Force-detected magnetic resonance in a field gradient of 250 000 Tesla per meter

Abstract

We report the detection of slice-selective electron spin resonance with an external magnetic field gradient comparable to local interatomic gradients, using the techniques of magnetic resonance force microscopy. An applied microwave field modulated the spin-gradient force between a paramagnetic DPPH sample and a micrometer-scale ferromagnetic tip on a force microscope cantilever. A sensitivity equivalent to 184 polarized electron moments in a one-Hertz detection bandwidth was attained. We mapped the tip magnetic field with a resonant slice thickness of order one nanometer, thereby demonstrating magnetic resonance on length scales comparable to molecular dimensions.

Advances in mechanical detection of magnetic resonance

“…47 Even the first demonstration of "magnet on tip" MRFM, though a remarkable development, employed a large enough magnetic tip that the gradient was not much larger than in early experiments. 48 By gluing magnets as small as 5 m to commercial silicon nitride cantilevers and operating the cantilevers within a few tens of nanometers of a surface, Bruland et al 49 were able to detect electron spin resonance using magnetic field gradients as large as 2.5ϫ 10 5 T / m. The sensitive slice in these experiments was only a nanometer thick, on the order of molecular dimensions. Remarkably, at a temperature of 77 K, Bruland et al achieved a magnetic moment sensitivity of ϳ200 e / ͱ Hz.…”

“…This effect was not present in the magneton-cantilever ESR experiment of Bruland et al because the polarizing field was zero and the tip field was used to polarize the sample. 49 In an NMR experiment, working near zero field would lead to unacceptably low Curie-law magnetization.…”

Nuclear magnetic resonance imaging with 90-nm resolution

“…To quantitatively characterize the magnetic tips, we performed nanometer-scale magnetometry by measuring the MRFM signal as a function of tip-sample distance and applied magnetic field 15,20 . The sample was a 100 nm-thick film of CaF 2 (99.99%) evaporated onto the end of a mass-loaded silicon cantilever ( Fig.…”

“…Unlike the permanent magnet tips previously used for MRFM detection of electron spin resonance 11,15 , the present tips are based on a thin film of magnetic material that has high magnetic moment, but is magnetically soft. In particular, we use tips fabricated with sputter-deposited Co 70 Fe 30 having magnetization µ 0 M = 2.3 T. This magnetization is somewhat higher than that of iron and more than twice as high as the permanent magnetic material SmCo 5 , which is commonly used for MRFM 11,15 .…”

“…In particular, we use tips fabricated with sputter-deposited Co 70 Fe 30 having magnetization µ 0 M = 2.3 T. This magnetization is somewhat higher than that of iron and more than twice as high as the permanent magnetic material SmCo 5 , which is commonly used for MRFM 11,15 . A soft magnetic material is suitable for nuclear spin MRFM because nuclear spin experiments are typically performed in a strong magnetic field, which conveniently magnetizes the tip.…”