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

T. Hingant, J.-P. Tetienne, L. J. Martínez, K. Garcia, D. Ravelosona, J.-F. Roch, and V. Jacques

Phys. Rev. Applied 4, 014003 – Published 13 July 2015

Abstract

We present an approach to infer the surface density of magnetic moments $I_{s}$ 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 $I_{s}$ 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 $I_{s}$ induced by local irradiation with ${He}^{+}$ ions in an ultrathin ferromagnetic wire. This method offers a route to study variations of magnetic properties at the nanoscale.

Received 2 March 2015

DOI:https://doi.org/10.1103/PhysRevApplied.4.014003

Authors & Affiliations

T. Hingant¹, J.-P. Tetienne¹, L. J. Martínez¹, K. Garcia², D. Ravelosona², J.-F. Roch¹, and V. Jacques^1,*

¹Laboratoire Aimé Cotton, CNRS, Université Paris-Sud and ENS Cachan, 91405 Orsay, France
²Institut d’Electronique Fondamentale, Université Paris-Sud and CNRS UMR 8622, 91405 Orsay, France

^*Corresponding author. vjacques@ens-cachan.fr

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Vol. 4, Iss. 1 — July 2015

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Images

Figure 1
(a) Schematic view of the magnetization $M$ at the edge of a ferromagnetic material with PMA. The gray arrows indicate the resulting magnetic field lines. The perpendicularly magnetized film can be considered as two planes of opposite magnetic charges (red) located at $z = \pm (t / 2)$ . (b),(c) Magnetic field components $B_{x}^{ed} (x)$ and $B_{z}^{ed} (x)$ calculated at a distance $d = 100 nm$ using Eq. (1) for $M_{s} = 10^{6} A / m$ and $t = 1 nm$ , corresponding to $I_{s} = M_{s} t \approx 100 μ_{B} / {nm}^{2}$ , which is a typical value for the ferromagnetic samples considered in this work. The edge is placed at $x = 0$ .
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Figure 2
(a) Principle of the experiment. Magnetic-field measurements across a ferromagnetic wire (width $w$ ) are performed with a scanning-NV magnetometer operating in tapping mode. The inset indicates the quantization axis of the NV defect’s electronic spin $u_{NV}$ , characterized by the spherical angles $θ$ and $ϕ$ in the $(x y z)$ reference frame. (b),(c) Zeeman-shift profile measured by scanning the NV defect across a $1 − μ m$ -wide wire of $Ta − {Co}_{20} {Fe}_{60} B_{20} (1 nm) − MgO$ . The blue curve is the simultaneously recorded AFM topography of the sample. Two NV probes with different orientations $(θ, ϕ)$ are used in (b) and (c). The red solid lines are data fitting from which the parameters $(I_{s}, d)$ are extracted. The uncertainties, corresponding to 1 standard deviation, are evaluated by following the analysis described in the main text.
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Figure 3
(a) AFM image and (b) corresponding Zeeman-shift distribution recorded while scanning an NV center above a square dot etched in a $Ta − {Co}_{20} {Fe}_{60} B_{20}$ $(1 nm) − MgO$ film. Scale bar: 100 nm. (c) Line cut extracted from the white dashed line in (b) together with the simultaneously recorded topography of the sample (blue curve). The red solid line is data fitting, yielding $I_{s} = 98 \pm 3 μ_{B} / {nm}^{2}$ and $d = 54 \pm 3 nm$ . (d) Calculation of the full Zeeman-shift distribution above the square dot using these parameters. Scale bar: 100 nm.
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Figure 4
(a) Schematic drawing illustrating the decrease in magnetic moment density induced by local irradiation with ${He}^{+}$ ions. (b) AFM image of a $1 − μ m$ -wide wire of $Ta − {Co}_{40} {Fe}_{40} B_{20} (1 nm) − MgO$ . The irradiation region is indicated in yellow between the black dashed line. (c) Corresponding Zeeman-shift distribution recorded with scanning-NV magnetometry. The NV probe is characterized by $θ = 100 °$ and $ϕ = 48 °$ . Scale bar is 200 nm. (d) Line cuts extracted from the three white dashed lines in (c). The ratio between the field maxima above the edges directly indicates a relative decrease of $I_{s}$ by $\sim 40 %$ in the irradiated region. The solid lines are data fitting. (e) Calculation of the full Zeeman-shift distribution above the irradiated wire using the parameters extracted from the fit. Scale bar is 200 nm.
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Figure 5
(a) Scheme illustrating the shift between the measured AFM topography, hence the NV center’s trajectory $z (x) = d + topo (x)$ , and the actual sample’s topography, hence the position of the magnetic wire. (d) Example of $topo (x)$ function measured on a $Ta − {Co}_{20} {Fe}_{60} B_{20} − MgO$ wire. For this sample, the etching height is $δ d \approx 53 nm$ .
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