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Schematic diagram of the feedback oscillator circuit. The top layer of the MTJ pillar shows the free layer, middle layer shows the tunnelling barrier, and bottom layer shows the pinned layer. The MTJ rests on the top of a waveguide which is electrically insulated from the MTJ. The wave guide is terminated into a resistance RT as shown. A fluctuating voltage is produced across the MTJ by the dc current and thermal fluctuations of the free layer. This drives a fluctuating current through the bottom waveguide, and exerts a fluctuating magnetic field on the free layer. The phase of the magnetic field with respect to the free layer oscillation can be adjusted by the adjustable delay. By choosing a suitable value of the phase, the free layer fluctuations can be amplified.
The spectral density of spectral density of my as a function of frequency at 300 K numerically calculated from Eq. (1) including the feedback term. The green, black, and red curves show the spectral density with −0.2 mA, 0 mA, and 0.2 mA dc current, respectively. The damping is enhanced for positive current and reduced for negative current. The blue curve shows the large spectral density obtained with −1 mA current. The spectral density corresponding to the blue curve is divided by 100 in the figure.
The critical voltage required for spontaneous oscillations as a function of the width of the free layer. The width of the feed-back waveguide is assumed to be equal to the width of the free layer. The length of the free layer is taken to be 4 times the width. Here, we have assumed a MTJ with resistance-area product = 3 Ω(μm)2, α = 10−2, γ = 2.21 × 105 m/(A s), MR = 100%, RT = 50 Ω, and resonance frequency = 2.8 GHz.
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