Dougherty W M, Bruland K J, Chao S H, Garbini J L, Jensen S E, Sidles J A
Department of Mechanical Engineering, University of Washington, Seattle, Washington 98195, USA.
J Magn Reson. 2000 Mar;143(1):106-19. doi: 10.1006/jmre.1999.1994.
We report theory and observations of paramagnetic resonance in a measured field gradient of 44,000 T per meter by the technique of magnetic resonance force microscopy (MRFM). Resonance was induced in a dilute solid solution of diphenylpicrylhydrazyl in polystyrene at 77 and 10 K by an amplitude-modulated microwave field. This modulated the force between resonant sample spins and a micrometer-scale SmCo magnetic tip on a force microscope cantilever. The force signals were typically of order 10 fN, and were detected above a thermal noise floor of 80 aN per root hertz at 10 K, equivalent to a magnetic moment noise of 200 micro(B) per root hertz of bandwidth. Resonance saturation was readily observed. Starting with the Bloch equations, we derived simple analytic expressions for the predicted cantilever signal amplitudes and T(1)-dependent phase lags, valid at low microwave power levels. For power levels below saturation, the data were in good agreement with the Bloch equation predictions, while above saturation the measured force increased more slowly with power than predicted. Several ESR mechanisms which might lead to non-Bloch dynamics in the MRFM environment are reviewed. Spin-relaxation mechanisms are also reviewed. A detailed description of the experimental apparatus is offered.
我们通过磁共振力显微镜（MRFM）技术报告了在每米44,000 T的测量场梯度中的顺磁共振理论和观测结果。在77 K和10 K下，通过调幅微波场在聚苯乙烯中的二苯基苦味酰基肼稀固溶体中诱导共振。这调制了共振样品自旋与力显微镜悬臂上微米级SmCo磁尖端之间的力。力信号通常为10 fN量级，在10 K下，在每根号赫兹80 aN的热噪声本底之上被检测到，相当于每根号赫兹带宽200微玻尔磁子的磁矩噪声。共振饱和很容易被观察到。从布洛赫方程出发，我们推导出了预测悬臂信号幅度和与T(1)相关的相位滞后的简单解析表达式，这些表达式在低微波功率水平下有效。对于低于饱和的功率水平，数据与布洛赫方程预测结果吻合良好，而在饱和以上，测量到的力随功率的增加比预测的要慢。本文回顾了可能导致MRFM环境中出现非布洛赫动力学的几种电子自旋共振机制。自旋弛豫机制也进行了回顾。还提供了实验装置的详细描述。
