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Nano–Scale Magnetoelectric Motors

Project Objective

This project aims to develop practical electric motors on the order of one micron in size (about the same as a red blood cell) that have power density comparable to large–scale motors. To achieve this, we employ experimental multiferroic/magnetoelectric composites that couple electrical polarization to magnetization to allow the manipulation of magnetic fields at the nanoscale.

figure 1

Figure 1: A concept sketch of a nanomotor. The rotor is a permanent magnet that rotates to follow the changing magnetic field of the ring underneath.

Project Description

The development of nanoscale magnetic actuators lies at the intersection of mechanical engineering, physics, and materials science. Nanomotor development relies on combining piezoelectric and magnetostrictive materials into novel structures, designing and fabricating machine parts at the nanoscale, and modeling the dynamics accurately enough to design control algorithms.

Glossary of Terms

  • Piezoelectric, electrostrictive:  Materials that change shape when exposed to an electric field
  • Magnetostrictive:  A material that changes its magnetization under stress
  • Magnetoelectric effect:  Coupling between electric field and magnetization in certain materials
  • Multiferroic:  Combining ferromagnetism with inherent electrical polarization

The Need for a Nanomotor

Examples of popular micro-electromechanical systems (MEMS) include accelerometers, gyroscopes, pressure sensors, mirror arrays, and printer nozzles. Many require actuation, but the options are very limited: electrostatic actuators are bulky and generate small forces at high voltages, piezoelectric actuators operate at high frequencies but create very small displacements, and thermal actuators work slowly while dissipating large amounts of power as heat. With better actuators, MEMS technology could be applied to fields like micro-robotics, active medical devices, optics, micro-valves, and precision machinery.

Electromagnets, used in electric motors and solenoids, make excellent macro-scale actuators due to their large forces and displacements, high speed, and good efficiency. The magnetic field strength depends on the electric current flowing through the magnet coil. Smaller electromagnets must use thinner wire with higher resistance and less ability to dissipate heat, which drastically reduces their capacity for current. At the micro-scale, fabricating a coil at all becomes impractical. Effective small-scale magnetic actuators require an alternative to electromagnet coils.

Multiferroics and Magnetoelectric Composites

Ferromagnetic materials, like iron and nickel, exhibit persistent magnetization that responds to magnetic fields. Ferroelectric materials, like PZT, exhibit persistent electrical polarization that responds to electric fields. A multiferroic material does both and often couples the two, giving rise to the magnetoelectric effect where applying an electric field causes changes to the magnetization or vice versa. Such materials are rare, but the effect can be duplicated by tightly bonding a piezoelectric or electrostrictive material with a magnetostrictive material. Applying voltage across the piezoelectric creates an electric field that makes it expand, which stresses the attached magnetostrictive layer and changes the magnetic field it produces (Fig. 2).

figure 2

Figure 2: Simulation in COMSOL of a piezoelectric material called PZT (the square base layer) with a ring of magnetostrictive nickel attached to the surface. A voltage is applied at the electrodes (yellow) so that the PZT deforms and forces the nickel to change shape, altering its magnetic field (not shown). Color indicates strain.

Motor Concept

The magnetic orientation of the stator needs to rotate through 360 degrees to make the rotor spin continuously. The stator is a symmetric ring of nickel, Terfenol–D, or other magnetostrictive because it can take on a directional magnetization state that shifts easily (Fig. 3). The magnetic orientation stays perpendicular to the applied tensile stress created by activating electrodes on opposite sides of the ring. Modeling the system's behavior will help optimize the pattern of electrodes and the sequence of applied voltages to maximize the motor's torque and speed.

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Figure 3: Electrodes positioned at 60-degree intervals (right) to achieve controlled rotation of the nano-ring onion magnetization state (A–F).

Current and Future Work

  •  Strain simulations using finite element analysis (FEA) in COMSOL
  • Magnetic field and torque simulations using FEA in ANSYS
  • Dynamics and control simulations in MatLab
  • Design and prototyping of a free-spinning nanoscale rotor using CNF facilities

Funding Agencies

The National Science Foundation has funded an Engineering Research Center for Translational Applications of Nanoscale Multiferroic Systems (TANMS). TANMS is headquartered at UCLA and includes Cornell, Berkeley, and Cal State Northridge. Other research groups are studying the basic materials and possible applications in computer memory and antennas. The LIMS group focuses on the dynamics and control of a nanomotor, as well as some design optimization and prototyping.