Measuring the Magnetic Moment Density in Patterned Ultrathin Ferromagnets with Submicrometer Resolution

Abstract

We present a new approach to infer the surface density of magnetic moments Is in ultrathin ferromagnetic films with perpendicular anisotropy. It relies on quantitative stray field measurements with an atomic-size magnetometer based on the nitrogen-vacancy center in diamond. The method is applied to microstructures patterned in a 1-nm-thick film of CoFeB. We report measurements of Is with a few percent uncertainty and a spatial resolution in the range of (100 nm) 2 , an improvement by several orders of magnitude over existing methods. As an example of application, we measure the modifications of Is induced by local irradiation with He + ions in an ultrathin ferromagnetic wire. This method offers a new route to study variations of magnetic properties at the nanoscale.Ultrathin ferromagnetic films with perpendicular magnetic anisotropy (PMA) have attracted considerable interest over the last years both for fundamental studies in nanomagnetism and for the development of a new generation of low power spintronic devices [1][2][3]. In this context, it is crucial to determine accurately the surface density of magnetic moments I s in such ultrathin ferromagnets, which can not be simply inferred from tabulated bulk values owing to significant interface effects [4]. To this end, macroscopic magnetometry methods like superconducting quantum interference devices (SQUIDs) or vibrating-sample magnetometers have become ubiquitous owing to simplicity of use. However, these conventional techniques are prone to parasitic magnetic signals, leading to an intrinsic background on the order of 10 . To overcome this background, the ferromagnetic signal needs to be averaged over a large sample, thus limiting drastically the spatial resolution of the measurement. As an example, we consider a ferromagnetic film with a typical thickness t = 1 nm and a saturation magnetization of M s = 10 6 A/m, corresponding to a surface density of magnetic moments I s = M s t ≈ 100 µ B /nm 2 . In order to reach a signal-to-background ratio of ∼ 10, the signal must be averaged over a surface larger than (1 mm) 2 . Different approaches have been used to tackle this issue. Notably, a recent improvement of the spatial resolution down to the order of (10 µm) 2 has been achieved by measuring the dipolar repulsion of magnetic domain walls with a magneto-optical Kerr microscope [9]. The use of micro-and even nano-SQUID also allows for a significant gain in sensitivity and resolution. However it remains difficult with such devices to determine accurately the link between magnetic flux and magnetization, and thus to extract precise values of I s [10].In this work, we use a single nitrogen-vacancy (NV) center in diamond as an atomic-size magnetometer to probe the stray field above ultrathin ferromagnets and infer I s on a scale smaller than (100 nm) 6 A/m and t = 1 nm, corresponding to Is = Mst ≈ 100 µB/nm 2 , which is a typical value for the ferromagnetic samples considered in this work. The edge is placed at x = 0. out applying any external magnetic field, which en...

Probing magnetism in 2D materials at the nanoscale with single-spin microscopy

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“…2, lower panels). Assuming a purely out-of-plane magnetisation, analytical fits [2] to these data allow for the quantitative determination of both σ z and z NV . Potential rotations of the magnetisation away from z in the vicinity of the edge due to, e.g.…”

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