The Effect of Temperature Variations in Thermal Noise Magnetometry

Abstract

Magnetic nanoparticles are powerful tools in biomedical applications, where they are employed in both diagnosis and therapy. A prerequisite for the efficiency of these applications are precisely characterized particles. Several magnetic measurement techniques are established, which all have in common that they measure the magnetic response of the particles exposed to an externally applied magnetic field, which may change the magnetic state of the particles [1]. Therefore, Thermal Noise Magnetometry (TNM) has been developed, a magnetic nanoparticle characterization technique that does not rely on an external magnetic excitation [2]. It detects amplitude variations in the net magnetic moment of the sample caused by thermal fluctuations. TNM measurements have been proven to be feasible, and complementary to other characterization techniques due to their diminutive impact on the sample [2, 3].Two mechanisms are responsible for the directional fluctuations of the magnetic moments. Brownian fluctuations occur if the particles are free to move in the sample, i.e. when they are suspended in a solution. The hydrodynamic size of the particles and the viscosity of the fluid influence the Brownian fluctuation rate. In parallel, the magnetization of each particle can also switch in the frame of the particles themselves, called Néel switching. The Néel switching rate is determined by the volume of the particles’ magnetic core and anisotropy constant.Additionally, the timescales of both mechanisms also depend on the temperature since the fluctuations are induced by the thermal energy in system. TNM experiments are typically performed at room temperature, but the particles are characterized for biomedical applications at body temperature. From preliminary results, a considerable effect of temperature variations in TNM is visible, influencing the particles’ fluctuation rate distributions and noise spectrum. As expected from the theory, the fluctuation rates shift towards higher values for higher temperatures, which leads to a decrease in the noise power in the considered frequency range.  Power spectrum densities for Perimag® particles at different temperatures.