Isaac Corinne E, Gleave Christine M, Nasr Paméla T, Nguyen Hoang L, Curley Elizabeth A, Yoder Jonilyn L, Moore Eric W, Chen Lei, Marohn John A
Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853-1301, USA.
Phys Chem Chem Phys. 2016 Apr 7;18(13):8806-19. doi: 10.1039/c6cp00084c.
We report achieving enhanced nuclear magnetization in a magnetic resonance force microscope experiment at 0.6 tesla and 4.2 kelvin using the dynamic nuclear polarization (DNP) effect. In our experiments a microwire coplanar waveguide delivered radiowaves to excite nuclear spins and microwaves to excite electron spins in a 250 nm thick nitroxide-doped polystyrene sample. Both electron and proton spin resonance were observed as a change in the mechanical resonance frequency of a nearby cantilever having a micron-scale nickel tip. NMR signal, not observable from Curie-law magnetization at 0.6 T, became observable when microwave irradiation was applied to saturate the electron spins. The resulting NMR signal's size, buildup time, dependence on microwave power, and dependence on irradiation frequency was consistent with a transfer of magnetization from electron spins to nuclear spins. Due to the presence of an inhomogeneous magnetic field introduced by the cantilever's magnetic tip, the electron spins in the sample were saturated in a microwave-resonant slice 10's of nm thick. The spatial distribution of the nuclear polarization enhancement factor ε was mapped by varying the frequency of the applied radiowaves. The observed enhancement factor was zero for spins in the center of the resonant slice, was ε = +10 to +20 for spins proximal to the magnet, and was ε = -10 to -20 for spins distal to the magnet. We show that this bipolar nuclear magnetization profile is consistent with cross-effect DNP in a ∼10(5) T m(-1) magnetic field gradient. Potential challenges associated with generating and using DNP-enhanced nuclear magnetization in a nanometer-resolution magnetic resonance imaging experiment are elucidated and discussed.
我们报告了在0.6特斯拉和4.2开尔文的磁共振力显微镜实验中，利用动态核极化（DNP）效应实现了增强的核磁化。在我们的实验中，一根微线共面波导输送无线电波以激发核自旋，并输送微波以激发250纳米厚的氮氧化物掺杂聚苯乙烯样品中的电子自旋。电子和质子自旋共振均表现为附近带有微米级镍尖端的悬臂梁机械共振频率的变化。在0.6 T时，根据居里定律磁化无法观测到的核磁共振信号，在施加微波辐射使电子自旋饱和时变得可观测。所得核磁共振信号的大小、建立时间、对微波功率的依赖性以及对辐射频率的依赖性，均与磁化从电子自旋转移到核自旋的情况一致。由于悬臂梁磁性尖端引入的非均匀磁场的存在，样品中的电子自旋在一个10纳米厚的微波共振切片中被饱和。通过改变所施加无线电波的频率，绘制了核极化增强因子ε的空间分布。对于共振切片中心的自旋，观测到的增强因子为零；对于靠近磁体的自旋，ε = +10至 +20；对于远离磁体的自旋，ε = -10至 -20。我们表明，这种双极核磁化分布与在约10(5) T m(-1)磁场梯度中的交叉效应DNP一致。阐明并讨论了在纳米分辨率磁共振成像实验中产生和使用DNP增强的核磁化所面临的潜在挑战。
