Yuri K. Fetisov

Piezoelectric single crystal langatate and ferromagnetic composites: Studies on low-frequency and resonance magnetoelectric effects

Mechanical strain mediated magnetoelectric (ME) effects are studied in bilayers and trilayers of piezoelectric single-crystal lanthanum gallium tantalate (LGT) and magnetostrictive permendur (P). The ME voltage coefficient ranges from 2.3 V/cm Oe at 20 Hz to 720 V/cm Oe at bending resonance and is higher by an order of magnitude than in composites with ferroelectric lead zirconate titanate or lead magnesium niobate-lead titanate. The low-frequency magnetic noise for P-LGT-P is a factor of 2-10 smaller than for ferroelectrics based composites. Langatate is free of ferroelectric hysteresis, pyroelectric effects, and phase transitions up to 1450 °C and is of interest for ultrasensitive, high temperature magnetic sensors.

Nonlinear ferromagnetic resonance and foldover in yttrium iron garnet thin films-inadequacy of the classical model

A thin-film resonator structure has been used for quantitative measurements of ferromagnetic resonance foldover and the associated bistable power response for yttrium iron garnet (YIG) thin films. The resonator consisted of a 1-mm by 1-mm-square, 4.9 /spl mu/m-thick epitaxial YIG film on top of a 50 /spl mu/m-wide, 3-mm-long microstrip transducer. A static magnetic field of 3200 Oe was applied perpendicular to the film. Low- order magnetostatic forward volume wave standing modes were excited at low power levels in the -20-dBm range and detected as resonance dips in reflected power versus frequency spectra over the range 4-5 GHz. At powers in the 0- to +15-dBm range, these dips showed foldover and bistable response characteristics for increasing and decreasing frequency or power sweeps. The use of 1-10-/spl mu/s-wide pulses instead of continuous-wave (CW) excitation resulted in the consistent disappearance of the foldover and bistability characteristics. The frequency sweep pulse data at fixed power reproduced the down-sweep CW results, and the pulse data for both increasing and decreasing power at fixed frequency reproduced the increasing-power CW results. A quantitative theoretical analysis demonstrates that observed foldover and bistable response characteristics are much weaker than predicted from the classical precession foldover mechanism proposed by Anderson and Suhl, in which the decrease in the static component of the magnetization drives the response. The up-sweep and down-sweep foldover frequency jumps both occur sooner than predicted by this classical mechanism and the calculated foldover profiles are much more severe than the data show.

Microwave resonators based on single-crystal yttrium iron garnet and lead magnesium niobate-lead titanate layered structures

Electrically and magnetically tunable microwave resonators based on ferrite-piezoelectric layered structure have been investigated. The structure consisted of bonded layers of single-crystal yttrium iron garnet film and single-crystal lead magnesium niobate-lead titanate (PMN-PT) slab. The frequency of the resonator was tuned over a wide frequency range, from 3to10GHz, by varying the bias magnetic field (magnetic tuning) and over a narrow range, up to 45MHz, by the application of an electric field (electric tuning) to the PMN-PT layer. Performance characteristics of the resonator are discussed. The data are in good agreement with theory.

Microwave bistability in a magnetostatic wave interferometer with external feedback

Microwave bistability has been obtained for the first time in a magnetostatic wave (MSW) interferometer with external magnetic field feedback. The interferometer contained a magnetostatic surface wave transmission line with a 7.5-/spl mu/m thick epitaxial film of yttrium iron garnet magnetized with an external static magnetic field of 1048 Oe, a variable attenuator in a parallel reference channel, and external feedback in the form of an additional static field derived from the line output. Bistable and multistable output power versus input power responses are derived from the frequency dependence of the interferometer transmission and the MSW wave number dependence on the static magnetic field. Typical bistable and multistable output versus input power hysteresis loops were obtained under constant frequency operation in the range 4.9 to 5.2 GHz, and for input powers from -30 to +10 dBm. The bistable response depends on the frequency of the microwave signal, the attenuation in the reference channel, and the gain of the feedback loop. An analysis based on linear MSW theory agreed with experiment. Applications of MSW microwave bistability include power limiter devices and basic microwave logic elements.

Multiferroic bending mode resonators and studies on temperature dependence of magnetoelectric interactions

Temperature dependence of the magneto-electric voltage u and resonance frequency are measured at bending mode over T = 220–350 K for Ni-PZT bilayers fabricated by epoxy bonding and by electro-deposition. Both samples show a general decrease in u due to T-dependent permittivity. In epoxy bonded resonators, a further decrease in u occurs due to weakening of mechanical coupling originating from softening of the epoxy. Bending mode frequency varies by 4.5%–16% and is attributed to changes in the Young’s modulus and differential thermal expansion. The results are of importance for composite based devices such as magnetic sensors.

