A Novel Spinel Ferrite-Hexagonal Ferrite Composite for Enhanced Magneto-Electric Coupling in a Bilayer with PZT.

The magnetoelectric effect (ME) is an important strain mediated-phenomenon in a ferromagnetic-piezoelectric composite for a variety of sensors and signal processing devices. A bias magnetic field, in general, is essential to realize a strong ME coupling in most composites. Magnetic phases with (i) high magnetostriction for strong piezomagnetic coupling and (ii) large anisotropy field that acts as a built-in bias field are preferred so that miniature, ME composite-based devices can operate without the need for an external magnetic field. We are able to realize such a magnetic phase with a composite of (i) barium hexaferrite (BaM) with high magnetocrystalline anisotropy field and (ii) nickel ferrite (NFO) with high magnetostriction. The BNx composites, with (100 - x) wt.% of BaM and x wt.% NFO, for x = 0-100, were prepared. X-ray diffraction analysis shows that the composites did not contain any impurity phases. Scanning electron microscopy images revealed that, with an increase in NFO content, hexagonal BaM grains become prominent, leading to a large anisotropy field. The room temperature saturation magnetization showed a general increase with increasing BaM content in the composites. NFO rich composites with x ≥ 60 were found to have a large magnetostriction value of around -23 ppm, comparable to pure NFO. The anisotropy field HA of the composites, determined from magnetization and ferromagnetic resonance (FMR) measurements, increased with increasing NFO content and reached a maximum of 7.77 kOe for x = 75. The BNx composite was cut into rectangular platelets and bonded with PZT to form the bilayers. ME voltage coefficient (MEVC) measurements at low frequencies and at mechanical resonance showed strong coupling at zero bias for samples with x ≥ 33. This large in-plane HA acted as a built-in field for strong ME effects under zero external bias in the bilayers. The highest zero-bias MEVC of ~22 mV/cm Oe was obtained for BN75-PZT bilayers wherein BN75 also has the highest HA. The Bilayer of BN95-PZT showed a maximum MEVC ~992 mV/cm Oe at electromechanical resonance at 59 kHz. The use of hexaferrite-spinel ferrite composite to achieve strong zero-bias ME coupling in bilayers with PZT is significant for applications related to energy harvesting, sensors, and high frequency devices.

Electric field tuning of a nickel zinc ferrite resonator by non-linear magnetoelectric effects.

The nature of nonlinear magnetoelectric (NLME) effect has been investigated at room-temperature in a single-crystal Zn substituted nickel ferrite. Tuning of the frequency of magnetostatic surface wave (MSSW) modes under an applied pulsed DC electric field/current has been utilized to probe the effect. The frequencies of the modes at 8-20 GHz were found to decrease by ~ 400 MHz for an applied DC power P of ~ 100 mW and the frequency shift was the same for all of the MSSW modes and linearly proportional to P. A model is proposed for the effect and the NLME phenomenon was interpreted in terms of a reduction in the saturation magnetization due to the DC current. The decrease of magnetization with applied electric power, estimated from data on mode frequency versus P, was - 2.50 G/mW. The frequency tuning efficiency of the MSSW modes due to NLME effects in the ferrite resonator was found to be 4.1 MHz/mW which is an order of magnitude higher than the shift reported for M-type strontium and barium hexaferrite resonators investigated earlier. The spinel ferrite resonator discussed here has the potential for miniature, electric field tunable, planar microwave devices for the 8-20 GHz frequency range.

Y-type hexagonal ferrite-based band-pass filter with dual magnetic and electric field tunability.

