Visualizing spin torque in nanoscale magnetic devices using ultrafast x-ray microscopy

Spin momentum transfer is one of the most actively studied phenomena in nanoscale magnetic devices. This phenomenon is widely believed to be critical for enhancing the density of a next generation of magnetic random access memory devices, as well as for the realization of future on-chip microwave oscillators for signal processing. The response of ultrathin film magnetization in multilayer heterostructures, patterned into submicron pillars, to short-pulse (picosecond to nanosecond) spin polarized currents, determines the device performance.

Intellectual merits:

In the proposed research, new insight into the magnetization dynamics and operation of spin torque devices will be gained through synchrotron-based, stroboscopic x-ray transmission microscopy and magnetometry. The penetration depth of x-rays into metals, combined with the magnetic circular dichroism of absorption at Fe, Co, and Ni edges, makes buried layer magnetization selectively accessible; pump-probe (stroboscopic) measurement, synchronized with radio frequency driving fields, illuminates their dynamics. Otherwise-unobtainable spatial resolution of 40 nanometers, down to 15 nanometers, can be attained through the short wavelengths of soft x-rays and the use of focusing optics; innovative techniques to enhance both temporal resolution (to 2 picoseconds) and magnetic contrast have been demonstrated in prior work by the PI and coworkers on unpatterned films, and will be applied in the proposed research on nanostructures. The effects of spin torque will then be visualized at picosecond time scales and ~40 nanometer length scales, allowing device designers to 'take movies' of operation, both in device switching and precessional magnetization dynamics, in real devices. Technical goals include the clarification of free and fixed layer dynamics, the first imaging of spin wave propagation away from a point contact, and of phase locking in arrays of nanopillars. Observed results can be compared directly with detailed micromagnetic simulations, using observed inhomogeneity as an input for the first time.

Broader impacts:

The proposed research will provide a unique and broad graduate training experience for

two Ph.D. students, who will have the opportunity to work on site at both a startup company and national laboratories. Research activities will be closely integrated with outreach activities to local K-12 students; Columbia will provide a 50% cost match for the support of one graduate research assistant who will be actively involved in the maintenance of the Hayden K-12 outreach program, with 'Science Saturdays' enrichment for students typically drawn from Harlem. The proposed activities will enhance research infrastructure at national laboratories, developing novel synchrotron-based instrumentation for the study of magnetization dynamics. Benefits to society are possible by providing understanding of, and assisting in the development of, novel magnetoelectronic products.