Nanoscale magnetometry of a synthetic three-dimensional spin texture

TL;DR

This work tackles the challenge of non-invasively imaging and quantifying complex 3D spin textures in thick multilayer SAFs at the nanoscale. By leveraging NV-SPM as a quantitative vector-field magnetometer under ambient conditions, the authors map static and dynamic spin textures with multiple NV orientations to separate field components and detect GHz-range magnetic noise via T1 relaxometry. They reveal a 3D spin texture featuring ferromagnetic cores around wiggles in domain walls and demonstrate nanoscale domain-wall shifts, supported by quantitative micromagnetic simulations. The approach provides a powerful, non-invasive pathway to study interlayer coupling, spin-wave modes, and DW stability in layered magnetic materials, with implications for advanced 3D magnetic architectures.

Abstract

Multilayered synthetic antiferromagnets (SAFs) are artificial three-dimensional (3D) architectures engineered to create novel, complex, and stable spin textures. Non-invasive and quantitative nanoscale magnetic imaging of the two-dimensional stray field profile at the sample surface is essential for understanding the fundamental properties of the spin-structure and being able to tailor them to achieve new functionalities. However, the deterministic detection of spin textures and their quantitative characterization on the nanoscale remain challenging. Here, we use nitrogen-vacancy scanning probe microscopy (NV-SPM) under ambient conditions to perform the first quantitative vector-field magnetometry measurements in the multilayered SAF [(Co/Pt)$_5$/Co/Ru]$_3$/(Co/Pt)$_6$. We investigate nanoscale static and dynamic properties of antiferromagnetic domains with boundaries hosting ``one-dimensional'' ferromagnetic stripes with ~ 100 nm of width and periodic modulation of the magnetization. By employing NV-SPM measurements in different imaging modes and involving NV-probes with various crystallographic orientations, we demonstrated distinct fingerprints emerging from GHz-range spin noise and constant stray fields on the order of several mT. This provides quantitative insights into the structure of domains and domain walls, as well as, into magnetic noise associated with thermal spin-waves. Our work opens up new opportunities for quantitative vector-field magnetometry of modern magnetic materials with tailored 3D spin textures and stray field profiles, and potentially novel spin-wave dispersions--in a quantitative and non-invasive manner, with exceptional magnetic sensitivity and nanometer scale spatial resolution.

Figures (5)

• Figure 1: Magnetic imaging of a multilayered synthetic antiferromagnet. (a) A single NV center in a scanning diamond probe is used for sensing the magnetic field and noise of the sample at a given NV-sample distance $d_{\text{NV}}$. The NV spin state is optically initialized with a green laser (515 nm) and read out by detecting the NV-PL signal (650-850 nm) after interaction with the sample. Qualitative magnetic imaging is possible via MW-free NV-PL maps with and without applying external magnetic field. For ODMR measurements, MW excitation in the GHz range is required to promote the electron spin transition. The sample stray field and noise are sensed along and perpendicular to the NV-axis, respectively. The angles $(\theta_{\text{NV}},\varphi_{\text{NV}})$ define the orientation of the NV-axis in the laboratory frame. (b) Schematic structure of the multilayered SAF [[Co(0.55 nm)/Pt(0.70 nm)]$_{5}$/Co(0.55 nm)/Ru(0.75 nm)]$_{3}$/[Co(0.55 nm)/Pt(0.70 nm)]$_{6}$. (c) 6 $\mu$m x 6 $\mu$m MFM image of the multilayered SAF. (d) 6 $\mu$m x 6 $\mu$m NV-PL quenching map recorded with a (100)-oriented NV probe and without applying external magnetic field, pixel size 24 nm/px and acquisition time 24 ms/px. The intensity is normalized with respect to the average counts on the AF domains (bright areas).
• Figure 2: Effects of an external off-axis magnetic field on the contrast of NV-PL maps. (a) Schematic illustration of the relative orientation between the (100)-oriented NV probe and the sample magnetization during NV-SPM measurements. The direction of the NV-axis is determined by the coordinates $(\theta_{\text{NV}},\varphi_{\text{NV}}) = (113\degree,270\degree )$ in the laboratory frame. (b) 3 $\mu$m x 3 $\mu$m NV-PL quenching map recorded in presence of an external off-axis field $B_{\text{ext}}$ = 20 mT applied along $(\theta_{\text{ext}},\varphi_{\text{ext}}) = (54.8\degree,180\degree )$. Pixel size 100 nm/px and acquisition time 25 ms/px. (c) High resolution NV-PL quenching maps (500 nm x 720 nm) on a FM DW recorded at different value for the external field. Pixel size 4 nm/px and acquisition time 25 ms/px. (d) Normalized and vertically shifted ODMR spectra measured on the AF domain and FM DW without external field.
• Figure 3: Resolving the internal structure of the FM stripe. (a) Schematic illustration of the relative orientation between the NV-axis of the (111)-oriented NV probe and sample magnetization during NV-SPM measurements. The direction of the NV-axis is determined by the coordinates $(\theta_{\text{NV}},\varphi_{\text{NV}}) = (180\degree,0\degree )$ in the laboratory frame. (b) NV-PL quenching map recorded without external field. (c) Normalized and vertically shifted ODMR spectra measured on the AF domain and FM DW. (d)-(f) Dual Iso-B contour images. The size, pixel size and acquisition time for all the images shown in this figure are 1.5 $\mu$m x 1.0 $\mu$m, 4 nm/px and 20 ms/px, respectively.
• Figure 4: Stray field maps, DWs and FM cores. (a) Stray field map of the component projected along the NV-axis, $B_{z}= - B_{\text{NV}}$, pixel size of 3.5 nm/px and acquisition time of 100 ms/px. Reconstructed stray field map of the components along (b) the x-axis and (c) the y-axis. Micromagnetic simulated images of the stray field map produced by a square AF domain along (d) the z-axis, (e) the x-axis and (f) the y-axis. Field values are calculated at 75 nm from the sample surface. (g) Profiles of the OOP and IP components of the measured stray fields across the DW. (h) Overlap of the simulated remanent OOP magnetization of the top and second FM blocks to visualize the alternating left and right DW-shift that give rise to the FM cores around the DW with magnetic moments pointing up and down. The inset figure illustrates the 3D magnetic texture of the multilayered structure.
• Figure 5: $T_1$ relaxometry and magnetic noise (a) Pulse sequence for all-optical $T_1$ measurements. A green laser pulse of 3 $\mu$s is sent to initially polarize the NV spin state in $m_s = 0$, after some time $\tau$ a second laser pulse is used for reading out the final NV spin state. The signal is read out during a time window of 200 ns (between the first 200-400 ns) of the second pulse and normalized by the signal during the last $\mu$s (reference signal). (b) $T_1$ measurements performed on a (111)-NV probe with the tip withdrawn from the sample, and after positioning the tip on the AF domain and FM wall. (c) Comparison of $T_1$ measurements on diamond probes with different crystallographic orientations.