John P. Domann

Strain-mediated 180° switching in CoFeB and Terfenol-D nanodots with perpendicular magnetic anisotropy

A micromagnetic and elastodynamic finite element model is used to compare the 180° out-of-plane magnetic switching behavior of CoFeB and Terfenol-D nanodots with perpendicular magnetic easy axes. The systems simulated here consist of 50 nm diameter nanodots on top of a 100 nm-thick PZT (Pby[ZrxTi1-x]O3) thin film, which is attached to a Si substrate. This allows voltage pulses to induce strain-mediated magnetic switching in a magnetic field free environment. Coherent and incoherent switching behaviors are observed in both CoFeB and Terfenol nanodots, with incoherent flipping associated with larger or faster applied switching voltages. The energy to flip a Terfenol-D memory element is an ultralow 22 aJ, which is 3–4 orders more efficient than spin-transfer-torque. Consecutive switching is also demonstrated by applying sequential 2.8 V voltage pulses to a CoFeB nanodot system with switching times as low as 0.2 ns.

Theoretical model of a piezoelectric composite spinal fusion interbody implant

Failure rates of spinal fusion are high in smokers and diabetics. The authors are investigating the development of a piezoelectric composite biomaterial and interbody device design that could generate clinically relevant levels of electrical stimulation to help improve the rate of fusion for these patients. A lumped parameter model of the piezoelectric composite implant was developed based on a model that has been utilized to successfully predict power generation for piezoceramics. Seven variables (fiber material, matrix material, fiber volume fraction, fiber aspect ratio, implant cross-sectional area, implant thickness, and electrical load resistance) were parametrically analyzed to determine their effects on power generation within reasonable implant constraints. Influences of implant geometry and fiber aspect ratio were independent of material parameters. For a cyclic force of constant magnitude, implant thickness was directly and cross-sectional area inversely proportional to power generation potential. Fiber aspect ratios above 30 yielded maximum power generation potential while volume fractions above 15% showed superior performance. This investigation demonstrates the feasibility of using composite piezoelectric biomaterials in medical implants to generate therapeutic levels of direct current electrical stimulation. The piezoelectric spinal fusion interbody implant shows promise for helping increase success rates of spinal fusion.

Strain powered antennas

This paper proposes the creation of strain powered antennas that radiate electromagnetic energy by mechanically vibrating a piezoelectric or piezomagnetic material. A closed form analytic model of electromagnetic radiation from a strain powered electrically small antenna is derived and analyzed. Fundamental scaling laws and the frequency dependence of strain powered antennas are discussed. The radiation efficiency of strain powered electrically small antennas is contrasted with a conventional electric dipole. Analytical results show that operating at the first mechanical resonance produces the most efficient strain powered radiation relative to electric dipole antennas. A resonant analysis is exploited to determine the material property space that produces efficient strain powered antennas. These results show how a properly designed strain powered antenna can radiate more efficiently than an equally sized electric dipole antenna.

Stacked macro fiber piezoelectric composite generator for a spinal fusion implant

A manufacturing method was developed to create a piezoelectric 3-layer stacked, macro fiber composite generator operating in d33 mode to promote bone growth in spinal fusion surgeries. A specimen of 9 × 17 × 9 mm thick was constructed from 800 μm diameter PZT fibers and medical grade epoxy. Electromechanical testing was performed at three stages of manufacturing to determine the influence of these processes on power generation. An average peak power of over 335 μW was generated in the heat-treated specimen during simulated human body loads. The work provides insights into manufacturing methods for lowered source impedance power generation for a variety of applications.

Composite piezoelectric spinal fusion implant: Effects of stacked generators

Spinal fusion surgeries have a high failure rate for difficult-to-fuse patients. A piezoelectric spinal fusion implant was developed to overcome the issues with other adjunct therapies. Stacked generators were used to improve power generation at low electrical load resistances. The effects of the number of layers on average maximum power and the optimal electrical load resistance were characterized. The effects of mechanical preload, load frequency, and amplitude on maximum power and optimal electrical load resistance were also characterized. Increasing the number of layers from one to nine was found to lower the optimal electrical load resistance from 1.00 GΩ to 16.78 MΩ while maintaining maximum power generation. Mechanical preload did not have a significant effect on power output or optimal electrical load resistance. Increases in mechanical loading frequency increased average maximum power, while decreasing the optimal electrical load resistance. Increases in mechanical loading amplitude increased average maximum power output without affecting the optimal electrical load resistance.

