Browsing by Author "Srinivasan, Gopalan"
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- Converse magnetoelectric effects in composites of liquid phase epitaxy grown nickel zinc ferrite films and lead zirconate titanate: Studies on the influence of ferrite film parametersZhou, Peng; Popov, M. A.; Liu, Ying; Bidthanapally, Rao; Filippov, D. A.; Zhang, Tianjin; Qi, Yajun; Shah, P. J.; Howe, B. M.; McConney, M. E.; Luo, Yongming; Sreenivasulu, Gollapudi; Srinivasan, Gopalan; Page, M. R. (2019-04-09)The interactions between electric and magnetic subsystems in a ferroelectric-ferromagnetic composite occur through mechanical forces. Here we discuss results of a systematic investigation on the strength of the magnetic response of the composite to an applied electric field, known as the converse magnetoelectric (CME) effect, and its dependence on the ferroic order parameters and volume fraction for the two phases. Studies were carried out on composites of lead zirconate titanate and 2-30-mu m-thick nickel zinc ferrite (NZFO) films grown by liquid phase epitaxy on lattice matched (100) and (111) MgO substrates. Ferromagnetic resonance was utilized to determine the strength of CME from data on electric field E induced shift in the resonance frequency and its dependence on ferrite film orientation and thickness as well as MgO substrate thickness. The CME coupling coefficient A was found to be a factor of 2 to 4 higher in samples with NZFO films with (100) orientation than for (111) films. A decrease in A was measured with increasing ferrite film thickness and a very significant enhancement in the strength of CME was measured for decreasing MgO thickness. A model for CME that takes into consideration the influence of nonferroic MgO substrate was developed, and estimated A values are in very good agreement with the data. The results presented here are also of importance for a new class of electric field tunable ferrite microwave devices.
- Enhancing the sensitivity of magnetoelectric sensors by increasing the operating frequencyPetrie, Jonathan; Viehland, Dwight D.; Gray, David; Mandal, Sanjay; Sreenivasulu, Gollapudi; Srinivasan, Gopalan; Edelstein, Alan S. (American Institute of Physics, 2011-12-15)We present a field modulation technique that increases the operating frequency of magnetoelectric ( ME) sensors so that it can match the mechanical resonance frequency of the sensor. This not only improves the sensitivity but also reduces the effect of 1/f noise that is inherent at low frequencies. The technique, which is shown to apply to both symmetric and asymmetric ME sensors, relies on the strong, nonlinear magnetic field dependence of the magnetostriction. The combination of a lower 1/f noise and enhanced response at resonance has increased the signal to noise ratio of a symmetric sensor by two orders of magnitude. The detection limit of this sensor was lowered from 90 to 7 pT/root Hz at 1Hz in a magnetically unshielded environment. (C) 2011 American Institute of Physics. [doi:10.1063/1.3668752]
- Multiferroic Core-Shell Nanofibers, Assembly in a Magnetic Field, and Studies on Magneto-Electric InteractionsSreenivasulu, Gollapudi; Zhang, Jitao; Zhang, Ru; Popov, Maksym; Petrov, Vladimir M.; Srinivasan, Gopalan (MDPI, 2017-12-23)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 CompositesSreenivasulu, Gollapudi; Qu, Peng; Petrov, Vladimir M.; Qu, Hongwei; Srinivasan, Gopalan (MDPI, 2016-02-20)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.
- Shifting the operating frequency of magnetoelectric sensorsPetrie, Jonathan; Gray, David; Viehland, Dwight D.; Sreenivasulu, Gollapudi; Srinivasan, Gopalan; Mandal, Sanjay; Edelstein, Alan S. (American Institute of Physics, 2012-04-01)A method is presented for increasing the operating frequency of symmetric and asymmetric magnetoelectric (ME) sensors so that the operating frequency can be equal to the mechanical resonance frequency of the sensor. This increase improves the signal to noise ratio of a symmetric sensor by at least two orders of magnitude because it mitigates the effect of 1/f noise and the sensor has an increased response at its resonant frequency. The method is based on the strong, nonlinear magnetic field dependence of the magnetostriction. Our method has lowered the detection limit to 4 pT/root Hz at 1 Hz in a magnetically unshielded environment. (C) 2012 American Institute of Physics. [doi:10.1063/1.3677840]
- Three-dimensional left-handed material lensPetrov, R. V.; Srinivasan, Gopalan; Bichurin, Mirza I.; Viehland, Dwight D. (AIP Publishing, 2007-09-01)A model is provided for the image formation by a three-dimensional lens with negative index of refraction (n) and compared with results for an array of split-ring resonators (SRRs) and wires. For n=-1, a linear decrease in image distance r is expected with increasing object distance l. A nonlinear r vs l is anticipated for other n values. A lens of an array of SRRs and wires was made and characterized over 1-18 GHz and n=-1, and a gain of similar to 8 dB at 11.25 GHz was achieved. Results on r vs l are in excellent agreement with the model. (C) 2007 American Institute of Physics.