Dynamics and Noise in Individual Mesoscale Magnetic Particles

University Of Minnesota-Twin Cities, Minneapolis MN

Investigators

Abstract

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.