Spin-transfer Magnetic Random Access Memory Research Articles

Sub-nanosecond spin-torque switching of perpendicular magnetic tunnel junction nanopillars at cryogenic temperatures

Spin-transfer magnetic random access memory devices are of significant interest for cryogenic computing systems where a persistent, fast, low-energy consuming, and nanometer scale device operating at low temperature is needed. Here, we report the low-temperature nanosecond duration spin-transfer switching characteristics of perpendicular magnetic tunnel junction (pMTJ) nanopillar devices (40–60 nm in diameter) and contrast them to their room temperature properties. Interestingly, the characteristic switching time decreases with temperature, with the largest reduction occurring between room temperature and 150 K. The switching energy increases with decreasing temperature, but still compares very favorably with other types of spin-transfer devices at 4 K, with <300 fJ required per switch. Write error rate (WER) measurements show highly reliable switching with WER ≤ 5 × 10–5 with 4 ns pulses at 4 K. Our results demonstrate the promise of pMTJ devices for cryogenic applications and show routes to further device optimization.

Spin-transfer magnetic random access memory devices with an orthogonal polarizing layer

Spin-transfer magnetic random access memory (MRAM) devices with a polarizing layer magnetized perpendicular to the free and the reference layers are believed to improve the writing performance by inducing a large spin-transfer torque on the free layer at the beginning of the switching process. Experimental realizations of such devices, both with all-metal structures and magnetic tunnel junctions have been made. Faster switching with less energy cost in the orthogonal devices has been achieved comparing to their collinear counterparts. In addition, the processional switching process in such devices has been demonstrated in both statistical and time-resolved measurements. Although further theoretical and material studies are needed for thorough understanding of the switching process and improving the device performance, the orthogonal MRAM devices hold great potential for memory applications operating at low temperatures as well as those that require fast writing speed.

Annular Spin-Transfer Memory Element

An annular magnetic memory that uses a spin-polarized current to switch the magnetization direction or helicity of a magnetic region is proposed. The device has magnetic materials in the shape of a ring (1 to 5 nm in thickness, 20 to 250 nm in mean radius and 8 to 100 nm in width), comprising a reference magnetic layer with a fixed magnetic helicity and a free magnetic layer with a changeable magnetic helicity. These are separated by a thin non-magnetic layer. Information is written using a current flowing perpendicular to the layers, inducing a spin-transfer torque that alters the magnetic state of the free layer. The resistance, which depends on the magnetic state of the device, is used to read out the stored information. This device offers several important advantages compared to conventional spin-transfer magnetic random access memory (MRAM) devices. First, the ring geometry offers stable magnetization states, which are, nonetheless, easily altered with short current pulses. Second, the ring geometry naturally solves a major challenge of spin-transfer devices: writing requires relatively high currents and a low impedance circuit, whereas readout demands a larger impedance and magnetoresistance. The annular device accommodates these conflicting requirements by performing reading and writing operations at separate read and write contacts placed at different locations on the ring.

Ultrafast switching in magnetic tunnel junction based orthogonal spin transfer devices

Orthogonal spin-transfer magnetic random access memory (OST-MRAM) 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.