Volume 91, Issue 10, 15 May 2002
Index of content:
91(2002); http://dx.doi.org/10.1063/1.1447530View Description Hide Description
In this work a synthetic ferrimagnet structure as a spin dependent tunnel junction free layer is proposed as a magnetic random access memories cell. The CoFe/Ru/CoFe and CoFe/NiFe/Ru/NiFe/CoFe systems show antiferromagnetically coupling range >1000 Oe and rotate rigidly anti-aligned with coercive fields below 30–40 Oe. Tunnel junctions were patterned to 4 μm×2 μm rectangles and ellipses using standard CoFe/Ni and synthetic ferrimagnet free layers. Identical signal levels are measured (17.8% and 16.3% as-deposited). Switching fields are dominated by material anisotropy for the synthetic ferrimagnet structure. Both free layer types show reversal by domain wall motion. Less switching dispersion is found for synthetic ferrimagnet free layer (≈10 Oe), with magnetization reversal occurring mostly in sharp single steps.
Effects of two in-plane fields on the magnetization reversal mechanism in magnetic tunnel junction elements91(2002); http://dx.doi.org/10.1063/1.1452264View Description Hide Description
One of the potential applications of the spin tunnel junction is as magnetic random access memory (MRAM) elements. For MRAM application purposes, two in-plane perpendicularly applied fields are required during magnetization reversal. In this article, Lorentztransmission electron microscopy has been used to study the magnetization reversal mechanism of tunnel junction elements under the influence of two in-plane perpendicular fields. Four hundred elements with differing shapes and sizes have been observed. When an in-plane Y field was applied to the hard axis followed by sweeping the X field along the easy axis, it was found that the number of elements in which 360° domain walls are observed to form decreases as the strength of the Y field increases, and that the magnetization reversal mechanism in the tunnel junction elements gradually changes from a domain wall motion mechanism to a moment rotation mechanism as the Y field increases. As expected, the reversal field (X field) reduces as the Y field increases.
91(2002); http://dx.doi.org/10.1063/1.1454977View Description Hide Description
In this article, the effect of thermal fluctuations on switching field at short and long time scales have been investigated by micromagnetic simulation with the Langevin and the time-temperature scaling methods. When the film thickness and/or the aspect ratio are reduced, the switching field decreases substantially owing to the decrease of shape anisotropy and the increase of thermal fluctuation. In the soft magnetic film with a dimension of the calculated switching field at a pulse width of 10 ns is 32 Oe, but estimated retention time is less than 1000 s. In such a thermally unstable film, thermally activated switching of magnetization occurs by complex behaviors with creations and annihilations of small and slightly tilted magnetic clusters.
91(2002); http://dx.doi.org/10.1063/1.1452265View Description Hide Description
The thermal decay of nominally rectangle bar arrays with different sizes and and thicknesses (7.9, 10.0, 18.3, and 43.5 nm) has been studied. Observation of the remanent domain structure using magnetic force microscopy showed that the bars were uniformly magnetized with simple end curling in the thinnest samples, with simple end curling and end vortices in the intermediate samples, and with only end vortices in the thickest samples. The 1 μm wide elements switched abruptly from the “up” polarized domain to the “down” polarized domain state during reversal. Time-dependent coercivitymeasurements were analyzed using Sharrock’s formula to extract the intrinsic switching fields and the thermal sensitivity factors. The latter increased from 113 to 243 in the 7.9-nm-thick samples to 167–628 in the 18.3 nm samples, but decreased precipitously to 66–94 for the 43.5 nm samples. This behavior is correlated to the element’s size and the end domain structure.
91(2002); http://dx.doi.org/10.1063/1.1447180View Description Hide Description
The development of magnetic random access memory requires very reliable magnetic tunnel junctions (MTJs). However these devices are prone to dielectric breakdown. To investigate the reliability of the MTJs, we performed constant voltage stress tests. This study shows the area scaling and the voltage dependence of the time-to-breakdown of exchange biased, naturally oxidized MTJs. The fraction of broken devicesF is plotted on a Weibull scale. The Weibull fit of our data shows clearly that scales with the area, meaning that the breakdown sites are randomly distributed. Moreover, the Weibull distributions have a constant slope β (=0.35) and show no tails. This suggests that only one breakdown mode is observed. It is not clear yet whether an intrinsic or extrinsic failure mechanism is causing the breakdown, but considering the small Weibull slope, extrinsic breakdown caused by randomly distributed process-induced defects in the oxide is most plausible. Second, the voltage dependence of is studied. To fit the data, a linear dependence of on stress voltage is assumed. It is however not yet proven whether this model is applicable for ultrathin barriers.