Thermal microwave foldover and bistability in ferromagnetic resonance

We investigated microwave foldover and bistability responses for a microwave resonator structure containing a 100-/spl mu/m-thick yttrium iron garnet single-crystal film. We applied a static field of 3100 Oe perpendicular to the film plane, with continuous-wave and pulse microwave excitation at 4 to 5 GHz, and powers ranging from below 0 dBm up to 25 dBm. For input powers below 0 dBm, the main ferromagnetic resonance line was Lorentzian, centered at f/sub 0/=4540 MHz, and had a frequency width at half power of 17.6 MHz. Input powers above 0 dBm yielded nonlinear effects. For powers from 0 to 7 dBm, frequency sweeps gave distorted lines but no foldover. From 7 to 20 dBm, a hysteretic foldover response developed, with a gradual rise and a steep drop in absorbed power for up sweep and a cusp response on down sweep. We observed a corresponding nonlinear and bistability response when power sweep measurements were made at fixed frequency. The threshold frequency for bistability was about 15 MHz above f/sub 0/. The frequency shifts with power and the effect of changes in the sweep rate or pulse duty cycle indicate a thermal origin of these power-dependent nonlinear responses. All of the results could be accurately modeled though a resonance analysis in which the mode frequency f/sub r/ changed with the absorbed power P/sub a/ according to f/sub r/=f/sub 0/+BP/sub a/, with B=1.83 MHz/mW from the frequency shift data.

Magnetic field tunable acoustic resonator with ferromagnetic-ferroelectric layered structure

High overtone acoustic resonator with yttrium iron garnet/zinc oxide layered structure was theoretically considered, fabricated, and experimentally investigated. The theory of the resonator, containing an arbitrary number of magnetic and nonmagnetic dielectric/ferroelectric layers, placed in a transverse magnetic field is presented. The simulation shows the possibility to tune the resonant frequency in the range of ±1 MHz by magnetic field. This tuning is due to the resonance magnetoelastic interaction in the saturated ferrite film and the total phase shift of acoustic wave in the structure. The experiment proves the magnetic field influence on resonance frequencies and attenuation of transverse wave with polarization vector quasicollinear with the field direction. The tuning about 0.25 MHz near the acoustic resonant frequency 2 GHz was obtained in the field 260 Oe. This frequency is close to the ferromagnetic resonance frequency in ferrite film, corresponding to the field applied.

Suppression of microwave magnetic envelope solitons by continuous wave magnetostatic wave signals

The influence of continuous wave (cw) magnetostatic wave signals on microwave magnetic envelope soliton pulse formation and propagation in magnetic films has been examined. Pulsed and cw microwave signals were applied to the input of a single crystal yttrium–iron–garnet film magnetostatic wave delay line. The nominal operating frequency was 4.8 GHz. The pulse signals served to form solitons with no cw power present. Under suitable conditions, the cw signal served to inhibit or eliminate the soliton formation and propagation. The suppression effect was measured as a function of the cw signal frequency and power. The suppression is maximized when the cw signal frequency coincides with the pulse carrier frequency. At this frequency, an input cw power of 80 mW is sufficient to suppress completely a soliton pulse formed from a 10 ns wide, 500 mW peak power input pulse.

A magnetoelectric sensor of threshold DC magnetic fields

A multiferroic magnetic field sensor capable of producing an output for threshold magnetic fields has been fabricated and characterized. The sensor consists of a trilayer composite of piezoelectric X-cut lanthanum gallium tantalate and magnetostrictive Metglas placed inside a solenoid and a wide-band amplifier. The composite plays two distinct roles in the device; it forms the feedback loop of an oscillator and sets the frequency of sustained oscillations. The sensor generated an output of 2.5 V at the longitudinal acoustic resonance frequency of 87.5 kHz for the trilayer for DC magnetic fields H = 0.3 to 50 Oe parallel to the composite plane. The device functions as a threshold magnetic field sensor for this H-interval, and the threshold ON and OFF H-values for an ac voltage output could be controlled electronically or with a proper choice of the ferromagnetic phase in the composite.

A magnetoelectric composite based signal generator

Self-oscillations in an active loop consisting of a wide-band amplifier and a magnetoelectric composite in the feedback circuit have been observed. The composite with a ferroelectric lead zirconate titanate bimorph and ferromagnetic Metglas serves as a resonator that determines the frequency of oscillations and provides the feedback voltage. Under amplitude balance and phase matching conditions, the device generated signals at 2.3 kHz, at the bending resonance frequency of the composite. The oscillations were observed over a specific range of magnetic bias H. The shape of the signal generated is dependent on electrical circuit parameters and magnitude and orientation of H.