility of about 8.2 × 10-6 emu/g·Oe at room temperature. This work suggests that BTO-substitution can enhance dielectric response and reduce th...

Magnetic properties and microstructure of Fe∕Pt multilayer films capped with SiO2 amorphous layer for magnetic recording use

Fe∕Pt multilayer films capped with an amorphous SiO2 layer have been fabricated in order to study the microstructure and magnetic properties via a molecular beam epitaxy technique at 400°C. The formation of granularlike FePt films was obtained with this process due to the interpenetration of SiO2 which has a lower surface energy. Studies of angular dependent coercivity show a tendency of a domain-wall motion weaken toward rotation of reverse-domain type upon thickness of SiO2 capping layer atop the Fe∕Pt multilayer films. The intergrain interaction was confirmed from the Kelly–Henkel plot that indicated the exchange coupling between neighboring grains in the uncapped FePt films. On the other hand, negative δM value was obtained when the FePt films capped with SiO2 layer, indicating that the SiO2 capping layer can lead to the reduction of intergrain exchange coupling in SiO2/FePt films.

Permittivity Enhancement of

Co inserted layer with different thickness on the permittivity of Ta₂O₅(60 nm)/Co(x nm)/Ta₂O₅(60 nm) trilayer films fabricated on the B270 glass substrates by the reactive sputtering technique was studied. The variation of dielectric constant of the samples is dependent on the thickness of the Co inserted layer observed from 1 kHz and 30 MHz. The dielectric constant of the Ta₂O₅(60 nm)/Co(x nm)/Ta₂O₅(60 nm) thin films was varied roughly from 7.9 up to 90.6 with the thickness (x) of the Co interlayer varied from 0 nm to 20 nm. The value of the dielectric constant is roughly near 7.9 for samples with x between 0 and 2 nm. However, it increases abruptly for samples with x larger than 2 nm, this large enhancement behavior of the dielectric constant could be explained due to the growth mechanism of the Cobalt inter-layer from island clusters to continuous Co layer for samples with x between 2 and 3 nm. For the magnetic induced ferroelectric variation, the variation of the dielectric constant increased with thickness of Co for samples with x larger than 3 nm. However this increase behavior is roughly saturated for applied magnetic field roughly above 20 Oe. This magnetic tunability of the dielectric constant is clearly attributed to the Co layer in the films. From the electric consideration, the adding of a Co inter-layer in Ta₂O₅/Co/Ta₂O₅ structures redistributes the interface charges between Co and Ta₂O₅ layers, and that enhances both the intrinsic polarization and its dielectric constant. The magnetoelectric properties in Ta₂O₅/Co/Ta₂O₅ films are manifested and it has potential for a ferroic sensor application.

\hbox{Ta}_{2}\hbox{O}_{5}/\hbox{Co/Ta}_{2}\hbox{O}_{5}

Dielectric permittivity and loss of BiFeO3 (BFO) and 5 mol% lanthanum-substituted BFO [(Bi0.95La0.05)FeO3 or BFO-5% La] ceramics have been carried out as functions of temperature and frequency. A frequency-dependent and broad dielectric shoulder and maximum were observed in BFO and BFO-5% La near 600-700 K. These dielectric responses are likely due to the magnetoelectric coupling while the antiferromagnetic-paramagnetic transition takes place near the Néel temperature. As an approximation, a barrier model with intrinsic barriers B (in temperature unit) every lattice constant a and extrinsic barriers B +Δ every distance d is introduced to describe the low-frequency upturn in dielectric loss in the high-temperature region. Good qualitative fits are obtained for BFO and BFO-5% La. This work suggests that 5 mol% La substitution can enhance dielectric response and considerably reduce electric conductivity.

Trilayer Films

Magnetization, grain size, structure, and temperature-dependent dielectric permittivity of BiFeO3, (BiFeO3,0.95(BaTiO3)0.05 -(BaTiO3)0.05(5%BTO-BFO), and (Bi0.95Ba0.05)(Fe0.95Ti0.05)O3[5%(Ba, Ti)-BFO] ceramics have been carried out. BFO and 5%BTO-BFO exhibit an typical antiferromagnetic behavior, whose magnetization curve is linear with magnetic field at room temperature. The magnetic hysteresis loop of 5%(Ba,Ti)-BFO ceramic shows a weak ferromagnetic behavior with a small remanent magnetization of about 0.01 emu/g possibly due to a higher density in the grain matrix. This result also implies that the A-and B-site substitutions with Ba2+ and Ti4+ could efficiently suppress the spiral magnetic modulation. The magnetic susceptibilities of BFO, 5%BTO-BFO, and 5%(Ba,Ti)-BFO ceramics are about 1.9 × 10-5, 5.43 × 10-6, and 8.67 × 10-6 emu/gOe, respectively. Frequency-dependent dielectric maxima were observed in BFO, 5%BTO-BFO, and 5%(Ba,Ti)-BFO ceramics upon heating. This dielectric response is likely triggered by the antiferromagnetic-paramagnetic transition near the Neel temperature.