This work is on the design, fabrication and characterization of a hexagonal ferrite band-pass filter that can be tuned either with a magnetic field or an electric field. The filter operation is based on a straight-edge Y-type hexagonal ferrite resonator symmetrically coupled to the input and output microstrip transmission lines. The Zn2Yfilter demonstrated magnetic field tunability in the 8-12 GHz frequency range by applying an in-plane bias magnetic field H0 provided by a built-in permanent magnet. The insertion loss and 3 dB bandwidth within this band were 8.6 ± 0.4 dB and 350 ± 40 MHz, respectively. The electric field E tunability of the pass-band of the device was facilitated by the nonlinear magnetoelectric effect (NLME) in the ferrite. The E-tuning of the center frequency of the filter by (1150 ± 90) MHz was obtained for an input DC electric power of 200 mW. With efforts directed at a significant reduction in the insertion loss, the compact and power efficient magnetic and electric field tunable Zn2Y band-pass filter has the potential for use in novel reconfigurable RF/microwave devices and communication systems.

Strain induced anisotropy in liquid phase epitaxy grown nickel ferrite on magnesium gallate substrates.

This work focuses on the nature of magnetic anisotropy in 2.5-16 micron thick films of nickel ferrite (NFO) grown by liquid phase epitaxy (LPE). The technique, ideal for rapid growth of epitaxial oxide films, was utilized for films on (100) and (110) substrates of magnesium gallate (MGO). The motivation was to investigate the dependence of the growth induced anisotropy field on film thickness since submicron films of NFO were reported to show a very high anisotropy. The films grown at 850-875 C and subsequently annealed at 1000 C were found to be epitaxial, with the out-of-plane lattice constant showing unanticipated decrease with increasing film thickness and the estimated in-plane lattice constant increasing with the film thickness. The uniaxial anisotropy field Hσ, estimated from X-ray diffraction data, ranged from 2.8-7.7 kOe with the films on (100) MGO having a higher Hσ value than for the films on (110) MGO. Ferromagnetic resonance (FMR) measurements for in-plane and out-of-plane static magnetic field were utilized to determine both the magnetocrystalline the anisotropy field H4 and the uniaxial anisotropy field Ha. Values of H4 range from -0.24 to -0.86 kOe. The uniaxial anisotropy field Ha was an order of magnitude smaller than Hσ and it decreased with increasing film thickness for NFO films on (100) MGO, but Ha increased with film thickness for films on (110) MGO substrates. These observations indicate that the origin of the induced anisotropy could be attributed to several factors including (i) strain due to mismatch in the film-substrate lattice constants, (ii) possible variations in the bond lengths and bond angles in NFO during the growth process, and (iii) the strain arising from mismatch in the thermal expansion coefficients of the film and the substrate due to the high growth and annealing temperatures involved in the LPE technique. The LPE films of NFO on MGO substrates studied in this work are of interest for use in high frequency devices.

High-resolution magnetic sensors in ferrite/piezoelectric heterostructure with giant magnetodielectric effect at zero bias field.

A dielectric AC magnetic sensor in layered ferrites/piezoelectric composites was fabricated and developed, whereby its high magnetodielectric (MDE) effects, the typical magnetic-sensing parameters, were systematically characterized at zero bias. Polycrystalline ferrites were synthesized by the solid-state sintering technique with a composition of Ni0.7Zn0.3Tb0.02Fe1.98O4, and the desired spinel structure and soft magnetic properties were confirmed by x-ray diffraction and VSM, respectively. The field-induced charge order insulating state in piezoelectric ceramics accounts for the suppressed permittivity, which enables the possibility of a highly sensitive magnetic sensor at zero bias field. Experimental results exhibit that a small variation in H as low as 100 mOe can be clearly distinguished with a favorable nonlinearity of 2.24%. Meanwhile, the output stability of the presented sensor under 2h of constant and continuous excitation was tested within a favorable fluctuating tolerance range of 6.14-6.28 nF, and the estimated uncertainty of ∼0.063 038 nF was verified by statistical analysis. The presented ferrite/piezoelectric magnetic sensors exhibiting a high MDE response without the requirement for an external magnetic bias are of importance for use in bio-magnetic field detection due to metrics of miniaturization, high sensitivity, and favorable stabilities.

Converse magneto-electric effects in a core-shell multiferroic nanofiber by electric field tuning of ferromagnetic resonance.

