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 spin-polarized electrical currents. The results of this work will help to improve the uniformity and reliability of future electronics based on magnetic nanodevices. 

The motion of the magnetization in magnetic nanostructures is at the core of important technologies such as computer hard drives and magnetic memory chips. Additionally, emerging technologies such as magnetic logic and second-generation spin-torque memory chips write and read “bits” of information by switching and sensing the magnetization state in nanostructures.

In this project we develop measurement methods to aid the development of new nanomagnet-based devices. We focus our efforts in two areas: (1) the development of a ferromagnetic resonance force microscope that allows us to probe, manipulate and image the dynamic behavior of individual nanostructures, and (2) measuring the “spin torque” effects of electron spins carried by electrical currents.

The ferromagnetic resonance force microscope addresses a number of challenges facing the development of nanomagnet-based future electronics. One such challenge is that a few small defects can ruin a product such as a memory chip that relies on the uniform operation of many nanodevices. While a number of existing tools, including electron microscopy, are available for characterizing structural and chemical properties, the magnetic properties are the primary concern. The ferromagnetic resonance microscope allows us to measure magnetic properties on a device-by device basis, or even at different locations within a device, by measuring the frequency of precession (magnetic vibration) of the magnetization.. The figure shows a modeled series of standing spin-wave modes (wave-like precession modes) in a magnetic thin film where the waves are trapped by the field from the nearby magnetic tip on a cantilever. Different parts of the film can be tested simply by moving the tip and noting changes in the resonance frequenices.

Our second focus, measuring spin torque effects, provides an important method to manipulate and detect magnetization states in nanomagnetic devices using electrical currents. Electrons carry electrical charge in a current, but they also carry quantum mechanical spin, and it is the spin that gives rise to magnetism in materials. In a limited way, moving the electrons in a ferromagnetic metal actually also moves the magnetization with the flow of electrons. We measure the drift velocity of the magnetization using spin waves, in which the precession takes the form of traveling waves. The motion of the magnetization in the electrical current causes an effective Doppler shift of the spin waves, and that shift allows us to determine the magnetic polarization of the current, which quantifies how “magnetized” the electric current is. The polarization data for different ferromagnetic metals is useful for the design of spin-torque devices, and also for interpreting experiments that measure other aspects of spin-torque phenomena.

Nanoscale Magnetic Media Diagnostics by Rippling Spin Waves

CNST Researchers Use Spin Waves to Measure Magnetic Polarization of Electrical Current

• Spectroscopic defect imaging in magnetic nanostructure arrays, H.-J. Chia, F. Guo, L. M. Belova, and R. D. McMichael, Applied Physics Letters 101, 042408 (2012).
  NIST Publication Database        Journal Web Site
• Nanoscale spin wave localization using ferromagnetic resonance force microscopy, H.-J. Chia, F. Guo, L. M. Belova, and R. D. McMichael, Physical Review Letters 108, 087206 (2012).
  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