We investigate the origin of the experimentally observed varying current-frequency nonlinearity of the propagating spin wave mode in nanocontact spin torque oscillators. Nominally identical devices with 100 nm diameter are characterized by electrical microwave measurements and show large variation in the generated frequency as a function of drive current. This quantitative and qualitative device-to-device variation is described in terms of continuous and discontinuous nonlinear transitions between linear current intervals. The thin-film grain microstructure in our samples is determined using atomic force and scanning electron microscopy to be on the scale of 30 nm. Micromagnetic simulations show that the reflection of spin waves against the grain boundaries results in standing wave resonance configurations. For a simulated device with a single artificial grain, the frequency increases linearly with the drive current until the decreased wavelength eventually forces another spin wave antinode to be formed. This transition results in a discontinuous step in the frequency versus current relation. Simulations of complete, randomly generated grain microstructures additionally shows continuous nonlinearity and a resulting device-to-device variation in frequency that is similar to the experimental levels. The impact of temperature from 4 to 300 K on the resonance mode-transition nonlinearity and frequency noise is investigated using simulations and it is found that the peak levels of the spectral linewidth as a function of drive current agree quantitatively with typical levels found in experiments at room temperature. The impact of the grain microstructure on the localized oscillation modes is also investigated.

中文翻译：

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晶粒内自旋波反射对纳米接触自旋扭矩振荡器的影响

我们研究了纳米接触自旋扭矩振荡器中传播自旋波模式的实验观察到的变化的电流频率非线性的起源。直径为 100 nm 的名义上相同的设备通过电微波测量进行表征，并且显示出作为驱动电流函数的生成频率的巨大变化。这种定量和定性的设备到设备变化是根据线性电流间隔之间的连续和不连续非线性转换来描述的。我们样品中的薄膜晶粒微观结构是使用原子力和扫描电子显微镜确定的，其尺度为 30 nm。微磁模拟表明自旋波对晶界的反射导致驻波共振配置。对于具有单个人造颗粒的模拟设备，频率随驱动电流线性增加，直到减小的波长最终迫使形成另一个自旋波波腹。这种转变导致频率与电流关系的不连续阶跃。完整的、随机生成的晶粒微观结构的模拟还显示出连续的非线性和由此产生的与实验水平相似的设备到设备的频率变化。使用仿真研究了 4 到 300 K 的温度对谐振模式转换非线性和频率噪声的影响，发现作为驱动电流函数的谱线宽的峰值水平与实验中发现的典型水平在数量上一致室内温度。