We investigate the effect of temperature on the thermal magnetic noise signal of magnetic nanoparticle (MNP) systems as models for non-interacting macrospins. An analytical expression for the amplitude of the fluctuations in a magnetic field is derived for the Brownian and the Néel fluctuation mechanisms and compared with numerical results at different temperatures. To experimentally validate our findings, magnetic noise spectra of two commercially available polydisperse MNP systems (Ferucarbotran and Perimag) were measured at different, biomedically relevant temperatures. A distinctive effect of temperature on the power spectral noise densities is measurable already for 5 K temperature differences and, within the bandwidth of the experiment, higher noise amplitudes are found for lower temperatures. However, a crossing of the spectra at higher frequencies is revealed in simulations so that the total fluctuation amplitude is conserved. These findings contribute to a profound understanding of temperature influences on MNP fluctuation and relaxation mechanisms.

中文翻译：

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温度对磁性纳米粒子系统热波动的影响

我们研究了温度对磁性纳米粒子 (MNP) 系统的热磁噪声信号的影响，作为非相互作用宏观自旋的模型。为布朗和内尔波动机制导出了磁场波动幅度的解析表达式，并与不同温度下的数值结果进行了比较。为了通过实验验证我们的发现，在不同的生物医学相关温度下测量了两种市售多分散 MNP 系统（Ferucarbotran 和 Perimag）的磁噪声谱。对于 5 K 温差，温度对功率谱噪声密度的显着影响已经可以测量，并且在实验的带宽内，发现较低温度的噪声幅度较高。然而，在模拟中揭示了较高频率的光谱交叉，因此总波动幅度得以守恒。这些发现有助于深入了解温度对 MNP 波动和弛豫机制的影响。