Coherent microwave generation by spintronic feedback oscillator

Abstract

The transfer of spin angular momentum to a nanomagnet from a spin polarized current provides an efficient means of controlling the magnetization direction in nanomagnets. A unique consequence of this spin torque is that the spontaneous oscillations of the magnetization can be induced by applying a combination of a dc bias current and a magnetic field. Here we experimentally demonstrate a different effect, which can drive a nanomagnet into spontaneous oscillations without any need of spin torque. For the demonstration of this effect, we use a nano-pillar of magnetic tunnel junction (MTJ) powered by a dc current and connected to a coplanar waveguide (CPW) lying above the free layer of the MTJ. Any fluctuation of the free layer magnetization is converted into oscillating voltage via the tunneling magneto-resistance effect and is fed back into the MTJ by the CPW through inductive coupling. As a result of this feedback, the magnetization of the free layer can be driven into a continual precession. The combination of MTJ and CPW behaves similar to a laser system and outputs a stable rf power with quality factor exceeding 10,000.

Figure 1

(a) Schematic diagram of the feedback oscillator. The top layer of the MTJ pillar shows the free layer, middle layer shows the tunneling barrier, and the bottom layer shows the pinned layer. A coplanar waveguide (CPW) rests on top of the free layer and is electrically insulated from the MTJ. A DC current is passed through MTJ via Bias-T. The oscillating voltage produced across the MTJ, due to the oscillations of free layer magnetization, is split into two paths using a power splitter. One part is amplified, using an amplifier in the feedback circuit, and fed into the CPW. The second part of oscillating voltage is observed on the spectrum analyzer. The oscillating current in the CPW creates an ac magnetic field on the free layer, which acts as the feedback. The phase between the free layer magnetization oscillation and the ac magnetic field can be adjusted by the phase shifter. (b) Frequency of the peak in the noise spectrum as a function of magnetic field applied along the y-direction, for I_dc = −2 mA. The inset shows the noise spectrum obtained for H = 70 Oe and I_dc = −2 mA. The noise spectra were measured by disconnecting the feedback waveguide. The data is taken for sample A.

Figure 2

(a) Power spectral density (PSD) as a function of dc current ranging from −2.2 mA to −2.7 mA, with an applied magnetic field of 58 Oe along the y-axis. The amplifier gain was set to +24 dB. Inset shows the power spectral density for low dc current values, ranging between −1.7 mA to −2.1 mA. As the dc bias current increases, the peaks grow in amplitude and become sharper. The PSD for different currents shown in the inset are multiplied by various factors for clear visibility. (b) Variation of frequency and line width as a function of bias current. The narrow line width of 200 kHz obtained at −2.7 mA corresponds to a quality factor of ~12800. (c) The total power output as a function of dc bias current. The data is taken for Sample A.

Figure 3

(a) Power spectra of the plots shown in Fig. 2(a) in log scale. The side peaks can be clearly seen. (b) Power spectral density for the case of no feedback (red curve) and for case when gain of the amplifier is +10 dB (green curve). The green curve has been shifted vertically upwards for clarity. It shows some side peaks along with the fundamental peak. (c) Power spectral density when gain of the amplifier is +20 dB. The side peaks are more evident in this case. (d) Power spectral density for amplifier gain of +33 dB. In this case the intensity of fundamental peak is enhanced greatly and consequently the side peaks are not visible. The linewidth of the peak decreases with increasing gain. The data for graphs 3(b–d) is taken using sample B with H = 92 Oe and I_dc = 1 mA.

Figure 4. Comparison of the amplification processes in a laser and feedback oscillator.

(a) Schematic diagram of amplification of photons by stimulated emission in a laser (b) Schematic diagram of the amplification of microwave photons by a combination of MTJ powered by a dc bias current and a co-planar wave guide (CPW). A microwave photon incident on the CPW, excites magnons in the free layer. The magnons in the free layer, generate microwaves due to the TMR effect and a dc bias current. If the TMR effect is large (or if dc bias current is large), the incident microwaves can be amplified.

Figure 5

Simulation results: (a) The spectral density of m_x for various dc current values. A magnetic field of 60 Oe was applied along y axis and amplifier with gain of +21 dB was assumed in the feedback line. As the dc bias current is increased the peaks in the spectral density grow in amplitude and become sharper indicating improvement in the linewidth of the peak. (b) The magnetization of each cell on the major axis of the ellipse was recorded as a function of time. The cross-correlation (at 0 lag) between the m_x at the center and m_x along the axis is plotted for different values of dc bias current. For low values of bias currents, the cross-correlation as a function of distance decays to zero rapidly. For larger bias currents, where the amplitude of peak in spectral density is large, the cross-correlation remains large even near the sample edges. Thus the entire sample oscillates coherently for large currents.

Figure 6. Power output as a function of dc bias current for amplifier gain of +29 dB and H = 70 Oe.

The data is taken using sample D.