Point Contact Microwave Oscillations

The point contact nano-oscillators fabricated in this work were observed to generate microwave emission upon the application of a direct current. The setup used to assess RF emission was introduced in Section 3.4.3. The RF measurements discussed here were performed on a 100 nm (design) point contact device with a nominal resistance of 19.4 Ω due to the increased series resistance of the CPW electrodes in a two-probe measurement and featuring a magnetoresistance change of 40 mΩ at 2 mA sense current, as is displayed in Figure 5-19.

Figure 5-19: Magnetoresistance change associated with the free magnetic layer switching its direction is approximately 40 mΩ for a 100 nm point contact oscillator.

Although no convincing high frequency RF oscillations were observed on the fabricated samples, interestingly, emission was discovered in the low frequency spectrum, ranging from 200 to 400 MHz, for out-of-plane external magnetic fields (0 to 346 mT). Figure 5-20 displays a set of RF emission spectra as a function of DC

current in the 220 to 420 MHz frequency range, which were recorded for a 301 mT out-of-plane external field (i.e. perpendicular to the thin film plane). Current-tunability of the low frequency emission peaks is demonstrated as the peaks blue-shift (i.e. towards higher frequency) with increasing current. Additionally, the linewidth of the emission peaks increases considerably for increasing current. Emission is only observed for one particular direction of the current through the magnetic multilayer.

Therefore, a positive current is defined to flow from the fixed to the free magnetic layer, corresponding to electrons flowing from the free to the fixed layer. For the application of an out-of-plane external field, emission was only observed for negative currents.

Figure 5-20: Low frequency RF power spectra of a 100 nm point contact device for a 301 mT out-of-plane field and varying negative DC current. The low frequency emission peaks shift towards higher frequencies for increasing current. At the same time, linewidth broadening of the peaks for increasing current can be observed.

Figure 5-21 displays point contact device RF emission spectra in the 200 to 1400 MHz frequency range (x-axis) for negative currents (y-axis) as a function of increasing external field (top to bottom panel). The applied field is stepped between 80, 147, 222, 301 and 346 mT. For the lowest field, no RF emission is observed (the features at 890 MHz are artifacts resulting from cellular network bleed into the measurement setup). For slightly higher fields, emission is observed with a current threshold that decreases with applied field. Harmonics of the fundamental mode are also clearly visible. For the highest field, oscillations start at currents around 22 mA.

For each distinct field setting, the emission frequency increases with DC current, as was already observed in Figure 5-20. Additionally, the emission frequency seems to decrease with increasing applied field, most notably when going from 222 mT to 301 mT. Linewidth broadening is typically observed for increasing currents. Although the measurements indicate a field dependence of the threshold current related to the onset

of the dynamics, it is not easy to quantify its value due to the typical current- and field-hysteretic nature of the magnetization dynamics [47]. For example, it is observed that once the precession has been started, either or both field and current can be reduced considerably, while the dynamics pertain.

Figure 5-21: RF emission from a point contact device in the 200 to 1400 MHz frequency range as a function of applied field and DC current. In this experiment, a negative current is applied to the point contact device, corresponding to electrons flowing from the fixed to the free magnetic layer. The field is applied out-of-plane and is varied from top to bottom between 80, 147, 222, 301 and 346 mT. For the lowest field, no RF emission is observed, while for higher fields, a current threshold is noticed. For the highest fields, oscillations start at currents around 22 mA. The emission frequency seems to decrease with applied field, featuring additional broadening of the high current peaks for the highest field value.

The observation of low frequency dynamics in point contact devices is relatively new and has only recently also been reported by another group [47]. However, the suspected mechanism that causes the low frequency oscillations is thought to be understood for some time from predictions [48] and observations [49][50] of the gyrotropic motion of a vortex core in patterned micrometer size NiFe disks.

80 mT

147 mT

222 mT

301 mT

346 mT

According to micro-magnetic simulations, a spin-transfer current is also able to move a vortex core around its equilibrium position [51], which supports that the observed low frequency oscillations may indeed be due to the current-driven motion of a vortex core domain state under the point contact area.

Although emission in the higher frequency range may be more interesting for the application of point contact devices in wireless applications, especially when the observation of spin waves in optical BLS experiments is aimed for, the current samples only revealed interesting phenomena in the low frequency range. It is suggested that the low frequency vortex modes dominate in the current samples, due to the larger than expected point contact sizes (and thus, increased Oersted fields, which act as a restoring force for the non-uniform vortex mode). Consequently, to eliminate these vortex modes, the point contact fabrication process should be further refined to deliver smaller point contacts. This may require the consideration of a different etching technique, or the existing PMMA process should be further optimized. The combination of a thinner SiO2 passivation layer and a thinner resist layer (currently 300 nm PMMA) could increase the resolution of the definition process and improve the point contact sizes, so that eventually the low frequency vortex oscillations can be suppressed in favor of high frequency phenomena.