Route Towards Efficient Magnetization Reversal Driven by Voltage Control of Magnetic Anisotropy (original) (raw)
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Current-induced magnetization reversal in nanopillars with perpendicular anisotropy
Nature Materials, 2006
Devices that show a magnetic anisotropy normal to the film surface hold great promise towards faster and smaller magnetic bits in data-storage applications. We describe an experimental demonstration of current-induced magnetic reversal of nanopillars with perpendicular anisotropy and high coercive fields. The best results are observed for Co/Ni multilayers, which have higher giant magnetoresistance values and spin-torque efficiencies than Co/Pt multilayers. The reference layers were designed to have significantly higher anisotropy allowing a complete current-field phase diagram of the free-layer reversal to be explored. The results are compared to micromagnetic modelling of the free layer that, depending on the bias current and applied field, details regions of irreversible magnetic switching, coherent and incoherent spin waves, or static non-uniform magnetization states. This ability to manipulate high-anisotropy magnetic elements could prove useful for a range of spintronic applications.
AIP Advances
In this paper we present a detailed numerical study of magnetization switching in shape-anisotropic thin-film nanoelements. These elements are at present of the major interest for the applied solid state magnetism as main components of a new generation of conventional and spintransfer-torque (STT) magnetic random access memory (MRAM) cells. To conduct this study, we have developed a highly efficient method for massively parallel micromagnetic simulations of the magnetization reversal in small-size nanoelements, which allows to fully exploit the large performance gain available on the GPU architecture (usually achievable only for large systems). We apply our method to the spin-torqueinduced magnetization switching in elliptical nanoelements in presence of thermal fluctuations. Being able to compute simultaneously the reversal of up to 1000 such elements, we obtain the dependence of (i) the average switching time and (ii) the distribution density of switching times for individual elements on the element size with a high statistical accuracy. Analysis of these dependencies provides important insights into the physics of magnetization reversal in such systems. Comparison with analogous simulations in the macrospin approximation allows to determine the validity limits of the macrospin model. Our methodology can be applied for the optimization of the MRAM design regarding the information life time and significantly improve the prediction accuracy of write and read error rates of conventional and STT-based MRAM cells.
Voltage-Controlled Magnetic Anisotropy in Spintronic Devices
SPIN, 2012
Electric-field-control of magnetism can dramatically improve the energy efficiency of spintronic devices and enhance the performance of magnetic memories. More generally, it expands the range of applications of nonvolatile spintronic devices, by making them energetically competitive compared to conventional semiconductor solutions for logic and computation, thereby potentially enabling a new generation of ultralow-power nonvolatile spintronic systems. This paper reviews recent experiments on the voltage-controlled magnetic anisotropy (VCMA) effect in thin magnetic films, and their device implications. The interfacial perpendicular anisotropy in layered magnetic material stacks, as well as its modulation by voltage, are discussed. Ferromagnetic resonance experiments and VCMA-induced high-frequency magnetization dynamics are reviewed. Finally, we discuss recent progress on voltage-induced switching of magnetic tunnel junction devices and its potential applications to magnetic random a...
Precessional Switching of Thin Nanomagnets with Uniaxial Anisotropy
Topics in Applied Physics, 2006
This review describes the evolution of the magnetization of uniaxial thin magnets when subjected to fast-rising magnetic field pulses. We report detailed "all electrical" experimental investigations of precessional switching on soft uniaxial micron-sized thin magnets, and we discuss them using a comprehensive, mostly analytical framework. General criteria are derived for the analytical assessment of the switching ability of any arbitrary set of experimental parameters. For this, we start from the Landau-Lifshitz equation and first consider the precessional switching in a much idealized macrospin, easy-plane loss-free system. We then test the main outputs of this model with time-resolved experiments on advanced MRAM cells. Using applied fields above the anisotropy field Hk, we prove the quasi-periodic nature of the magnetization trajectory and we demonstrate experimental conditions ensuring a sub-200ps ballistic magnetization reversal. We then upgrade our model accuracy by taking into account the uniaxial anisotropy and the behaviour in hard axis fields of the order of Hk. We derive a simple though reliable estimate of the switching speed; its limiting factors highlight the experimental poor switching reproducibility when close to the minimal hard axis reversal field Hk/2. The latter field does not correspond to the minimal energy cost of the reversal, whose prospective evolution in the future generations of MRAM is predicted. Small departures from the macrospin state are discussed. The effect of damping is modelled using perturbation theory. Finite damping alters the precessional motion periodicity and puts some constraints on the field rise time. A special focus is dedicated to the relaxation-dominated precessional switching: the minimal hard axis field triggering the switching is shown to be above Hk/2 by an extra field cost linked to the damping constant times the square root of MSHk. Finally, the selective addressing and the direct-write of a magnetic cell with combined easy axis and hard axis fields are studied. We introduce the concept of bounce and revisit the dynamical astroïd to derive the related characteristic reversal durations and their margins. We propose a field timing that is immune to the delay jitter between the combined addressing fields. We finish by investigating shortly the challenges and the promises of the "precessional" strategy for future MRAM generations.
