Direct measurement of interfacial Dzyaloshinskii-Moriya interaction in $X|\text{CoFeB}|\text{MgO}$ heterostructures with a scanning NV magnetometer $(X=\text{Ta},\text{TaN}, \text{and W})$

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Direct measurement of interfacial Dzyaloshinskii-Moriya interaction in $X | CoFeB | MgO$ heterostructures with a scanning NV magnetometer $(X = Ta, TaN, and W)$

I. Gross, L. J. Martínez, J.-P. Tetienne, T. Hingant, J.-F. Roch, K. Garcia, R. Soucaille, J. P. Adam, J.-V. Kim, S. Rohart, A. Thiaville, J. Torrejon, M. Hayashi, and V. Jacques

Phys. Rev. B 94, 064413 – Published 11 August 2016

Abstract

The Dzyaloshinskii-Moriya interaction (DMI) has recently attracted considerable interest owing to its fundamental role in the stabilization of chiral spin textures in ultrathin ferromagnets, which are interesting candidates for future spintronic technologies. Here we employ a scanning nanomagnetometer based on a single nitrogen-vacancy defect in diamond to locally probe the strength of the interfacial DMI in $CoFeB | MgO$ ultrathin films grown on different heavy metal underlayers $X = Ta$ , TaN, and W. By measuring the stray field emanating from domain walls in micron-long wires of such materials, we observe deviations from the Bloch profile for TaN and W underlayers that are consistent with a positive DMI value favoring right-handed chiral spin structures. Moreover, our measurements suggest that the DMI constant might vary locally within a single sample, illustrating the importance of local probes for the study of magnetic order at the nanoscale.

Received 23 May 2016

DOI:https://doi.org/10.1103/PhysRevB.94.064413

Physics Subject Headings (PhySH)

Domain walls Dzyaloshinskii-Moriya interaction Spin texture

Ferromagnets Magnetic multilayers Nitrogen vacancy centers in diamond Ultrathin films

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

I. Gross^1,2, L. J. Martínez¹, J.-P. Tetienne², T. Hingant², J.-F. Roch², K. Garcia³, R. Soucaille³, J. P. Adam³, J.-V. Kim³, S. Rohart⁴, A. Thiaville⁴, J. Torrejon⁵, M. Hayashi⁵, and V. Jacques^1,*

¹Laboratoire Charles Coulomb, Université de Montpellier and CNRS, 34095 Montpellier, France
²Laboratoire Aimé Cotton, CNRS, Université Paris-Sud, ENS Cachan, Université Paris-Saclay, 91405 Orsay Cedex, France
³Institut d'Electronique Fondamentale, CNRS, Université Paris-Sud, Université Paris-Saclay, 91405 Orsay, France
⁴Laboratoire de Physique des Solides, CNRS, Université Paris-Sud, Université Paris-Saclay, 91405 Orsay, France
⁵National Institute for Materials Science, Tsukuba 305-0047, Japan

^*vincent.jacques@umontpellier.fr

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Vol. 94, Iss. 6 — 1 August 2016

