Y. D. Yao

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 350u2005°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 850u2005°C in BFO upon heating. BFO–5%BT exhibits a R–C transition near 830u2005°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 150u2005°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 (xu2009=u20090.0, 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 8.2u2009×u200910-6 emu/g·Oe at room temperature. This work suggests that BTO-substitution can enhance dielectric response and reduce th...

Investigation on electromagnetic and microwave absorbing properties of La0.7Sr0.3MnO3−δ/carbon nanotube composites

In this study, the influence of the addition of carbon nanotube (CNT) (0, 0.5, 1.0, and 2.0 wtu2009%) on complex permittivity, complex permeability, and reflection loss for La0.7Sr0.3MnO3−δ (LSMO)/polymer composites was investigated. Microwave absorbing composites were prepared by molding and curing a mixture of LSMO powders, CNTs, and epoxy resin. The complex permittivity and complex permeability for LSMO/CNT composites were examined by a cavity perturbation technique. The experimental results showed that the complex permittivity increased with increasing CNT addition and the imaginary part of the permeability decreased with rising frequency. In the present study, a microwave absorbing composite filled with 80 wtu2009% LSOM powders and 2 wtu2009% CNTs exhibited the optimum performance, that is, the microwave absorbing peak was −22.85 dB at 9.5 GHz and the −10 dB absorbing bandwidth reached 3.3 GHz for the sample with a matching thickness of 3 mm.

Effect of initial stress/strain state on formation of (001) preferred orientation in L10 FePt thin films

The stress state of FePt thin films deposited at room temperature was controlled within the range from 1.01 GPa compressive to 0.18 GPa tensile before taking rapid thermal annealing (RTA). After the order–disorder transformation triggered by RTA at temperatures (Ta) from 650 to 800 °C for 5 min, the tensile-stressed films exhibit significant preferred orientation of (001) of L10 structure. However, the compressive-strained ones show isotropic texture. Strong (001) texture with high Lotgering orientation factor of 0.9 is obtained at Tau2009=u2009800 °C, resulting in enhanced perpendicular magnetic anisotropy. The results provide direct evidence of stress-induced (001) texture, which could be significant for future applications.

Magnetization Reversal Mechanism and Microstructure Refinement of the FePt (001) Nanogranular Films With SiO

Ordered FePt continuous thin films with an amorphous SiO2 single capping layer have been fabricated on MgO (001) single-crystal substrates by the electron beam deposition technique at 400degC in order to investigate the effects of SiO2 capping layer on microstructure and magnetic reversal process of the FePt (001) thin films. The formation of nanogranular-like FePt films was directly obtained with this process due to the interpenetration of SiO2 which has a lower surface energy than that of pure Fe or Pt. Studies of angular dependent coercivity show a tendency of a domain-wall motion weaken towards rotation of reverse-domain type upon thickness of SiO2 capping layer on the FePt thin films. The intergrain interaction was confirmed from the Kelly-Henkel (deltaM) plot that indicated the strong exchange coupling between neighboring grains in the FePt continuous films without SiO2 capping layer. On the other hand, negative deltaM value was obtained when the FePt films with SiO2 capping layer, indicating the SiO2 capping layer, can lead to the reduction of intergrain exchange coupling thus presence of dipole interaction in the SiO2/FePt nanocomposite thin film structures.

_{2}

In this paper, we report on domain wall (DW) pinning and depinning behavior using square notch and antinotch in NiFe nanowires. The presence of the DW in the samples was probed experimentally from focus magneto-optical Kerr effect (MOKE) measurement and anisotropic magnetoresistance (AMR). Pinning strengths as well as distribution of depinning field were measured. DW traps with antinotches were found to be more effective than notches, and the DW kept its structure during the depinning processes. Simulations of the magnetization orientation agreed with our results.

Capping Layer

Development of (001) texture in FePt thin films deposited on glass substrates with different thickness (t) treated by rapid thermal annealing (RTA) is studied. A critical thickness of 30 nm is characterized: below which the (001) preferred orientation of the films develops with increasing t ; when t >; 30 , the films become isotropic. Remarkable perpendicular anisotropy in magnetic properties is achieved in the 30 nm thick sample with the best (001) texture. Discontinuous changes are also observed in surface morphology, microstructure, magnetic domain structure, and internal stress. Direct evidences are presented relating the formation of (001) texture to abnormal grain growth. The internal stress/strain analysis indicates that the residual tensile (σ) stress is proportional to the degree of (001) preferred orientation. A large value of σ of about 8.9 GPa is obtained in the film with t = 30, suggesting the driving force forming the texture.

Magnetic Interaction in Domain Wall Depinning at Square Notch and Antinotch Traps

Room-temperature-deposited FePt thin films with thickness (t) ranged from 5 to 100 nm treated by rapid-thermal annealing (RTA) were studied. With annealing condition of 900°C for 60 seconds at heating rate of 80° C/sec., a metastable phase of FePt was observed in the films with t ≥ 40 nm. The phase is chemically ordered with a face-centered-cubic (fcc) structure. The lattice parameter of it is found identical to the planar spacing of L10 (100). The metastable structure is dominant in the film with t = 40 nm and gradually replaced by L10 phase with increasing t. The fcc phase is soft magnetic with saturation magnetization similar to that of disordered FePt. Drastic changes were also observed in surface morphology. The results infer the connection between the metastable transformation and internal strain. The formation of the fcc structure is sensitive to processing parameters except t . Slower heating rate, lower annealing temperature, and longer annealing time, tend to suppress the formation of it. The interesting findings provide additional knowledge for FePt thin films.

Critical Thickness of (001) Texture Induction in FePt Thin Films on Glass Substrates

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.

Rapid Thermal Annealing Induced Metastable Phase in FePt Thin Films

The microstructure, ordering parameter, and magnetic properties of multilayer [FePt(x)/Os]n films on glass substrate by dc-magnetron sputtering (with x being thickness in nm; Os with a fixed thickness 5 nm; n being the number of layers) have been studied as a function of the annealing temperatures between 300 and 900u2009°C. The grain size of multilayer films can be controlled by annealing temperature and thickness of the FePt layer with Os space layer. The coercivity as a function of the annealing temperature for samples with nu2009=u20091 and pure FePt behaves roughly saturated after annealing above 700u2009°C. However, for samples with nu2009>u20094 the value of Hc seems still increasing with increasing annealing temperature between 600 and 900u2009°C, and the ordering parameter decreases with increasing the number of Os layers. Our experimental results are reasonably well to describe the effect of strain-assisted transformation.