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Spin-transfer torque: A new turn on spins

Event Details

Event Dates: 

Friday, March 22, 2013 - 3:00am

Seminar Location: 

  • JILA X317

Speaker Name(s): 

Daniel Bürger

Speaker Affiliation(s): 

Peter Grünberg Institute
Seminar Type/Subject

Scientific Seminar Type: 

  • JILA Colloquium

Event Details & Abstract: 

A spin-polarized current entering into a ferromagnetic material exerts a torque on the magnetization by transferring spin angular momentum from the current to the ferromagnet. This so-called spin transfer torque (STT) gives rise to current-driven magnetization dynamics with unprecedented properties like the switching of the magnetization without applying an external field or the excitation of persistent large-angle precessions of the magnetization with frequencies in the GHz range, which are the basis of the so-called spin-transfer oscillators (STO). In the first part, I give a brief introduction to STT in magnetic multilayers with emphasis on developing a physical picture of the microscopic processes. The second part focuses on measurements and simulations of STT effects in nano pillars containing circular, single-crystalline Fe(001) nanomagnets (2-20 nm thick, 230 nm in diameter [1]), which form a magnetic vortex state. The lowest-energy dynamic mode of the vortices, the gyro tropic mode, can be excited by the STT. A direct comparison of the excitation characteristics of the uniform mode in high field and the gyrotropic mode in low field in the same nanopillar shows that the vortex excitation reveals superior properties. In particular, it maximizes the emitted high frequency power and shows a wider frequency tuning range [2]. Finally, locking of the gyro tropic mode to an external HF signal is demonstrated [3] as a first step towards the synchronization of many STOs, which is a prerequisite for achieving sufficient output power for applications, e.g. in communication technology. References[1] H. Dassow, R. Lehndorff, D.E. Bürgler et al., Appl. Phys. Lett. 89,222511 (2006).[2] R. Lehndorff, D.E. Bürgler et al., Phys. Rev. B 80, 054412 (2009).[3] R. Lehndorff, D.E. Bürgler et al., Appl. Phys. Lett. 97, 142503 (2010).