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Figure 1
Principle of the experiment. (a) A scanning NV magnetometer is used to measure the stray field $B^{ψ}$ produced by a DW in a perpendicularly magnetized ferromagnetic wire (black arrows). The bottom panels show top views of the magnetization for various DW structures, characterized by the angle $ψ$ between the in-plane magnetization (red arrows) and the $x$ axis. (b) Stray field component $B_{x}^{ψ} (x)$ calculated for different values of the DMI strength, corresponding to different $ψ$ angles. The calculation is performed at a distance $d = 100$ nm from a magnetic layer with thickness $t = 1$ nm, saturation magnetization $M_{s} = 1$ MA/m, and DW width $Δ = 20$ nm. With these parameters $D_{c} = 0.17$ $mJ / m^{2}$ . (c) Scanning electron microscope image of the sample used in this work, showing magnetic microwires and the gold stripline (yellow color) used both for DW nucleation and as a microwave antenna for scanning NV magnetometry.
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Figure 2
Ta as heavy metal underlayer. (a) Sketch of sample A. (b) AFM image and (c) corresponding Zeeman-shift distribution recorded by scanning the NV magnetometer above a DW isolated in a $1.5 − μ m$ -wide wire of sample A. Scale bar: 500 nm. (d) Linecut extracted from the white dashed line in (c). The markers are experimental data and the solid lines are the theoretical predictions for a Bloch (red), a Néel left (blue), and a Néel right (green) DW structure. The shaded areas include all the uncertainties in the theoretical predictions, which are dominated by uncertainties on the exchange constant $A$ (see main text). The quantization axis of the NV defect $u_{NV}$ is characterized by the spherical angles $(θ = 62^{\circ}, ϕ = - 25^{\circ})$ in the laboratory frame of reference $(x, y, z)$ , the probe-to-sample distance is $d = 123 \pm 3$ nm, and $M_{s} t = 930 \pm 30 μ A$ . We note that the Zeeman shift does not fall to zero far for the DW, because of the transverse zero-field splitting parameter of the NV defect $E$ [see Eq. (6)].
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Figure 3
Effect of nitrogen doping of the Ta underlayer. (a) Sketch of sample B. (b) Zeeman-shift distribution recorded by scanning the NV magnetometer above a DW isolated in a $1 − μ m − wide$ wire of sample B. Scale bar: 500 nm. (c) Linecut extracted from the white dashed line in (b). The data (markers) are well reproduced by a DW structure with $ψ = 75^{\circ}$ (black solid line). The quantization axis of the NV defect $u_{NV}$ is characterized by the spherical angles $(θ = 117^{\circ}, ϕ = 12^{\circ})$ in the laboratory frame of reference $(x, y, z)$ , the probe-to-sample distance is $d = 61 \pm 6$ nm, and $M_{s} t = 800 \pm 80 μ A$ . (d) Sketch of sample C with a 1-nm-thick TaN underlayer. (e) Zeeman-shift distribution above a DW isolated in a $1 − μ m − wide$ wire of sample C. Scale bar: 500 nm. (f) Linecut extracted from the white dashed line in (e). The probe-to-sample distance is $d = 80 \pm 5$ nm and $M_{s} t = 1040 \pm 50 μ A$ . In (c) and (f), the shaded areas include all the uncertainties in the theoretical predictions.
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Figure 4
W as heavy metal underlayer. (a) Sketch of sample D. (b)–(d) Magnetic field distributions recorded above three different DWs nucleated in 1- $μ m − wide$ wire of sample D. Scale bar: 500 nm. The linecuts are extracted from the white dashed line in (b), (c), and (d). The shaded areas include all the uncertainties in the theoretical predictions. The quantization axis of the NV defect $u_{NV}$ is characterized by the spherical angles $(θ = 110^{\circ}, ϕ = 12^{\circ})$ in the laboratory frame of reference $(x, y, z)$ , the probe-to-sample distance is $d = 114 \pm 10$ nm, and $M_{s} t = 820 \pm 100 μ A$ . We note that the tilt angle of the DW is taken into account in the theoretical predictions [34].
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Physical Review B

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Direct measurement of interfacial Dzyaloshinskii-Moriya interaction in $X | CoFeB | MgO$ heterostructures with a scanning NV magnetometer $(X = Ta, TaN, and W)$

I. Gross, L. J. Martínez, J.-P. Tetienne, T. Hingant, J.-F. Roch, K. Garcia, R. Soucaille, J. P. Adam, J.-V. Kim, S. Rohart, A. Thiaville, J. Torrejon, M. Hayashi, and V. Jacques

Phys. Rev. B 94, 064413 – Published 11 August 2016

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Physical Review B

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Direct measurement of interfacial Dzyaloshinskii-Moriya interaction in X|CoFeB|MgO heterostructures with a scanning NV magnetometer (X=Ta,TaN, and W)

I. Gross, L. J. Martínez, J.-P. Tetienne, T. Hingant, J.-F. Roch, K. Garcia, R. Soucaille, J. P. Adam, J.-V. Kim, S. Rohart, A. Thiaville, J. Torrejon, M. Hayashi, and V. Jacques

Phys. Rev. B 94, 064413 – Published 11 August 2016

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Direct measurement of interfacial Dzyaloshinskii-Moriya interaction in $X | CoFeB | MgO$ heterostructures with a scanning NV magnetometer $(X = Ta, TaN, and W)$