This report is on studies directed at the nature of magneto-electric (ME) coupling by ferromagnetic resonance (FMR) under an electric field in a coaxial nanofiber of nickel ferrite (NFO) and lead zirconate titanate (PZT). Fibers with ferrite cores and PZT shells were prepared by electrospinning. The core-shell structure of annealed fibers was confirmed by electron- and scanning probe microscopy. For studies on converse ME effects, i.e., the magnetic response of the fibers to an applied electric field, FMR measurements were done on a single fiber with a near-field scanning microwave microscope (NSMM) at 5-10 GHz by obtaining profiles of both amplitude and phase of the complex scattering parameter S11 as a function of bias magnetic field. The strength of the voltage-ME coupling Av was determined from the shift in the resonance field Hr for bias voltage of V = 0-7 V applied to the fiber. The coefficient Av for the NFO core/PZT shell structure was estimated to be - 1.92 kA/Vm (- 24 Oe/V). A model was developed for the converse ME effects in the fibers and the theoretical estimates are in good agreement with the data.

Probing magnon-magnon coupling in exchange coupled Y[Formula: see text]Fe[Formula: see text]O[Formula: see text]/Permalloy bilayers with magneto-optical effects.

We demonstrate the magnetically-induced transparency (MIT) effect in Y[Formula: see text]Fe[Formula: see text]O[Formula: see text](YIG)/Permalloy (Py) coupled bilayers. The measurement is achieved via a heterodyne detection of the coupled magnetization dynamics using a single wavelength that probes the magneto-optical Kerr and Faraday effects of Py and YIG, respectively. Clear features of the MIT effect are evident from the deeply modulated ferromagnetic resonance of Py due to the perpendicular-standing-spin-wave of YIG. We develop a phenomenological model that nicely reproduces the experimental results including the induced amplitude and phase evolution caused by the magnon-magnon coupling. Our work offers a new route towards studying phase-resolved spin dynamics and hybrid magnonic systems.

Highly efficient power conversion in magnetoelectric gyrators with high quality factor.

A high-Q magnetoelectric (ME) gyrator consisting of a trilayer laminate of nickel-iron-based constant elasticity alloy (Ni-Fe-Cr) and lead zirconate titanate with a coil wound around it has been developed and systematically characterized. Highly efficient magneto-mechanical-electric conversion can be achieved by means of the combination contributions of high quality factors from individuals, and much energy can be transferred through the gyration device. Under an electromechanical resonance frequency of 54.04 kHz, experimental results show that maximum efficiency reaches as high as 88.5% under an extremely low input density of 3.31 µW/cm3 with an optimum load resistance of 9.6 kΩ and a magnetic bias of 66 Oe. Such a highly efficient ME gyrator with a high Q factor can be beneficial or degrade the design goals that are likely to be achievable for practical applications in compact power transfer electronic devices.

Studies of Multiferroic Palladium Perovskites.

We have studied the atomic force microscopy (AFM), X-ray Bragg reflections, X-ray absorption spectra (XAS) of the Pd L-edge, Scanning electron microscopey (SEM) and Raman spectra, and direct magnetoelectric tensor of Pd-substituted lead titanate and lead zirconate-titanate. A primary aim is to determine the percentage of Pd+4 and Pd+2 substitutional at the Ti-sites (we find that it is almost fully substitutional). The atomic force microscopy data uniquely reveal a surprise: both threefold vertical (polarized out-of-plane) and fourfold in-plane domain vertices. This is discussed in terms of the general rules for Voronoi patterns (Dirichlet tessellations) in two and three dimensions. At high pressures Raman soft modes are observed, as in pure lead titanate, and X-ray diffraction (XRD) indicates a nearly second-order displacive phase transition. However, two or three transitions are involved: First, there are anomalies in c/a ratio and Raman spectra at low pressures (P = 1 - 2 GPa); and second, the c/a ratio reaches unity at ca. P = 10 GPa, where a monoclinic (Mc) but metrically cubic transition occurs from the ambient tetragonal P4 mm structure in pure PbTiO3; whereas the Raman lines (forbidden in the cubic phase) remain until ca. 17 GPa, where a monoclinic-cubic transition is known in lead titanate.

