Faculty of Engineering and Natural Sciences
State Memory: On The Road To Ultra-High Density Storage
Hitachi Global Storage Technologies,
San Jose Research Center,
Storing information in more and more compact ways, going well beyond what has been achieved in today’s technology, is a great challenge. There are two mainstream approaches that are being pursued to push the technological limitations on data storage density; one is the semiconductor device approach, such as flash technology, where information is stored as charge trapped on a gate electrode, and the other one being the magnetic storage, such as hard drive technology, where information is stored as small magnetic domains on a continuous medium.
Future improvements in the data storage density are limited in both technologies due to thermal stability issues in magnetic recording and charge leakage in flash memory devices making it very difficult to retain stored data for long periods of time. In this talk I will discuss the nanoscale physics of chalcogenide-based phase change memory devices and stacked magnetic nanowires developed to realize reliable data storage with ultra-high densities by utilizing the 3rd dimension. Dramatic and ultra-fast (nanosecond scale) changes in the physical properties such as electrical resistivity and optical reflectivity upon amorphization or crystallization makes chalcogenides ideal potential candidates for a universal non-volatile memory device, especially if, the device switching characteristics are highly scalable to nanometer dimensions. In the first part of my talk I will introduce a resistance mapping technique whereby we apply electrical pulses of various amplitudes, widths and trailing edges and deduce the resulting phase composition in nanoscale Ge2Sb2Te5 nanopillar devices. Such resistance map measurements provide a detailed picture of the scaling behaviour of the critical phase change conditions and the associated switching dynamics. A comparison of the observed characteristics with a three-dimensional finite element model (3D-FEM) of the electro-thermal physics enables a good assessment of the resulting device performance while providing physical insights into the nature of resistance switching behaviour.
In the second part of my talk I will discuss an alternative, magnetic approach using stacks of magnetic nanowires to store and transfer magnetic bits. We use the localized stray field from a magnetic tip to write single-domains in a CoNi/Pd magnetic layer by modulating its coercivity via instantaneous and localized heating from a current pulse. Our studies have shown threshold current densities on the order of ~107 A/cm2 for 10 ns write times. I will present our detailed investigation of the resulting domain structure in nanowires with various widths as a function of the current pulse amplitude and width to vary the local temperature and the tip height to vary the local field. I will also discuss the nature of current-induced demagnetization limit and thermally-induced domain transfer between magnetic layers and compare the results with 3D finite element simulations of the thermal distribution profile.
November 26, 2008, 13:40,