A spin-transfer magnetization switching technique is a promising candidate as a writing mechanism for a high-density magnetic random access memory because of its scalability. The required switching current Ic, however, is still too large for this technique to be applied to MRAM using tunneling magnetoresistive devices. Here, it is demonstrated that reducing the saturation magnetization Ms of magnet cells is an effective way to decrease Ic. Use of a CoFeB film with μ0Ms of 0.75T as a magnet cell reduced Ic measured with a continuous current by an order of magnitude. We changed the duration of a writing current pulse from 1μs to 5s to investigate thermal effects on the switching process, and predicted that CoFeB magnet cells with low Ic can be compatible with the thermal durability required for MRAM applications.
Sub-ns magnetization switching has been triggered by spin momentum transfer in pulsed current in pillar shaped CoFe∕Cu∕CoFe trilayers. By analyzing the change in magneto-resistance induced after the application of individual short current pulses (100ps–10ns), we measured the probability of magnetization reversal as a function of the current pulse magnitude, polarity and duration, at various temperatures between 150 and 300K. At all studied temperatures, the reversal process can take place within a few 100ps. The energy cost of the reversal scales favorably with the switching speed and decreases in the 1pJ range when using 100ps current pulses at 300K. Significantly higher switching speeds are obtained at lower temperatures, which is opposite to a thermal activation of the reversal.
We study the speed of the magnetization switching resulting from spin transfer in pillar-shaped CoFe∕Cu∕CoFe spin valves and the temperature dependence thereof. The switching speed was investigated with current pulses of durations from 100ps to dc while the temperature was varied from 50to300K. Quasistatic loops indicate that the reversal events imply transition states with reduced remanences. Their interval of occurrence shrinks gradually to almost null when the temperature is raised to 300K. The curvature of resistance versus current hysteresis loops is different in the antiparallel and parallel branches, which evidences the influence of the Ampere field on the quasistatic micromagnetic configuration. In the dynamical regime, the pulse-induced parallel to antiparallel transition speed is not much temperature dependent from 50to300K. In contrast, the pulse-induced antiparallel to parallel transition is thermally disfavored and much faster at 150K than at 300K. We model the experimental behavior by a competition between thermal fluctuations and the Ampere-field-related C-like bending of the magnetization in the free layer. The contribution of the Ampere field dominates in most cases. These contributions are amplified or damped together by the spin-transfer torque, but since the C-like bending is a response to a magnetic field, it sets in at a gradual pace ruled by the classical Gilbert relaxation. Most of the difference between quasistatic switching and pulse-induced switching results from this complete or incomplete alignment with the total effective field. Our demonstration of 100-ps switching validates spin-transfer switching for fast memory applications.
A'procedure is described for analysing the fine structure of intermediate structure resonances in which the matrix elements connecting the special state with the ordinary states are extracted directly. The procedure was tested numerically, and found to be satisfactory for cases in which background contributions to the widths are small. The method is applied to several narrow subthreshold fission resonances.
NUCLEAR REACTIONS Analysis of intermediate structure fine structure;Np(n, f), 238pu(n, f), w ( n , f); deduced spreading widths of subthreshold fission I resonances. 23'? Permanent address: AERE, Harwell, England.
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