Self-assembly of multiferroic core-shell composites using DNA functionalized nanoparticles.

Ferrite-ferroelectric core-shell nanoparticles were prepared by deoxyribonucleic acid (DNA) assisted self-assembly and the strained mediated magneto-electric (ME) interactions between the ferroic phases were studied. The nanoparticle type and size were varied and the DNA linker sequence was also varied. Two kinds of particles, one with 600 nm barium titanate (BTO) core and 200 nm nickel ferrite (NFO) shell and another with 200 nm BTO core and 50 nm nickel cobalt ferrite (NCFO) shell were prepared. The particles were linked by three different oligomeric DNA containing 19, 18 or 30 base pairs. The core-shell structure was evident from electron microscopy and scanning microwave microscopy images. Films and disks of the core-shell particles were assembled in a magnetic field and used for measurements of low frequency ME voltage coefficient (MEVC) and magnet-dielectric effect. The MEVC data on films indicate that particles assembled with DNA with 30 base pairs exhibit the strongest ME coupling suggesting a more fully integrated heterogenous nanocomposite and the weakest interaction for DNA with 18 base pairs. These results indicate that the longer linker region in DNA is the key factor for forming better composites. This result may be due to the irregular shape of the nanoparticles. Longer DNA strands would be able to bridge better generating more linkages. Shorter strands would not able to bridge the irregularly shaped particles as well and therefore result in linkages and less heterogeneity in the composites.

Switchable 3-0 magnetoelectric nanocomposite thin film with high coupling.

A mixed precursor solution method was used to deposit 3-0 nanocomposite thin films of PbZr0.52Ti0.48O3 (PZT) and CoFe2O4 (CFO). The piezoelectric behavior of PZT and magnetostrictive behavior of CFO allow for magnetoelectric (ME) coupling through strain transfer between the respective phases. High ME coupling is desired for many applications including memory devices, magnetic field sensors, and energy harvesters. The spontaneous phase separation in the 3-0 nanocomposite film was observed, with 25 nm CFO particle or nanophases distributed in discrete layers through the thickness of the PZT matrix. Magnetic-force microscopy images of the nanocomposite thin film under opposite magnetic poling conditions revealed in-plane pancake-like regions of higher concentration of the CFO nanoparticles. The constraints on the size and distribution of the CFO nanoparticles created a unique distribution in a PZT matrix and achieved values of ME coupling of 3.07 V cm-1 Oe-1 at a DC bias of 250 Oe and 1 kHz, increasing up to 25.0 V cm-1 Oe-1 at 90 kHz. Piezo-force microscopy was used to investigate the ferroelectric domain structure before and after opposite magnetic poling directions. It was found that in this nanocomposite, the polarization of the ferroelectric domains switched direction as a result of switching the direction of the magnetization by magnetic fields.

Multiferroic Core-Shell Nanofibers, Assembly in a Magnetic Field, and Studies on Magneto-Electric Interactions.

Ferromagnetic-ferroelectric nanocomposites are of interest for realizing strong strain-mediated coupling between electric and magnetic subsystems due to a high surface area-to-volume ratio. This report is on the synthesis of nickel ferrite (NFO)-barium titanate (BTO) core-shell nanofibers, magnetic field assisted assembly into superstructures, and studies on magneto-electric (ME) interactions. Electrospinning techniques were used to prepare coaxial fibers of 0.5-1.5 micron in diameter. The core-shell structure of annealed fibers was confirmed by electron microscopy and scanning probe microscopy. The fibers were assembled into discs and films in a uniform magnetic field or in a field gradient. Studies on ME coupling in the assembled films and discs were done by magnetic field (H)-induced polarization, magneto-dielectric effects at low frequencies and at 16-24 GHz, and low-frequency ME voltage coefficients (MEVC). We measured ~2-7% change in remnant polarization and in the permittivity for H = 7 kOe, and a MEVC of 0.4 mV/cm Oe at 30 Hz. A model has been developed for low-frequency ME effects in an assembly of fibers and takes into account dipole-dipole interactions between the fibers and fiber discontinuity. Theoretical estimates for the low-frequency MEVC have been compared with the data. These results indicate strong ME coupling in superstructures of the core-shell fibers.