Thermally assisted spin-transfer torque magnetization reversal in uniaxial nanomagnets
Applied Physics Letters, 2012
We simulate the stochastic Landau-Lifshitz-Gilbert (LLG) dynamics of a uniaxial nanomagnet out to sub-millisecond timescales using a graphical processing unit based micromagnetic code and determine the effect of geometrical tilts between the spin-current and uniaxial anisotropy axes on the thermally assisted reversal dynamics. The asymptotic behavior of the switching time (I → 0, τ ∝ exp(−ξ(1−I) 2)) is approached gradually, indicating a broad crossover regime between ballistic and thermally assisted spin transfer reversal. Interestingly, the mean switching time is shown to be nearly independent of the angle between the spin current and magnet's uniaxial axes. These results have important implications for modeling the energetics of thermally assisted magnetization reversal of spin transfer magnetic random access memory bit cells.
Nonuniform switching of the perpendicular magnetization in a spin-torque-driven magnetic nanopillar
Physical Review B, 2011
Time-resolved scanning transmission x-ray microscopy (STXM) measurements were performed to study the current-induced magnetization switching mechanism in nanopillars exhibiting strong perpendicular magnetic anisotropy (PMA). This technique provides both short time (70 ps) and high spatial (25 nm) resolution. Direct imaging of the magnetization demonstrates that, after an incubation time of ∼ 1.3 ns, a 100 × 300 nm 2 ellipsoidal device switches in ∼ 1 ns via a central domain nucleation and opposite propagation of two domain walls towards the edges. High domain wall velocities on the order of 100 m/s are measured. Micromagnetic simulations are shown to be in good agreement with experimental results and provide insight into magnetization dynamics during the incubation and reversal period.
Ultra-fast magnetization reversal in magnetic nano-pillars by spin-polarized current
Journal of Magnetism and Magnetic Materials, 2005
We study the speed limitations of the magnetization switching resulting from spin transfer in pillar-shaped CoFe/Cu/ CoFe spin valves. The quasi-static critical currents are I cÀ ¼ À2 mA for the antiparallel (AP) to parallel (P) configuration and I cþ ¼ þ4:6 mA for the P to AP transition. Current pulses of duration down to 100 ps and amplitude of 4I c trigger switching at 300 K. The switching is probabilistic for lower current pulses. The P to AP transition speed is not much temperature dependant from 50 to 300 K. In contrast, the AP to P transition is thermally inhibited and is much faster at 150 K than at 300 K. This thermal inhibition highlights the importance of the macrospin coherency and of the thermally excited spin waves with finite wave vector parallel to the magnetization. Our results validate spintransfer switching for fast memory applications.
Precessional switching of thin nanomagnets: analytical study
The European Physical Journal B - Condensed Matter, 2003
We study analytically the precessional switching of the magnetization of a thin macrospin. We analyze its response when subjected to an external field along its in-plane hard axis. We derive the exact trajectories of the magnetization. The switching versus non switching behavior is delimited by a bifurcation trajectory, for applied fields equal to half of the effective anisotropy field. A magnetization going through this bifurcation trajectory passes exactly along the hard axis and exhibits a vanishing characteristic frequency at that unstable point, which makes the trajectory noise sensitive. Attempting to approach the related minimal cost in applied field makes the magnetization final state unpredictable. We add finite damping in the model as a perturbative, energy dissipation factor. For a large applied field, the system switches several times back and forth. Several trajectories can be gone through before the system has dissipated enough energy to converge to one attracting equilibrium state. For some moderate fields, the system switches only once by a relaxation dominated precessional switching. We show that the associated switching field increases linearly with the damping parameter. The slope scales with the square root of the effective anisotropy. Our simple concluding expressions are useful to assess the potential application of precessional switching in magnetic random access memories.