Dielectric Permittivity and Magnetoelectric Coupling in Multiferroic

The effect of Co inserted layer with thickness below 20 nm on the dielectric permittivity of SiO2 (60 nm)/Co(x nm)/SiO2 (60 nm) thin films fabricated on glass B270 substrates by the reactive sputtering technique was studied. The dielectric constant is around 7.4 for B270 glass substrate and SiO2 /B270 film. However, it is rapidly raised up to roughly 55 for all the SiO2/Co/SiO2 samples with the thickness of Co inserted layer larger than 2 nm. From the cross section TEM pictures of the SiO2 /Co/SiO2 films with 1 and 2 nm Co thickness, we have experimentally demonstrated that this enhancement behavior of the dielectric constant is due to the growth mechanism of the Cobalt inter-layer from island clusters to continuous Co layer for samples with x larger than 2 nm. The adding of a Co inter-layer redistributes the interface charges between Co and SiO 2 layers, and that enhances both the intrinsic polarization and its dielectric constant. For the magnetic induced dielectric variation, the variation of the dielectric constant also increased with thickness of Co for samples with x larger than 2 nm. A direct observation of a 0.04-0.20% dielectric variation is induced by external magnetic field. However this increase behavior is roughly saturated for applied magnetic field roughly above 60 Oe. The magnetodielectric properties in SiO2/Co/SiO2 films are manifested and it has potential for a ferroic sensor application.

{\hbox {BiFeO}}_{3}

In situ X-ray diffraction of BiFeO3 (BFO) ceramic and thin film has been carried out as a function of temperature (25degC-900degC) upon heating. The BFO thin film was deposited on Pt/TiOx/SiOx/Si(100) substrate by radio-frequency magnetron sputtering at 650degC. BFO thin film exhibits a structural transformation from rhombohedral to cubic in the region of 700-750degC upon heating. The magnetic property of BiFeO3/Co/BiFeO3 multilayer structure changes from diamagnetic to ferromagnetic phases with increasing thickness of Co layer. The dielectric permittivities (real and imaginary parts) of BiFeO3 ceramic exhibit a frequency-dependent relaxation-like behavior and its magnitude (real part) is higher than 1000 for f les 1 MHz. Surface morphology shows that the root-mean-square roughness of BFO thin film varies with sputtering power.

and

Phase transitions, dielectric permittivity, and conductivity of (Bi1-xBax)(Fe1-xTix)O3 (x = 0.05 and 0.1) [BFO-(Ba,Ti)] multiferroic ceramics have been studied as functions of temperature and frequency. In situ synchrotron x-ray diffraction revealed rhombohedral–cubic transitions in the temperature ranges 760–780°C in BFO-5%(Ba,Ti), and 720–750°C in BFO-10%(Ba,Ti). A one-dimensional barrier model with intrinsic barriers B every lattice constant a and extrinsic barriers B + Δ is introduced to describe the dielectric response and conductivity. This work revealed that (Ba,Ti) substitutions can enhance the intrinsic barriers and reduce the hopping rate of charge carriers, thereby giving the desired effect of decreasing the conductivity.

({\hbox {Bi}}_{0.95}{\hbox {La}}_{0.05}){\hbox {FeO}}_{3}

Co and Ta inserted layers with thickness x varied from 0 to 20 nm on the dielectric permittivity of BaTiO3 (60 nm)/Co(x nm)/BaTiO3 (60 nm) (BCB) and BaTiO3 (60 nm)/Ta(x nm)/BaTiO3 (60 nm) (BTB) films fabricated on glass B270 substrates by the reactive sputtering technique was studied. The dielectric constant is around 7.6 for B270 glass substrate and BaTiO3 /B270 film. However, it is rapidly raised up to roughly 84.3-89.4 for BCB, and 75.5-88.2 for BTB samples with the thickness of the Co or Ta inserted layer larger than 2 nm. This large enhancement of the dielectric constant could be explained due to the growth mechanism of the metallic inserted layer from island clusters to continuous layer for samples with x roughly near 3 nm, and the conducting property of the metallic inserted layer. The adding of a Co or Ta interlayer redistributes the interface charges between metal and ferroelectric layers, and that enhances both the intrinsic polarization and its dielectric constant. For the magnetic induced dielectric variation, after comparing the variation of the dielectric properties of both Co and Ta inter layers, the magneto dielectric properties in BaTiO3 /Co/BaTiO3 films are manifested. This magnetic tunability of the dielectric constant is clearly attributed to the magnetic metallic Co layer in the films. From this study, the magneto dielectric properties are manifested in BTO/Co/BTO films and it has the potential for a ferroic sensor application.