Nanomagnet Dynamics

We develop techniques for measuring magnetization dynamics in magnetic nanostructures with a particular emphasis on the emerging needs of future electronics. The dynamics of nanomagnets underpin existing commercial products such as computer hard drives and magnetic random-access memory (MRAM) chips.  Nanomagnet dynamics are also at the heart of a wide array of devices currently under development or proposed for future electronics.
For these applications, it will be important to understand and control variations in material properties as a function of device geometry. At the CNST, we are developing dynamic measurement techniques to characterize the properties of magnetic nanostructures and their interactions with electrical currents. The results of this work will help to improve the uniformity and reliability of future electronics based on magnetic nanodevices.  

Hard drives with bit-patterned media, magnetic memory chips, and virtually every magnetism-based form of future electronics that has been proposed will require large arrays of magnetic nanostructures with highly uniform magnetic characteristics.  Unfortunately, uniformity is difficult to achieve, because within a chip there are always device-to-device variations in material properties and nanoscale variations in the geometry of the key components. The consequences of these small variations may not be important in large structures, but are significant in nanoscale devices.

To characterize magnetic nanostructures, we measure the dynamics of the magnetization. Both modeling and experiment have shown that the normal modes of magnetization vibration are sensitive to variations in a structure’s geometry and material properties. By analogy, a bowl struck with a spoon will sound different depending on whether it is cracked or not, and if it is made of ceramic or glass.  Similarly, the ferromagnetic resonances of magnetic nanostructures are sensitive to defects both in their material properties and in their structure.

While the analogy with tableware acoustics is useful, ferromagnetic resonance, which involves precession of the magnetization vector around an equilibrium direction at microwave frequencies is more complicated. The resonance frequencies are determined by the applied magnetic field, material properties, exchange interactions, and dipole-dipole interactions, all of which depend on the geometry of the device. Because ferromagnets feature strong exchange and dipolar interactions, ferromagnetic resonance in a nanostructure is best described in terms of spatially extended normal modes, like vibrations of a drumhead.  An example of normal modes in ferromagnetic resonance are shown in the figure for a 120 nm × 110 nm × 10 nm ellipse of Ni80Fe20 alloy in 50 mT applied field. These modes were calculated using NIST’s object oriented micromagnetic framework (OOMMF).

In this project, we exploit the properties of ferromagnetic resonance in nanostructures to develop measurement techniques for magnetism-based future electronics. We are developing measurements of the normal modes in small structures as diagnostics of nonuniformity in nanostructure arrays, which is critical for error-free data storage.  As devices are made smaller, the properties of the edge are expected to become increasingly important; therefore, we are also using measurements of localized edge modes to determine the magnetic properties at thin film edges. Finally, we are conducting measurements of spin wave propagation in current-carrying stripes to determine the ratio of up-spin electron current to down-spin electron current in metallic ferromagnets. This “spin wave Doppler” method yields basic materials parameters that are critical for the development of spin-torque-based devices.

The techniques developed in this project yield a wealth of information on the magnetic properties of nanostructures and nanodevices—from anisotropy and damping to edge properties and current polarization. We are striving to provide the critical measurement science to support development of magnetism-based future electronics.ure electronics.

 

• Temperature dependence of magnetization drift velocity and current polarization in Ni₈₀Fe₂₀ by spin-wave doppler measurements, M. Zhu, C. L. Dennis, and R. D. McMichael, Physical Review B 81, 140407 (2010).
  NIST Publication Database          Journal Web Site
• Effect of interactions on edge property measurements in magnetic multilayers, M. Zhu and R. D. McMichael, Journal of Applied Physics 109, 043904 (2011).
  NIST Publication Database          Journal Web Site
• Enhanced magnetization drift velocity and current polarization in (CoFe)1-xGex alloys, M. Zhu, B. D. Soe, R. D. McMichael, M. J. Carey, S. Maat and J. R. Childress, Applied Physics Letters 98, 072510 (2011).
  NIST Publication Database          Journal Web Site