The effect of thermal fluctuations on spin-transfer switching has been studied for a broad range of time scales (sub-ns to seconds) in a model system, a uniaxial thin film nanomagnet. The nanomagnet is incorporated into a spin-valve nanopillar, which is subject to spin-polarized current pulses of variable amplitude and duration. Two physical regimes are clearly distinguished: a long pulse duration regime, in which reversal occurs by spin-transfer assisted thermal activation over an energy barrier, and a short time large pulse amplitude regime, in which the switching probability is determined by the spin angular momentum in the current pulse.

Orthogonal spin-transfer magnetic random access memory ͑OST-MRAM͒ uses a spin-polarizing layer magnetized perpendicularly to a free layer to achieve large spin-transfer torques and ultrafast energy efficient switching. We have fabricated and studied OST-MRAM devices that incorporate a perpendicularly magnetized spin-polarizing layer and a magnetic tunnel junction, which consists of an in-plane magnetized free layer and synthetic antiferromagnetic reference layer. Reliable switching is observed at room temperature with 0.7 V amplitude pulses of 500 ps duration. The switching is bipolar, occurring for positive and negative polarity pulses, consistent with a precessional reversal mechanism, and requires an energy of less than 450 fJ.

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Spin-transfer switching with short current pulses has been studied in spin-valve nanopillars with perpendicularly magnetized free and reference layers. Magnetization switching with current pulses as short as 300 ps is demonstrated. The pulse amplitude needed to reverse the magnetization is shown to be inversely proportional to the pulse duration, consistent with a macrospin spin-transfer model. However, the pulse amplitude duration switching boundary depends on the applied field much more strongly than predicted by the zero temperature macrospin model. The results also demonstrate that there is an optimal pulse length that minimizes the energy required to reverse the magnetization.

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