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Project
Spin transfer torque induced magnetization dynamics.
In the field of spintronics the electron spin is used as an extra degree of freedom for device design. The possibility to induce magnetization dynamics by spin polarized currents is a very promising application studied heavily for memory applications and more recently for nanoscale oscillators. Of particular interest are MgO-tunnel junction based nanopillaroscillators because of their compatibility with present day electronics, their efficiency and their scalability.
In this work we have studied dynamics induced in nanopillar spin transfer torque oscillators (STO)from MgO-based tunnel junctions. To that end we designed, validated andpatented a redeposition-free process for the fabrication of nanopillarsfrom MgO-based tunnel junctions, based on a specifically designed bilayer e-beam process.
We have performed RF, noise spectroscopy, experiments on a number of nanopillar STOs and found clear spin transfer torqueinduced oscillations evidenced by typical threshold behavior. Study of the modes and comparison to our macromagnetic model shows three types ofoscillations can be excited. We find free layer oscillations and acoustic and optic oscillation modes of the synthetic ferrimagnet system. Thislast oscillation type persists in zero externally applied magnetic field which is indispensable for practical applications. On theoretical grounds two types of spin torque are predicted in tunnel junctions, Slonczewski and field-like torque. From quasistatic measurements of nanopillar astroids and from the tunability of the induced modes with bias we find no significant effects of heating nor field-like torque.
Spin transfer torque ferromagnetic resonance (STT-FMR) experiments allow the study of the amplitudes of the different spin torque components. Comparison to a noise spectroscopy experiment shows the similarities in finding and determining the nature of the present modes. Fits of the STT-FMR peakshape of the lowest order free layer mode for hard axis applied fields show a field like torque that is significantly smaller than the Slonczewski type torque. However the field-like torque is non-zero contrary to our previous experiments.
Tunnel barriers in general, moreover nanometer sized devices with MgO tunnel barriers, are very sensitive to the voltage applied and tend to breakdown prematurely if too high voltages are applied. We have investigated the reliability and find that for the larger sized nanopillars the shape of both the ramped (RVS) and constant stress (CVS) tests indicate intrinsic hard breakdown behavior, ruling out process induced, extrinsic failure. The Weibull slope for these devices is however rather low, likely due to the small thickness of the barrier, reducing the statistics to a single defect breakdown.
Finally the STOs created in this work from MgO-based tunnel junctions were compared to all-metallic systems and to published values for STOs. The tunability and the possibility to engineer the oscillating frequency are very appealing, as is the possibility to produce oscillators that operate in zero field. Nevertheless the output power needs to be increased by several orders of magnitude in single STO devices. To that end the MgO-based system requires research towards improving reliability and increasing the angleof oscillation. Towards that goal, perpendicular polarizers and/or phase locking mechanisms seem to be the only route.
In this work we have studied dynamics induced in nanopillar spin transfer torque oscillators (STO)from MgO-based tunnel junctions. To that end we designed, validated andpatented a redeposition-free process for the fabrication of nanopillarsfrom MgO-based tunnel junctions, based on a specifically designed bilayer e-beam process.
We have performed RF, noise spectroscopy, experiments on a number of nanopillar STOs and found clear spin transfer torqueinduced oscillations evidenced by typical threshold behavior. Study of the modes and comparison to our macromagnetic model shows three types ofoscillations can be excited. We find free layer oscillations and acoustic and optic oscillation modes of the synthetic ferrimagnet system. Thislast oscillation type persists in zero externally applied magnetic field which is indispensable for practical applications. On theoretical grounds two types of spin torque are predicted in tunnel junctions, Slonczewski and field-like torque. From quasistatic measurements of nanopillar astroids and from the tunability of the induced modes with bias we find no significant effects of heating nor field-like torque.
Spin transfer torque ferromagnetic resonance (STT-FMR) experiments allow the study of the amplitudes of the different spin torque components. Comparison to a noise spectroscopy experiment shows the similarities in finding and determining the nature of the present modes. Fits of the STT-FMR peakshape of the lowest order free layer mode for hard axis applied fields show a field like torque that is significantly smaller than the Slonczewski type torque. However the field-like torque is non-zero contrary to our previous experiments.
Tunnel barriers in general, moreover nanometer sized devices with MgO tunnel barriers, are very sensitive to the voltage applied and tend to breakdown prematurely if too high voltages are applied. We have investigated the reliability and find that for the larger sized nanopillars the shape of both the ramped (RVS) and constant stress (CVS) tests indicate intrinsic hard breakdown behavior, ruling out process induced, extrinsic failure. The Weibull slope for these devices is however rather low, likely due to the small thickness of the barrier, reducing the statistics to a single defect breakdown.
Finally the STOs created in this work from MgO-based tunnel junctions were compared to all-metallic systems and to published values for STOs. The tunability and the possibility to engineer the oscillating frequency are very appealing, as is the possibility to produce oscillators that operate in zero field. Nevertheless the output power needs to be increased by several orders of magnitude in single STO devices. To that end the MgO-based system requires research towards improving reliability and increasing the angleof oscillation. Towards that goal, perpendicular polarizers and/or phase locking mechanisms seem to be the only route.
Date:1 Jan 2008 → 31 Dec 2011
Keywords:Nano pillar, Spin torque, Oscillator, Magnetoresistance
Disciplines:Applied mathematics in specific fields, Classical physics, Optical physics, Nanotechnology, Design theories and methods
Project type:PhD project