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Nanomagnetism at NIST: Magnetic Dynamics
The dynamics of the magnetization are crucial to the performance of nanomagnetic devices. For example, in memory applications, the ease of switching the magnetization and the speed at which it can be done are of crucial importance. For sensing applications, the ease with which the magnetization can be moved at small angles without fluctuations determines the sensitivity of the device. A number of groups at NIST study magnetization dynamics and develop techniques to measure it. Historically, Ferromagnetic Resonance (FMR) has been a widely used technique. At NIST, it has been used to study the dynamics of ferromagnets coupled to antiferromagnets, a phenomenon known as exchange bias. Recent technical developments have enabled its use to study the effect of the edges on nanomagnetic structures. The magnetic modes are identified by tracking the resonance frequencies as a function of field. The properties of the edge modes indicate the quality of the edges of the nanostructures.
A number of novel techniques for measuring magnetic dynamics have been developed at NIST. These techniques complement the information gained in the resonance measurements. One such technique is Pulsed Inductive Microwave Magnetometry (PIMM). While FMR is typically limited to small amplitude magnetization dynamics, PIMM has measured the dynamics at the large amplitudes typically encountered during magnetization reversal. The Frequency-Resolved MagnetoOptic Kerr Effect (FR-MOKE) uses optics instead of microwaves to detect magnetization dynamics. FR-MOKE has been used to study large amplitude magnetic precession and magnetic normal modes of magnetic nanostructures.
It is difficult to measure the magnetization dynamics with nanoscale spatial resolution. For these situations, calculations can be a particularly valuable augmentation of the measurements described above. Calculations can provide both time and space resolution. Comparisons between measurements and simulations allow inference of the spatial dependence from agreement of the calculated and measured dynamics. The Object-Oriented MicroMagnetics Framework (OOMMF) code, developed at NIST, is widely used for this purpose. The OOMMF code is designed for magnetic nanostructures. However, it is not well suited for nanostructures embedded in a larger magnetic system. An example is a spin transfer torque nano-oscillator, which requires much larger active areas in simulations. For these systems, specialized codes have been developed to simulate the dynamics.
Most of the parameters that enter into a micromagnetic simulation are inferred from experiment. This approach works well for materials that are well characterized but is difficult for newer materials. Part of the theory effort at NIST is to develop the capability of computing these parameters, like the magnetocrystalline anisotropy and the damping parameter, from first principles to develop the ability to predict magnetic properties.