自旋力矩振子中的非白频率噪声及其对谱线宽度的影响
Non-white frequency noise in spin torque oscillators and its effect on spectral linewidth
摘要：
我们测量的功率谱密度的频率波动在nanocontact自旋扭矩振荡器的时间尺度高达50 ms。我们使用混频器转换范围从10 GHz到40 GHz的振荡器信号到70 MHz附近的频带，然后数字化的时域波形。我们分析的波形使用过零时间戳和滑动傅立叶变换，讨论这两种方法的不同的局限性和优点，并将它们结合起来，以获得一个频率噪声频谱跨越五十多年的傅立叶频率$f$。对于具有由单个Ni $_{\text{}80}$ Fe $_{\text{}20}$层或Co/Ni多层组成的自由层的器件，我们发现频率噪声谱在大$f$处为白色，并且在小$f$处随\emph{$1/f$}变化。交叉频率范围从$\approx\unit[10^{4}]{Hz}$到$\approx\unit[10^{6}]{Hz}$，并且多层器件中的$1/f$分量更强。通过实际的和模拟的频谱分析仪测量，我们表明，$1/f$频率噪声的振荡器的谱线的测量时间增加的原因都扩大和形状的变化。我们的研究结果表明，长期稳定的自旋力矩振荡器不能准确地预测模型的基础上的热（白）噪声源。
英文摘要：
We measure the power spectral density of frequency fluctuations in nanocontact spin torque oscillators over time scales up to 50 ms. We use a mixer to convert oscillator signals ranging from 10 GHz to 40 GHz into a band near 70 MHz before digitizing the time domain waveform. We analyze the waveform using both zero crossing time stamps and a sliding Fourier transform, discuss the different limitations and advantages of these two methods, and combine them to obtain a frequency noise spectrum spanning more than five decades of Fourier frequency $f$. For devices having a free layer consisting of either a single Ni$_{\text{}80}$Fe$_{\text{}20}$ layer or a Co/Ni multilayer we find a frequency noise spectrum that is white at large $f$ and varies as \emph{$1/f$} at small $f$. The crossover frequency ranges from $\approx\unit[10^{4}]{Hz}$ to $\approx\unit[10^{6}]{Hz}$ and the $1/f$ component is stronger in the multilayer devices. Through actual and simulated spectrum analyzer measurements, we show that $1/f$ frequency noise causes both broadening and a change in shape of the oscillator's spectral line as measurement time increases. Our results indicate that the long term stability of spin torque oscillators cannot be accurately predicted from models based on thermal (white) noise sources.
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