Strain-mediated multiferroic control of spontaneous exchange bias in Ni-NiO heterostructures

This paper presents the measurement of strain-mediated multiferroic control of spontaneous exchange bias (SEB) in magnetostrictive nickel/nickel oxide (Ni/NiO) bilayers on ferroelectric lead magnesium niobate-lead titanate (PMN-PT). Electric field control of a positive to negative exchange bias shift was measured, with an overall shift of 40.5 Oe, corresponding to a 325% change. Observed changes in coercivity are also reported and provide insight into the role of competing anisotropies in these structures. The findings in this paper provide evidence that magnetoelastic anisotropy can be utilized to control spontaneous exchange bias (SEB). This control of SEB is accomplished by modifying a bulk anisotropy (magnetoelasticity) that adjusts the mobility of interfacial anti-ferromagnetic spins and, therefore, the magnitude of the exchange bias. The demonstrated magnetoelastic control of exchange bias provides a useful tool in the creation of future magnetoelectric devices.

Magnetoelastic shockwave response (Conference Presentation)

Hypervelocity impacts generate shockwaves causing catastrophic damage and failure of surrounding material and structures. Adaptive materials have been previously studied to mitigate shockwaves by providing diagnostics tools, wave steering, and dissipating mechanical energy. Numerous ferroelectric studies have been conducted, with less focus on ferromagnets, or magnetoelasticity. This presentation explores the response of magnetoelastic Galfenol (Fe81.6Ga18.4) to high strain-rate, high stress amplitude loading. Experimentally, 2 GPa rarefacted shockwaves are generated in Galfenol using a Nd:YAG laser. Magnetization changes are recorded using inductive coils along the sample length and interferometry is used to infer the stress amplitude at the specimen’s back surface. The experimental results highlight how the shockwave evolves as it travels, including the onset of lateral release waves. Magnetic field control of the mechanical wave speed by 20% is observed, accompanied by large control of the measured magnetization changes. These changes highlight the coupled magnetoelastic nature of the effect. Furthermore, it is observed that the magnetization more strongly couples to lateral release waves than the incident compressive pulse. Last, magnetization changes are seen to precede the propagating mechanical wave, indicating dipolar coupling can transfer energy ahead of the mechanical wave front. A numerical model has been developed to provide further insight into the experimental study. This model fully couples elastodynamics with magnetostatics using a nonlinear magnetoelastic constitutive behavior. The nonlinear model captures the main findings of the experimental study, including wave evolution, and strong magnetoelastic coupling to the release waves.

Strain powered magnetic antennas (Conference Presentation)

Electric antennas are still large structures (approximately 1m for 300 MHz operation), and have so far eluded the miniaturization trend common in the electronics industry. This is due in large part to an impedance mismatch with free space, and an increase in system losses from ohmic heating as antenna dimensions shrink. Recent work has proposed using multiferroic heterostructures to create small energy efficient antennas. This idea was first explored by Rowen’s 1961 paper on electromagnetic (EM) radiation from YIG, and Mindlin’s 1973 paper on radiation from quartz. Since then limited work has looked at EM radiation from adaptive materials, and there are currently no analytical models describing such a device. This presentation provides an analytical model to examine small mechanically powered energy efficient antennas. An analytical framework is provided that couples elastodynamics and electrodynamics using piezoelectric and piezomagnetic constitutive behavior. This approach uses an eigenmode expansion of the undamped longitudinal vibrations in a prismatic rod to describe EM radiation from each harmonic mode. The problem is reduced to examination of damped harmonic oscillators, and EM radiation is shown equivalent to an effective volumetric strain-rate dependent damping. This approach provides the frequency response of a mechanical antenna, and demonstrates important scaling behavior relative to conventional antennas. Resonant analysis leads to simple closed form expressions for antenna efficiency, and leads to a metric directly comparing mechanical and conventional antennas, facilitating both material selection and device design. To summarize, this presentation provides a first look at the strain-mediated control of wireless communications.

Strain mediated multiferroic motors (Conference Presentation)

As traditional electric motors scale down, the available power density rapidly decreases. For a motor occupying a volume of 1 micron cubed, the available power density is roughly six orders of magnitude lower than a 1 millimeter cubed motor. Strain-mediated multiferroic heterostructures have recently been proposed to create high power density, micron scale, magnetic motors. These motors leverage magnetoelastic anisotropy to rotate the magnetic moment of a small disk, and use dipolar forces to couple rotors or beads to the stray magnetic field. A key challenge to the creation of these motors is to deterministically control magnetization rotations without the need for complex fabrication or control schemes. This presentation demonstrates how controlling the relative orientation of magnetic exchange bias and magnetoelastic anisotropies can be used to deterministically control motor rotations over a broad frequency range. A Stoner-Wohlfarth magnetic macrospin model is created that couples a single domain magnetic disc to a [011] cut PMN-PT substrate. This model accounts for magnetoelastic, shape, and exchange anisotropy energies. The exchange anisotropy is rotated relative to the biaxial strain created by the PMN-PT substrate. Results demonstrate precessional magnetization dynamics deterministically controlled with an oscillating voltage on the PMN-PT substrate. This approach enables 360 degree rotations over a broad frequency range. The frequency response is provided up through ferromagnetic resonance, and power density calculations are made with comparison to existing micromechanical motors.