Feiming Bai

Destruction of spin cycloid in (111)c-oriented BiFeO3 thin films by epitiaxial constraint: Enhanced polarization and release of latent magnetization

In BiFeO3 films, it has been found that epitaxial constraint results in the destruction of a space modulated spin structure. For (111)c films, relative to corresponding bulk crystals, it is shown (i) that the induced magnetization is enhanced at low applied fields; (ii) that the polarization is dramatically enhanced; whereas, (iii) the lattice structure for (111)c films and crystals is nearly identical. Our results evidence that eptiaxial constraint induces a transition between cycloidal and homogeneous antiferromagnetic spin states, releasing a latent antiferromagnetic component locked within the cycloid.

Push-pull mode magnetostrictive/piezoelectric laminate composite with an enhanced magnetoelectric voltage coefficient

A magnetoelectric (ME) laminate composite consisting of a symmetric longitudinally poled piezoelectric Pb(Mg1∕3Nb2∕3)O3–PbTiO3 crystal and two longitudinally magnetized magnetostrictive Tb1−xDyxFe2 layers has been developed that has a notably superior ME voltage coefficient, relative to previous laminate configurations. The symmetric nature of the longitudinally poled piezoelectric layer allows for operation in a push-pull mode that optimizes elastic coupling between layers. Our small laminate has a giant ME voltage coefficient of ∼1.6V∕Oe at low frequencies, a significant enhancement of this coefficient to ∼20V∕Oe under resonance drive, and an exceptional low-level magnetic field sensitivity of ∼10−12T at f=f0.

X-ray and neutron diffraction investigations of the structural phase transformation sequence under electric field in 0.7Pb(Mg1∕3Nb2∕3)-0.3PbTiO3 crystal

The structural phase transformations of 0.7Pb(Mg1∕3Nb2∕3)O3-0.3PbTiO3 (PMN-30%PT) have been studied using x-ray diffraction (XRD) and neutron scattering as a function of temperature and electric field. We observe the phase transformational sequence (i) cubic (C)→ tetragonal (T)→ rhombohedral (R) in the zero-field-cooled (ZFC) condition; (ii) C→T→ monoclinic (MC)→ monoclinic (MA) in the field-cooled (FC) condition; and (iii) R→MA→MC→T with increasing field at fixed temperature beginning from the ZFC condition. Upon removal of the field, the MA phase is stable at room temperature in the FC condition, and also in the ZFC condition with increasing field. Several subtleties of our findings are discussed based on results from thermal expansion and dielectric measurements, including (i) the stability of the MA phase, (ii) a difference in lattice parameters between inside bulk and outside layer regions, and (iii) a difference in the phase transition temperature between XRD and dielectric data.The structural phase transformations of 0.7Pb(Mg1∕3Nb2∕3)O3-0.3PbTiO3 (PMN-30%PT) have been studied using x-ray diffraction (XRD) and neutron scattering as a function of temperature and electric field. We observe the phase transformational sequence (i) cubic (C)→ tetragonal (T)→ rhombohedral (R) in the zero-field-cooled (ZFC) condition; (ii) C→T→ monoclinic (MC)→ monoclinic (MA) in the field-cooled (FC) condition; and (iii) R→MA→MC→T with increasing field at fixed temperature beginning from the ZFC condition. Upon removal of the field, the MA phase is stable at room temperature in the FC condition, and also in the ZFC condition with increasing field. Several subtleties of our findings are discussed based on results from thermal expansion and dielectric measurements, including (i) the stability of the MA phase, (ii) a difference in lattice parameters between inside bulk and outside layer regions, and (iii) a difference in the phase transition temperature between XRD and dielectric data.

Domain hierarchy in annealed (001)-oriented Pb(Mg1∕3Nb2∕3)O3-x%PbTiO3 single crystals

The domain structures of annealed (001)-oriented Pb(Mg1∕3Nb2∕3)O3-x%PbTiO3(PMN-x%PT) crystals for x=10, 20, 30, 35, and 40at.% have been investigated by polarized optical microscopy and scanning force microscopy in the piezoresponse mode. The results demonstrate the presence of a domain hierarchy on various length scales ranging from 40nm to 0.1mm, which varies with x.

