Dynamics and Noise in Individual Mesoscale Magnetic Particles

非技术摘要：在大多数情况下，噪音被认为是一种滋扰或问题，例如参与谈话或检测到信号。然而，一种被称为随机共振(SR)的现象是，在某些条件下，噪声使人们更容易听到对话或检测到信号。这一现象已被用于模拟大脑功能和医疗设备。为了充分发挥其潜力，我们需要使用模型系统来研究潜在的物理。首席研究员开发了这样一个模型系统，该模型系统由小至200个原子的磁性纳米/介观粒子组成。磁状态，即北极和南极(NS)的方向，可以通过连接到每个粒子的四根导线测量的电学特性来确定。颗粒非常小，以至于NS取向因热噪声而波动。理解当施加一个小的依赖时间的磁场时，NS方向是如何随时间变化的，这是基本理解SR的关键。这一基本理解为SR的更大应用和先前应用的扩展提供了基础。此外，纳米/介观粒子是与我们对基本噪声理论的理解有关的其他悬而未决的物理问题的理想模型系统。这项研究是由研究生和本科生进行的，作为他们在经济上重要的先进技术和物理的技术培训的一部分。技术摘要：这项研究探索了随机共振(SR)的基本模型和未经测试的属性，即1/f噪声是由随机电报噪声(RTN)振荡器的集合造成的。这两个漫长的物理问题都可以用首席调查员(PI)开发的技术来实现。这包括制造小到40 nm的单个磁性颗粒，并连接四个非磁性引线，用于各向异性磁阻的四个终端电阻测量。PI之前的研究已经测量了单个纳米/中尺度磁性颗粒的RTN。对于SR研究，显示RTN的单个点受到小交流磁场的影响。交流磁场不足以驱动磁化通过产生RTN的能量格局。然而，热噪声可以实现状态之间的转换，这是SR的核心。考察了磁化强度随交流场大小、温度、粒子势垒的变化情况，对SR模型进行了检验。1/f噪声来自一组不相关的RTN振荡器的假设通过链接显示RTN的各个点并测量链中和整个链中的各个点的噪声来验证。相关性对噪声的影响是通过减小点之间的间隔来测量的，这会增加点之间的磁相互作用。

Nontechnical abstract:Noise is considered to be a nuisance or problem in most situations such as a engaging in conversation or detecting a signal. However a phenomenon known as stochastic resonance (SR) is where noise makes it easier to hear a conversation or detect a signal under certain conditions. This phenomenon has been used to model brain function and in medical devices. To fully realize its potential, we need to investigate the underlying physics using a model system. The Principal Investigator has developed such a model system consisting of magnetic nano/mesoscale particles as small as 200 atoms on a side. The magnetic state, the orientation of the North and South poles (NS) can be determined by electrical properties measured by four wires attached to each particle. The particles are so small that the NS orientation fluctuates from thermal noise. Understanding how the NS directions change with time when a small time dependent magnetic field is applied is the missing key to a fundamental understanding of SR. This fundamental understanding provides the foundation for greater applications of SR and extensions of it previous applications. In addition the nano/mesoscale particles are an ideal model system for other outstanding physics questions related to our understanding of fundamental noise theory. The research is conducted by both graduate students and undergraduates as part of their technical training in economically important advanced technologies and physics.Technical abstract:The research explores both the fundamental model of stochastic resonance (SR) and the untested attribution that 1/f noise is due to a collection of random telegraph noise (RTN) oscillators. Both of these long physics questions are within reach with the technology developed by the Principal Investigator (PI). This consists of the manufacture of individual magnetic particles as small as 40nm with four nonmagnetic leads attached for four terminal resistance measurements of the anisotropic magnetoresistance. Previous research by the PI has measured RTN in individual nano/mesoscale magnetic particles. For the SR research a single dot exhibiting RTN is subjected to a small ac magnetic field. The ac field is not sufficient to drive the magnetization through the energy landscape giving rise to the RTN. The thermal noise, however, can enable the transition between states which is at the heart of SR. Exploring the response of the magnetization as functions of ac field magnitude, temperature, particle energy barriers tests the models of SR. The hypothesis that 1/f noise evolves from a collection of uncorrelated RTN oscillators is tested by chaining individual dots exhibiting RTN and measuring the noise of both the individual dots in the chain and the total chain. The effect of correlations on the noise is measured by decreasing the separation between the dots that increases their magnetic interactions.