Multilevel cell is an important concept to improve the density of spin transfer torque memory. We demonstrated two-bit multilevel cell spin transfer switching using MgO-based magnetic tunnel junctions. Two types of cell structural design are discussed. Multiple resistance levels are depicted in a current-driven spin transfer switching loop, which shows a switching current density of ∼2×106A∕cm2. Reversible transitions between high and low states of the soft bit are achieved using minor-loop spin transfer switching. The influence of external magnetic field to spin transfer switching is also discussed.

The exchange bias between an antiferromagnet (AF) and a ferromagnet (FM) vanishes at a temperature called the “blocking” temperature. Measurements are presented that show that the blocking temperature decreases with decreasing the thickness of the AF. This behavior is explained on the basis of a thermal fluctuation model, from which an analytic expression for the blocking temperature as a function of the antiferromagnetic thickness is obtained. This study also gives a quantitative understanding of the relationship between the blocking temperature and the AF grain size.

Measurements of noise properties of MgO-based magnetic tunnel junctions were performed in the frequency range from 100MHzto18GHz. A pronounced narrow peak at the frequency of around 7GHz with a full width at half-maximum as low as 21MHz has been observed. The peak amplitude and 1∕f noise have a threshold dependence on the dc bias current. The narrow peak is present only at one current polarity for an antiparallel orientation of the magnetic layers adjacent to the barrier, which is consistent with spin-momentum transfer theoretical predictions.

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We have investigated high-quality MgO tunnel junctions with a range of barrier thickness in order to identify the underlying physical mechanism responsible for dielectric breakdown. Two types of dielectric breakdown (“soft” and “hard”) were observed. Soft breakdown was observed in a few percent of the devices. This breakdown mode is not intrinsic and is attributed to tunnel junction imperfections. The hard breakdown occurs because a critical electric field is reached across the tunnel barrier. Other possible breakdown mechanisms, such as thermally driven mass diffusion or charge trapping, were not consistent with the hard dielectric breakdown data and were ruled out.