Fe–Ga/Pb(Mg1/3Nb2/3)O3–PbTiO3 magnetoelectric laminate composites

We have found large magnetoelectric (ME) effects in long-type laminate composites of Fe–20%Ga magnetostrictive alloys and piezoelectric Pb(Mg1∕3Nb2∕3)O3–PbTiO3 single crystals. At lower frequencies, the ME voltage coefficient of a laminate with longitudinally magnetized and longitudinally polarized (i.e., L-L mode) layers was 1.41V∕Oe (or 1.01V∕cmOe). Near the natural resonant frequency (∼91kHz) of the laminate, the ME voltage coefficients were found to be dramatically increased to 50.7V∕Oe (36.2V∕cmOe) for the L-L mode. In addition, the laminate can detect a minute magnetic field as low as ∼2×10−12T at resonance frequency, and ∼1×10−10T at lower frequencies.

Magnetostrictive and magnetoelectric behavior of Fe–20 at. % Ga/Pb(Zr,Ti)O3 laminates

The magnetostrictive and magnetoelectric (ME) properties of laminate composites of Fe–20at.%Ga and Pb(Zr,Ti)O3 (PZT) have been studied for laminates of different geometries. The results show that (i) a long-type magnetostrictive Fe–20at.%Ga crystal plate oriented along ⟨001⟩c and magnetized in its longitudinal (or length) direction has higher magnetostriction than a disk-type one; and consequently (ii) a long-type Fe–20at.%Ga∕PZT laminate has a giant ME effect, and is sensitive to low-level magnetic fields.

Domain engineered states over various length scales in (001)-oriented Pb(Mg1/3Nb2/3)O3-x%PbTiO3 crystals : Electrical history dependence of hierarchal domains

The dependence of the domain structure on poling for (001)-oriented Pb(Mg1∕3Nb2∕3)O3-x%PbTiO3 crystals with x=20, 30, 35, and 40 has been investigated by scanning force microscopy in the piezoresponse mode and by polarized optical microscopy. The results demonstrate a domain hierarchy on various length scales, ranging from nanometers to millimeters, which is notably dependent upon the electrical history of the specimen. The results provide important insights into the nature of the domain engineered state in these crystals.

Intermediate ferroelectric orthorhombic and monoclinic M B phases in [110] electric-field-cooled Pb ( Mg 1 ∕ 3 Nb 2 ∕ 3 ) O 3 –30% Pb Ti O 3 crystals

Office of Naval Research under Grant Nos. N000140210340, N000140210126, MURI N0000140110761

Magnetic and magnetoelectric properties of as-deposited and annealed BaTiO3-CoFe2O4 nanocomposite thin films

The magnetic and magnetoelectric (ME) properties of as-deposited and annealed BaTiO3?CoFe2O4 nanocomposite films with CoFe2O4 (CFO) nano-rods embedded in a BaTiO3 matrix have been studied and compared. The BaTiO3 phase in the as-deposited film changed from (0?0?1) preferred orientation to (1?0?0) orientation after annealing at 950??C for 2?h. In the meantime, the magnetization increased dramatically from 200 to 380?emu?cm?3. Distinct changes in magnetic contrast of the CFO nano-rods were observed by magnetic force microscopy under application of an electric field to the composite film. ME voltage coefficient as large as 4.9 ? 10?9?s?m?1 or 1.2?V?Oe?1?cm?1 was estimated for the annealed film under 10?MV?m?1, although the value was much lower for the as-deposited film. It is shown that higher ME effects can be found in films with good phase homogeneity, poly-domains and unrelaxed stress.

Low-loss NiCuZn ferrite with matching permeability and permittivity by two-step sintering process

Magneto-dielectric materials with matched permeability and permittivity are promising candidates as loading materials to reduce the physical dimensions of low-frequency antennas. In this study, NiCuZn ferrites were prepared by both the traditional final-stage sintering process and the two-step sintering process to obtain the magneto-dielectric materials. It was found that the samples sintered by the two-step sintering process tended to obtain microstructure with dense, homogeneous, and small average grain size, which was favorable to obtain low magnetic and dielectric losses. The sample sintered by the two-step sintering process with high temperature 950 °C and holding temperature 900 °C could obtain almost equal permeability and permittivity of around 11.8. And the magnetic and dielectric loss tangents were lower than 0.015 in a frequency range from 10 to 100 MHz. These properties make the material useful to the design of miniaturized antennas.