Sensitivity Enhancement in Magnetic Sensors Based on Ferroelectric-Bimorphs and Multiferroic Composites.

Multiferroic composites with ferromagnetic and ferroelectric phases have been studied in recent years for use as sensors of AC and DC magnetic fields. Their operation is based on magneto-electric (ME) coupling between the electric and magnetic subsystems and is mediated by mechanical strain. Such sensors for AC magnetic fields require a bias magnetic field to achieve pT-sensitivity. Novel magnetic sensors with a permanent magnet proof mass, either on a ferroelectric bimorph or a ferromagnetic-ferroelectric composite, are discussed. In both types, the interaction between the applied AC magnetic field and remnant magnetization of the magnet results in a mechanical strain and a voltage response in the ferroelectric. Our studies have been performed on sensors with a Nd-Fe-B permanent magnet proof mass on (i) a bimorph of oppositely-poled lead zirconate titanate (PZT) platelets and (ii) a layered multiferroic composite of PZT-Metglas-Ni. The sensors have been characterized in terms of sensitivity and equivalent magnetic noise N. Noise N in both type of sensors is on the order of 200 pT/√Hz at 1 Hz, a factor of 10 improvement compared to multiferroic sensors without a proof mass. When the AC magnetic field is applied at the bending resonance for the bimorph, the measured N ≈ 700 pT/√Hz. We discuss models based on magneto-electro-mechanical coupling at low frequency and bending resonance in the sensors and theoretical estimates of ME voltage coefficients are in very good agreement with the data.

Probing the local strain-mediated magnetoelectric coupling in multiferroic nanocomposites by magnetic field-assisted piezoresponse force microscopy.

The magnetoelectric effect that occurs in multiferroic materials is fully described by the magnetoelectric coupling coefficient induced either electrically or magnetically. This is rather well understood in bulk multiferroics, but it is not known whether the magnetoelectric coupling properties are retained at nanometre length scales in nanostructured multiferroics. The main challenges are related to measurement difficulties of the coupling at nanoscale, as well as the fabrication of suitable nano-multiferroic samples. Addressing these issues is an important prerequisite for the implementation of multiferroics in future nanoscale devices and sensors. In this paper we report on the local measurement of the magnetoelectric coefficient in bilayered ceramic nanocomposites from the variation in the longitudinal piezoelectric coefficient of the electrostrictive layer in the presence of a magnetic field. The experimental data were analyzed using a theoretical relationship linking the piezoelectric coefficient to the magneto-electric coupling coefficient. Our results confirm the presence of a measurable magnetoelectric coupling in bilayered nanocomposites constructed by a perovskite as the electrostrictive phase and two different ferrites (cubic spinel and hexagonal) as the magnetic phases. The reported experimental values as well as our theoretical approach are both in good agreement with previously published data for bulk and nanostructure magnetoelectric multiferroics.

REST regulates distinct transcriptional networks in embryonic and neural stem cells.

The maintenance of pluripotency and specification of cellular lineages during embryonic development are controlled by transcriptional regulatory networks, which coordinate specific sets of genes through both activation and repression. The transcriptional repressor RE1-silencing transcription factor (REST) plays important but distinct regulatory roles in embryonic (ESC) and neural (NSC) stem cells. We investigated how these distinct biological roles are effected at a genomic level. We present integrated, comparative genome- and transcriptome-wide analyses of transcriptional networks governed by REST in mouse ESC and NSC. The REST recruitment profile has dual components: a developmentally independent core that is common to ESC, NSC, and differentiated cells; and a large, ESC-specific set of target genes. In ESC, the REST regulatory network is highly integrated into that of pluripotency factors Oct4-Sox2-Nanog. We propose that an extensive, pluripotency-specific recruitment profile lends REST a key role in the maintenance of the ESC phenotype.