Lithographic engineering of anisotropies in (Ga,Mn)As

The focus of studies on ferromagnetic semiconductors is moving from material issues to device functionalities based on novel phenomena often associated with the anisotropy properties of these materials. This is driving a need for a method to locally control the anisotropy in order to allow the elaboration of devices. Here we present a method which provides patterning induced anisotropy which not only can be applied locally, but also dominates over the intrinsic material anisotropy at all temperatures

Exploiting Locally Imposed Anisotropies in (Ga,Mn)As: a Non-volatile Memory Device

Progress in (Ga,Mn)As lithography has recently allowed us to realize structures where unique magnetic anisotropy properties can be imposed locally in various regions of a given device. We make use of this technology to fabricate a device in which we study transport through a constriction separating two regions whose magnetization direction differs by 90 degrees. We find that the resistance of the constriction depends on the flow of the magnetic field lines in the constriction region and demonstrate that such a structure constitutes a non-volatile memory device

TurboSMARTS: Accurate microarchitecture simulation sampling in minutes

Recent research proposes accelerating processor microarchitecture simulation through statistical sampling. Prior simulation sampling approaches construct accurate model state for each measurement by continuously warming large microarchitectural structures (e.g., caches and the branch predictor) while emulating the billions of instructions between measurements. This approach, called functional warming, occupies hours of runtime while the detailed simulation that is measured requires mere minutes. To eliminate the functional warming bottleneck, we propose TurboSMARTS, a simulation framework that stores functionally-warmed state in a library of small, reusable checkpoints. TurboSMARTS enables the creation of the thousands of checkpoints necessary for accurate sampling by storing only the subset of warmed state accessed during simulation of each brief execution window. TurboSMARTS matches the accuracy of prior simulation sampling techniques (i.e., ±3% error with 99.7% confidence), while estimating the performance of an 8-way out-of-order superscalar processor running SPEC CPU2000 in 91 seconds per benchmark, on average, using a 12 GB checkpoint library

An Evaluation of Stratified Sampling of Microarchitecture Simulations

Recent research advocates applying sampling to accelerate microarchitecture simulation. Simple random sampling offers accurate performance estimates (with a high quantifiable confidence) by taking a large number (e.g., 10,000) of short performance measurements over the full length of a benchmark. Simple random sampling does not exploit the often repetitive behaviors of benchmarks, collecting many redundant measurements. By identifying repetitive behaviors, we can apply stratified random sampling to achieve the same confidence as simple random sampling with far fewer measurements. Our oracle limit study of optimal stratified sampling of SPEC2K benchmarks demonstrates an opportunity to reduce required measurement by 43x over simple random sampling. Using our oracle results as a basis for comparison, we evaluate two practical approaches for selecting strata, program phase detection and IPC profiling. Program phase detection is attractive because it is microarchitec- ture independent, while IPC profiling directly minimizes stratum variance, therefore minimizing sample size. Unfortunately, our results indicate that: (1) program phase stratification falls far short of optimal opportunity, (2) IPC profiling requires expensive microarchitecture- specific analysis, and (3) both methods require large sampling unit sizes to make strata selection feasible, offsetting reductions in sample size. We conclude that, without better stratification approaches, stratified sampling does not provide a clear advantage over simple random sampling

Lateral magnetic anisotropy superlattice out of a single (Ga,Mn)As layer

We use lithographically induced strain relaxation to periodically modulate the magnetic anisotropy in a single (Ga,Mn)As layer. This results in a lateral magnetoresistance device where two non-volatile magnetic states exist at zero external magnetic field with resistances resulting from the orientation of two lithographically defined regions in a single and contiguous layer.Comment: 5 pages, 7 figure

Simulation sampling with live-points

Current simulation-sampling techniques construct accurate model state for each measurement by continuously warming large microarchitectural structures (e.g., caches and the branch predictor) while functionally simulating the billions of instructions between measurements. This approach, called functional warming, is the main performance bottleneck of simulation sampling and requires hours of runtime while the detailed simulation of the sample requires only minutes. Existing simulators can avoid functional simulation by jumping directly to particular instruction stream locations with architectural state checkpoints. To replace functional warming, these checkpoints must additionally provide microarchitectural model state that is accurate and reusable across experiments while meeting tight storage constraints. In this paper, we present a simulation-sampling framework that replaces functional warming with live-points without sacrificing accuracy. A live-point stores the bare minimum of functionally-warmed state for accurate simulation of a limited execution window while placing minimal restrictions on microarchitectural configuration. Live-points can be processed in random rather than program order, allowing simulation results and their statistical confidence to be reported while simulations are in progress. Our framework matches the accuracy of prior simulation-sampling techniques (i.e., ±3% error with 99.7% confidence), while estimating the performance of an 8-way out-of-order superscalar processor running SPEC CPU2000 in 91 seconds per benchmark, on average, using a 12 GB live-point librar

Detailed transport investigation of the magnetic anisotropy of (Ga,Mn)As

This paper discusses transport methods for the investigation of the (Ga,Mn)As magnetic anisotropy. Typical magnetoresistance behaviour for different anisotropy types is discussed, focusing on an in depth discussion of the anisotropy fingerprint technique and extending it to layers with primarily uniaxial magnetic anisotropy. We find that in all (Ga,Mn)As films studied, three anisotropy components are always present. The primary biaxial along ([100] and [010]) along with both uniaxial components along the [110] and [010] crystal directions which are often reported separately. Various fingerprints of typical (Ga,Mn)As transport samples at 4 K are included to illustrate the variation of the relative strength of these anisotropy terms. We further investigate the temperature dependence of the magnetic anisotropy and the domain wall nucleation energy with the help of the fingerprint method

SimFlex: a fast, accurate, flexible full-system simulation framework for performance evaluation of server architecture

The new focus on commercial workloads in simulation studies of server systems has caused a drastic increase in the complexity and decrease in the speed of simulation tools. The complexity of a large-scale full-system model makes development of a monolithic simulation tool a prohibitively difficult task. Furthermore, detailed full-system models simulate so slowly that experimental results must be based on simulations of only fractions of a second of execution of the modelled system. This paper presents SimFlex, a simulation framework which uses component-based design and rigorous statistical sampling to enable development of complex models and ensure representative measurement results with fast simulation turnaround. The novelty of SimFlex lies in its combination of a unique, compile-time approach to component interconnection and a methodology for obtaining accurate results from sampled simulations on a platform capable of evaluating unmodified commercial workload