20 septembre 2009

Classical and precessional reversal

Spintronics created magnetic memories and radio-frequency oscillators based on similar stacks of magnetic and non-magnetic layers. In an oscillator, a continuous current of spin-polarized electrons flowing across the stack produces a steady magnetization precession of one layer, at the origin of the radio-frequency emission. With a current pulse of controlled width, it is possible to switch the direction of magnetization, i.e., to write information in a memory cell. Spintec is investigating this mechanism to create ultra-fast, non-volatile memories.


The idea is to switch the storage layer direction of the memory cell, by producing a half-precession. This is achieved by applying a current pulse through the structure for a total duration that is equal to a half-period precession. Starting from an initial configuration, where magnetization of the reference and storage layers are parallel, a pulse of 400ps makes the storage layer magnetization direction turn 180°. A pulse of 800ps produces a full 360° rotation. One could continue to 1200 and 1600ps, but the progressive loss in precession coherence reduces the switching probability. At low temperature, it is possible to achieve 6 half-turns.


Increasing the pulse current amplitude increases the switching/rotation speed. This opens the way for ultra-fast switching of MRAM cells. Also, a write time below 1 nanosecond, close to that of MOS transistors, would allow for new functionalities in Magnetic Tunnel Junctions, notably for non-volatile magnetic logic applications.


Further reading: J. Herault, et al., Journal of Applied Physics 106 (2009) 014505; C. Papusoi, et al., Applied Physics Letters 95 (2009) 072506


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